摘要:

Russian Chemical Reviews 71 (1) 1 ± 31 (2002) Molecular polymer ± polymer compositions. Synthetic aspects A D Pomogailo Contents I. Introduction II. Modern approaches to the description of the properties of polymer ± polymer compositions III. In situ preparation of polymer ± polymer compositions in the catalytic polymerisation of olefins IV. Synthesis of polar ingredients in the presence of polyolefins V. On some synthetic methods for preparation of polyolefin ± polyolefin compositions VI. Improvement of compatibility of polymeric components by introduction of compatibilisers VII. Polyolefin-based polymer blends prepared by reactive blending VIII. In situ preparation of diblock chemical hybrids IX. Preparation of conducting composites using the principle `compatibilisation via polymerisation' X.Formation mechanism and the structure of in situ prepared polymer ± polymer compositions XI. Fields of application of in situ prepared polymer ± polymer compositions XII. Conclusion Abstract. of compatibility the improving of ways main The The main ways of improving the compatibility of polymer as polyolefins containing compositions polymer ± polymer ± polymer compositions containing polyolefins as one one component to given is attention Considerable considered. are component are considered. Considerable attention is given to the the preparation ingredient an of presence the in polyolefins of preparation of polyolefins in the presence of an ingredient bound bound to consecutive) (including catalytic the in catalyst the to the catalyst in the catalytic (including consecutive) polymer- polymer- isation the and blends reactor so-called The olefins.of isation of olefins. The so-called reactor blends and the synthesis synthesis and are composites conducting and diblock of structure and structure of diblock and conducting composites are described. described. The of polymers of compatibilisation of role important The important role of compatibilisation of polymers of different different nature out. pointed is component third a of use the with nature with the use of a third component is pointed out. The The formation the compositions, molecular of mechanism formation mechanism of molecular compositions, the structure structure and fields main the and layer intermediate the of properties and properties of the intermediate layer and the main fields of of application compositions polymer ± polyolefin synthetic of application of synthetic polyolefin ± polymer compositions are are discussed.references 335 includes bibliography The discussed. The bibliography includes 335 references. I. Introduction In recent decades, the assortment of composite materials pro- duced from industrially important polymer blends or alloys has much expanded. Blending of two or more thermoplastics, an intricate physicochemical process which proceeds under the action of mechanical forces and high temperatures, is an efficient method for the preparation of novel materials with desired properties. Most pairs of polymers are incompatible; however, targeted modification of their morphology allows preparation of polymer blends with reasonable operating characteristics.Molecular design of multicomponent polymeric systems is a rather complicated problem. That is why communications con- cerning various physicochemical, structural and other aspects of this problem have been reported only recently.1±13 One of the reasons preventing direct blending of polyolefins (PO) with engineering plastics is the non-polar character of the PO. In this A D Pomogailo Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation. Fax (7-095) 515 35 88. Tel. (7-096) 524 50 20. E-mail: adpomog@icp.ac.ru Received 23 May 2001 Uspekhi Khimii 71 (1) 5 ± 38 (2002); translated by A M Raevsky #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n01ABEH000681 1249 10 13 16 18 20 22 25 26 connection design of multicomponent thermoplastic composite materials requires that considerable attention be given to the structural and chemical modification of ingredients (compounds).To achieve compatibility means to minimise the interfacial stress and to improve the interfacial adhesion. This allows the prepara- tion of materials with good deformation ± strength characteristics, high impact viscosity, thermal stability, are readily reprocessed, etc., without essential modification of their structure. In partic- ular, improvement of the operating characteristics (e.g., mechan- ical strength, crack resistance, light stability, colorability and printing sensitivity, reduction of electrifiablness and sweating, etc.) of PO and, first of all, low-density polyethylene (LDPE) and polypropylene (PP) requires that they be modified with various additives including those of high-molecular-mass.Modification of the PO components of polymer ± polymer compositions (PPC) is performed in both the formation (copolymerisation) stage by post-modification (graft polymerisation, modification of the sur- face, etc.) and immediately during the preparation of PPC (the so- called reactor blends or in situ preparation of PPC). Such modified PO act as compatibilisers (interfacial agents) and improve the compatibility of ingredients. Yet another reason for increased interest in PPC is the fact that these types of polymer blends are often formed as processing waste and cheap, degraded polymeric products can be re- used.14 ± 16 For both economic and environmental safety reasons, the volume of utilised polymer blends and thermoplastic alloys has been substantially increased since the early 1980s.Currently, requirements imposed on the environmental safety of processing and consumption of polymeric products are continuously increas- ing. This gives an impetus to researchers to place the problems of recycling of such materials as PO (60% of the total volume of polymeric waste), polystyrene (PS), poly(vinyl chloride) (PVC), poly(methyl methacrylate) (PMMA), poly(ethylene terephtha- late) (PETP), poly(tetrafluoroethylene) (PTFE), polyurethane (PU), polyamide (PA), rubber, cross-linked polyethers, cellulose- containing and multilayered materials as a focus of their efforts.After development of efficient methods for separation, reprocess- ing and/or modification, these materials would be a cheap and important source of polymeric raw materials, since the countries with well-developed economies produce 20 vol.%, or 8 mass %,2of nearly 200 million tons of solid household waste per year. However, different compositions of such complex raw materials are responsible for the formation of some incompatible ingre- dients in the recycling. This causes a deterioration in the physical properties of the end products and the materials become brittle.Moreover, some environmentally hazardous components of poly- mer blends favour the destruction of other blend components (in particular, PVC causes destruction of PETP and other polymers). Recycling and, especially, sorting expenditures aimed at separat- ing raw materials into individual, compatible types of polymers lead to a substantial increase in the cost : property ratio compared to that for the processes based on the use of primary polymer blends. Different types of plastics, e.g., thermoplastics, thermoset- ting plastics, multilayered blend-based composite materials, etc., require a different approach. A pair of polymers is considered compatible if its components have similar polarities, have similar functional groups or can interact under blending conditions to form donor ± acceptor, hydrogen and other types of bonds, as well as if they are structurally similar and have similar solubilities.However, meet- ing these conditions is sometimes insufficient, so the number of compatible polymers is small. This especially concerns such non- polar polymers as PO. For instance, LDPE and high-density polyethylene (HDPE) are poorly compatible; co-crystallisation of linear low-density polyethylene (LLDPE) and LDPE results in the formation of separate crystal aggregates on the micrometre level;17 and PE and PP are immiscible on the molecular level. Moreover, even such a pair as isotactic and syndiotactic PP is also incompatible on the molecular level.Therefore, when analysing polymer compatibility, one should take into account not only thermodynamic, but also chemical aspects, namely, the interaction between macromolecules. There are two approaches to the design of compatible polymer ± polymer systems.4 These are (i) linking of macromolecules by chemical bonds (the synthesis of block copolymers, interpenetrating poly- mer networks, cross-linking of blend components) and (ii) mod- ification of the chemical structure of polymers to obtain negative free energy of mixing (if polymer macromolecules contain func- tional groups that can strongly interact, some of the components are either modified, thus changing the chemical structure of the monomer units, or copolymerised).Thermodynamic incompati- bility of polymer blend components prevents the formation of a common crystal lattice (i.e., no mixed crystals are formed). Nevertheless, an intermediate, interfacial layer between the ingre- dients of a PPC can exist in this case due to the formation of common supramolecular structures, specific packing of the poly- mer chains and the absence of well-defined interfaces between the elements of supramolecular order even for crystalline polymers. This to a great extent determines the strength and nature of adhesive forces in the system, which strongly affects the surface area of true interface (phase contact), microrheological processes and contact reactions at the interface. Specific features of the adhesive interactions also depend on the preparation conditions of PPC: e.g., mechanical mixing of components leads to the formation of intermediate layers only as a result of physical interparticle interaction.A way to design novel materials with improved properties involves the preparation of microheterogeneous compositions with controllable structural inhomogeneity. This class of materi- als comprises compositions containing PO as one of the ingre- dients. The results of economic and environmental analysis 16 of the `life cycle' of polymers, which includes their synthesis, proc- essing and modification, usage, waste separation, recycling, re-use and final utilisation of waste allowed one to rank (i) PO first among the widely used plastics and (ii) PP first among all PO.This choice is also determined by the variety of PO-based polymer blends, alloys and composites (including the in situ prepared compositions). This review concerns main approaches to the PO-containing PPC. Particular emphasis is placed on the synthetic aspects of the A D Pomogailo problem. Often, though not always, the use of such systems allows compatibility of components on the molecular level. The defini- tion of such compositions as `molecular compositions' (see the title of this review) should be considered to some extent conven- tionally since the preparation of PPC involves blending of components not only on the molecular level, but also on the level of microphase aggregates consisting of a large number of like macromolecules.In addition, sometimes it is impossible to clearly distinguish between different procedures for the preparation of PPC because many of them can be formed following different mechanisms. This also makes formal classification of PPC diffi- cult. II. Modern approaches to the description of the properties of polymer ± polymer compositions To gain an insight into specific features of PPC-containing systems, we have to briefly outline the methods for analysis of their thermodynamic and physicomechanical properties. It does not always happen that the use of simple physical polymer mixtures consisting of components with complementary properties leads to desired results. The reason is thermodynamic incompatibility of most polymer pairs, which is largely because of a small entropy of mixing.Therefore, all polymer compositions to some extent undergo separation into different phases that are characterised by weak adhesion, which deteriorates the properties of the compositions. Usually, `compatibility' of polymers refers to their mutual solubility. In the case of polymer blends, the term `miscibility' is most often used. A necessary, but not sufficient, condition for thermodynamic compatibility of polymers is neg- ative free energy of mixing, Gm: DGm=DHm7TDSm . Due to the high molecular mass of polymers, the entropy of mixing of two macromolecules, DSm, changes only slightly. Hence, compatibility of a pair of polymers requires that the enthalpy of mixing, DHm, be negative, zero or very slightly positive.In addition, specific interactions between miscible poly- mers must occur (see above). Methods for studying and improving the thermodynamic compatibility of polymers have been reviewed by Tager and Blinov.18 According to their concept, the best miscibility is characteristic of those polymers whose molecules contain func- tional groups with opposite functionalities, capable of forming hydrogen, donor ± acceptor, ionic, p-bonds and other types of bonds, rather than of structurally (or compositionally) similar polymers. The following simple equation for calculating the free energy of mixing of polymers was proposed by Tager:19 7Dgx=DGIII7(oIDGI+oIIDGII), where Dgx is the mean free energy of mixing of polymers with each other per gramme of the blend; DGI, DGII and DGIII are the Gibbs energies of mixing of polymers I, II and their blend dissolved in the same solvent, respectively; and oI and oII are the mass fractions of polymers I and II in the PPC.Compatibility of polymers is strongly affected by not only the molecular nature and molecular masses of the components, but also their molecular mass distri- bution.20 However, often, `compatibility' of polymers means the achievement of good mechanical properties rather than compat- ibility on the thermodynamic level. Keeping this in mind, it should be noted that practically even compatible polymers can appear to be immiscible due to problems arising during the preparation of PPC.There are three main methods for polymer blending. They include mechanical mixing, film casting from common (the same) solvent and in situ blending, i.e., polymerisation of a monomer in the presence of the other (most often, preliminarily swollen) polymer. Each of these methods has specific advantages and drawbacks. For instance, the simplest method, mechanical mix- ing, cannot always be realised due to limited thermal stability of aMolecular polymer ¡¾ polymer compositions. Synthetic aspects particular ingredient in specified temperature and homogenisa- tion time ranges. Other polymers, being compatible at room temperature, can be incompatible at higher temperatures. Some- times, casting from common (the same) solvent is precluded by solubility problems and by immiscibility of particular polymers in the presence of specific solvents.Often, the only way of over- coming these obstacles is to use in situ polymerisation. However, this is also not a universal procedure since during the polymer- isation one monomer can become a solvent for the other polymer and this can induce phase separation of the components. For instance, polymerisation of vinyl chloride in the presence of chlorinated PE can result in monophase or two-phase products depending on the initial reagent ratio.21 Nevertheless, in situ polymerisation is considered as a potentially promising industrial method for the preparation of polymer blends. Numerous PPC prepared in this manner have been patented.This method allows the preparation of thermodynamically stable PPC; however, they can become unstable on heating, which means that such PPC have lower critical solution temperatures (LCST) and, hence, should be prepared at temperatures below LCST. Considerable attention has been given to the problems of design of PPC from ingredients with complementary properties. Unfortunately, no general equations for the description of the dependence of a particular property of a PPC on its chemical composition have been proposed so far. For instance, the problem of calculations of the modulus of elasticity, EPPC, for a polymer blend from the known moduli of elasticity of particular compo- nents or phases is still to be solved.22, 23 The moduli of two-phase blends are often estimated using the Coran ¡¾ Patel approach 24 based on the assumption that the magnitudes of the moduli of elasticity of a two-phase polymer blend (Young's modulus or shear modulus) must lie between the effective upper (Eu) and low (El ) moduli calculated from the models of parallel and series linking of the components.The moduli Eu and El are calculated as follows: Eu=j1E1+j2E2 , ¢§1, El= E2 j1 a j2 E1 where E1 and E2 are the moduli of elasticity of the starting components 1 and 2 and j1 and j2 are their volume fractions (j1+j2=1). The modulus of the two-phase blend, E, can be written in the form E=fEu + (17f )El , where 0<f<1. The maximum and minimum value of the modulus of elasticity corresponds to f=1 and f=0, respectively.Considering PPC as specific filled systems, one can use an approach known as the mean field approximation for spherulites in the matrix:25 Ec Em a O1 a njUE0 O1 ¢§ jUE 0m a On a jUEr , m a nO1 ¢§ jUEr where Ec , Em , Er are the moduli of the composite, matrix and an elastic inclusion with the volume fraction j, E0m is the real part of the elastic modulus of the matrix, n*=2(47nm)/(775nm) and nm is the Poisson coefficient of the matrix. This expression imposes no limitations on both the volume fractions of ingredients and the ratio of the modulus of elasticity of the matrix to that of the inclusion. If the moduli of elasticity of the matrix (Em) and inclusion are substantially different, it is appropriate to use the simplified Guth relationship which takes into account the pair interaction between rigid particles:26 EPPC=Em(1+2.5j+14.1j2). At j<0.4, the results of calculations using this formula coincide with experimental data.3 There are many equations derived based on different models of matrix compositions (e.g., the Smallwood and Takayanagi equations, etc.); however, they are rarely used. All of them use the volume fraction of a component, j, as the only structural parameter, though the experimental data show that the physico- mechanical properties of PPC are also affected by salient features of their morphology. For instance, the properties of ultrahigh- molecular-mass PE (UMMPE) depend not only on the volume fraction of crystallites, but also the ratio of the fractions of crystallites with compacted and aligned chains and on the type of mutual arrangement of these crystallites.27 Therefore, PPC should be described using such molecular models that take into account additional morphological parame- ters. For instance, if a model implies an interaction between components 1 and 2,28 we get EPPC=E1+E2(17j1)+DE.The sign preceding DE points to the presence (a `plus' sign) or the absence (a `minus' sign) of an interaction. For instance, the modulus of elasticity of a PP ¡¾HDPE composition can be larger than those of the individual components at a particular compo- nent ratio and at rapid cooling of melt. In other words, the addition of the first ingredient improves the properties of the second (this is also called positive synergism of the polymeric constituents).Recently, the elastic properties of partially crystalline material were studied theoretically based on the scaling concept and very good agreement with experimental data was achieved (see, e.g., Ref. 29). If the glass transition temperature, Tg, of the first component of a PPC approaches that of the second ingredient of the PPC, these polymers are partially compatible.30 The degree of compat- ibility, D, is defined as the ratio of the difference between the Tg values of the blend components to the difference between the Tg values of pure polymers:31 D a T bg1 ¢§ T bg2, 0<D<1, T pg1 ¢§ T pg2 where Tbg and T pg is the glass transition temperature of the blend component and of the pure polymer, respectively.The glass transition temperature of the PPC of compatible polymers is intermediate between the Tg values of the blend components and is determined using the Fox equation (the so-called blend rule) 32 1 Tg a j1 a j2 . Tg2 Tg1 Large deviations from the Fox equation are often considered as an indication of strong interpolymer interaction.33 According to the Hoffmann equation,34 the melting temper- T 1 ¢§ 1 Tm , d m a 1d Tc a ature of a non-isothermally crystallised PPC (Tm) increases linearly as the crystallisation temperature of the ingredients (Tc) increases where d is the crystallite morphological factor and T m is the equilibrium melting temperature.For instance, a PPC consisting of poly(but-1-ene) (PB) and hydrogenated oligo(cyclopentadiene) (HOCP) has a 1/d value of *0.34, while the ternary PP ¡¾ PB ¡¾HOCP blend (50 : 30 : 20) { is characterised by Tm= 456 K.35 Szabo et al.36 showed that the interpolymer interaction between such blend components as poly(isobutylene) (PIB) and linear LDPE obtained by copolymerisation of ethylene with { Hereafter, the concentrations of blend components are given in per cent by mass.4 1 Tm 1 j a ¢§ RV2 w12O1 ¢§ j2U2, ¢§ 1 Tm DH2V1 1 a-olefins (LLDPE) can be estimated using the modified Flory ¡¾ Huggins equation 37 where j1 and j2 are the volume fractions of the polymers 1 and 2, V1 and V2 are the molar volumes of repeating units of the polymers 1 and 2, respectively, DH2 is the enthalpy of melting of PE, and w12 is the Flory ¡¾ Huggins interaction parameter.The DH2 is 4.1 kJ mol71 and the parameter w12 is 70.022 at 393 K. This proves that even relatively weak dispersion forces can make two polymers compatible. Interesting information for estimation of the interaction between the PPC components can be obtained from the results of measurements of the surface (g) and interfacial tension. g=gd+gp , where gd and gp are the components of the surface tension of the polymer, which are due to the dispersion and dipole interaction, respectively. These characteristics of intermolecular interaction are immediately related to the cohesive energy of the system under study.The surface tension and its components depend on the nature of PPC ingredients and decrease upon their modification (Table 1).38 In addition, the higher the temperature the smaller the g values (Table 2). The surface tension at the interface between two immiscible polymers in a PPC (g12) can be calculated using the following equation:40 gp1 a gp2 g12 a g1 a g2 ¢§ 4gd1gd2 ¢§ 4gp1gp2 , gd1 a gd2 where subscripts 1 and 2 correspond to the first and second components, respectively. Table 1. Surface tension and its components (dyn cm71) for PP and PA-6.38 Polymer g gp gd 40.8 53.0 35.8 40.8 PP PA-6 PP a PA-6 a 28.3 32.1 26.3 30.1 12.5 20.9 9.5 10.7 a Modified by grafting maleic anhydride (MA, 1%).Table 2. Surface tension g (dyn cm71) for some polymers at different temperatures.39 PMMA HDPE LDPE PVAa T /K 41.1 32.0 28.9 36.5 28.6 25.9 35.3 27.3 24.6 293 413 453 35.3 28.8 26.5 aPVA stands for poly(vinyl acetate). Next, we present the surface tension (g12) values at the inter- face between the first polymer and the second polymer in the PPC at 423 K.41 LDPE PP 1.3 PE PVA (25%) 1.3 HDPE PVA 11.0 LDPE PVA 9.8 First polymer Second polymer g12 /dyn cm71 The surface tension at the interface can also be approximated using the Wendt ¡¾ Owens equation 42 A D Pomogailo 1 2 U2. g a Og1=2 ¢§ g1=2 Thus, there are some modern methods that often allow a quite reasonable description of the properties of real PPC.However, many aspects of this problem remain unclear as yet. III. In situ preparation of polymer ¡¾ polymer compositions in the catalytic polymerisation of olefins The method for the preparation of molecular PPC (hereafter, we will denote them `the in situ prepared PPC') in the catalytic polymerisation or copolymerisation of a-olefins (ethylene, propy- lene, butene), diene monomers (butadiene, isoprene) and mono- mers of the acetylene type (acetylene, phenylacetylene, 3-bromopropyne, etc.) on metal complex catalysts involves the following stages.43 First, this is a chemical or physical immobili- sation of the catalyst components on the `polar' ingredient of the PPC to be prepared.Usually, the catalyst components represent macromolecular metal complexes based on PMMA, PVC, PTFE, poly(vinylidene chloride) (PVDC), polyformaldehyde (PFA), PVA, poly(vinyl alcohol) (PVAlc), poly(4-vinylpyridine) (P4VP), cellulose and other polymers. The second stage consists of a liquid-phase or gas-phase polymerisation or copolymerisation of corresponding monomer resulting in the desired PPC. In the text below we consider the main approaches to the synthesis of catalyst components (of the Ziegler ¡¾ Natta type), some features of the polymerisations catalysed by these, in essence, polymer-immobilised 44 catalysts and, finally, selected experimental data on the structure of particular systems. 1. Synthesis of catalyst components Polymers containing functional groups (L) are macroligands that can bind the components of catalytic systems, viz., a transition metal compound MXn [TiCl4 , VCl4 , VOCl3 , VCl3 , TiCl3 , Ti(OBu)4 , CoCl2 , MoCl5, etc.], an organometallic (organoalumi- nium, organolithium, organomagnesium, organozinc) com- pound, etc.A general scheme of metal complex immobilisation taking into account the formation of chemical bonds can be represented as follows: L L LL L L MXn MXn L L L L +MXn MXn72 MXn71 L L LL L L L L L LMXn MXn L L L L The penetration depth of the active catalyst into the near- surface layer of the component of a PPC can vary depending on the reaction conditions. This can be accompanied by both mono- dentate and polydentate coordination of MXn by the intramolec- ular or intermolecular mechanism.44 The optimum immobilisation procedures can involve either the use of a suspen- sion of the PPC component or gas-phase coordination of the complexes.Many transition metal halides are relatively strong Lewis acids capable of forming ionic complexes. It is this type ofMolecular polymer ± polymer compositions. Synthetic aspects bonding that is used for the coordination ofMXn to PVC, PVDC, PTFE and poly-a-bromostyrene. + CH CH2 CH [MXnCl]7. +MXn CH2 Cl Yet another approach, applicable to crystalline compounds MXn (e.g., TiCl3 , VCl3), is based on the formation of carboxylic acid salts. For instance, TiCl3 is coordinated by chlorinated polymers (PVC, chlorinated PP, natural rubber) by the following mechanism:45 C C TiCl3+TiCl4 .+2 TiCl3 Cl Joint grinding of TiCl3 and PVC leads to complete disappear- ance of the crystalline phase. Nearly complete oxidation of Ti3+ into Ti4+ is evidence for essential transformations in the TiCl3 ± PVC system. Special methods for immobilisation of compounds MCl3 (with retention of their submicrostructure) on polymers have been developed. The most efficient procedure involves solid- phase mechanochemical synthesis in the presence of polymeric constituents of PPC (most often, PO). Experimentally, this is achieved by activation of the reacting components in the vibrat- ing, ball or differential agate mills (usually, in an inert atmos- phere).46, 47 The surface of polymers is also modified using cationic polymerisation,48 which results in macromolecular analogues of stable carbonium salts.To immobilise metal complexes on the surface of PO (the grafted layer is 10 to 30A thick), the surface of the polymer is functionalised by performing graft polymerisation of polar mono- mers.46 In particular, the surface of PE, PP or ethylene/propylene copolymer can be modified by grafting, e.g., acrylic acid, 4-vinyl- pyridine, acrylonitrile, allyl alcohol, vinylpyrrolidone, etc. R1 R2 R1 R2 Initiation of polymerisation in the presence of monomer C C + C C R3 Y R3 Y n Carbochain polymer surface R1=H, Me; R2 =H, Me, C6H5; Y=OH, NHR, CH2NH2 , SR, CH2OH, CH2SR2, CO2H, OCOMe; R3=H, Me, C2H5 , CH2CH CH2, C6H5 .In the text below we will show that graft polymerisation products can also be used individually as the third components (compatibilisers) to improve the compatibility of components of binary PPC. Table 3 lists some characteristics of catalyst components MXn that are used for the in situ preparation of PPC. Polymer ± polymer compositions can also be prepared in situ by anionic-coordination or ionic polymerisation using an alter- native approach which involves immobilisation of organometallic compound on the polar component. In this connection, it is of interest to consider the modification of the surface of PETP or PA-66 via 1,2-polyaddition of compounds LiR to carbonyl groups.49 Organomagnesium compounds are immobilised on graft-functionalised polymers.50 Addition reactions of AlR3 or AlR2H to the multiple bonds of polymers, resulting in the formation of reactive organometallic centres have been reported.In particular, addition products of dialkylaluminium hydrides to random styrene (25%)/butadiene copolymers have been studied in detail.51, 52 AlR2 . CH2 CH2 CH CH2+AlR2H 5 Table 3. Characteristics of catalyst components used for in situ prepara- tion of PPC.46 Polymer MXn Component Immobilisation conditions of MXn on the polymer surface Immobil- ised metal content /mg g71 time /h solvent T /8C PVC PVDC PMMA PFA Cellulose PTFE HDPE PP 20 ± 40 2.0 50 ± 70 2.0 30 ± 35 2.0 20 ± 25 1.5 2.0 20 ± 25 1.5 20 ± 25 1.5 20 ± 25 1.5 20 ± 40 0.5 20 ± 40 1.0 20 ± 40 1.0 20 ± 40 1.0 20 ± 40 0.5 20 ± 40 0.5 1.0 1.0 1.0 1.0 1.0 1.0 VCl4 TiCl4 VCl4 VCl4 TiCl4 VCl4 VCl4 VCl4 VCl3 TiCl3 VCl3 TiCl3 PE ± g-P4VP TiCl3 VCl3 PE ± g-PAA b Ti(OBu)4 PE ± g-PAAc c Ti(OBu)4 Ni(acac)2 VCl4 PE ± g-PVP d CoCl2 VCl4 CCl4 heptane CCl4 CCl4 heptane 60 CCl4 CCl4 CCl4 see a """""heptane 60 " 60 toluene 40 CCl4 25 methanol 40 CCl4 25 0.8 1.1 1.5 0.5 0.8 1.0 1.2 0.6 10.0 10.0 10.0 10.0 10.0 10.0 0.5 1.0 8.0 7.5 8.0 6.0 a Immobilisation is performed by grinding a mixture of components in a ball mill in the absence of solvent.bPAA stands for poly(allyl alcohol).c PAAc stands for poly(acrylic acid). d PVP stands for polyvinylpyrroli- done. Organoaluminium compounds can undergo insertion into the multiple bonds of functional groups of the polymeric constituents of PPC.44 Most likely, immobilisation of alumoxanes proceeds by the same mechanism. Both components of the metal complex system can be immobilised on the same polymer. The coordination mechanism of polymers to transition metal compounds (one-component catalysts of olefin polymerisation) is much simpler. In particular, immobilisation of ZrR4 (R�benzyl, p-allyl) and Cr(p-allyl)3 on the surface of polymer fibres proceeds via 1,2-polyaddition to the carbonyl groups of PETP or the reaction with amido groups of PA-66.49 O R3ZrO d7 d+ O C O C R ZrR3+ R Probably, organometallic zirconocene derivatives known as the most efficient catalysts of olefin polymerisation, are immobi- lised on polymers following the same mechanism.Currently, a new method for immobilisation of two different reactive centres on the same polymeric constituent of PPC has been developed. It allows bifunctional catalysis, i.e., simultaneous polymerisation of monomers of different chemical nature or conversion of one monomer into products of different structure. This method will be considered below. 2. In situ preparation of polymer ± polymer compositions: general kinetic rules The composition of PPC prepared in situ in the presence of catalyst components can be controlled by varying the polymer- isation conditions and, first of all, duration of polymerisation and the amount of monomer consumed.The rate of ethylene polymerisation (a PE-based, in situ prepared PPC) is to a great extent dependent on the nature of the polar component and the type of its bonding to the catalyst6 3 30 20 1 10 2 0 60 30 Time /min 90 Figure 1. Catalytic activity of VCl4 . L± AlBui2Cl systems in the polymer- isation of ethylene for different typeof immobilisation. Ligand (L): (1) vinyl acetate; (2) PVA; (3) PVAlc. components. Generally, immobilisation ofMXn or AlRxCl37x by covalent bonds is considered optimum. The component playing the role of macroligand stabilises the process,53 which manifests itself as a steady-state polymerisation (except for the initial stage).It should be noted that polymerisation at 313 K in the presence of the traditionally used system VCl4 ± AlR2Cl is essentially a non- steady-state process. Here, the maximum activity is achieved immediately after blending the components; however, the rate of ethylene polymerisation reduces by a factor of 45 after 30 min.54 In contrast to this, catalytic activity of the systems based on macrocomplexes reduces by 3 to 8 times after the same time interval. Systems in which VCl4 is covalently bound to, e.g., PVAlc, exhibit higher catalytic activity compared to macrocom- plexes with PVA (Fig. 1). Catalyst components formed in the reactions of TiCl4 or VCl4 with PVC or PVDC also demonstrate a stabilising effect.Kinetic studies revealed 55 that the rate constants for the chain propagation and chain termination (by monomer or organo- aluminium compound) stages and the activation energies for the PE chain propagation in the presence of both the macrocomplex- based systems under consideration and traditionally used Zie- gler ± Natta catalysts have similar values. However, the formation of the reactive centres proceeds by different mechanisms, which is responsible for the stabilising effect of the polymeric catalysts. The molecular mass of PE formed depends on the nature of the catalyst component and on the polymerisation conditions. Standard procedures for controlling the molecular mass of PE include addition of hydrogen. The in situ prepared PPC are isolated and treated using the same procedures as those used in the low-pressure polymerisation of olefins.This type of PPC can also be prepared under conditions of the gas-phase polymerisation of ethylene (in this case, the catalyst component is dispersed in the suspension layer).56 Using components with specific molecular shape also allows preparation of granulated PPC. Binary polyolefin compositions (e.g., PE ± PP) can be synthes- ised using the catalyst components obtained by co-grinding of PO and MCl3 (e.g., the size of TiCl3 submicrocrystals co-ground with PE is *1 nm).57 The stereosequence of PP formed on such catalysts is determined by conventional factors. This method also allows the in situ preparation of such PPC as PE ± PB, PE ±polycyclohexene, PE ± PS, etc.The PE ± polybutadiene (PBd) and PE ± polyisoprene (PI) compositions are prepared by polymerisation of the diene monomer on the PE ± g-P4VP . CoCl2 ± AlEt2Cl system.57 In these catalysts each Co atom pro- duces the polydiene chain, the molecular mass of PI lies between 370 000 and 1 300 000 and the content of 1,4-cis-PI exceeds 80%. In situ preparation of conducting compositions PE ± polyace- tylene (PAc) involves immobilisation of the catalytic system Ti(OBu)4 ± AlR3 on the surface of modified PE films followed by gas-phase or liquid-phase polymerisation of acetylene 58 and Polymerisation rate /litre min71 A D Pomogailo results in the formation of trans-PAc similarly to the process in homogeneous systems. Thus, the use of catalyst components allows the preparation of PPC based on polyolefins and various polymers in the anionic- coordination polymerisation. Let us consider the structure of particular PPC in more detail.3. HDPE±PMMA Compositions Main characteristics of the properties of the in situ prepared polymer ± polymer compositions HDPE±PMMA and polymer blends of HDPE and PMMA (with similar molecular mass characteristics) are listed in Table 4. Table 4. Density (r), yield stress (s), elongation at break (e) and glass transition temperature of polymer ± polymer compositions HDPE± PMMA.60 Parameter PMMA content in composition b /volume fraction Starting compo- nents a 0.29 0.14 0.07 0.04 r /kg m73 1.019 1.017 0.983 0.965 0.968 0.965 0.959 0.962 1.190 0.948 7 s /MPa 3500 1150 3155 1200 3200 2700 5000 2500 7 e (%) 180 45 210 70 250 80 2 ±3 370 Tg /K 221 221 221 7 217 203 225 221 221 221 a The data for PMMA are listed in the numerator and the data for HDPE are given in the denominator.b The data for molecular (in situ prepared) compositions are presented in the numerator and the data for polymer blends are listed in the denominator. An increase in the volume fraction of PMMA (j) in the synthetic PPC is accompanied by a decrease in the relative elongation at break (e) and an increase in the yield stress (s). Compared to the in situ prepared PPC of similar chemical composition, polymer blends are characterised by smaller e and s values (by 3 to 4 times for e and 1.5 to 2 times for s).The glass transition temperatures of the components are virtually independ- ent of the procedure for the preparation of PPC, which suggests low thermodynamic compatibility. Both components of such PPC are linear polymers whose chemical composition and structure can be studied by selective extraction. This method is often used for analysing polymer blends and, in particular, interpenetrating polymer networks (IPN).59 Polymer blends prepared without the formation of a complex with VCl4 in the ethylene polymerisation using the VCl4 ± AlEt2Cl system exhibit a ready and complete separation into the starting components. In the case of in situ prepared PPC the content of non-extractable PMMA (j=0.007 ± 0.14) varies between 4.4 mass% and 6.7 mass %.60 Most likely, this is evidence that a fraction of PMMAmolecules is chemically grafted onto PE.This assumption is confirmed by the results of various physicochemical studies. For instance, new bands with maxima at 1740 and 1300 cm71, corresponding to stretching vibrations of carbonyl and ester groups, appear in the IR spectra of non-extractable products along with the main absorption bands typical of the PE backbone. The assumption that the integrated intensity of X-ray diffrac- tion reflections is proportional to the content of the crystalline component in a polymer blend consisting of amorphous and crystalline phases holds for any blend including polymer blends based on crystalline PE and amorphous PMMA.Hence it follows that if the structure of the PE constituent of a PPC is independent of the chemical composition of the PPC, the height of the most intense peak of PE, corresponding to the (110) reflection, can serveMolecular polymer ± polymer compositions. Synthetic aspects h(110) /mm 200 160 1 120 2 80 400 0.4 0.2 Figure 2. Empirical correlation between the height of the (110) peak of HDPE and the chemical composition of polymer blends of HDPE and PMMA (1) and molecular polymer ± polymer compositions HDPE±PMMA (2).60 as a quantitative characteristic of the chemical composition of the PPC. For mechanically mixed polymer blends, this correlation corresponds to curve 1 in Fig.2, whereas for in situ prepared PPC the plot is shifted to the left. This looks like an apparent reduction of the volume fraction of PE and does not correspond to the true chemical composition of the in situ prepared PPC determined independently (from the density, material balance of polymer- isation, etc.). This contradiction can be readily eliminated assum- ing that some structural domains of PE formed do not contribute to the (110) peak intensity. Indeed, in contrast to polymer blends, the X-ray diffraction patterns of the in situ prepared PPC display a clearly seen peak labelled by an arrow in Fig. 3. This peak corresponds to a new a Intensity bcd 21 27 Figure 3. X-Ray diffraction patterns of PMMA (a), HDPE (b), in situ prepared composition HDPE±PMMA (c) and a blend of HDPE and PMMA (d ) (j=0.19).65 0.8 0.6 j (rel.u.) 15 9 2y /deg 7 interplanar distance (*3A) in the structure of PPC and is indicative of the interaction between PE and PMMA on the molecular level and of a local distortion of the phase boundaries. The latter is due to the fact that chain propagation (the growth) of the PE macromolecules is accompanied not only by crystallisa- tion, but also by interpenetration of the first component into the bulk of the second component (due to diffusion of macromole- cules or their segments). As the distance from the surface of PMMAincreases, this interaction, which determines the thickness of the intermediate layer, weakens.If the intermediate layer is sufficiently thick, it can form an individual phase. In addition, a salient feature of this system is the propagation of the PE chains accompanied by mixing of the components, which also improves their compatibility and the physicomechanical properties of in situ prepared PPC (the so-called operating compatibility) compared to the properties of conventional polymer blends (see Table 4). This type of PPC, prepared by copolymerisation of propylene and CO on phosphinepalladium complexes resulting in a propylene ±CO alternating copolymer and followed by blending with PMMA, is thought to be miscible.61 Experiments 62 on multicomponent PPC containing LDPE and small amounts of hydrophilic polymers [PVA or poly(vinyl acetal), 2% to 6%] showed that, as a rule, in the presence of surfactants the macromolecular coils of the additives undergo insertion into the LDPE domains in both the amorphous phase and intermediate layer.4. HDPE±PVC Compositions PVC-Based materials exhibit some valuable properties. At the same time, they are characterised by low thermal stability and thermal plasticity in processing and by low impact toughness at negative temperatures. Sometimes, these drawbacks can be elim- inated by modifying PVC with thermoplastic polymers or copoly- mers, and the resulting PPC combine the advantages of the starting components. This is also favoured by miscibility of PVC and a broad spectrum of polymers containing appropriate func- tional groups. Such composite materials can be used for the preparation of, e.g., polymer films for enclosing greenhouses.63 As applied to theHDPE±PVC systems, this method for in situ preparation of PPC involves immobilisation of VCl4 , VOCl3 or TiCl4 on the surface of PVC latex or suspension PVC (Mn=5.56104, the specific surface area is 6 m2 g71 and the particle diameter lies between 50 and 100 mm) via the formation of a macromolecular carbonium salt following the above-mentioned mechanism.49, 64 ± 66 Similarly to the case of the HDPE±PMMA systems, the chemical composition of PPC is controlled by varying the consumption of ethylene for polymerisation.Using selective extraction with boiling THF in a Soxhlet apparatus for 24 h, the fraction of graft PVC was estimated (Table 5).The larger the volume fraction of PE in synthetic PPC, the larger the amount of non-extractable PVC due to an increase in the number of polymer Table 5. Percentage of PVC grafted onto PE in the suspension polymer- isation of ethylene using the PVC. VCl4 ± AlEt2Cl system. PVC content in composition Percentage of grafted PVCa (mass %) volume fraction mass% 3.3 3.0 2.3 2.4 1.8 1.0 5.6 17 25 32 46 60 80 56 b 0.12 0.19 0.25 0.38 0.52 0.76 0.25 a The content of PVC that could not be extracted with boiling THF in a Soxhlet apparatus for 24 h. b The polymer ± polymer composition pre- pared by gas-phase polymerisation of ethylene.8chains or their segments that enter the reaction. The IR spectra of such graft polymers exhibit new bands at 615, 640, 833, 968 and 1260 cm71 corresponding to PVC in addition to the absorption bands of PE.We will consider possible mechanisms of grafting in the next sections of the review. Similarly to the HDPE±PMMA compositions, it was found that some of the structural domains of PE in the HDPE±PVC systems are X-ray silent. The plot of the empirical dependence of the height of the (110) peak on the chemical composition of the HDPE±PVC system is shifted to the left relative to the plot of analogous dependence for mechanically mixed polymer blends. The proportion of structural domains that do not contribute to the intensity of the (110) peak of PE is*20% (estimated from the decrease in j value taking into account the non-extractable fraction of PVC and also from the results of optical measure- ments).This value is in reasonable agreement with the decrease in the proportion of the `X-ray responsive' PE. Unlike polymer blends, a salient feature of the structure of in situ prepared PPC is the appearance of a new interplanar distance (*4A). Therefore, grafting of the PE chains or their segments onto PVC macro- molecules occurs in the loose, intermediate near-surface layer. The mechanics of two-component systems treats this type of PPC as compositions with a framework or interpenetrating structure and adhesive interaction between the components over a wide range of component ratios. According to the cubic model which ignores any polymer ± polymer interactions,67 the yield stress of PE (s0) and PPC (s) must be related by a simple formula: = 17j2/3.0 s sIndeed, the experimental and theoretical data for polymer blends are in reasonable agreement (Fig. 4 a), which points to the absence of interactions between the HDPE matrix and PVC particles. No agreement between experiment and theory is observed for the in situ prepared PPC. This serves as additional evidence of the polymer ± polymer interactions which favour a 60% (on the average) increase in the yield stress, s, of such compositions. Polymer compositions containing a heat-treated component (e.g., crystalline HDPE) become sensitive to the sample preparation conditions. In particular, the points corre- sponding to the samples prepared at low and high melt cooling rates are grouped near different theoretical curves (see Fig.4 b). Table 6 lists the numerical values of the main physicomechan- ical characteristics of different polymer ± polymer compositions HDPE± PVC. As can be seen, the in situ prepared PPC are characterised by greater ultimate strength at elongation, relative Table 6. Numerical values of physicomechanical characteristics of polymer ± polymer compositions HDPE± PVC.66 Starting components Property PVC content in the in situ prepared PPC (mass%) 23 17 PVC HDPE 23.8 24 42.0 30.1 30 0.6 30 80 0.7 19 23 1.0 30 40 0.7 52 0 780 780 7 see c 70 98 770 928 0 777 Ultimate strength at elongation /MPa Relative elongation (%) Rigidity /N Tear resistance /kN m71 Impact cold resistance /8C High-frequency welding b time /s weld strength (%) weld cold resistance /8C aRoom-temperature cracking.b High-frequency currents. cNo welding. dNo welding; surface combustion occurs. A D Pomogailo b a 1073 E /MPa 1073 s /MPa 1 2.5 6 2.0 4 2 1.5 2 1.0 0 0 0.2 0.4 0.6 0.8 j 0.1 0.2 0.3 0.4 j Figure 4. Theoretical (solid curves) and experimental dependences of the yield stress (a) and Young's modulus (b) for polymer blends of HDPE and PVC (open circles) and corresponding in situ prepared PPC HDPE± PVC (solid circles) on the volume fraction of PVC.65 Stress rate 0.03 s71; the cooling rate is 1 (1) and 20 ± 30 K min71 (2).elongation and tear resistance. The impact toughness of the film samples obtained was estimated from the impact cold resistance to a standard load (a 0.5 kg ball) fallen from a height of 0.5 m.68 Polymer blends of analogous chemical composition exhibit poorer physicomechanical characteristics in the whole range of HDPE/PVC ratios. Yet another important feature of the in situ prepared poly- mer ± polymer compositions HDPE± PVC, viz., their ability to undergo high-frequency welding, should be pointed out. This is apparently due to the improved compatibility of the components and higher homogeneity of the PPC. As the PVC content increases from 23 mass% to 55 mass %, the duration of the welding process is reduced from 70 down to 18 s; however, the impact toughness of the PPC reduces at high PVC content.If the content of PVC is lower than 20%, high-frequency welding of molecular composites becomes impossible. Polymer blends do not undergo high-frequency welding in the whole range of component ratios, which is evidence of high inhomogeneity of their structure. Comparison of strain ± stress diagrams of conventional poly- mer blends and synthetic PPC showed that the latter behave as ductile materials and are not subject to brittle fracture both at room temperature and at 203 K. This allows the use of the in situ prepared PPC at low temperatures. The optimum content of PVC in such compositions is nearly 45 mass %. It should be noted that the formation of a grafted layer from the polymer formed was also observed during the cationic polymerisation of styrene or isobutylene in a suspension of PVC with immobilised Et2AlCl (in a heptane ± 1,2-dichloroethane PVC content in mechanical mixtures (mass %) 55 44.5 30 23 17 55 44 30 5.0 14.4 24.2 16.8 20.4 21.3 7 7 0.75 50.75 3.5 8.5 0.6 3.0 13 0.7 31 62.5 0.7 25 20 0.7 17 7 7 70.6 7 7 0.7 3.5 see a see a see a see a see a 760 780 780 see d see d see d see d see d 18 55 730 30 65 770 56 98 770Molecular polymer ± polymer compositions.Synthetic aspects mixture at 253 K).48 Depending on the polymerisation conditions, the degree of heterophase grafting varied from 2 vol.% to 13 vol.% for PS and from 7 vol.% to 15 vol.% for PIB (with n-pentane as selective extracting agent).In the latter case, the 250 mm PVC particles were coated with a separable PIB layer 50 mm thick. Probably, flaking of the grafted layer from PVC is due to large differences in the surface energy (immiscibility) and mechanical properties of PIB and PVC. The concentration of chlorine in the exterior of the grafted layer is much lower than in the interior and sharply decreases as the thickness of the grafted layer increases. Thus, synthetic methods make it possible to prepare PPC with improved physicomechanical and operating characteristics. Sim- ple blending of the corresponding components does not allow improvement of these characteristics. IV. Synthesis of polar ingredients in the presence of polyolefins An alternative approach to the in situ preparation of PPC implies the synthesis of the second (hereafter, `polar') component in the presence of a PO.In the reaction bulk, the PO has little effect on either the radical and ionic polymerisation or on the polyconden- sation of the component precursors. Polyolefins can be modified by mechanochemical treatment with the monomer in the presence of Lewis acids.69 For instance, the preparation of a layered material based on PE fibre (glass fibre) involves stacking of aligned fibres that were preliminarily soaked with methyl methacrylate (MMA) in a sheet die followed by bulk polymerisation of MMA at room temperature 70, 71 with the amine ± peroxide system as initiator.72 The polymerisation is conducted until thickening of the material; shrinkage control is achieved by adding excess monomer.The final preparation stage of the composition involves heating to 328K (this temperature corresponds to complete conversion of MMA). Compatibility of particular pairs of polymers (e.g., PO and PS) can be improved by performing in situ cationic polymerisation of styrene in the presence of corresponding polyolefins with Lewis acids (e.g., AlCl3) as catalysts.73 The reaction mechanism is similar to the benzene ring alkylation according to Friedel ± Crafts. CH CH2 CH CH2 Catalyst + PO PS Graft copolymer The degree of grafting depends on the nature of the Lewis acid and reaches a maximum value (15.7%) for AlCl3 . The procedure for preparation of polymer ± polymer blends based on crystalline polymers (HDPE, PP, PA-12) after their deformation via delocalised crazing by drawing in liquid mono- mers (styrene, MMA, etc.) involves drawing a polymer film (e.g., PP) in MMA containing the necessary amount of both initiator and cross-linking agent.74 ± 76 As the desired degree of drawing is achieved, in situ polymerisation of the monomer in the PP matrix is carried out.Deformation is accompanied by continuous filling of the newly formed, porous polymer structure with the poly- merised blend. Generally, delocalised crazing allows substantial enhancement of the swelling of the polymer (swelling of PP or PE in conventional monomers is at most 10 vol.% ± 15 vol.%, since the low-molecular-mass component can penetrate only the amor- phous domains of the polymer structure while crystallites remain intact).From the standpoint of the preparation procedure such PPC can be classified as IPN. Experiments with the HDPE±PMMA system showed that complete separation of the blend components by successive selective extraction with chloro- form at 333 K for 6 h (to dissolve PMMA) and with n-decane at 9 393 K for 2 h (to remove HDPE) is also impossible, while the homopolymers are completely dissolved under these conditions. The content of the insoluble (non-extractable) product is 25 mass%± 30 mass %. This is due to either or both chemical grafting of the polymer chains and their interpenetration on the molecular level.Thus, despite thermodynamic incompatibility of their compo- nents, the in situ prepared polymer ± polymer compositions are to some extent compatible and the degree of compatibility is higher than in the case of simple mixing of polymer melts. Nevertheless, such PPC represent two-phase systems, which is confirmed by the data of calorimetric studies, according to which the glass tran- sition temperatures of the components remain unchanged. It is of considerable interest to consider the formation of the PU phase in PE in more detail. Thermoplastic PU is widely used in blends with other polymers including PO.77 For instance, the formation of in situ compositions containing 85% PE and 15% PU from diisocyanates (diphenylmethane-4,4 0-diisocyanate or dicyclohexylmethane-4,4 0-diisocyanate) and diols (polyetherdiol or 1,4-butanediol) in PE melts (413 K) in the presence of catalyst was reported.78 During the process the molecular mass of the dispersed phase changes in nearly the same manner as in the case of bulk polymerisation of virgin PU under analogous conditions (Fig.5). Since the solubility of the isocyanate monomer in PE melts is rather high, while the solubility of the diol is low, the stoichiometry ratio between them violates, which imposes some limitations on the degree of polymerisation by in situ blending. { A number of procedures which facilitate the formation and favour strengthening of the interfacial layers was developed. As a rule this is achieved by introducing various additives (compati- bilisers) which improve adhesive interactions between the starting components and the in situ formed phases.1073Mn 40 2 30 1 20 100 40 20 10 30 t /min Figure 5. Formation kinetics of PU (curing at 413 K) in the dispersed phase (1) and in the bulk (2) under comparable conditions.78 For instance, consecutive anionic polymerisation of styrene and 4-vinylpyridine in the presence of BuLi at 195 K resulted in block copolymers which induce phase separation.80 Block copoly- mers of styrene and 2-vinylpyridine were synthesised using con- secutive polymerisation of the monomers with potassium cumene as initiator.81 Atactic ethylene/propylene diblock copolymers were prepared by hydrogenation of poly(butadiene-b-2-methyl- 1,3-pentadiene), polybutadiene and poly(2-methyl-1,3-penta- diene).82, 83 The preparation of a partially deuterated ethylene/ propylene diblock copolymer based on perdeuterated butadiene used as co-monomer was also reported.{ Semi-interpenetrating polymer networks based on cross-linked PU and linear poly(butyl methacrylate) are prepared by introducing the latter component in the monomeric form followed by performing radical polymerisation simultaneously with cross-linking.7910 V. On some synthetic methods for preparation of polyolefin ± polyolefin compositions Multicomponent, PO-based polymer blends represent the most abundant group of PPC; most often, they serve as materials for recycling. This group of polymer compositions comprises blends of normal PE and high-molecular-mass or ultrahigh-molecular- mass HDPE, blends of different types of PE (LDPE, medium density PE and HDPE), etc.84, 85 These materials exhibit some valuable properties, such as improved impact toughness, resist- ance to external factors including cold resistance, have good optical and rheological characteristics, high crystallisation rate, etc.86, 87 These parameters of the PPC are nonlinear functions of their chemical composition, which is due to the influence of the interfacial layer and to the changes in the structure and properties of the constituent phases in the blends caused by the interaction with other components.One of the main goals of research into such PPC is the development of methods for improvement of processability of PO.1. Systems based on different types of polyethylene Design of novel materials with improved properties involves the preparation of microheterogeneous composites as an integral part. Microheterogeneous composites represent (i) PPC based on different types of PE (high density and low density PE) and (ii) highly oriented systems containing LDPE and HDPE.88 Low density PE, with its long-chain branchings, and linear HDPE (LHDPE) are of similar chemical nature. Therefore, one could expect that they will form homogeneous blends which, never- theless, represent microheterogeneous systems. An increase in the concentration of HDPE in the blend leads to a decrease in the crystallite size and to an improvement of the structural homoge- neity of PPC.If the content of HDPE exceeds 30%, phase separation occurs, HDPE forms a dispersed phase and interfacial interaction between the polymeric constituents is observed. Fur- ther increase in the concentration of HDPE causes the formation of ordered HDPE domains that are structurally similar to the HDPE crystallites. Polymer ± polymer compositions containing 80% HDPE have superiority in mechanical properties compared to the starting components. This is thought to be a result of the highest possible structural homogeneity of the polymer composi- tions due to the maximum compatibility of the components. Neutron scattering studies showed that the melts of HDPE and LDPE are compatible in the whole range of their concentra- tions.89 A key factor that governs the extent of co-crystallisation of HDPE and LDPE is the similarity of crystallisation rates of each component.The amount of co-crystallisate formed increases as the isothermal crystallisation temperature reduces under quenching conditions.90 The co-crystallisation is also influenced by the branching composition of the constituent macromolecules, as was shown taking a blend of linear PE (1% to 20%) and branched PE as an example.91 These systems can also exhibit partial compatibility like, e.g., a blend of LLDPE (1-octene co- monomer), HDPE and LDPE.92 As the content of LDPE in its blends with the ethylene ± pro- pylene ± 1,4-hexadiene terpolymer increases, the Tg value is shifted towards higher temperatures (i.e., towards the region of b-relax- ation of PE, which is due to an increase in the mobility of small independent structural fragments of the macromolecules).93 This confirms that the blend components are miscible on the molecular level.In addition, LDPE and ethylene/propylene copolymers are characterised by close values of the density of cohesive energy in the amorphous phase; these polymers also seem to be compatible on the molecular level. The addition of small amounts of LDPE (merely 2.5%) to LLDPE (1-hexene co-monomer with a density of 0.900 g cm73) leads to the formation of well-defined spherulites and to a substantial decrease in their size.94 Hence, LDPE is a nucleating agent for the crystallisation of LLDPE and the addition of LDPE shifts the a-relaxational processes towards higher temperatures or longer times.The table presented below A D Pomogailo lists the melting temperatures (Tm), densities (r), crystallite volume fractions (x) and spherulite radii (r) for individual LLDPE and for the LLDPE component of the LLDPE±LDPE composition containing 2.5% LDPE.94 x (%) Sample Tm /K r /mm r /g cm73 6.5 0.7 33.2 34.3 0.9031 0.9048 370.3 372.5 LLDPE LLDPE±LDPE Linear LDPE has superiority in mechanical properties (e.g., elongation at break) compared to LDPE. LLDPE-based blends exhibit intermediate properties (e.g., torque 95) between those of the starting polymers. Studies on both film and melt behavior of such blends revealed no severe problems originating from polymer incompatibility. On the other hand, non-polar and low-polar crystalline polymers are poorly miscible with one another and with non-crystalline polymers.In this respect, a classical example is provided by blends of PE and PIB.96 Polymer ± polymer compositions prepared from solutions of polyolefins are also used, though much more rarely. For instance, linear PE (Mw=53 000, Mn=16 000) and branched PE (Mw=76 800, Mn=11 300) are mixed in predetermined ratios from 1% solution in xylene and then precipitated with metha- nol.91 Studies on the properties of such PPC allows the develop- ment of methods for compatibilisation of these polymers for their blending, processing and further use of the material. Crystallisation of blends of HDPE and LLDPE has been the subject of intensive research (see, e.g., Refs 97 and 98).The morphology and mechanical properties of films prepared from blends of UMMPE with a viscosity-average molecular mass of 66106 and low-molecular-mass PE (46104) by gelation } or crystallisation from solutions are determined by the degree of entanglement of the macromolecules of both polymers. Here, LDPE has no time to form crystals since it crystallises at a much lower rate than HDPE. The modulus of elongation of such PPC is as high as 50 GPa, which is a quite appropriate value for conven- tional usage. These PPC exhibit lower viscosity, which favours substantial improvement of the performance of the equipment used for fabrication of blend-based films.Structural peculiarities of the blend-based PPC under consid- eration determine 99 ± 101 the tensile strength of the PE blends,102 their operating characteristics 103 and dielectric and other proper- ties. For instance, isothermal crystallisation of blends of the linear and branched PE at 397 Kresults in compact branched structures. Carrying out the process at 288 K leads to the formation of open, lamellar spherulite structures; quenching favours the appearance of a continuous spherulite texture.91 The dielectric properties of PPC depend, first of all, on their morphology, heat treatment conditions and the content of linear PE. This also concerns the PPC based on industrially used LDPE and LLDPE.104 At the same time, HDPE and LLDPE are compatible in the whole range of their concentrations provided that quenching occurs at low melt cooling rate.105 In the presence of HDPE the blend morphol- ogy is governed by the distribution of lamellae (crystallites), which is due to higher mobility of the HDPE molecules.The macro- molecules of LLDPE form mixed crystals (co-crystallise) with the HDPE molecules. The lattice parameters of the crystalline phase increase as the number of side chain branchings increases. Vol'fson 16 constructed a `map' of the morphology behaviour of the LDPE±LLDPE blends with the predominant `shish-kebab'- like morphology, containing 0.5% to 5.0% LDPE and quenched at different temperatures in the range from 393 to 463 K.Polymer ± polymer compositions with low content of LDPE are characterised by a higher degree of crystallinity of LLDPE, higher melting temperature and rigidity.94 The ultimate strength decreases while the yield stress increases as the content of LDPE increases (Fig. 6). } For gelation, polymer solutions in decalin, prepared at 408 K, are poured in an aluminium pan and the solvent is evaporated.Molecular polymer ± polymer compositions. Synthetic aspects 1073 s /MPa su /MPa 1.0 80 0.8 60 0.6 40 0.4 20 0.2 60 [LDPE] (mass%) 40 20 0 Figure 6. Yield stress (s) and ultimate strength (su) of compositions LLDPE±LDPE as functions of the component ratio at 296 K.94 Information on `ternary' PE-based PPC is scarce. Donatelli 99 studied ternary films prepared from low, medium and high density PE and showed that two polymers can be compatible within rather narrow limits of the content of branched units while they separate into individual crystalline phases at higher concentration of branchings. 2.Polyethylene ± polypropylene systems and other compositions Low-temperature impact brittleness of PP originates from the relatively high glass transition temperature of this compound (*263 K). An increase in the cold resistance of PP-based materi- als is a topical problem. Blends of PP and HDPE have been studied in most detail. At particular chemical compositions, these PPC are characterised by high breaking strength, modulus of elasticity and impact toughness and low brittleness temperature.The structural-dynamical parameters of the amorphous domains, the supramolecular structure and physicomechanical properties of such systems consisting of polymeric constituents with different molecular-mass characteristics have been studied. The properties of PPC prepared from isotactic PP and PE also depend on which catalyst was used for the polymerisation of ethylene, namely, a heterogeneous (Ziegler ± Natta type) or a one-centre homogene- ous metallocene catalyst.106 Isotactic PP and HDPE are thermo- dynamically incompatible and form different types of crystal lattices. Since the melting temperatures of these polymers are different, they form individual structures.107 In situ preparation of PPC from HDPE and PP by polymer- isation at low pressure involves immobilisation of the catalyst components on either PP or PE.(This procedure was used in the polymerisation of, e.g., propylene with the product of co-grinding of HDPE and TiCl3 as the catalyst component.46) The newly formed PPC can have virtually any chemical composition and exhibits improved strain ± stress properties and cold resistance as compared to conventional polymer blends. It should be noted that the polymerisation method allows one to control the properties of the PP polymerisate, its stereosequence and molecular-mass parameters. Not only PE, but also other polymers and copolymers can be used for the preparation of the catalyst component. In particular, blends of PE and synthetic ethylene ± propylene rub- ber (SKEP), PP and polybutene, PP and poly(4-methylpentene), etc., were prepared.The structure of this type of PPC is shown in Fig. 7. Co-grinding of the catalyst (MCl3) and HDPE leads to coating each catalyst submicroparticle with the polymer layer (`skin'). Probably, sub-microcrystals (e.g., TiCl3 ) are inserted into the amorphous phase of PE. Nucleation of the PP phase with the globular structure gradually transforming into a lamellar one occurs in the bulk of the catalyst component particle. Micro- heterogeneous mixing of the growing PP phase with the PE phase is thought to result in the formation of IPN. 11 1 2 3 Figure 7. Structure of a submicroparticle of composition HDPE± PP. (1) Catalyst microparticle; (2) HDPE `skin'; (3) PP phase.Binary compositions containing PP (70%) and HDPE (30%) are prepared by in situ polymerisation on binuclear metal complex catalysts immobilised on the SiO2 surface.108 The mechanical characteristics of this material (Young's modulus, tensile strength) are 1.5 to 2 times higher than the corresponding char- acteristics of `pure' PP. This is due to the very complex and unusual morphology of the material. Considerable attention has been given to the studies of polymer blends based on PP and some other polymers such as elastomers, different crystalline and amorphous thermoplastics [PETP, polycarbonates, PS and acrylonitrile ± butadiene ± styrene (ABS) terpolymers]. Gupta et al.109 analysed the rheological properties of binary and ternary blends of PP with ABS and LDPE.Ethylene/propylene block copolymers exhibit improved impact toughness and low brittleness temperature (down to 256 K). At the same time, the strain ± stress properties of such blends prepared from, e.g., isotactic PP and LDPE110, 111 are to a great extent dependent on the blending temperature and duration of blending of the components. The diffraction patterns of the HDPE± PP blends with different chemical compositions represent superpositions of the diffraction patterns of the starting compo- nents, which indicates that no co-crystallisation occurs. The average crystallite size in the blends is nearly the same as those in the non-blended components (150 and 170 A, respectively).112 However, the specific volume of the blend exceeds the value obtained using the additivity scheme.This points to the formation of a loose structure resulting from an increase in the content of the amorphous phase. This conclusion was also confirmed by the results of radiothermoluminescence studies, according to which the glass transition temperatures of the components in blends of LDPE and PP decrease (Fig. 8). Such a negative synergistic effect of the blend components can be rationalised by different rates and Tg /K 2 253 243~~ 233 1 223 213 100 0 80 40 PP content (vol.%) Figure 8. Glass transition temperature of the LDPE (1) and PP (2) phase as a function of the component ratio of LDPE ±PP blend.11212 temperatures of their crystallisation, by their thermodynamic incompatibility and by the absence of co-crystallisation in the blend.The rate of crystallisation of isotactic PP blended with LLDPE insignificantly decreases as the content of LLDPE increases.113 Dissolution of chain segments leads to the formation of an extended amorphous interfacial layer which precludes crystallisation of PP. The LDPE phase can be considered as a poor solvent for the PP phase (and vice versa).3 High density PE is incompatible with isotactic PP under steady-state conditions. However, compositions prepared from these polymers (PP :HDPE=60 : 40) by injection moulding form a phase with regularly separated structure and an identity period of 0.15 mm. The regular structure cannot be formed by simple melt mixing but only by the spinodal decomposition (at zero shear rate).114 In the compositions consisting of isotactic PP and HDPE (0.1% ± 3%), phase separation occurs on gradual cooling from 473 down to 443 K.115 The average spherulite diameter is less than 10 mm for HDPE and greater than 100 mm for PP.24 The propor- tion of ordered polymer chains in the amorphous phase of PP increases upon the addition of HDPE (10 mass% to 20 mass%) even for isotropic blends of these polymers prepared by melt mixing.116 The modulus of elasticity of the blends exhibits a synergistic effect due to the strengthening of the intercrystallite links in both the PP phase and in the interfacial layer.It is interesting that, unlike the PPC based on syndiotactic PP and random ethylene/propylene copolymer } (a 80 : 20 blend, 4.9 mass%ethylene units), isotactic and syndiotactic PP synthes- ised on zirconocene catalysts are incompatible even upon heating up to 513 K (a 50 : 50 blend).118 The mechanical properties of PP ± PE compositions can be substantially improved by usingUMMPEinstead of conventional PE.119 Such blends were prepared by coprecipitation of polymers from 1% xylene solutions at 398 K with excess methanol.Improved compatibility of PP and UMMPE can be most likely rationalised by the interpenetration of polymer chains in the amorphous phase due to the low degree of crystallinity of UMMPE. A study on the morphology of blends of amorphous-crystal- line block copolymers with amorphous homopolymers carried out taking the system consisting of ethylene/propylene diblock copolymer and atactic PP as an example showed that blending of the block copolymer responsible for the formation of the lamellar structure of microdomains and atactic PP leads to changes in the morphology of the microdomain structures and to microphase separation.120, 121 If the atactic PP chains are shorter than the block length, morphological transformations from lamellar struc- ture into a bicontinuous cylindrical one, then into a discrete cylindrical and, finally, into a spherical structure occur.If the PP chain is longer than the block length, microphase separation occurs and a morphological change from lamellar to bicontinuous cylindrical structure is observed. In melts, these morphological transformations are accompanied by changes in the blend crys- tallisation kinetics.In the blend of isotactic PP and SKEP the morphology of the ethylene domains depends only slightly on the viscosity ratio of the blend components and exhibits a strong dependence on the content of ethylene.122 The microstructure and chemical compo- sition of SKEP (the dispersed phase) affect the rheology, mor- phology and properties of such compositions prepared by injection moulding.123 The microstructure of a composition of isotactic PP and SKEP (60 : 40) essentially depends on which (a titanium- or vanadium-containing) catalyst it was prepared. Blends of isotactic PP and random SKEP, synthesised on vana- } For comparison, mention may be made that a study on the texture of a blend of syndiotactic and atactic PS revealed that the density of cross-link points strongly depends on temperature and exhibits relatively weak dependence on the initial chemical composition of noncrystallisable materials and on the degree of crystallinity of lamellar stacks.117 A D Pomogailo dium-containing catalysts exhibit substantially improved impact toughness at low temperatures.The structure of the SKEP component affects both the crystallisation kinetics and the inter- nal structure of spherulites of isotactic PP (the thickness of the crystalline lamellae and amorphous layer). Among other PO± PO compositions, we will dwell on blends of isotactic PP and PIB.Problems of co-crystallisation and compatibility of blend components were considered by Cham et al. 124 Blends of PP and PIB are compatible in the amorphous phase and can even undergo co-crystallisation. Figure 9 presents the glass transition temperature and the Flory ± Huggins interfa- cial interaction parameter of the PE ± PIB compositions as func- tions of their chemical composition.125 The maximum shift of Tg is 8.6 K for PE and 5.9 K for PIB (at 90% PE). This means that the solubility of PIB in PE is higher than that of PE in PIB. According to calculations, the solubility of PIB in the PE phase of the PE ± PIB (10 : 90) composition is *17% by mass, whereas the content of the PE phase in the PIB phase of the PE ± PIB (90 : 10) composition is only 6 mass %.Low values of the Flory ± Huggins interaction parameters w12 and w21 (see Fig. 9 b) point to some degree of compatibility of the blend components despite the absence of specific interactions between their macromolecules. For unknown reasons, the w12 value differs from w21.{b a Tg /K w12 , w21 2 0.015 1 0.010 1 2 0.005 230 210 190 170 150 80 0 80 100 20 40 60 0 20 40 60 PIB content (mass%) Figure 9. Glass transition temperatures (a) and Flory ± Huggins para- meters (b) of compositions PE ±PIB as a function of the component ratio: (1) PE phase; (2) PIB phase. Interesting results were obtained in studies of PIB ± PS poly- mer networks containing from 10% to 70% PS.127 Such blends were prepared by radical polymerisation of styrene preliminarily swollen in the PIB matrix.Electron microscopy studies did not reveal the formation of PS particles; however, cloudiness and rougher surfaces of the samples observed with increasing the content of PS are evidence of phase separation. Mechanical properties of PS are strongly dependent on its molecular mass. A decrease in the molecular mass probably favours compatibility of PS and PIB and an increase in the degree of grafting of the PS chains onto PIB, which manifests itself as increased ultimate strength, elongation at break and the fracture energy of the samples (at close PS content). Information on polymer blends based on higher PO is scarce and fragmentary. 3. Preparation of polyolefin ± polyolefin compositions by consecutive polymerisation In the preceding section of this review we considered the forma- tion of PPC in the course of the synthesis of one of the components.However, the in situ approach is also efficient if two or more polyolefin components are formed during the syn- thesis. This type of PPC is prepared by consecutive polymerisation of monomers on metal complex catalysts, which can be done { This was also observed for polymer ± polymer compositions PS ±PMMA (the Flory ± Huggins parameter for this system is 0.0350.010).126Molecular polymer ± polymer compositions. Synthetic aspects 3 2 1 Figure 10. Formation of nascent morphology in the preparation of polyolefin compositions under conditions of polymerisation compatibil- ity.(1) Catalyst subparticle; (2) polymer prepared in the first stage of polymer- isation (phase 1); (3) polymer prepared in the second stage of polymer- isation (phase 2).128 immediately in the polymerisation reactor using the Ziegler ± Natta catalysts. The dispersity of the polyolefin compositions obtained is determined by both the size of the catalytic species (most often, TiCl3) and peculiarities of the polymer morphology. Salient features of such PPC, differing from polymer blends of identical chemical compositions, are high molecular masses of the components and the presence of corresponding random and block copolymers localised within the interfacial layers.128 A scheme illustrating the preparation of this type of PPC is presented in Fig.10. The growing PP chains linked into crystalline fibrillae form a specific `skin' around the TiCl3 microcrystal. For instance, after completion of the polymerisation of propylene and removal of excess PP down to the specified post-polymerisation content, the reactor is filled with ethylene and then copolymerisa- tion of ethylene with propylene on `living' PP chains is carried out, followed by the homopolymerisation of ethylene. In this case, nucleation of the polymeric phase of PE occurs in the bulk of the globular PP particles formed in the first stage. The growing PE phase causes deformation of the `skin' of the first submicroparticle and penetrates the PP layer. High-molecular-mass PE dispersed in the PP matrix represents particles of submicron size and is strongly amorphised. Hence, compatibilisation via polymerisa- tion involves fine mixing of the components, which is accompa- nied by the formation of interpenetrating phases. The properties of the resultant product are determined by balance between its rigidity and strength (impact toughness).Semicrystallinity of the phase of isotactic PP has a crucial effect on the mechanical properties of these materials. The chemical composition of the elastomeric component and its content in particular PPC is also of great importance. Polymer ± polymer compositions prepared using this approach consist of crystallisable polymers and are called polyallomers.129 Studies on the morphology of polyolefins synthesised by consecutive polymerisation of propylene and ethylene in the presence of the TiCl3 ± AlEt2Cl catalytic system showed 130, 131 that the PE phase formed in the second stage causes deformation of the preliminarily formed PP layer followed by its disturbance and penetrates to the particle surface. A similar phenomenon was also observed for the polymerisation of propy- lene in the second stage of the process.Here, the PP phase penetrates the PE or SKEP layer formed in the first stage.132 The same is true for polymerisation of the butene ± ethylene pair. Consecutive polymerisation also allows preparation of homoge- neous ternary polyallomers containing, e.g., the PP, PE and PB phases.133 VI. Improvement of compatibility of polymeric components by introduction of compatibilisers The most widely used strategies of improvement of the compati- bility of immiscible polymeric components involve such approaches that initiate strong specific interactions (ion ± ion and ion ± dipole interactions, hydrogen bonds, etc.), formation of IPN, cross-linking, etc.Compatibility of the blend components is improved due to strengthening of the interfacial layers. Compa- tibilisation of different classes of polymers is achieved by intro- ducing polymeric additives (specific `concentrates' of the interface 13 phases) that favour manifestation of the above-mentioned com- patibilising factors. These additives favour the formation of different types of bonds between the blend components and improvement of microheterogeneity of the systems in the stage of dispersion blending.Compatibilisers represent reactive copoly- mers containing chain segments that are compatible or miscible with both components of the PPC. These additives are of consid- erable interest as an alternative to conventional or graft copoly- mers. They can both be synthesised in the polymerisation stage and provide a means for in situ modification of the systems under study. This approach allows controlling the surface tension at the interface and the interfacial adhesion, thus providing more homogeneous distribution of the dispersed phase. The third component also acts as plasticiser. It has been known for long 134 that block copolymers (in regard to polymer blends, they are often considered as `oil-in-oil'- type emulsifying agents 135) can emulsify polymer dispersions.This prevents phase separation. For instance, the addition of an AB diblock copolymer to a binary blend of polymers A and B causes a marked compatibilisation effect.136 Moreover, a diblock copolymer AC can be used as a link between immiscible polymers A and B provided that polymer C is miscible with polymer B.137 Yet another version of this approach involves blending of two incompatible polymers A and B using a diblock copolymer CD provided that both A and B are miscible with both C and D.138, 139 Blending of the diblock copolymer with the homopolymers having identical monomer units is a thermal process. However, if blend- ing of a diblock copolymer consisting of blocks with different chemical nature with two immiscible polymers is accompanied by exothemic interaction between the blending pairs of polymers, this interaction can become an additional factor of compatibility of the immiscible components.The most widely used compatibilisers represent additives containing anhydride (most often, maleic anhydride, or MA),140 ± 145 carboxylic,146, 147 epoxy 148, 149 and oxazoline 150 functional groups which react with the amino or carboxy groups of various thermoplastic polymers. Binary or ternary blends are also compatibilised using commercial products. Most often, these are PE-g-poly(maleic anhydride), ethylene ±butyl acrylate ±MA terpolymers (91 : 8 : 1) or poly(ethylene-co-vinyl acetate) (91 : 9) modified by grafting ofMA(1%).Here, we restricted ourselves to the typical examples. 1. Compatibilisation of polyolefin blends Weak interfacial interaction deteriorates the properties of compo- site materials based on PO blends. Compatibilisation of PO± PO block copolymers allows improvement of the miscibility of blend components and affects the properties of the interfacial layer in heterogeneous PO blends. If the molecular mass of a block copolymer is sufficiently large, it can act as an anchor at both sides of the interface. In particular, compatibility of a blend of isotactic PP and PE is often improved by using SKEP as the interfacial modifier.151 ± 153 The cohesive energies of LDPE and ethylene/propylene copolymers in the amorphous phase are similar;93 therefore, they are miscible on the molecular level.The addition (even in small amounts) of a random ethylene/propylene copolymer to the blend of HDPE and PP improves the impact toughness of the PPC;154 that is, a portion of SKEP is dissolved in the amorphous domains of PE and PP, thus forming the interface layer. Multicomponent blends of LDPE, HDPE, isotactic PP, PB and poly(methylpentene) and recycled PO± PO blends can also be compatibilised by melt blending with ethylene/propylene block copolymers used as additives. An even more pronounced compa- tibilising effect is achieved by using synthetic ethylene ± propyle- ne ± diene terpolymers (SKEPT) as additives to these polymeric compositions.155 Grafting of compatibilisers onto a particular blend component (PE or PP) is also widely used.This approach allows improvement of blend processing conditions and an increase in the performance of the equipment used.14 The mechanical properties of the PPC based on different types of PE (LDPE, HDPE, LLDPE and hydrogenated polybutadiene) and melt-mixed PS exhibit a strong dependence on the content (usually 2% to 10%) and molecular mass (60 000 to 270 000) of copolymer G [G stands for poly(hydrogenated butadiene- b-styrene)] used as an additive (see Fig. 11).156, 157 The hydro- genated triblock styrene ± ethylene ± butadiene ± styrene terpoly- mer characterised by the total molecular mass of 35 000, the molecular mass of the central block of 7500 and the PS content of 29% is an efficient interfacial modifier for the HDPE± PS systems.158 ± 160 Stability of polyolefin blends is often improved using ternary blends.For instance, the ternary blend of PP, polybutene and hydrogenated oligocyclopentadiene (50 : 30 : 20) 35 has one glass transition temperature, 289 K (cf. a value of 286 K obtained from calculations using the Fox equa- tion), which confirms compatibility of the blend components. a s /MPa 40 30 2 1 20 20 40 60 80 100 0HDPE content (mass%) c e (%) 100 50 2 10 1 51 20 40 60 80 100 0 HDPE content (mass%) Figure 11. Yield stress (a), tensile strength (b) and relative elongation at break (c) plotted as functions of the component ratio of composition HDPE± PS without (1) and with addition of 10% copolymer G (2).Graft, triblock and star copolymers have high emulsifying ability and are even more efficient compatibilisers. These types of blends based on PP, HDPE and SKEPT (as a rule, with ethyl- idene-2-norbornene as the third monomer) are of commercial value and have therefore been the subject of intensive research.161 ± 163 If some fraction of the elastomeric constituent (SKEPT) of the blend of PP, SKEPT and PE is replaced by PE, this system can exhibit a higher or lower impact toughness depending on the blending conditions of the components, their rheological proper- ties and viscosities.164 Sometimes, the interfacial adhesion is improved by carrying out the reactions of dynamic selective cross-linking (see below).165 2.Compatibility of polyolefin ± polyamide systems Polyamides (PA-6, PA-66, PA-12, etc.) exhibit high mechanical strength, rigidity and good processability and are therefore widely used in the automotive industry and for production of textile fibres. They have high melting temperatures and oil resistance. However, PA are characterised by ready deformation at low temperatures, cracking and high moisture absorption, which imposes limitations on possible applications of these materials. b sb /MPa 40 2 30 20 1 100 20 40 60 80 100 HDPE content (mass%) A D Pomogailo Polyamides are often used in the form of blends with low-module polymers, such as PO and, in particular, LDPE, which exhibits excellent elastic properties at low temperatures and low moisture absorption.Using such blends reduces the cost of materials. The high commercial value of the systems based on PA-6 and LDPE has attracted considerable attention.166 Interest in the compati- bility problem of PO and PA is also due to the fact that the polyolefins and polyamides represent two main groups of materi- als for recycling.167, 168 The components of the two-phase system LDPE± PA-6 are incompatible; therefore, this blend is characterised by weak interfacial adhesion and high dispersity of the LDPE domains. The properties of such PPC are determined by the size and shape of the dispersed phase; it should be noted that these parameters depend on the elasticity of the components.10, 169 The addition of interfacial modifiers, viz., block or graft copolymers (their content is 0.5% to 1%) leads to a decrease in both the degree of crystallinity of LDPE and the crystal lattice parameters and to improvement of the mechanical properties (especially, yield stress, Young's modulus, impact toughness, relative deformation).The components of this system were compatibilised by the introduc- tion of the block copolymer of styrene, ethylene and butylene to the modified extrudate.170 Based on the analysis of the shape of thermomechanical curves of the blends, it was suggested that PA-6 was nearly molecularly dispersed in the LDPE matrix in the presence of 5% of the ternary block copolymer.In the case of melt mixing of blend components the introduc- tion of a third component can facilitate the blending and improve the mutual solubility of the components, thus providing predom- inant interaction of functional groups with a particular compo- nent. The most detailed studies on the problems of graft functionalisation of PO with MA or acrylic acid were carried out taking compositions of PO and PA as examples.171 ± 173 In situ grafting in the extruder at 473 K in the presence of dicumyl peroxide (DP) (PP :MA:DP=94 : 6 : 0.3) was followed by gran- ulation. Free-radical melt grafting of MA, glycidyl methacrylate and diethyl maleate (DEM) onto PO was also studied in detail.174, 175 In situ preparation of PPC based on PA-6 and LDPE that was graft functionalised with MA or DEM led to interesting results.176 The addition of PP ±MA compatibilisers improved the interfacial adhesion and the impact toughness of the compositions of PP and PA-6.177, 178 Probably, the interfacial adhesion becomes stronger due to the formation of hydrogen bonds between the end amino groups of PA and grafted MA; this leads to nearly complete homogeneity of polymer blends.141 It is appropriate to use functionalised PO, e.g., carboxy derivatives for compatibilisation of polyamide matrices and the dispersed PO phase.179, 180 As a rule, grafting of carboxy or anhydride groups onto the PO chains is performed during melt mixing.Reactions of the amino or amido groups of the PA phase with functional groups of the PO phase result in the formation of in situ prepared copolymers characterised by weakened interfacial tension and improved interfacial adhesion of their components.181 Improvement of fracture toughness is often achieved by energy dissipation within the interfacial layer of immiscible components of PPC.The introduction of a block copolymer additive should result in a decrease in both the interfacial tension and phase dispersity; in many instances this depends on the number of `stitches' between the two phases.182 Nearly perfect compatibility was observed, e.g., upon addition of small amounts of the PP/MA copolymer to blends of PE and PA-6.183 Park et al.184 studied the morphology, thermal and rheological properties of the composition of PP, PA-6 and PP/MA (5%) including the dependence of its viscosity on the chemical composition at low shear rate (Fig.12). The results obtained point to the negative deviations from the additivity rule for the simple system PP ± PA-6 and to positive deviations for the blends of PP, PA-6 and PP/MA. These results were used for assessing the texture of the PPC and evaluating the chemical interaction between the starting polymers. Currently, the composition of SKEP, PA-6 and the melt-preparedMolecular polymer ± polymer compositions. Synthetic aspects a Z /P 104 103 102 b 104 103 102 103 10 102 Figure 12. Shear viscosity (Z) of compositions PP ± PA-6 (a) and PP ± PA-6 ± (PP/MA) (b) plotted vs. shear rate. (a) PP (1), PP ± PA-6=75 : 25 (2), PP ± PA-6=50 : 50 (3), PP ± PA-6= 25 : 75 (4) and PA-6 (5); (b) PP ± PA-6 ± (PP/MA)=75 : 25 : 5 (1), PP ± PA-6 ± (PP/MA)= 50 : 50 : 5 (2 ) and PP ±PA-6 ±(PP ±MA)=25 : 75 : 5 (3).graft copolymer of carboxy- or anhydride-modified SKEP and PA-6 has been the subject of intensive research. Sometimes, more complex interfacial modifiers are used. For instance, in situ modification of the blend of PP and MA (the degree of grafting was 0.3 mass% or 0.8 mass %) with poly(pro- pylene glycol)-bis-(2-propyamine) with Mw=230, 400 and 2000 was reported.185, 186 This was done based on the fact that polymers containing ether groups [e.g., poly(ethylene glycol) or poly(pro- pylene glycol) (PEG or PPG)] are compatible with PA due to the formation of hydrogen bonds187 and resulted in the PP±MA± co- PPG composition which exhibits good compatibility with PA-6 due to hydrophobic-hydrophilic interactions between the ether Table 7.Mechanical properties and melt flow indices of PPC modified in a different manner.94 PPC PA-6 ± PP PA-6 ± PP ±PP-g-MA (0.3%) PA-6 ±PP ±PP-g-MA ± co-PPG (400) b PA-6 ±PP ±PP-g-MA ± co-PPG (2000) b PA-6 ±PP ±PP-g-MA (0.8%) PA-6 ±PP ±PP-g-MA ± co-PPG (400) b PA-6 ±PP ±PP-g-MA ± co-PPG (2000) b 11.9 13.8 15.3 19.4 25.6 21.4 38.6 a Dimensionality: grammes of polymer flown over a period of 10 min at 513 K and a standard load of 21.2 N. b Figures in parentheses denote the molecular mass of PPG. 12345123 u /s71 Component ratio (by mass) 30 : 70 30 : 70 : 5 30 : 70 : 5 30 : 70 : 5 30 : 70 : 5 30 : 70 : 5 30 : 70 : 5 15 groups of PPG and amido groups of PA-6.Some characteristics of mechanical properties of the PP ± PA-6 compositions prepared in different manner are listed in Table 7. It should be noted that all the compatibilised compositions have lower melt flow indices than the simple polymer blend. Increasing the degree of grafting ofMA and the chain length of PPG leads to improvement of the strength properties of the compositions. Schematically, the formation of such a PPC can be represented as follows: O Me Me O NH2 +H2N CH CH2 (OCH2CH)n O PP O Me Me PA-6 NH2 (OCH2CH)n NCHCH2 O O Me Me NHCO(PA-6) (OCH2CH)n NCHCH2 PPG O n=2± 3, 5± 6, 33.The limiting factor in the design of compatible systems with optimum characteristics is the concentration of reactive groups. Compatibilisation of immiscible polymers using isocyanate groups which react not only with the amino or carboxy groups, but also with hydroxy groups has been rarely reported. For instance, blending of LDPE and PA-6 involves the modification of LDPE by grafting the product of the interaction of 2-hydroxy- methyl methacrylate and isophoronediisocyanate (the reaction is carried out in xylene at 393 K in the presence of 1% DP in N2 atmosphere) followed by mixing the modified LDPE and PA-6 in N2 atmosphere at 518K for 4 h.188 In the course of blending the NCO groups interact with the terminal amino groups.Chemical bonding between the butyl acrylate or MA groups and terminal amino groups is formed in the compositions based on PA-6 and PE ±MA, in the ethylene ± butyl acrylate ±MA (91 : 8 : 1) ter- polymer and in the ethylene/vinyl acetate (91 : 9) copolymer modified by grafting MA (1%).41 O CO2H H2N PA-6 CH C CH2 7C4H9OH OC4H9 CH2 Strength properties /MPa Melt flow index a fracture toughness tensile strength Unnotched impact toughness /MPa 2.2 2.5 3.3 3.8 7.7 8.0 11.5 25.0 30.2 31.3 32.3 34.1 34.4 35.6 19.6 9.3 10.0 6.7 6.7 9.6 6.416 CO2H CH2 OC.NH PA-6 CH CH2 or Me O C CO CH2 CO2H H2N PA-6 O CH O CO CH2 CH CH2 CH2 CH CH2 CO2H CH2 NH PA-6 CO CH CO2HMe O C CH CH2 CH CH2 CH2 O CH2 3.Third component in the compatibilisation reactions of polyolefins with other polymers Compatibility of PVC and chlorinated PE with ethylene/vinyl acetate copolymers (EVAC) is often used to improve the mechan- ical properties of PPC.78 Such a modification can also be performed in the polymerisation stage. For instance, blends of styrene and acrylonitrile or styrene and MMA are selectively grafted onto chlorinated PE using TiCl3.79 The addition of PE ± g- PS (merely 2.5 mass %) to the PE ± PS films substantially improves their strain ± stress properties.80 The LDPE±EVAC± PETP system has considerable advant- age over the LDPE± PETP composition, namely, the presence of polar functional groups in the copolymer provides the possibility of controlling the contact reactions, i.e., the chemical interaction at the interface.189 It is solely the polar interaction of the PP ±MA additive (4 mass% MA) and the carbonyl groups of the blend of PP and the liquid-crystalline polymer that is responsible for improvement of the mechanical properties of this system.190 A number of reaction compatible blends of PO [with PE ± g-poly(glycidyl methacrylate) or PE ± glycidyl acrylate (GA) as compatibiliser] and polycarbonate (PC), PETP and other polymers was reported.142, 191, 192 The composition of PP and PC can be compatibilised using the PP ±GA graft copolymer (2.5% ± 20%), which favours weakening of the interfacial inter- action, improves the mechanical properties of the composition, increases the crystallisation temperature of PP and changes the blend morphology.In the molten state (in both the internal blending chamber and twin screw extruder) the epoxy groups of the third component react with the terminal groups of PC by the mechanism shown below C PC PP CH CH2 +HO O CH2 CH CH2 O O Me O PC C O CH2 CH CH2 PP CH CH2 OH O Me Interestingly, the coupling reaction between the polymeric constituents makes extraction of the entire PC phase from the ternary blend impossible, as in the case of compositions of PE and PMMA, PE and PVC, LLDPE and PA-6, etc. This method allows compatibilisation of LLDPE modified by grafting MA193 and PVAlc (the component ratio is 70 : 30). Another example is provided by the analogously modified com- position of PP and liquid-crystalline polymer, which exhibits improved strength, deformation characteristics and the modulus of elasticity.194 Numerous methods for the introduction of reactive polymeric reagents capable of in situ compatibilising of PO and PS have been reported.149, 195 One of them involves generation of reactive radicals on the surface of PO or PS to form the grafted A D Pomogailo layer 196, 197 (detailed consideration of this approach is presented below).The morphology and properties of PPC based on PP and ethylene/dimethylaminoethyl methacrylate copolymers contain- ing up to 27 mass% dimethylaminoethyl methacrylate units by mass have been studied in detail.198 The components of this PPC are incompatible and form two-phase structures.As the content of the copolymer increases, the compositions become brittle and the interfacial adhesion between the constituent phases weakens substantially. The third component, e.g., PS ± b-poly(ethylene-co-bute- ne) ± b-PS (PSBECB) is often used as plasticiser, especially for improvement of the impact toughness of brittle polymers.199 This compatibiliser represents a microheterogeneous thermoplastic elastomer in which the PS end blocks (as a rule, le sethan 35 mass%) are linked into rigid domains that form a lattice in the flexible matrix of the ethylene/butene copolymer (EBC). It is used if one component of a PPC is compatible with the EBC blocks while the other component is compatible with the PS blocks.This additive allows the preparation of polymer blends containing not only PP, HDPE, LHDPE, but also the ABS plastics and other components.200, 201 In studies of ternary systems containing, e.g., PA, a modified additive, PSBECB ±MA, is used.202 It is thought that the EBC blocks are compatible with PP and the interfacial adhesion between PA and PSBECB is provided by the formation of imides in the reaction of the terminal amino groups with anhydride. In this case, the formation of domains with rigid polyamide core and flexible elastomer `skin', which favours improvement of the strength and viscosity without changing the polymer rigidity, cannot also be ruled out.203 Finally, the PSBECB constituent of ternary blends containing impact-resistant PS reduces the degree of crystallinity of PP.204 The modulus of elasticity and yield stress of PP ± PSBECB blends decrease as the content of the block copolymer increases.205 Good compatibility of PSBECB and PP can be due to the random nature of the copolymer in the EBC block.The effect of repulsion 206 between different polymer chain segments in EBC (though the repulsion between the ethylene and butene segments must be very weak) can improve the compati- bility of the PE ± PP pair. It was shown that PSBECB is a good compatibiliser for polymer blends based not only on isotactic PP, but also syndiotactic PP.207 In both cases, the additive, PSBECB, favours a decrease in the modulus of elasticity, yield plateau and an increase in the yield stress.The use of the compatibiliser consising of PSBECB and MA allows reduction of the tear resistance for the block copolymer and isotactic PP. Summing up, we considered the most widely used strategies of using polymeric compatibilisers for improvement of compatibility of polymer blend components. It shoud be mentioned that polymeric supports for metal complex catalysts, obtained by the grafting of corresponding, in situ polymerisable monomers onto PO (see the grafting scheme presented in Section III.1) contain the interfacial modifier which improves compatibility of the compo- nents of PPC. VII. Polyolefin-based polymer blends prepared by reactive blending Many polymeric compositions considered above are prepared by reactive blending in extruder reactors.Since this process is accompanied by numerous chemical reactions between the com- ponents or specially introduced compatibilisers, it would be reasonable to call the blends obtained `reactor blends' or `extru- sion blends'. However, in this review we will use the term `reactor blends' in a somewhat narrower sense. As a rule, functionalisation of polymers by graft poly- merisation represents an individual stage of the process and is followed by their blending. Currently, extruder reactors are widely used as continuous reactors combining the stages of dispersion of blend components and chemical reactions betweenMolecular polymer ± polymer compositions. Synthetic aspects them.180, 208 ± 210 In particular, chemical modification of thermo- plastics in a twin screw extruder is thought to be a promising procedure for production of novel materials.Extrusion combines the in situ functionalisation of blend components and the formation of block or graft copolymers at the interface. Inter-chain copolymers prepared by extrusion are often used in statu nascendi for compatibilisation of thermo- dynamically incompatible blend components; this particularly concerns the PE ± PA-6 blends modified using the third compo- nent.211 In these cases, at low concentrations of the interfacial modifier and initiator the additives favouring improvement of the relative elongation at break and the impact toughness perform three functions.First, they act as reactive interfacial modifiers that improve compatibility of the components in the molten state. Second, they are grafted onto PO by the radical mechanism. Third, their functional groups react with the terminal groups of the components (e.g., PA), thus forming covalent or hydrogen bonds. Most often, reactive blending is performed in the presence of peroxide (0.5% to 2.0%) with MA, bifunctional or multifunc- tional monomers as interfacial modifiers. The additives having long hydrocarbon chains (e.g., alkylmaleic monoamide or alkyl- maleic monoether) are more efficient compatibilisers than the short-chain additives. Non-monomeric additives are used more rarely. For instance, the addition of stearic acid (5%) to the blend of LDPE and PA-6 (75 : 25) allows control of the phase morphol- ogy.212 The effect of in situ reactive blending in an extruder reactor was also observed in studies of blends of LLDPE modified by grafting of maleic acid or MA with PA-6 or PA-11.213 ± 215 In situ mechanochemical preparation of self-compatible poly- mer blends by solid-phase extrusion grinding of immiscible poly- olefin constituents represents a variant of the approach in question.16, 216, 217 The process is accompanied by scission of polymer chains and, hence, generation of free radicals capable of entering the reactions with another polymer chain or undergoing recombination with other macroradicals (this was experimentally proved taking the PE ± PP system as an example), thus forming a block copolymer or a graft copolymer.The latter acts as an in situ generated compatibiliser. This procedure is particularly conven- ient when performing recycling since the process proceeds at a low temperature, so both the mechanical and oxidative degradation are reduced to a minimum. Nearly three decades ago a new product, viz., a thermoplastic blend based on SKEP finely dispersed in the PP matrix appeared in the world market. Such polyolefin blends are prepared by dynamic vulcanisation and called thermoplastic elastomers or thermoplastic vulcanisates.218 They represent multicomponent polymer blends which possess the mechanical properties of elastomers at room temperature and the rheological properties of thermoplastic polymers at elevated temperatures. A salient feature of the procedure for their preparation is the combination of blending and vulcanisation (with SKEPT as the third compo- nent). This leads to the formation of PPC with specific morphol- ogy, namely, two-phase systems in which the particles of the cross- linked elastomer of size 1 to 10 mm are dispersed in a continuous thermoplastic polymer matrix and the content of the elastomeric component exceeds that of the thermoplastic polymer. Usually, such systems are cured with DP.Dynamic vulcanisation can be performed in two ways (Fig. 13). Thermoplastic elastomers offer improved processability, e.g., their melts behave as thermoplastics despite the presence of the cross-linked elastomeric constituent.219 The thermoplastic nature of the blend can be due to the curing dynamics features which prevent the formation of a three-dimen- sional rubber network in the dispersed phase.However, the melt viscosity of the PPC enriched with PP increases as the content of SKEPT increases up to 25%. The crystal structure of the starting PP can either change or remain unchanged depending on the procedure for preparation. The structure and properties of this type of dynamically cured polymer blends, peculiarities of their 17 Calendering of SKEPT and DP (353 K, 10 min) Pre-blending of PP and HDPE (363 K, 15 min) Dynamic vulcanisation of SKEPT and DP (363 K, 15 min) Dynamic vulcanisation of pre-blended PP and HDPE in the presence of SKEPT and DP (363 K, 15 min) Blending of pre-blended PP and HDPE in the presence of SKEPT (363 K, 15 min) Vulcanised blend Tests Figure 13.A scheme of dynamic vulcanisation process. rheology and structural characteristics have been the subject of detailed studies (see, e.g., Refs 220 and 221). The deformation mechanism of thermoplastic elastomers based on isotactic PP and SKEPT involves the deformation of the structure of the compo- sition in the first stage followed by simultaneous deformation of both the cross-linked SKEPT domains and PP macromolecules; the strength and elongation at break are determined by the tension of the cross-linked elastomer.222 Blends of PO and SKEPT can be prepared using numerous methods that are not only of scientific, but also practical value.One of these procedures involves the formation of blends of cross- linked SKEPT (ternary SKEP) and PO using roll mills or peroxide-initiated dynamic vulcanisation in an extruder. This method was first proposed for PPC;223 however, at present it has found wide industrial application for production of plastics and thermoplastic elastomers. It offers such advantages as short blending time, short-term processing cycle, small energy expendi- ture for recycling and the possibility of easy control over the properties of products by varying the component ratio. Main conditions for the preparation of materials based on dynamically vulcanised polymer blends with optimum properties have been formulated and a number of factors determining the structure and properties of such blends were pointed out.224 For instance, polymeric compositions with high content of rubber can be used as thermoplastic elastomers, while polymer blends with high content of plastic can be used as rigid plastics `enriched' with rubber.Peculiarities of their morphology improve processability despite the presence of the cross-linked elastomeric component.199 Dynamic vulcanisation also allows the preparation of such PPC as, e.g., SKEPT± PP, SKEPT± PP ± HDPE, LDPE± SKEPT, etc.225, 22618 VIII. In situ preparation of diblock chemical hybrids In this section we will consider diblock polymeric compositions in which one of the block materials does not belong to the poly- olefins.From the standpoint of structure and properties this group of PPC comprises diblock copolymers, namely, graft copolymers in which one component (the polymeric support) represents a macromolecular organometallic compound. These copolymers can be prepared using the Ziegler ± Natta catalysts and by consecutive polymerisation of olefins. Lithium-containing block copolymers of ethylene and buta- diene 227 and of ethylene and isoprene228 seem to be the first representatives of this group of PPC. The molecular mass of the PE blocks varied from 13 000 to 36 000. Since ethylene polymer- isation was accompanied by cross-linking of 1,2-butadiene units, this method was found to be more appropriate for the preparation of ethylene/styrene diblock copolymers 229 or blends consisting of PS blocks and blocks of ethylene/propylene copolymer (EPC).230, 231 The synthetic procedure involves treatment of the organolithium compound with TiCl4 to replace lithium by tita- nium, reduction of titanium and the formation of the catalyst component for polymerisation of ethylene (the molecular mass was 7000 ± 33 000 for PE blocks, 10 000 ± 11 000 for PS blocks and 33 000 for EPC blocks).The PS ± b-poly(ethylene-co-propylene) composition was also synthesised using the PS ± Li macroanion and TiCl4.232 The molecular mass of the EPC blocks was as high as 500 000, the Mw :Mn ratio was 2.24 to 4.23, the degree of crystallinity varied from7%to 20% and the content of ethylene units in the block was 70% to 80%.The efficiency of the catalyst, expressed as the number of polymer chains per Ti7C bond was rather high (0.88). According to the results of differential scanning calorimetry (DSC) studies, the thermogram (Fig. 14) exhibits a small peak of PS at Tg=369 K. Both the melting temperature (410 K) and relatively low molecular mass (Mn=63 000, Mw :Mn=1.4) point to a linear structure of the PE blocks.{ 413 T /K 393 373 353 Tg (PS) Tm(PE) DHendo Figure 14. A DSC scan of block copolymer PS ± PE.232 Among numerous methods for the in situ preparation of diblock PPC in the radical polymerisation of a particular compo- { For comparison, mention may be made that consecutive anionic copolymerisation of equivalent amounts of styrene and 4-vinylpyridine on LiBu at 195 K resulted in a PS ± b-P4VP (PS : P4VP=59 : 41) compo- sition (Mn=62 300),80 which was used as compatibiliser for blends of polar and non-polar polymers belonging to different classes.Anionic polymerisation of styrene in the presence of polymers is accompanied by chain entanglement of newly formed linear macromolecules and the polymer network of cross-linked chains 233 and results in macroscopically heterogeneous gels. The formation of cross-linked or grafted chains was also observed in the anionic polymerisation (using LiBu, AlEt3) of diisocyanates in the presence of butadiene, isoprene and butadiene-styrene rubbers.234 In the last-named case the formation of graft polymers occurs due to the involvement of the side double bonds in positions 3 and 4.A D Pomogailo nent, two procedures are most widely used. The first method (preparation of IPN) is in essence a radical polymerisation of vinyl monomers in the presence of soluble or swollen polymers (here, a typical example is provided by the polymerisation of styrene in the presence of polybutadiene). The second procedure involves the use of polymeric peroxide or azo initiators (see, e.g., Refs 235 and 236) whose concentrations are comparable with those of the components. In situ preparation of diblock PPC can proceed either by conventional polymerisation mechanisms or as a stepwise (con- secutive) process involving `switching' of the polymer formation mechanisms (in this case, each block is formed on the active centres of different chemical nature).The latter can be exemplified by the preparation of diblock copolymers based on olefins and vinyl monomers in the presence of the TiCl3 ± AlEt3 ± ZnEt2 system.237 The stage of ethylene polymerisation involves the formation of polymer chains with terminal zinc ± carbon bonds that provide the possibility for the next stage, TiCl3-catalysed polymerisation of vinyl monomers, to proceed. By choosing the most appropriate combinations of the olefin and vinyl monomers one can prepare block, graft or block/graft copolymers represent- ing either the end products (PPC) or compatibilisers of PO and vinyl polymers. Methods for the preparation of modified poly-a-olefins by switching of polymerisation mechanisms (anionic-coordination mechanism changes to cationic or anionic mechanism, anionic mechanism is replaced by the radical one, etc.) have long been developed.A typical example is as follows:238 first, polymerisa- tion of propylene is performed on the TiCl3±Et2AlCl catalytic system. After some time, the unreacted monomer is separated and 4-vinylpyridine and SnCl4 or BunLi are added. This procedure can also be used for preparation of polyolefin chains with the MMA, vinyl ketone, acrolein and other fragments. Isotactic PP synthes- ised on the ethylenebis(tetrahydroindenyl)zirconium dichloride (C2H4[H4Ind]2ZrCl2) ± methylalumoxane catalytic system by suc- cessive treatment with the borane ± dimethyl sulfide complex and pentane-1,2-diyldimagnesium dibromide was transformed into PP with the MgBr terminal groups.239 The modified PP was then used as anionic initiator of polymerisation of MMA at tem- peratures between 195 and 318 K.1H NMR and gel permeation chromatography studies confirmed the formation of the PP ±b-PMMA system (Mn=5300 ± 3000) which can be used as polymeric additive to blends of PP and PMMA. It was established experimentally that the polymerisation mechanism changes from anionic-coordination to radical during the preparation of diblock copolymers of PE and PMMA.240 The addition of MMA to the VCl4 ± AlR2Cl catalytic system in the course of anionic-coordination polymerisation of ethylene leads to its inhibition and initiation of the polymerisation of MMA (probably, as a result of radical induced cleavage of the V7C bond).The process results in the PE ±b-PMMA system. CH2 nCH2 V R VCl4+AlR2Cl MMA CH2)nR V (CH2 mMMA CH2 R (CH2 CH2)n71CH2 CH CH2 CH2)n R (CH2 C O OMe m It is known that both homopolymerisation and copolymerisa- tion of polar monomers on metal complex catalysts proceed solely by the coordination mechanism in the absence of electron-donor additives and can follow both coordination and radical pathways in the presence of such additives. It cannot be ruled out that in many instances the polar monomer itself acts as electron donor.Molecular polymer ± polymer compositions. Synthetic aspects Kinetic studies confirmed that graft copolymerisation ofMMAis radical initiated.`Living' } coordination polymerisation of propy- lene on the V(acac)3 ± AlEt2Cl system was used for the synthesis of a diblock copolymer of PP and PMMA.241 The polymerisation proceeds at 195 K; raising the temperature up to 298K in the presence of MMA leads to the formation of a radical centre as a result of homolytic cleavage of the V7C bond. CH V3+ CH2 MMA 195 to 298 K Me Me MMA C V3+ CH2 CH CH2 298 K Me C O OMe PP ±b-PMMA The macroradical is stabilised by the vanadium ion, which provides the possibility for `living' polymerisation of syndiotactic PMMA blocks to proceed; chain termination in the radical polymerisation occurs on the organoaluminium compound. Diblock copolymers of propylene and methyl vinyl ketone were synthesised using the stereospecific Ziegler ± Natta cata- lysts.238 In first stage, a conventional anionic-coordination poly- merisation of propylene proceeds, while the second stage of the process (in contrast to the radical polymerisation of MMA) represents an anionic polymerisation of methyl vinyl ketone catalysed by terminal alkylaluminium groups. CH CH2 298 K P+nMe C O CH2 Et2Al CH2 P CH CH2 Et2AlCH2CH2 Me C O n No additional treatment is required to `switch' between the catalytically active centres in the schemes presented above since MMA itself acts as the `switching' agent.Transformation of the anionic process into a radical one was also observed by Souel et al.242 who studied polymerisation of isoprene in the presence of butyllithium.Treatment of the product, `living' oligomeric iso- propenyllithium, with Me3PbCl results in an adduct whose decomposition in the presence of styrene leads to the formation of PI ± b-PS. Let us consider yet another procedure for in situ preparation of PPC based on polymers belonging to different classes, namely, polyolefins and polydienes. We will dwell on the copolymerisation of olefin and diene monomers under the action of bifunctional immobilised catalysts. For instance, copolymerisation of ethylene and butadiene can be performed using complexes of Co2+ and V4+ immobilised on the PE ± g-poly(vinylpyrrolidone) (PE ± g-PVP) system.46 Alkylation of this heterometallic complex with AlR2Cl leads to the formation of active centres initiating the polymerisation of ethylene (V-centres) and butadiene (Co-centres).CH CH2 N +AlR2Cl CoCl2 O VCl4 ethylene CH CH2 N n butadiene CoO CH CH CH2 CH2 CH2 V CH2 n } Polymerisation proceeds until complete consumption of monomer; however, the addition of a fresh portion of the monomer leads to a restart of the process at the same rate.In the text below the combination of Co- and V-centres will be shown in a simplified form as coexist in the presence of ethylene and butadiene and retain their catalytic activity despite essential modification. Butadiene is known to be a strong inhibitor of ethylene polymerisation. The rate of copolymerisation of ethylene and butadiene on vanadium- containing active centres is low.Indeed, the rate of ethylene polymerisation decreases in the presence of butadiene (Fig. 15); however, inhibition of the reaction proceeding in the presence of heterometallic cobalt ± vanadium catalysts is less pronounced. In turn, the rate of butadiene polymerisation on the Co-centres also somewhat decreases in the presence of ethylene 243 whose mole- cules undergo insertion into the polymer chain (1 or 2 ethylene molecules per macromolecule), thus reducing the molecular mass of polybutadiene. The mechanism of this process is similar to that of chain transfer to polymer and consists in decomposition of the less stable s-alkyl cobalt derivative that is formed from the p-allylic product upon addition of ethylene. 0.15 0.10 0.050 40 Figure 15.Kinetic curves for the synthesis of HDPE± PBd compositions under comparable conditions. (1) Ethylene polymerisation on monometallic catalyst; (2) copolymerisa- tion of ethylene and butadiene on bimetallic catalyst; (3) copolymerisation of ethylene and butadiene on monometallic catalyst. CH2 Polymerisation rate /litre min71 CH CH2 CoCl+CH2 CH R R CH CH CH2 C4H6 A B RCH CH CH2 CH CH2 Here, route A corresponds to chain transfer while route B corresponds to copolymerisation of ethylene and butadiene. However, spatial proximity of both active centres and co-oper- ative chain propagation processes make migrations by, e.g., the inter-chain exchange mechanism 244 via transfer of the polybuta- diene chain to PE (route C) or recombination of polymer chains (D) quite probable.D P0 P00 Co P0 Co V P00 C V P0 P00 If these reactions are accompanied by chain disproportiona- tion, the P 0 ±P00 diblock copolymer is formed; persistence of 19 Co7P0. These active centres V7P00 123 t /min 80 CoCl CH2 CH2 CH2 CoCl CHCH C4H6R CH2 CH220 Table 8. Formation of PE ± polybutadiene block copolymers in the PPC prepared by copolymerisation of ethylene and butadiene on bifunctional cobalt ± vanadium catalyst.46 Content of polybutadiene units a in product (mass%) Ethylene consumption for polymerisation /g 2.7 (7) 38 (95) 24 (90) 14 (90) 0.5 b 0.5 1.0 2.0 a The value in parentheses is the content of 1,4-cis units.b For copolyme- risation of ethylene and butadiene on monofunctional catalyst (VCl4 on PE ± g-PVP). ethylene polymerisation after chain transfer results in a polyblock copolymer. Such a reaction mechanism is few and far between. Obtaining convincing proof of this mechanism presents consid- erable difficulties, since analysis of the structure of such `chemical hybrids' characterised by possible formation of interpenetrating polymer chains is a rather complicated problem. However, experi- ments with 14C-labelled butadiene allowed one to collect proof of the formation of block copolymers (Table 8). Quantitatively, the formation of the random copolymer was estimated by comparing its radioactivity with that of the ethylene/butadiene copolymer synthesised on the Co-containing monometallic catalyst.The stereosequence of polybutadiene units (mostly 1,4-cis-butadiene) in the block copolymers is such that mutual influence of the monomers is weak. Probably, this method for in situ preparation of diblock chemical hybrids will appear to be useful in the case of PPC based on polyolefins and polyallene and on other polymers.} Synthesis of binary polymer networks belongs to new avenues of current research. Binary networks consist of two chemically different polymer chains linked by one covalent bond, whereas IPN comprise two independent networks that cannot be separated without cleavage of covalent bonds.246 The distinctions between these systems are illustrated in Fig. 16. a b 1 1 2 2 Figure 16.The structure of two-component (a) and interpenetrating (b) polymer networks; polymer A (1) and polymer B (2). As applied to polyolefins, binary polymer networks were prepared for, e.g., the poly(isobutylene) ± poly(dimethylsiloxane) system.247 Allyl-tritelechelic PIB was synthesised by `living' poly- merisation of isobutylene at 193 K using the tricumyl chloride ± TiCl4 ±N,N-dimethylacetamide catalytic system. Allylation at the end groups was performed using allyltrimethylsilane. Allyl-tri- telechelic PIB undergoes cross-linking with SiH-ditelechelic poly- } It should be noted that copolymerisation is also performed using other catalytic systems. For instance, the microblock copolymer of butadiene and styrene is formed when the system LiR ±KOR is used.245 In this case, butadiene undergoes polymerisation of the Li centres, while styrene is polymerised on the K centres.The microblock size is determined by the frequency of C7Li>C7K transitions. A D Pomogailo (dimethylsiloxane) by hydrosilylation under the action of Pt-containing catalysts. This method was used for preparation of novel thermoplastic elastomers PS ± b-PIB.248 Thus, in situ prepared PPC can be synthesised from various classes of incompatible monomers following any polymer forma- tion mechanism. The diblock copolymers considered in this section are most often used as polymeric additives that improve compatibility of immiscible polymers rather than individually. IX. Preparation of conducting composites using the principle `compatibilisation via polymerisation' Mixing of conducting and dielectric particles is a well known procedure used for preparation of conducting polymers.249 The synthesis of a conducting polymer surrounded by a dielectric matrix is often used to improve the strength and electrical conductivity of polymeric materials.250, 251 According to percola- tion theory,252 an increase in the content of the conducting polymer to a particular critical value causes a sharp decrease in its electrical resistance (the so-called percolation threshold is reached).The method for in situ preparation of molecular compositions containing a polyolefin as one component can also be extended to the conjugated polymers (in particular, doped PAc).Conducting PAc films are very brittle, which is detrimental to their operating characteristics, and unstable (film destruction occurs with time). Therefore, the preparation of materials that can be processed in a conventional manner requires modification of such films. An attempt at preparing a composite material with good electrical properties by simple dispersion of powdered PAc obtained by catalytic polymerisation of acetylene according to Shirakava using the Ti(OBu)4 ± AlEt3 system in the PE matrix failed,253 since only large PAc aggregates of size 10 to 100 mmwere formed. Blending of components on a molecular level can be achieved by performing direct polymerisation of acetylene on any surfaces moistened with the catalyst solution.In particular, such a polymerisation can be carried out on 300 mm-thick LDPE films impregnated with Ti(OBu)4 and alkylated with AlR3.253 The reaction on the surface of the PE powder impregnated with the same catalytic system is carried out in the liquid phase (in dibutyl ether) 254 or in the gas phase.255 Sometimes, polymerisation is performed from the gas phase in viscous media (e.g., silicon oil) in the presence of PE-supported catalysts.256, 257 Polystyrene solu- tions (5% ± 20%) in toluene can be considered as examples of such high-viscosity (10 ± 400 sP) media. Suspension polymerisation allows the preparation of materials with good electrical and mechanical properties. High density PE films containing PAc (6 mass%± 10 mass %) remain flexible and elastic and exhibit high strength.The conductivity of the compositions prepared by doping such films with iodine or lithium lies between 10 and 100 O71 cm71 and increases upon film drawing due to alignment effects.255 Polyacetylene obtained by polymerisation of acetylene on a PE matrix at 383 K has a trans-structure stereosequence and undergoes cross-linking with time, thus forming a three-dimen- sional structure in the bulk of the PE matrix. Such composite materials are characterised by a conductivity of 1 to 5 O71 cm71. High-molecular-mass PAc latex particles are formed on the Luttinger catalysts in the presence of surfactants, mainly copoly- mers of styrene or PE blends and poly(ethylene oxide) (PEO).258 To improve the adhesion between the conducting components and the polymeric support (i.e., films based on PE, PP, SKEP, PS, etc., and shaped as necessary), the latter is modified by grafting of functional groups [poly(acrylic acid) (PAAc) or poly(allyl alcohol) (PAA)] that serve as `immobilisers' of the catalyst of acetylene polymerisation.58 After polymerisation these supports become coated with a dark PAc film 10 ± 50 mm thick, which acquires conducting properties (see Table 9) after treatment with iodine vapours or other electron acceptor dopants such as SbF5 , AsF5 , FeCl3 and perchlorates.Molecular polymer ± polymer compositions.Synthetic aspects Table 9. Properties of conducting films prepared by polymerisation of acetylene in the presence of catalytic system Ti(OBu)4 ± AlEt3 and immo- bilised on dielectric supports.58 Support Specific thickness conduc- /mm Polymerisa- PAc film tion time /h Titanium content /mmol g71 tivity a PE PE ± g-PAA PE ± g-PAAc PE ± g-PAAc PE ± g-PAA PTFE ±g-PAA PVAlc 70.05 0.016 0.02 0.025 0.01 0.02 161077 6.2 4.2 14.4 6.7 2.4 3.9 3221332 traces 0.04 0.06 0.10 0.115 0.4 0.20 a Dimensionality is O71 cm71.In this case, we deal with the surface conductivity of the in situ prepared PPC, while the polymeric supports remain dielectrics. This method can also be used for the preparation of multi-layered PPC containing a conducting layer with controllable conductivity parameters. Materials based on this type of PPC are relatively easy to produce and retain their properties on long storage (more than three years).Probably, improved stability of these materials is due to the presence of a barrier preventing the oxidation of PAc. High-conductivity compositions, namely, block copolymers of PAc and PS are also prepared by `switching' the polymerisation mechanisms,259, 260 that is, the polymerisation of the styrene oligomer (Mn=1000) obtained on LiBu in the presence of AlCl3 and Ti(OBu)4 additives is followed by the polymerisation of acetylene or propyne. It is noteworthy that an increase in the content of acetylene units in the copolymer leads to changes in its morphology, viz., fibrillar structures typical of PAc disappear and the polymer acquires a trans-structure.The syntheses of soluble block copolymers consisting of PAc blocks and polymeric support blocks (PS or polyisoprene) were reported.261, 262 In this case, CuCl2 in anhydrous THF at 193 K is used as catalyst. Polymeric support blocks exhibit dielectric properties; however, doping of the block copolymers allows the preparation of materials whose conductivity is comparable with the metal conductivity. This type of conducting composition can be synthesised using a terpolymer of, e.g., diene, ethylene and propylene and blends of thermoplastic elastomers as well.253, 255, 263 Bolognesi et al.264 reported an interesting method for prepa- ration of a comb-like copolymer in which the trans-PAc chains are grafted onto a polybutadiene or polyisoprene matrix [this was done using the Ti(OBu)4 catalytic system and a mixture of tetramethylenediamine and sec-butyllithium].264 A large body of patent literature exists on this subject.For instance, the procedure for preparation of a readily processable composition of PE and PAc involved treatment of a thin PE film with the catalyst followed by polymerisation of acetylene.265 The conductivity of the material obtained was 5 O71 cm71. To obtain doped, high- conductivity compositions based on PO and oriented PAc, plastic fibres or ribbons were washed with a solution of the Ziegler ± Natta catalyst and placed stretched in a vessel filled with acetylene gas.266, 267 Polymer blends based on conducting material were also prepared in another manner using porous films or mem- branes.268, 269 The newly formed macromolecules that fill porous matrices with uniaxially oriented pores were found to have a preferential orientation along the pore axis. The approach based on the use of porous HDPE films obtained by uniaxial drawing of the polymer in liquid media via delocalised crazing was used for the preparation of PE ± PAc compositions.270 The content of PAc was at most 8 mass% and the results obtained after selective extraction of PE indicated the formation of a rigid three-dimen- sional PAc network reinforcing the PE matrix.However, the formation of PAc chains grafted onto PE in such systems also cannot be ruled out. This will cause the orientation of both phases, 21 viz., the finely dispersed porous PE matrix and rigid-chain PAc macromolecules. Doping the polymer blend with iodine led to an increase in the bulk conductivity by 14 to 16 orders of magnitude, which points to a low percolation threshold for the conductivity of the structures with marked anisotropy of the mechanical and electrophysical properties.The conductivity of compositions con- taining up to 3% doped PAc was described in the framework of the simple percolation model. The structure and various (including electrical) properties of films based on PAc and polybutadiene have been studied in detail.271, 272 It is appropriate to prepare these compositions from diene ± ethylene ± propylene terpolymers and PAc.255 Poly- merisation of acetylene in the elastomeric matrix results in a product which exhibits not only high conductivity and resistance to aggressive media, but also reasonable operating characteristics.Cross-linking of these compositions leads to immobilisation of PAc. Graft polymers consisting of polydiene and PAc are rela- tively easy to prepare. In this case, the metal ± carbon bonds formed with participation of vinyl groups of the 1,2-polybuta- diene units in the first stage of the process are then involved in grafting of PAc in the second stage.273 C nC2H2 Ti(OBu)4±AlR3 R CC CCCTi CC R CC CC CC Soluble graft block copolymers of PAc (up to 5%) and atactic PS, soluble block copolymers of PAc and polyisoprene (including doped block copolymers) 274 and the PAc ± b-poly(norborne- ne) ± b-PAc triblock copolymers with different block lengths were synthesised.Graft copolymers are characterised by micro- phase separation. Polymer ± polymer compositions based on polyolefins and conjugated polymers can be prepared using not only PAc, but also such components as polyphenylacetylene, poly(propargyl alcohol) and poly(propargylamine), which are obtained by poly- merisation of the corresponding monomers on the Mo(V) com- plexes immobilised on PE, PP or PS.275 Polymerisations of crystalline diacetylenes in a microporous polymeric matrix resulted in compositions of isotactic PP and substituted poly- diacetylenes.276 Drawing the PP film in 20% solutions of sub- stituted 2,4-hexadiynes followed by removal of the solvent allowed the preparation of a film containing up to 20% diacety- lene units.Studies on the g-radiation-induced, solid-phase poly- merisation revealed an increase in the degree of conversion of the monomer in the PP matrix as compared to the reaction in solution. This was explained by a higher initiation rate rather than the mechanical effect of the matrix. A method for `internal' doping of the polyconjugated system with iodine that is partially separated during the polymerisation of the iodine-containing monomer was reported.277 Preparation of blends of polyconjugated polymers and UMMPE by gelation 278 ± 280 is also of interest. For instance, a UMMPE± poly[2-butyl-5-(2-ethylhexyl)-1,4-phenylenevinylene] (70 : 30) composition was prepared from solution (more correctly, a gel) in p-xylene.No information on the conducting properties of such PPC was reported. Products of the in situ polymerisation of pyrrole are also used as components of conducting PPC. The procedure for preparation of such PPC as PE ± PAc involves immobilisation of the catalyst on the surface of functionalised PE films. This method was also used for the synthesis of PE ± polypyrrole (PPr) compositions.281 To this end, a film consisting of PE and sulfonated PS was immersed in a solution containing FeCl2 and FeCl3 and then polymerisation of pyrrole was performed at 275 K for 30 min in22 Strain /kg 4 800 3 2 4000 1.0 Figure 17.`Strain ± displacement' curves for double film PE ± sulfonated PS (without PPr) (1) and ternary films PE ± sulfonated PS ± PPr (2 ± 4) with different conductivities prepared by electrochemical polymerisation.281 Conductivities (O71 cm71): (2) 3.8; (3) 7.5; (4) 14.9. the dark followed by post-polymerisation doping. The surface conductivity of the polymerisation product was*250 O71 cm71. Films prepared from this material exhibit good mechanical properties; however, the films characterised by higher conductiv- ity are less elastic and more rigid (Fig. 17). Conventional methods for the synthesis of conducting com- positions have some inherent drawbacks, such as high rigidity and dark colour of products, a complicated procedure for preparation of samples with specified conductivity and long duration of the formation of films thicker than 10 mm.To eliminate these draw- backs, it was proposed to prepare this kind of PPC in the form of PPr-based polymer alloys by chemical 282 or electrochemical 283 polymerisation of pyrrole on polymer supports. The method involves coating an electrode with a polymeric support film *20 mm thick. The PPr chains begin to grow from the film side adjacent to the electrode surface. Therefore, initially only the `electrode' side of the film is conducting. The opposite, `surface' side of the film acquires conducting properties only after being penetrated by the PPr chains. Polymer alloys prepared in this manner exhibit some attractive properties including high conduc- tivity (more than 300 O71 cm71), stability in air, good film- forming ability and high homogeneity.Werner 284 reported the synthesis of a number of PPr-based, conducting, thermoplastic and elastic in situ prepared PPC including those containing PB. Methods based on the use of monomers (e.g., pyrrole or aniline) as solvents for the polymerisation have also been developed.285 ± 287 Conducting compositions containing tailor-made PO can be prepared by the blending of components. In particular, micro- porous PE films synthesised by rapid melt extrusion followed by annealing exhibit a well-developed surface and can be used as supports for composite materials prepared by coating these films with a thin layer (4 ± 7 mm) of conducting polyaniline (PAn) from aqueous dispersion.288 This kind of material is characterised by strong adhesion between their components.Despite its very small thickness, the PAn layer has the same conductivity (0.2 O71 cm71) as that of a much thicker film cast from the PAn dispersion. PAn-Based conducting compositions are also prepared by polymerisation of monomers in porous polymeric matrices obtained by cold drawing of polymer films (including HDPE films) in reactive liquid media by a delocalised crazing mecha- nism.289, 290 Electrochemical polymerisation in thin pores of the PE matrix in the acid-aqueous electrolytes results in the formation of oxidised PAn (emeraldine salt). The specific surface conductiv- ity of the undoped product is 961073 O71 cm71 (the conductiv- ity of undoped PAn is only 10710 O71 cm71, see Ref.291). Doped PAn exhibits a conductivity of 5 O71 cm71 (see 1 Displacement /cm 2.0 A D Pomogailo Ref. 292). In finely dispersed porous matrices, PAn forms spher- ulites 60 ± 100 nm in diameter (cf. 300 nm in diameter fibrillae formed outside the matrices). The films are characterised by uniform dispersion of the PAn phase in the bulk and by complete phase separation of the components. It is noteworthy that the content of non-extractable (insoluble) PE retained as the constit- uent of the polymer blend upon selective extraction of PE in decalin at 418 K and in xylene at 408 K lies between 20% and 30% PE. A possible explanation is as follows. Since the growing polymer particles are much larger than the effective pore diameter of the matrix (4 ± 15 nm), the fibrillae of the matrix phase penetrate the particles of the PAn phase, thus preventing them from being accessed by the solvent molecules.In situ synthesis of conducting compositions can also be performed in the reverse order. In this case, the catalyst or initiator of polymerisation of the corresponding monomer is immobilised on the conducting component. For instance, doping of PAc with dihydronaphthalide sodium results in the formation of a charge- transfer complex 293 [Na0.057(CH)]x which is used as initiator for grafting of blocks of PS, polyisoprene, PMMA, PEO, etc., onto the PA chains.294 The possibility of immobilisation of metal complex catalysts on graphite followed by the in situ preparation of conducting PPC 295 makes this approach be particularly attractive.Here, the height of the potential barrier to the charge carriers injected into the dielectric polymeric component of the PPC reduces.296 The PE ± soot composition also exhibits conducting properties;297 a sharp decrease in the electrical resistance begins as the content of soot reaches 6 vol.%. The conductivity of this PPC is independent of the soot particle size (on the average, 62 nm in diameter) and the melt flow index of PE. As the content of soot changes from 16 vol.% to 28 vol.%, the maximum conductivity increases from 1 to 3 O71 cm71 at a current density of up to 15 A cm72. The current-vs.-voltage curve passes through an extremum. The decrease in the electric current is due to stress relaxation and to scission of soot chains caused by the Coulomb forces at high voltages.The HDPE-based composition exhibits higher conduc- tivity compared to the LDPE-based PPC; this is probably due to the lower volume fraction of the amorphous phase characterised by high concentration of soot in the former case. Thus, different synthetic procedures developed in the frame- work of the in situ approach to preparation of PPC are convenient and promising for the synthesis of conducting composite materi- als. X. Formation mechanism and the structure of in situ prepared polymer ± polymer compositions In the preceding sections of this review we have considered many aspects of the mechanism of formation of the in situ prepared PPC.In this section, we will sum up information on the morphology, formation mechanism of polyolefin molecular compositions and the role played by the interfacial layers. The strength of adhesive interaction between the components of PPC is an important characteristic determining their mechan- ical properties and operating characteristics. The properties of various two-phase polyolefin ± polymer compositions can essen- tially (and even abnormally) change irrespective of the nature of the polymeric constituents. Most often, the parameters of PPC are determined by peculiarities of the boundary (interfacial) layers characterised by effective interactions between the components of the PPC.In principle, the morphology of the interfacial layer can follow two patterns.7 First, the interfacial layer can comprise two regions adjacent to the one and only interface and having specific structures different from the structure of the bulk phases of each component (A and B). Second, a situation with two interfaces is possible. General and more detailed schemes illustrating the structure of the interfacial layers are shown in Fig. 18. If PPC are prepared by blending polymeric components that can be even virtually immiscible, the first pattern can be followed; however, inMolecular polymer ¡¾ polymer compositions. Synthetic aspects b a AA A AA A 1 2 2 1 II I 3II I Figure 18. The structure of intermediate layers in the formation of polymer blends (I ) and synthetic (II ) compositions (a) and the structure of the intermediate layers in polymer blends (I ) and in synthetic (II ) compositions iso-PP (1), triblock copolymer (2) and PE (3) (b).this case the interfacial layers are formed due to the physical interparticle interactions and self-diffusion of macromolecular segments. The latter process is a kind of segmental dissolution of polymers accompanied by migration of the low-molecular-mass homologues-components towards the interface (this provides an explanation for the important role of the molecular mass distri- bution of the starting polymers in the formation of PPC). As mentioned above, considerable attention is given to the methods for weakening the interface stress and improving adhe- sion between the immiscible phases.298 In the course of reactive blending (polymerisation of one monomer in the presence of the other component) such boundary layers are formed as a result of the appearance of interpenetrating polymer chains and, possibly, pseudointerpenetrating polymer networks, which favours the improvement of compatibility of the components.Often, the interface is represented as a conventional, two- dimensional geometrical surface. However, in real binary compo- sitions the interface can be rather thick and exhibit specific properties that differ from the properties of the bulk phases. These are not only lowered segmental mobility of macromole- cules, reduced number of possible macromolecular conformations and loosened macromolecular packing, but also violation of identity of the chain building blocks and changes in the transla- tional dislocation vectors and character of relaxation processes, etc.The properties of PPC are strongly dependent on how the interfacial layer is distributed within the compositions, i.e., is it localised only at the interface or does it extend to the bulk phases of the polymeric constituents? The interfacial layer has a complex structure. It includes the adsorbed near-surface layer with specific properties and some transition layers in which the degree of ordering of the macromolecular packing varies with the distance from the surface of the polymeric matrix. If the content of one component is low (5% to 10%), the so-called threshold space of the interfacial layer takes on particular significance, since in this case the phase continuity can be violated, thus providing the possibility of direct contact between the macromolecules of the polymeric constituents.Under certain conditions, one bulk phase can `penetrate' through the second bulk phase (Fig. 19), which leads to the formation of interfacial structures with improved adhesion. As a rule, the mechanical characteristics of the interfacial layer are related to its adhesive properties, that is, an increase in the modulus of elasticity corresponds to an increase in the adhesive energy and to thinning down of the interfacial layer. When estimating the contribution of the boundary layers to the proper- ties of a multiphase system, one should know the distance from the interface at which deviation of local properties from the bulk properties can be considered insignificant; an important point is, however, that the thickness of the interfacial layer determined by different methods depends on the sensitivity of a particular method.8 Nevertheless, the thermodynamic approach allows one to evaluate the minimum thickness of the interfacial layer (dmin), which meets the criterion for its stability.299 23 In situ prepared component Starting component Figure 19. A scheme illustrating interpenetration of polymer chains in compositions prepared by in situ polymerisation.The density of the interfacial layer changes exponentially with the distance from the interface and can be controlled in a targeted manner.The volume of the interfacial layer, V1 , can be found using the following equation 300 V1= r VO1 ¢§ eaNU, r1 where r and V are the density and volume of the polymer matrix, respectively; r1 is the effective density of the interfacial layer; a is the coefficient of proportionality; and N is the number of particles of a particular component. The density of the interfacial layers in the composition of HDPE and PVC was qualitatively assessed using positron anni- hilation technique.65 It was found that the in situ prepared PPC are characterised by dispersion of the PVC particles in disordered PE domains. A decrease in the density of macromolecular packing upon the formation of the interface was also pointed out by Chalykh et al.301 The PE component of this PPC was found to form one crystalline phase and two amorphous phases (the surface phase and the intermediate phase).302 NMR studies revealed the existence of domains characterised by different free induction decay parameters and relaxation times in the bulk of the PPC.The interaction between these domains is weaker than within each of them. The formation of the transition layer leads to loosening of the near-surface layer of the PE phase. The mean correlation time of intermolecular motions in this layer is 1078 s. Similarly to the filled systems, the interfacial layer in poly- mer ¡¾ polymer compositions serves three main interrelated func- tions.1 These are (i) stress redistribution from the phase contact region to the bulk phases (reduction of stress concentration in the more flexible component), or operation as a specific structural stress concentrator during the degradation of PPC; (ii) changes in the elastic and viscoelastic properties of PPC; and (iii) suppression of both cracking and formation of other defects (the interfaces prevent the samples from microcracking).The type of the phase structure and the sizes of corresponding structural domains depend on the nature of the polymers under study (the surface tension at the interface and the viscosities of the components and their ratios, the volume fraction of the compo- nents and blending conditions).It is these characteristics that determine the properties of PPC. In particular, the formation of amorphous-crystalline structure leads to improvement of the impact toughness (see, e.g., Ref. 303); PPC with fibrillar structure exhibit higher strength and improved strain ¡¾ stress properties (see, e.g., Refs 304 and 305); polymer compositions with lamellar structure are characterised by improved barrier properties 306 while continuous structure favours achievement of an optimum combination of the properties of both components.307 If the formation of dispersed structures has been studied in much detail, the conditions for the appearance of domains with24 continuous morphology have been little studied except, probably, for the PE ± PS system.308 Recently, Tseng et al.186 reported the results of scanning electron microscopy studies of the changes in the morphology of compatible and compatibilised incompatible PP ± PA-6 (30 : 70) systems.The incompatible PPC were charac- terised by high surface tension at the interface, which led to the formation of a coarse-grained product. Even the addition of a rather large amount of the PP/MA phase (5 parts by mass) was insufficient for inhibition of the formation of this structure. On the other hand, the addition of the PP/MA ± co-PPG copolymer (5 parts by mass) led to substantial reduction of the domain size of PA-6. The reduction magnitude was found to be a function of the diamine-modified PPG chain length. Finally, the reactions involving compatibilisers containing grafted MA (0.8 mass%) followed by the formation of the PP/MA ± co-PPG copolymer result in the compatible compositions for which the longer the PPG chains, the higher the dispersity.Similar results were also obtained in studies of a compatible system PP ± (PP/MA) ± PA-12 (65 : 10 : 25), in which all the MA groups enter into reactions with the terminal NH2 groups of PA-12 and the particle size lies between 0.1 and 0.5 mm or less.309 It should be noted that the particle size distribution of the dispersed phase in compatible systems is much narrower than in incompatible systems. In the latter, coarse-grained particles of PA-12 grow in the course of annealing at 453 K. This is accom- panied by grafting a larger number of the PA-12 molecules onto the PP/MA phase and by an increase in the thickness of the interfacial layer. An increase in the content of the PP/MA phase favours an increase in the dispersity of PA-12.Despite the presence of excess MA groups, the (PP/MA) ± g-PA-12 system is formed in these compositions. However, it should be kept in mind that thermomechanical treatment during processing of polymers has a strong effect on the mechanochemical processes in and, hence, on the morphology of these heterogeneous systems. In addition to the interaction between the functional groups of different components (including grafted ones) of an in situ prepared PPC, grafting of the newly formed polymeric constituent (a polyolefin, polydiene or polyacetylene) of the PPC onto the surface of the polar component also plays an important role in the formation of the interfacial layer in this type of PPC.Grafting can occur, first, in the initiation stage of polymerisation (the multiple bonds of the surface reactive groups can be cleaved and these groups can undergo insertion into macromolecules in the form of end groups); second, in the growth stage, when the multiple bonds of a particular component are involved in the copolymerisation; and, third, in the stage of chain transfer to the surface of the polar component (the reaction between the propagating chain end or a functional group of the polymer and the specific surface group, resulting in the formation of a covalent bond).Specific reaction mechanisms and the relative contributions of particular reaction stages depend on the nature of the constituents of PPC. Improvement of the properties of the PO± PO compositions prepared using the principle `compatibilisation via polymerisa- tion' as compared to polymer blends of the same chemical composition is rationalised by higher ordering of their morphol- ogy.128 The synthesis of the PO± PO compositions involves coat- ing of the catalyst with the polymeric constituents in rigorous order, that is, PP, SKEP and, finally, PE. In this case the adhesive, SKEP, is localised at the interface (cf. polymer blends with the same structural elements, in which SKEP is much more dispersed within the bulk of the dispersed phase).Therefore, the brittleness temperature of the compatibilised composition is reduced by 16 8. As a rule, one component of a polymer blend presents no obstacles to crystallisation of the other blend component.99 Nevertheless, there are some exceptions. For instance, the crys- tallisation temperature of the PA-6 phase in the PP ± PA-6 compositions is 2 ± 3 8 lower than that of the starting polymer, which points to a lower nucleation rate.186 In other words, the product of in situ polymerisaton, PP ± (PP/MA) ± PA-6, prevents crystallisation of PA-6.310, 311 On the other hand, the crystallisa- A D Pomogailo tion temperature of the PP phase of the composition is appreciably higher (by 6 ± 9 8) than the crystallisation temperature of the individual polymer.This means that PA-6 acts as a promoter for crystallisation of PP in the composition under consideration. No substantial changes in the melting temperatures of these semi- crystalline polymers in both compatible (437.5 K) and incompat- ible (494.5 K) PPC occur; however, the degree of crystallinity of the PA-6 phase in the compositions decreases. Probably, the presence of copolymers within the interfacial layer makes the crystallisation of PA-6 more difficult to proceed. On the other hand, the degree of crystallinity of the PP phase in compatible PP ± PA-6 compositions remains unchanged. Finally, yet another important structural feature of the interfacial layer should be pointed out. The macromolecules located within this layer adopt an extended conformation due to the orientational effect of the matrix.For instance, the macro- molecular segments adjacent to the PVC phase in the molecular compositions of PE and PVC are oriented.312 This can be rationalised by the effect of the PE chain growth conditions, namely, by high density of the growing PE chains in the thin near- surface layer. Polymer chains growing in this case in a co- operative manner adopt an extended conformation. It is assumed 7, 8 that crystallisation of one component near the phase boundary of the other component is accompanied by the for- mation of transcrystallites that are stronger and more rigid than spherulites. Interpenetration of the polymer blocks into the interfacial layer provides the possibility for the adhesive interac- tion (anchoring) between the constituent phases to occur.Such a submicrostructure contributes largely to improvement of the impact toughness of PPC. Orientation of macromolecules at the interface between glassy polymers leads to transition of the polymers into a state typical of liquid-crystalline polymers.313 Rigorous control of crystallisation conditions also allows the preparation of highly crystalline PE films (in this case, the polar component can act as nucleating agent for crystallisation) and polymer alloys. In particular, PE can undergo epitaxial crystallisation on oriented poly(oxymethylene) in spite of different crystal structures, macromolecular conforma- tions and polarity of both polymers.314 However, this is impos- sible if PP, cellulose, PVAlc or PA-6 are used instead of poly(oxymethylene). The formation rate of the in situ prepared PPC also plays a significant role.If macromolecules leave the reaction medium at a rate comparable with the chain propagation rate, no stacking occurs; rather, structures with extended con- formations of the macromolecules are formed.315 The so-called occlusion effect responsible for occlusion of the growing polymer chains within the `dead' polymer domains should also be taken into account. Thus, by using the `compatibilisation via polymer- isation' strategy one can prepare structures with the nonequili- brium orientation of macromolecules. From the aforesaid it follows that the interactions between the components of PPC at the interface, especially between crystalline polymers, should be considered in close relation to the changes in the molecular and supramolecular structure and taking into account the unique topology of such polymers. Not only physical immobilisation and chain entanglement, but also chemical inter- actions between different-type polymeric constituents can occur in the molecular compositions.The net effect of these factors leads to the formation of a common co-operative system, which includes several types of bonds and is characterised by additional structur- alisation of PPC, which favours the achievement of a high degree of homogenisation. This complex system of interactions at the interface in the in situ prepared PPC takes over the breaking load in the case of destruction of the composite material.Nevertheless, an ideal type of structural organisation of the PPC in question is provided by the IPN that represent molecular compositions formed in the in situ polymerisation of monomer 2 in preliminarily swollen polymer 1. The major difference between the classical IPN and the in situ prepared compositions based on crystalline polymers lies in the fact that the formation of IPNMolecular polymer ± polymer compositions. Synthetic aspects involves substantial swelling of the polymer in the monomer. One of the most important features of the IPN consists in extremely strong (in essence, on the molecular level) mutual dispersion of the components.The size, D, of the polymer 1 phase domains formed in the semi-IPN in the polymerisation of monomer 2 with the molecular mass M2 can be estimated using the semiempirical Donatelli equation.316 This equation takes into account the changes in the free energy of the system and is written as follows: D=RTn1âÖ1 ¡ o2Ü¡2=3 á o2=n1M2 ¡ 1=2ä , 2go2 where g is the surface tension at the interface, o2 is the mass fraction of the monomer and n1 is the density of cross-links in polymer 1. Under some assumptions we can estimate the domain size of the PS phase in the blends with PP, prepared by the in situ polymerisation of styrene in PP deformed via delocalised craz- ing.75 This size is nearly 100 nm, which is in agreement with the results of electron microscopy studies.The aggregative stability of these systems is provided by the strong system of entanglements between two macromolecular networks (the so-called catenane links), i.e., by a specific type of anchoring. Under optimum conditions this method allows one to achieve complete inter- penetration of the components on the molecular level even for thermodynamically incompatible polymers. Morphologically, the systems under study are also relatively close to the IPN. The point is that the second polymer phase is formed in the intercrystallite domains of partially crystalline PO. At temperatures below the melting temperature, PO exhibit rather high resistance to the action of the surface forces that cause phase separation, thus stabilising the newly formed structure.In other words, the crystal structure of PO, which plays the role of cross- links in the first network, reduces the possibility of the formation of extended phase domains of the second component and deter- mines the dispersity of the system. Indeed, the synthesis of the HDPE±PMMA compositions via delocalised crazing 76 resulted in the formation of the structures characterised by phase co- continuity (selective extraction of the components led to the formation of a continuous porous framework rather than to phase separation of the composition). The formation of the PO phase within the amorphous polar component occurs in a more complex manner. In this case, release of heat (its amount can be rather large) in the formation zone of the PO component leads to softening of the matrix and to penetration of the PO phase into the polar component matrix under the action of mechanical stress.A salient feature of the formation of IPN is the imposition of microphase separation that is due to the thermodynamic incom- patibility of the polymer networks formed on the chemical reactions.12 The chemical compositions of the separating phases depend on both the stoichiometry of the components and the IPN formation rate. The microphase structure is characterised by the degree of segregation of the two-component system in which phase separation into the polymeric constituents occurred (usu- ally, no equilibrium phase separation is reached and the phases with variable chemical composition are formed).The most sensitive characteristic of the phase separation in polymer blends is the relative elongation at break.84 Often, passage from compat- ibility to incompatibility is very sharp and occurs at a particular ratio of the components. Even if specific interactions between the macromolecules of PPC can occur at ambient temperatures, their thermodynamic compatibility deteriorates on heating and they are characterised by the lower critical solution temperature. Since the components of molecular PPC are thermodynami- cally incompatible, the compositions prepared from them belong to nonequilibrium systems. To convince oneself that this is true, it is sufficient to give, e.g., the PE ±PMMA system molecular mobility (by annealing), which leads to melting of the PE phase 25 and devitrification of the PMMA phase and is exactly the same as what occurs usually in the case of mixing of polymer melts.Thus, polyolefin-based PPC prepared by in situ polymer- isation of one component do not represent typical interpenetrat- ing polymer networks since they can be dissolved in appropriate solvents and are characterised by a wide range of variations in the yielding and creep. On the other hand, they also differ from conventional polymer blends. These PPC can be called molecular, pseudointerpenetrating, thermoplastic chemical polymer blends 317 that exhibit homogeneity on a microscopic level. Combinations of some specific properties of their components can lead to positive synergistic effects.XI. Fields of application of in situ prepared polymer ± polymer compositions Almost without exception PPC and their individual polymeric constituents have found the same fields of application, the use of the former being often more advantageous. Polymer ± polymer compositions belong to the class of hybrid composite materials with improved breaking strength. A salient feature of PPC is nonlinear dependence of their properties on the chemical compo- sition due to unique structural peculiarities. As a rule, these materials are characterised by improved impact toughness (cold resistance), strength, wear resistance and exhibit good dispers- ability and processability. Many of them (e.g., PVC± PE compo- sitions) are suitable for production of plastic pipes and sheets, various profiles, moulded products and antifriction materials.Polymer compositions based on syndiotactic PP exhibit radiation- damage stability and can be used instead of plasticised PVC.116 Improved thermal stability of the in situ prepared HDPE±PVC compositions is rather unusual and sets them apart from polymer blends or untreated PVC.49, 318 This property, which is also characteristic of the PVC± PS graft copolymers,319 is thought to be due to the fact that the mobile chlorine atoms present in the allyl groups as impurities or bound to the tertiary carbon atoms of PVC are removed in the course of grafting. This again confirms the formation mechanism of the structure of interfacial layers in the in situ prepared PPC.Polymer ± polymer compositions PVC ±HDPE can find applications as high-strength, cold resist- ant polymer films for enclosing greenhouses.63 These materials exhibit improved thermal stability and UV and IR radiation stability, i.e., they favour the `greenhouse effect'. The ability of the PE films used for enclosing greenhouses to keep unsweated and retain transparency for long periods under conditions of excess moisture content is also of great importance. Modification of PP with elastomeric and thermoplastic elas- tomer additives allows substantial improvement of the impact toughness of the materials and reduction of the brittleness temper- ature. Among conventional additives are PIB, butyl rubber, SKEP, SKEPT, polybutadiene rubbers, divinylstyrene-based thermoplastic elastomers, etc.Polymer ± polymer compositions based on PP and ternary SKEPT have superiority in cold resistance (down to 223 K) and impact toughness compared to the untreated PP.320 The rheological characteristics of these PPC allow them to be processed using conventional methods for processing of thermoplastics, viz., extrusion, injection moulding, blow moulding and pressing. Blends of different types of PE exhibit improved processabil- ity and higher impact toughness compared to the individual components. Often, this leads to enhancement of the performance of production equipment. The PE ± PB and PP ± PB compositions prepared using the method of `compatibilisation via polymer- isation' are characterised by high modulus of elasticity, strength, thermal stability, crack resistance and tensile creep.These materi- als are promising for production of hot water supply pipes, heat exchangers and cable plasticates. In addition, blends of engineer- ing polymers and commercially available thermoplastics are used for production of, e.g., bumpers.26 The thickness of the LDPE film used for production of containers should be at least 50 to 60 mm; these values are mainly determined by the low strength of the polymer. Compatibilisation of several types of polyolefins (e.g., LDPE and HDPE) allows substantial improvement of the properties of the polymer films and reduction of their thickness down to 20 to 30 mm without deterioration of the film strength.This result in more than doubling the performance of extruder reactors and reducing the specific consumption of materials.321 Packaging films are also produced from PPC based on blends of PE with atactic PP, PIB, etc. The content of the recycled materials used for production of PE packaging films and containers reaches 50%.16 Recycled materials are used for production in those cases where no heavy demands are placed on the structural strength of products, but their environmental safety characteristics should meet stringent requirements (e.g., decorative, insulating and sound-absorbing panels, etc.). To this end, one can use wood ± polymer composi- tions based on wood waste and recycled thermoplastics (in particular, sawdust mixed with SKEPT-modified recycled LDPE).322 Polyolefins, and first of all HDPE, LLDPE, LDPE, PP and its copolymers, as well as PB are the main components of the PPC used for production of pressure pipelines.The properties of polymer ± polymer compositions have a strong effect on the strength characteristics of pipes and the operating conditions. New types of LLDPE-based PPC were recommended 323 for production of hot water supply pipelines (323 K, a pressure of 5 MPa) with a service life of 50 years. Compositions based on UMMPE (Mw=4.56106) and low-molecular-mass PE (Mw=8.06104) are used for production of high-strength, high- modulus fibres by single-stage and two-stage drawing techni- ques.324, 325 Dilution of the blend with the main component is also used for recycling of multicomponent compositions.326 For instance, homogenisation of blends of LDPE, HDPE, PP and PVC fol- lowed by the addition of PP (*50%) resulted in phase inversion, so that the properties of the end product were mainly determined by the properties of multifilled PP. As mentioned above, PO and PA, with somewhat different polarities, represent a typical example of immiscible polymers.Blending of PO and PA in the presence of appropriate compati- bilisers improves the impact toughness of materials and provides a means of protecting the polymer surface from attack by atmos- pheric moisture and oxygen. The latter can be illustrated taking the in situ prepared PPC formed immediately on polymeric fibres as an example.49 The PETP or PA fibres become covered with a specific kind of PE `coat', which protects them from wear, thus improving the wear resistance of the materials, from moisture absorption, etc.(Fig. 20). In situ prepared polymer ± polymer compositions have found an unusual field of application as supports for metal complex catalysts.53 Specific properties of PPC based on HDPE and metal-containing polymers, e.g., titanium, nickel, cobalt and other metal polyacrylates should also be pointed out.327 Interpenetrating polymer networks can be formed not only within the interfacial layer, but also in the bulk of these PPC. This is due to the fact that not all unsaturated groups are consumed in the polymerisation of metal diacrylates in PE melts.Since the PPC are processed at temperatures above the polymerisation temperature, the unreacted functional groups can be involved in the chemical interaction to form IPN. In situ prepared polymer compositions can combine the advantages of thermoplastics and thermosetting polymers. Often, the in situ method allows recycling of thermoplastics. The use of filled PPC, e.g., a ternary system PP ±LDPE± short glass fibres (up to 30%± 40%) whose morphology was studied by Avalos et al., 328 is also expected to provide a way to substantially improve their properties. Stable conductivity, good mechanical characteristics, high homogeneity, protective effect of the matrix and good process- ability of the compositions of PAc and PO open the possibility of A D Pomogailo Figure 20.A SEM image of the surface of PETP fibre coated with a HDPE `skin'. their use for production of moulded products, e.g., semiconductor relays, thermistors, solar batteries, etc.329 The operating characteristics of PPC including their photo- degradation and photo-oxidation behaviour under natural con- ditions have been the subject of intensive research. For instance, it was found that the rate of ozone oxidation of oriented HDPE± PP compositions decreases as the content of PP increases.330 In studies of the blend of isotactic PP and SKEPT{ it was shown that an increase in the content of PP leads to enrichment of the interfacial layer with rubber and that the structure of the inter- facial layer is the determining factor in the thermal oxidation of blends and vulcanisates.331 This manifests itself as changes in both the mechanism and kinetics of radical steps in the initial stage of oxidation which can affect inhibition of the oxidation process.Photo-induced oxidation of PPC based on commercially available LDPE and LLDPE is accompanied by scission of the polymer chains and cross-linking of the molecules, as indicated by the changes in the molecular mass and molecular mass distribution. If PPC contain polymeric sensitiser constituents, chain scission reactions become more important, while cross-linking of polymer chains, which can also occur, appears to be less significant. No preferential degradation of a particular component was observed in studies of the blends of LLDPE332 or LDPE333, 334 and styrene ± butadiene copolymer.A unique possibility of controlling the properties of polymers has arisen after the preparation of blends of PO with non-olefin components by reactive blending. In particular, polyolefins co- calendered with PVC without using volatile plasticisers can be given such useful properties as elasticity and strength.335 Of course, the entire spectrum of applications of molecular polyolefin ± polymer compositions is much broader. XII. Conclusion The use of molecular polyolefin ± polymer compositions allows to a great extent the increasing industrial demands for novel materi- als to be met with reasonable cost : property ratio. Studies on interrelations between the compatibility of polymeric constitu- ents, structure and properties of these PPC are of great theoretical and practical importance.In addition, the preparation of this type of PPC by reactive blending opens the possibility of re-use of plastic waste and degraded polymeric materials. Compatibilisa- tion of the components of PPC requires improvement of inter- facial adhesion and an increase in the phase dispersity, which can often be achieved by using interfacial modifiers. PO-Based com- positions prepared by in situ polymerisation of one component do not belong to typical representatives of interpenetrating polymer { SKEPT composition is ethylidenenorbornene (4.0 mol %), propylene (32 mol %) and ethylene (the rest).Molecular polymer ± polymer compositions.Synthetic aspects networks; on the other hand, they differ from conventional polymer blends. They can be classified as molecular, pseudointer- penetrating, thermoplastic chemical polymer blends.317 In some optimum cases the in situ prepared PPC are characterised by homogeneity on a microscopic level, a valuable combination of specific properties of their constituents and by positive synergistic effects. The structure of a real particle of the in situ prepared PPC is shown in Fig. 21. The particle is composed of the catalyst component or another precursor (1) coated with the newly formed polymer (3) and the interfacial layer (2). The topology and phase structure of the PPC can be controlled in the design stage.Determination of the structure of the PPC and finding out the methods for its control will allow targeted design of polyolefin- based composite materials with desired properties. Unfortunately, no analytical expressions relating the molecular parameters and the composition of a system to its properties have been derived as yet. Would they be proposed, this will favour the refinement and generalisation of the experimental relationships and the develop- ment of a well-substantiated approach to the preparation of PO compositions with desired properties. A great advantage of the methods for preparation of PPC considered in this review is that they allow one to prepare opera- tionally compatible polymer compositions from thermodynami- b a 12 1 23 3 Figure 21.General view (a) and simplified scheme (b) of the structure of a particle of molecular PPC (for details, see text). cally incompatible components. These compositions exhibit stable properties and have superiority in some characteristics compared to traditionally used polymer blends. This approach follows a recent trend, according to which polymeric materials are given the desired properties by choosing appropriate combinations of the known components rather than by polymerisation of new mono- mers, since the potential of the organic synthesis of new monomers and polymers based on them are, after all, limited. This review has been written with the financial support of the Russian Foundation for Basic Research (Project No.01-03- 97018) and the Ministry of Industry and Science of the Moscow Region. References 1. E P Pludemann Interfaces in Polymer Matrix Composites (New York; London: Academic Press, 1974) 2. A G Sirota Modifikatsiya Struktury i Svoistva Poliolefinov (Modification of Structure and Properties of Polyolefins) (Leningrad: Khimiya, 1969) 3. V N Kuleznev Smesi Polimerov (Polymer Blends) (Moscow: Khimiya, 1980) 4. O Olabisi, L M Robertson, T H Shaw Polymer ± Polymer Miscibility (New York: Academic Press, 1981) 5. K Sols (Ed.) Polymer Compatibility and Incompatibility. Principles and Practices (New York: MMI Press, Harwood Academic Publishers, 1982) 6. L A Utracki Polymer Alloys and Blends, Thermodynamics and Rheology (MuÈ nich: Hanser, 1989) 27 7.A P Plochocki, in Polymer Blends Vol. 2 (EdsD R Paul, S Newman) (New York; San-Francisco; London: Academic Press, 1978) 8. D R Paul, L H Sperling (Eds) Multicomponent Polymer Materials (Washington, DC: Plenum, 1986) 9. D R Paul, J W Barlow, H Keskkula, in Encyclopedia of Polymer Science Engineering (Ed. J I Kroschwitz) (New York: Wiley, 1988) 10. C D Han (Ed.) Polymer Blends and Composites in Multiphase Systems (Washington, DC: American Chemical Society, 1984) 11. Yu S Lipatov Fizicheskaya Khimiya Napolnennykh Polimerov (Physical Chemistry of Filled Polymers) (Moscow: Khimiya, 1977) 12. Yu S Lipatov, L M Sergeeva Vzaimopronikayushchie Polimernye Setki (Interpenetrating Polymer Networks) (Kiev: Nukova Dumka, 1979) 13.A D Pomogailo, in Advances in Interpenetrating Polymer Networks Vol. 3 (Eds D Klempner, K C Frisch) (Lancaster; Basel: Technomic Publishers, 1991) p. 145 14. L Leidner Plastic Waste (New York: Marcel Dekker, 1981) 15. C R Leiduer, J W Barlow, D R Paul J. Appl. Polym. Sci. 26 9 (1981) 16. S A Vol'fson Vysokomol. Soedin., Ser C. 42 2000 (2000) a 17. M J Hill, C C Puig J. Appl. Polym. Sci. 65 1921 (1997) 18. A A Tager, V S Blinov Usp. Khim. 56 1004 (1987) [Russ. Chem. Rev. 56 579 (1987)] 19. A A Tager Vysokomol. Soedin., Ser. A 14 2690 (1972) a 20. S Enders, A Hinrichs, R Horst, B A Wolf J. Macromol. Sci.-Pure Appl. Chem. A 33 1097 (1996) 21. D J Walsh, G L Cheng Polymer 23 1965 (1982) 22.R Kristensen Vvedenie v mekhaniku kompozitov. Mir Moscow 1982 23. A A Berlin, S A Vol'fson, V G Oshmyan, N S Enikolopov Printsipy Sozdaniya Kompozitsionnykh Polimernykh Materialov (Principles of Creation of Polymer Composite Materials) (Moscow: Khimiya, 1990) 24. A Y Coran, R Patel J. Appl. Polym. Sci. 20 3005 (1976) 25. E N Kerner Proc. Phys. Soc. B 69 808 (1956) 26. E Guth J. Appl. Phys. 16 20 (1945) 27. M A Shcherbina, S N Chvalun, V A Aulov, V I Selikhova, N F Bakeev Vysokomol. Soedin., Ser. A 43 87 (2001) a 28. N A Erina, E V Prut Vysokomol. Soedin., Ser. A 42 269 (2000) a 29. V E Zgaevskii Dokl. Akad. Nauk 371 179 (2000) b 30. R E S Bretas, D G Baird Polymer 33 5233 (1992) 31. S C Tjong, Y Meng Polym. Int. 42 209 (1997) 32.T G Fox Bull. Am. Phys. Soc. 1 123 (1956) 33. F E Karasz,W J MacKnight Contemporary Topics in Polymer Sci- ence Vol. 2 (New York: Plenum, 1977) 34. J D Hoffmann, J J Weeks J. Chem. Phys. 37 172 (1962) 35. P L Beltrame, A Castelli, G Munaretto, M Canetti, A Seves J. Appl. Polym. Sci. 65 1659 (1997) 36. P Szabo, E Epacher, K Belina, B Pukanszky Macromol. Symp. 129 137 (1998) 37. W N Kim, C M Burns J. Polym. Sci., Polym. Phys. 28 1409 (1990) 38. M Canauz, A Castelli, P L Beltrame, A Siciliano, A Seves, in The 6th International Conference on Polymer Supported Reactions in Organic Chemistry (POC 94) (Abstracts of Reports), Venice, Italy, 1994 p. 63 39. M S Akutin, M L Kerber, E D Lebedeva, T N Kravchenko Plast. Massy (4) 20 (1992) 40. S Wu J.Colloid Interface Sci. 31 153 (1969) 41. P L Beltrame, A Castelli, M Di Pasquantonio,M Canetti, A Seves J. Appl. Polym. Sci. 60 579 (1996) 42. D V Van-Krevelen Svoistva i Khimicheskoe Stroenie Polimerov (Properties and Chemical Structure of Polymers) (Moscow: Khimiya, 1976) 43. USSR P. 395 407; Byul. Izobret. (35) 45 (1973) 44. A D Pomogailo Catalysis by Polymer-Immobilized Metal Complexes (Amsterdam: Gordon and Breach, 1988) 45. S Jerico,U Schuchardt, I Joekes,W Kaminski,A Noll J. Mol. Catal. A, Chem. 99 167 (1995) 46. A D Pomogailo, Doctoral Thesis in Chemical Sciences, Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow, 1981 47. S L Saratovskikh, A D Pomogailo, O N Babkina, F S D'yachkovskii Kinet. Katal. 25 464 (1984) c 48.A Vidad, J B Donnel, J P Kennedy J. Polym. Sci., Polym. Lett. Ed. 15 585 (1977)28 49. B Dobbs, R Jackson, J N Gaitskell, R F Gray, G Lynch, D T Thompson, R Whyman J. Polym. Sci., Polym. Chem. Ed. 14 1429 (1976) 50. A M Bochkin, A D Pomogailo, F S D'yachkovskii Kinet. Katal. 27 917 (1986) c 51. G Greber, G Egle Makromol. Chem. 53 206 (1962) 52. H Hulot, Y Landler, in Kinetics and Mechanism of Polyreactants (Preprint) (Budapest: Akademiai Kiado, 1969) Vol. 2, p. 215 53. A D Pomogailo, Kataliz Immobilizovannymi Kompleksami (Catalysis by Immobilised Complexes) (Moscow: Nauka, 1991) 54. A D Pomogailo, E B Baishiganov, U A Mambetov, G M Khvostik Vysokomol. Soedin., Ser. A 22 248 (1980) a 55. E B Baishiganov, A D Pomogailo, in Kompleksnye Metalloorgani- cheskie Katalizatory Polimerizatsii Olefinov (Complex Organometal- lic Catalysts of Polymerisation of Alkenes) (Chernogolovka: Institute of Chemical Physics, Academy of Sciences of the USSR, 1980) Vol.7, p. 46 56. A D Pomogailo, A A Torosyan, L N Raspopov, F S D'yachkovskii, N S Enikolopov Dokl. Akad. Nauk SSSR 252 1148 (1980) b 57. N D Golubeva, A D Pomogailo, A I Kuzaev, A N Ponomarev, F S D'yachkovskii Dokl. Akad. Nauk SSSR 244 89 (1979) b 58. G I Kozub, A D Pomogailo, L I Tkachenko, O N Efimov, A F Zueva, O S Roshchupkina, in Kompleksy s Perenosom Zaryada i Ion-Radikal'nye Soli (Tez. Dokl. VII Vsesoyuz. Soveshch.) Chernogolovka, 1988 [Charge Transfer Complexes and Radical ± Ion Salts (Abstracts of Reports of the Seventh All-Union Meeting), Chernogolovka, 1988] p.31 59. J M Widmaier, L H Sperling Macromolecules 15 625 (1982) 60. E B Baishiganov, A D Pomogailo, L N Raspopov, in Kompleksnye Metalloorganicheskie Katalizatory Polimerizatsii Olefinov (Complex Organometallic Catalysts of Polymerisation of Alkenes) (Chernogolovka: Institute of Chemical Physics, Academy of Sciences of the USSR, 1982) Vol. 8, p. 53 61. F Y Xu, J C W Chien Macromolecules 27 6589 (1994) 62. A F Nikolaev, V K Kryzhanovskii, V G Shibalovich, Ch Li, D Yu Efimova, N K Abramova, I V Nikitina, O V Konova Plast. Massy (3) 19 (2000) 63. S V Nerpin, T E Pashchenko Plast. Massy (11) 15 (1973) 64. L N Raspopov, G E Shakhova, A D Pomogailo, V A Vlasov, in Kompleksnye Metalloorganicheskie Katalizatory Polimerizatsii Olefinov (Complex Organometallic Catalysts of Polymerisation of Alkenes) (Chernogolovka: Institute of Chemical Physics, Academy of Sciences of the USSR, 1982) Vol.8, p. 31 65. A D Pomogailo, E B Baishiganov, A A Torosyan, L N Raspopov, in Polimer-Polimernye Sinteticheskie Sistemy (Preprint) ([Polymer ± Polymer Synthetic Systems (Preprint)] (Chernogolovka: Institute of Chemical Physics, Academy of Sciences of the USSR, 1983) p. 24 66. A D Pomogailo, F S D'yachkovskii, N S Enikolopyan, in Protsessy Polimerizatsii v Geterogennykh Sistemakh (Processes of Polymerisation in Heterogeneous Systems) (Leningrad: Okhta Research-and-Production Union `Plastpolimer', 1985) p. 108 67. N I Knunyants, L I Manevich, in Tezisy Dokladov II Vesoyuz. Konf.po Nelineinoi Teorii Uprugosti (Abstracts of Reports of the Second Conference on Nonlinear Theory of Elasticity) (Frunze: Ilim, 1985) p. 123 68. G Ya Poderyagina, E A Zhavoronkova, V K Maksimova, A A Torosyan Kozhevenno-Obuvnaya Promst (4) 29 (1987) 69. USSR P. 660 979; Byull. Izobret. (17) 82 (1979) 70. N Saha, A N Banerjee Polymer 37 4633 (1996) 71. N Saha, A N Banerjee, B C Mitra Polym. Adv. Technol. 6 637 (1995) 72. P Grosh, N Mukherjee Eur. Polym. J. 15 797 (1979) 73. Y-J Sun,W E Baker J. Appl. Polym. Sci. 65 1385 (1997) 74. A L Volynskii, L I Lopatina, N F Bakeev Vysokomol. Soedinen., Ser. A 28 398 (1986) a 75. A L Volynskii, L I Lopatina, L M Yarysheva, N F Bakeev Vysokomol. Soedin., Ser.A 39 1166 (1997) a 76. A L Volynskii, A Sh Shtanchaev, N F Bakeev Vysokomol. Soedin., Ser. A 26 2374 (1984) a 77. N A Gribanova Int. Polym. Sci. Technol. 23 47 (1996) 78. P Cassagnau, T Nietsch, M Bert, A Michel Polymer 40 131 (1999) 79. Yu S Lipatov, T T Alekseeva, V F Rosovitskii, N V Babkina Vysokomol. Soedin., Ser. A 35 652 (1993) a A D Pomogailo 80. W H Jo, B C Jo, J C Cho J. Polym. Sci. B, Polym. Phys. 32 1661 (1994) 81. K H Dai, E J Kramer J. Polym. Sci. B, Polym. Phys. 32 1943 (1994) 82. Z Xu, J W Mays, X Chen, N Hadjichristidis, F Schilling, E Bair, D S Pearson, L J Fetters Macromolecules 18 2560 (1985) 83. K Sakurai,W J MacKnight, D J Lohse, D N Schulz, J A Sissano Macromolecules 27 4941 (1994) 84. A J Lovinger,M L Williams J.Appl. Polym. Sci. 25 1703 (1980) 85. D W Bartlett, J W Barlow, D R Paul J. Appl. Polym. Sci. 27 2351 (1982) 86. C Booth, C Price (Eds) Comprehensive Polymer Science Vol. 2 (Oxford: Pergamon Press, 1989) 87. S K Bhateja, E H Andrews Polym. Eng. Sci. 23 888 (1983) 88. V A Artem'ev, A Ya Gol'dman,G D Myasnikov, L L Sul'zhenko Vysokomol. Soedin., Ser. A 27 1047 (1985) a 89. R G Alamo, J D Londono, L Mandelkern, F C Stehling, G D Wignall Macromolecules 27 441 (1994) 90. M J Galante, L Mandelkern, R G Alamo Polymer 39 5105 (1998) 91. I L Hosier, A S Vaughan, S G Swingler J. Mater. Sci. 32 4523 (1997) 92. H Lee, K Cho, T-K Ahn, S Choe, I-J Kim, I Park, B H Lee J. Polym. Sci. B, Polym. Phys. 35 1633 (1997) 93. H W Starkweather J.Appl. Polym. Sci. 25 139 (1980) 94. M Yamaguchi, S Abe J. Appl. Polym. Sci. 74 3153 (1999) 95. D Abraham,K E George,D J Francis Eur. Polym. J. 26 197 (1990) 96. Yu K Godovskii, N P Bessonov Vysokomol. Soedin., Ser. A 21 2293 (1979) a 97. S R Hu, T Kyu, R S Stein J. Polym. Sci. B, Polym. Phys. 25 71 (1987) 98. S K Rana J. Appl. Polym. Sci. 69 2599 (1998) 99. A A Donatelli J. Appl. Polym. Sci. 23 3071 (1979) 100. S Marinow, M May, K Hoffmann Plaste Kautsch. 30 620 (1983) 101. C Sawatari,M Matsuo Polymer 30 1603 (1989) 102. M R Shishesaz, A A Donatelli Polym. Eng. Sci. 21 869 (1981) 103. B Maxwell Polym. Eng. Sci. 22 280 (1982) 104. N D Datta, A W Birley Plas. Rubber Process. Appl. 3 237 (1983) 105. H E Graham Br. Polym. J. 18 88 (1986) 106.K A Chaffin, F S Bates, P Brant, G M Brown J. Polym. Sci. B, Polym. Phys. 38 108 (2000) 107. Yu S Lipatov (Ed.) Fizikokhimiya Mnogokomponentnykh Polimer- nykh Sistem. Polimernye Smesi i Splavy (Physical Chemistry of Multicomponent Polymer Systems. Polymer Alloys and Blends) (Kiev: Naukova Dumka, 1987) Vol. 2 108. E A Antipov, Yu V Kaufman, M L Kerber, M Shtamm, E V Fisher, in Tezisy Dokladov Mezhdunar. Konf. `Fundamental'nye Problemy Nauki o Polimerakh (K 90-letiyu Akademika V A Kar- gina)', Moskva, 1997 [Abstracts of Reports of the International Conference `Basic Problems of Polymer Science (Academician V A Kargin 90-year Anniversary)', Moscow 1997] p. 2/6 109. A K Gupta, A K Jain, S N Maiti J. Appl. Polym. Sci. 38 1699 (1989) 110.G Rizzo, G Spadaro Eur. Polym. J. 24 303 (1988) 111. G Spadaro, G Rizzo Eur. Polym. J. 25 1189 (1989) 112. B Vongdara, S A Kuptsov, I D Simonov-Emel'yanov Plast. Massy (10) 45 (1988) 113. Y Long, Z H Stachurski, R A Shanks Polym. Int. 26 143 (1991) 114. H Sano, H Yui, H Li, T Inoue Polymer 39 5265 (1998) 115. P Montes, Y A Rafiq,M J Hill Polymer 39 6669 (1998) 116. N A Erina, S G Karpova, O A Ledneva, L V Kompaniets, A A Popov, E V Prut Vysokomol. Soedin., Ser. B 37 1398 (1995) a 117. K M Kit, J M Schultz J. Polym Sci. B, Polym. Phys. 36 873 (1998) 118. J Shiomura, M Kohne, N Inoue, Y Yokote, M Akiyama, T Asanura, K Sugimoto, S Kimura,M Abe, in Catalyst Design for Tailor-Made Polyolefins (Tokio: Kodansha, 1994) p. 327 119. W Zhu, X Zhang, Z Feng, B Huang J.Macromol. Sci.-Phys. 35B 795 (1996) 120. K Sakurai,W J MacKnight, D J Lohse, D N Schulz, J A Sissano Macromolecules 26 3236 (1993) 121. K Sakurai,W J MacKnight, D J Lohse, D N Schulz, J A Sissano, J-S Lin,M Agamalyan Polymer 37 4443 (1996) 122. F Laupretre, S Bebelman, D Daoust, J Devaux, R Legras, J L Costa J. Appl. Polym. Sci. 74 3165 (1999) 123. L D'Orazio, C Mancarella, E Martuscelli, G Cecchin, R Corrieri Polymer 40 2745 (1999)Molecular polymer ± polymer compositions. Synthetic aspects 124. P M Cham, T H Lee, H Marand Macromolecules 27 4263 (1994) 125. P Szabo, B Pukanszky Macromol. Symp. 129 29 (1998) 126. J Kolarik, F Lednicky, B Pukanszky,M Pegoraro Polym. Eng. Sci. 32 886 (1992) 127. F-S Fu, J E Mark J.Appl. Polym. Sci. 37 2757 (1989) 128. V P Popov, Doctoral Thesis in Chemical Sciences, Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow, 1989 129. L H Sperling Interpenetrating Polymer Network and Related Materials (New York: Plenum, 1981) 130. V P Popov, I A Litvinov, Yu I Vol'f Vysokomol. Soedinen., Ser. A 22 1276 (1980) a 131. V P Popov,A L Gol'denberg, L A Netkach Vysokomol. Soedinen., Ser. A 23 750 (1981) a 132. Y Yokoyama, T Ricco J. Appl. Polym. Sci. 66 1007 (1997) 133. V P Popov, L A Netkach, P P Duvanova, V N Malikov Plast. Massy (5) 24 (1983) 134. G E Molau J. Polym. Sci. A, Polym. Chem. 3 1267 (1965) 135. J Periad, G Reiss Colloid Polym. Sci. 253 362 (1975) 136. W M Barentsen, D Heikens Polymer 14 579 (1973) 137.R Fayt, R Jerome, Ph Teyssie Polym. Eng. Sci. 27 328 (1987) 138. W H Jo, H C Kim, D H Baik Macromolecules 24 2231 (1991) 139. H C Kim, K H Nam, W H Jo Polymer 34 4043 (1993) 140. N G Gaylord,M Mehta J. Polym. Sci., Polym. Lett. 20 481 (1982) 141. F Ide, A Hasegawa J. Appl. Polym. Sci. 18 963 (1974) 142. R Greco,M Malinconico, E Martuscelli, G Ragosta, G Scarinzi Polymer 28 1185 (1987) 143. F C Chang, Y C Hwu Polym. Eng. Sci. 31 1509 (1991) 144. A Gonzalez-Montiel, H Kekkula, D R Paul Polymer 36 4605 (1995) 145. M Ulrich, C Gaze, P Laroche J. Appl. Polym. Sci. 67 201 (1998) 146. G Fairley, R E Prud'Homme Polym. Eng. Sci. 27 1495 (1987) 147. P M Subramanian, V Mehra Polym. Eng. Sci. 27 663 (1987) 148. T C Maa, F C Chang J.Appl. Polym. Sci. 49 913 (1994) 149. M K Akkapeddi, C D Mason, B Van Buskirk Polym. Mater. Sci. Eng. 67 317 (1992) 150. W E Baker,M Saleem Polym. Eng. Sci. 27 1634 (1987) 151. J Z Liang, C Y Tang,H C Man J. Mater. Proc. Tech. 66 158 (1997) 152. S J Mahajan, B L Deopura, Y Wang J. Appl. Polym. Sci. 60 1527 (1996) 153. H P Blom, J W The, A Rudin J. Appl. Polym Sci. 61 959 (1996) 154. L D'Orazio,R Grendo, E Martuscelli,G Ragosta Polym. Eng. Sci. 22 536 (1982) 155. B Pucanszky, A Kallo, G Bodor Polymer 30 1399 (1989) 156. R Fayt, R Jerome, Ph Teyssie J. Polym. Sci. B, Polym. Phys. 19 1269 (1981) 157. R Fayt, R Jerome, Ph Teyssie Macromol. Chem. 187 837 (1986) 158. C R Lindsey, D R Paul, J W Barlow J. Appl. Polym. Sci. 26 1 (1981) 159.N Mekhilef, B D Favis, P J Carreau J. Polym. Sci. B, Polym. Phys. 35 293 (1997) 160. D Bourry, B D Favis J. Polym. Sci. B, Polym. Phys. 36 1889 (1998) 161. W J Ho, R Salovey Polym. Eng. Sci. 21 839 (1981) 162. R C Thamm Rubber Chem. Technol. 50 24 (1977) 163. S Danesi, R S Porter Polymer 19 448 (1978) 164. I Fortelny, E Navratilova, J Covar Angew. Makromol. Chem. 188 195 (1991) 165. T Inoue, T Suzuki J. Appl. Polym. Sci. 59 1443 (1996) 166. C A Valenza, F P Lamantia J. Appl. Polym. Sci. 39 865 (1990) 167. G Illing, in Macromolecular Alloy Systems: A Contribution to the Determination of the Structure of Impact Resistant Polyamide ± Polyolefin-Alloy, Polymer Blends, Processing Morphology and Properties (Eds E Maartuscelli, R Palumbo,M Kryszewsky) (New York: Plenum, 1980) p.167 168. S Fuzessery, in International Forum Davos Recycle'93, Davos, 1993 p. 8/5-1 169. C D Han Multiphase Flow in Polymer Processing (New York: Academic Press, 1981) 170. B Jurkowski, Y A Olkhov J. Appl. Polym. Sci. 65 1807(1997) 171. W J MacKnight, R W Lenz Polym. Eng. Sci. 25 1124 (1985) 172. C-J Wu J. Appl. Polym. Sci. 52 1965 (1994) 173. R M Holsti-Miettinen, J U Seppala Polym. Eng. Sci. 34 395 (1994) 174. Y-J Sun, G-H Hu,M Lambla Angew. Makromol. Chem. 229 1 (1995) 29 175. C Rosales, R Perera, M Ichazo, J Conzales, H Rojas, A Sanches, A Diaz-Barrois J. Appl. Polym. Sci. 70 161 (1998) 176. A Lazzeri,M Malanima, M Pracella J. Appl. Polym. Sci. 74 3455 (1999) 177. S C Tjong J. Mater. Sci.32 4613 (1997) 178. C Marco, G Ellis,M A Gomez, J G Fatou, J M Arribas, I Campoy, A Fontecha J. Appl. Polym. Sci. 65 2665 (1997) 179. H Keskkula, D R Paul, in Nylon Plastics Handbook (Ed. M Kohan) (MuÈ nich: Hanser, 1994) 180. M Xanthos Reactive Extrusion. Principles and Practice (MuÈ nich: Hanser, 1992) 181. M van Duin, R J M Borgreve, in Reactive Modifiers for Polymers (Ed. S Al-Malaika) (London: Chapman and Hall, 1997) p. 133 182. C Creton, E C Kramer, C Y Hui, H R Brown Macromolecules 25 3075 (1992) 183. C C Chen, E Fontan,K Min, J White Polym. Eng. Sci. 28 69 (1988) 184. S J Park, B K Kim, H M Jeong Eur. Polym. J. 26 131 (1990) 185. US P. 5 959 032 (1999) 186. F-P Tseng, J-J Lin, C-R Tseng, F-C Chang Polymer 42 713 (2001) 187. P Jannasch, B Wesslen J.Appl. Polym. Sci. 58 753 (1995) 188. K Y Park, S H Park, K-D Suh J. Appl. Polym. Sci. 66 2183 (1997) 189. Ya A Kayak,A B Vainshtein,O I Kazachkova Plast. Massy (5) 25 (1985) 190. M Kozlowski, F P La Mantia J. Appl. Polym. Sci. 66 969 (1997) 191. D Y Chang, F C Chang Polym. Networks Blends 4 157 (1994) 192. D-L Xie, D Chen, B Jiang, C-Z Yang Polymer 41 3599 (2000) 193. S C Kim, S Y Lee, in The 4th International Symposium on Polymers for Advanced Technologies. PAT'97 (Book of Abstracts), Leipzig, 1997 p. I-IY. 6a 194. A Datta, H H Chen, D H Baird Polymer 34 759 (1993) 195. D Heikens, N Noen,W M Barentsen, P Piet, H Laden J. Polym. Sci., Polym. Symp. 62 309 (1978) 196. S Y Hobbs,M E J Dekkers, V H Watkins Polymer 29 1598 (1988) 197.V Flaris,W E Baker,M Lambea Polym. Networks Blends 6 29 (1996) 198. J Huang, J Bingzheng, Q Guo Eur. Polym. J. 25 61 (1990) 199. N R Legge, G Holden, H E Schroeder (Eds) Thermoplastic Elastomers (MuÈ nich: Hanser, 1987) 200. A K Gupta, K R Srinivasan J. Appl. Polym. Sci. 47 167 (1993) 201. M C Schwarz, J W Barlow, D R Paul J. Appl. Polym. Sci. 37 403 (1989) 202. J Rosch, T Duschek, R Mulhaupt Polym. Adv. Technol. 4 465 (1993) 203. J Rosch, R Mulhaupt Polym. Bull. 32 697 (1994) 204. D Hlavata, Z Horak Eur. Polym. J. 30 597 (1994) 205. A K Gupta, S N Purwar J. Appl. Polym. Sci. 30 1777 (1985) 206. G Brinke, F E Karasz, W J McKnight Macromolecules 16 1827 (1983) 207. S Setz, F Stricker, J Kressler, T Duschek, R Mulhaupt J.Appl. Polym. Sci. 59 1117 (1996) 208. W Michaeli, A Grefenstein, U Berghaus Polym. Eng. Sci. 35 1485 (1995) 209. S C Stinson Chem. Eng. News, Jpn. (15) 20 (1996) 210. E V Prut, A N Zelenetskii Usp. Khim. 70 72 (2001) [Russ. Chem. Rev. 70 65 (2001)] 211. R Li, R Zhang Chin. J. Polym. Sci. 15 205 (1997) 212. C C Chen, J L White Polym. Eng. Sci. 33 923 (1993) 213. S Lim, J L White Polym. Eng. Sci. 34 221 (1994) 214. J T Yeh, C C Fen-Chiang, M F Cho Polym. Bull. 35 371 (1995) 215. M Lambla, M Seadan Polym. Eng. Sci. 32 1687 (1992) 216. D Ahn, K Khait,M Petrich J. Appl. Polym. Sci. 55 1431 (1995) 217. A R Nesarikar, S H Carr, K Khait, F M Mirabella J. Appl. Polym. Sci. 63 1179 (1997) 218. A Y Coran, R Patel Rubber Chem. Technol. 53 141(1980) 219.A A Kanauzova, M A Yumashev, A A Dontsov Poluchenie Ter- moplastichnykh Rezin Metodom `Dinamicheskoi' Vulkanizatsii i ikh Svoistva (Tematicheskii Obzor) [Manufacture of Thermoplastic Rubbers by the Method of `Dynamic' Vulcanisation and Their Properties (Subject Review)] (Moscow: TsNIITEneftekhim, 1985) 220. C S Ha, D J Ihm, S C Kim J. Appl. Polym. Sci. 32 6281 (1986) 221. C S Ha, S C Kim J. Appl. Polym. Sci. 35 2211 (1988) 222. L V Kompaniets, N A Erina, L M Chepel', A N Zelenetskii, E V Prut Vysokomol. Soedin., Ser. A 39 1219 (1997) a 223. US P. 3 758 693 (1973)30 224. N A Erina, L M Chepel' E V Prut, in Khimicheskaya Fizika Poli- merov i Kompozitsionnykh Materialov (Chemical Physics of Poly- mers and Composite Materials) (Moscow: Institute of Chemical Physics, Russian Academy of Sciences, 2000) Vol.2, p. 298 225. C S Ha, Y S Hur, S C Kim J. Polym. Adv. Technol. 2 31 (1991) 226. K-H Kim, W-J Cho, C-S Ha J. Appl. Polym. Sci. 59 407(1996) 227. A Siove,M Fontanille Makromol. Chem. 181 1815 (1980) 228. P Cohen,M J M Abadie, F Schue,D H Richards Polymer 22 1316 (1981) 229. A Soum, A Siove, M Fontanille J. Appl. Polym. Sci. 28 961 (1983) 230. F Cansell, A Siove J. Polym. Sci., Polym. Chem. Ed. 25 675 (1987) 231. F Cansell, A Siove, G Belorgey J. Polym. Sci. B, Polym. Phys. 28 647 (1990) 232. X Zhang,W Li, B Huang Makromol. Chem. 191 1765 (1990) 233. J-M Widmaier Makromol. Chem. 179 1743 (1978) 234. J W Peruchin,W P Archirejew, E W Kusnezow Plaste Kautsch. 22 394 (1975) 235.B Hazer Eur. Polym. J. 27 775 (1991) 236. O Nuyken, R Weidner Adv. Polym. Sci. 73/74 147 (1986) 237. E Agouri, R Catte Chim. Ind., Gen. Chim. 104 1873 (1971) 238. Jpn. P. 32 412; Ref. Zh. Khim. 12 S 318p (1972) 239. T Shiono, Y Akino, K Soga Macromolecules 27 6229 (1994) 240. A D Pomogailo, E B Baishiganov, F S D'yachkovskii Vysokomol. Soedin., Ser. A 23 231 (1981) a 241. Y Doi, T Koyama, K Soga Makromol. Chem. 186 11 (1985) 242. T Souel, F Schue,MAbatie, D H Richards Polymer 18 1292 (1977) 243. D E Sterenzat, L M Davydova, B A Dolgoplosk, E N Kropacheva, B I Presman Vysokomol. Soedin., Ser. B 12 388 (1970) a 244. B A Rozenberg, V I Irzhak,N S Enikolopyan Mezhtsepnoi Obmen v Polimerakh (Interchain Exchange in Polymers) (Moscow: Khimiya, 1975) 245.I Ya Poddubnyi, V A Grechanovskii Vysokomol. Soedin. 6 64 (1964) a 246. L Sperling, in Interpenetrating Polymer Networks (Advances in Chemistry Series 239) (Eds D Klempner, L Sperling, L Utracki) (Washington, DC: American Chemical Society, 1995) 247. M A Sherman, J P Kennedy J. Polym. Sci. A, Polym. Chem. 36 1891 (1998) 248. J S Shim, S Asthana, N Omura, J P Kennedy J. Polym. Sci. A, Polym. Chem. 36 2997 (1998) 249. C C Ku, R Liepins Electrical Properties of Polymers (MuÈ nich: Hanser, 1987) 250. M-A DePaoli, R J Waltman, A F Diaz, J Bargon J. Chem. Soc., Chem. Commun. 1015 (1984) 251. M-A DePaoli, R J Waltman, A F Diaz J. Polym. Sci., Polym. Chem. Ed. 23 1687 (1985) 252. F Lux J. Mater. Sci. 28 285 (1993) 253. M E Galvin, G E Wnek J. Polym. Sci., Polym. Chem. Ed. 25 229 (1984) 254. V N Noskova, G P Belov, G I Davydova, L N Raspopov, P E Matkovskii Vysokomol. Soedin., Ser. B 33 602 (1991) a 255. K I Lee, H Jopson Macromol. Chem., Rap. Commun. 4 375 (1983) 256. H Naarmann, N Theophilou Synth. Met. 22 1 (1987) 257. N Theophilou, D B Swanson, A G McDiarmid, A Chakrabouty, H H S Javadi Synth. Met. 28 35 (1989) 258. J Edwards, R Fiaschen, B Vincent Macromol. Chem., Rap. Commun. 4 393 (1983) 259. A R Bishop Polymer 26 622 (1985) 260. G F Dandreaux,M E Galuin, G E Wnek J. Phys. C 44 3 (1983) 261. S P Armes, B Vincent, J M Write J. Chem. Soc., Chem. Commun. 1525 (1986) 262. S P Armes, B Vincent Synth. Met. 25 171 (1988) 263. M E Calwin, G T Wneck Am. Chem. Soc., Polym. Prepr. 23 99 (1982) 264. A Bolognesi, M Cafellani,M Destri, W Porzio Polymer 27 1127 (1986) 265. US P. 4 394 304; Ref. Zh. Khim. 3 S 537p (1984) 266. Jpn. Appl. 63-117 018; Ref. Zh. Khim. 10 S 370p (1989) 267. US P. 4 772 421; Ref. Zh. Khim. 16 T146p (1989) 268. W Liang, C R Vartin J. Am. Chem. Soc. 112 1118 (1990) 269. C R Martin Adv. Mater. 3 457 (1991) A D Pomogailo 270. L M Yarysheva, S A Saifullina, E A Rozova, A I Sizov, B M Bulychev, A L Volynskii, N F Bakeev Vysokomol. Soedin., Ser. A 36 363 (1994) a 271. M F Rubner, S K Tripathy, J Georger Jr , P Cholewa Macromolecules 16 870 (983) 272. E K Sichel, M F Rubner J. Polym. Sci., Polym. Phys. Ed. 23 1629 (1985) 273. S Destri, M Catellani, A Bolognesi Macromol. Chem., Rap. Com- mun. 5 353 (1984) 274. J Kerr, J W White Synth. Met. 41 ± 43 911(1991) 275. A D Pomogailo, Zh S Kiyashkina, A I Kuzaev, S B Echmaev, I N Ivleva, F S D'yachkovskii Vysokomol. Soedin., Ser. A 27 707 (1985) a 276. A P Orlenko, E N Teleshov, G N Gerasimov Vysokomol. Soedin., Ser. A 33 1504 (1991) a 277. S M Fomin, G N Gerasimov, A I Stash, V M Vozzhennikov, I V Bulgarovskaya, E N Teleshov Vysokomol. Soedin., Ser. A 33 1511 (1991) a 278. T W Hagler,K Pakbaz, J Moulton, F Wuldl, P Smith, A J Heeger Polym. Commun. 32 339 (1991) 279. C Weder, C Sarwa, C Bastiaasen, P Smith Adv. Mater. 9 1035 (1997) 280. C Y Yang, A J Heeger, Y Cao Polymer 41 4113 (2000) 281. B Zinger, D Kijel Synth. Met. 41 ± 43 1013 (1991) 282. H L Wang, J E Fernandez Macromolecules 25 6179 (1992) 283. O Niwa,M Kakuchi, T Tamamura Macromolecules 20 749 (1987) 284. W Werner Synth. Met. 41 ± 43 843 (1991) 285. E Ruckenstein, J S Park Polym. Compos. 12 289 (1991) 286. A Mohammadi, D W Paul, O Inganas, J O Nilsson, I Lundstrom J. Polym. Sci., Polym. Chem. Ed. 32 495 (1994) 287. J W Byun, S S Im Synth. Met. 55 ± 57 3501 (1993) 288. G K El'yashevich, A G Kozlov, N Gospodinova, P Mokreva, L Terlemezyan Vysokomol. Soedin., Ser. B 39 762 (1997) a 289. S A Saifullina, L M Yarysheva, A V Volkov, A L Volynskii, N F Bakeev Vysokomol. Soedin., Ser. A 38 1172 (1996) a 290. S Yu Ermusheva, L M Yarysheva, A L Volynskii, N F Bakeev Vysokomol. Soedin., Ser. A 38 1179 (1996) a 291. J C Chiang, A G McDiarmid Synth. Met. 13 193 (1986) 292. A G McDiarmid, J C Chiang, A F Richter, N L D Somasiri, A J Epstein, in Conducting Polymers (Ed. L Alcacer) (Dordrecht: Reidel, 1987) p. 105 293. I Kminek, J Trekoval Macromol. Chem., Rap. Commun. 5 53 (1984) 294. J A Stowell, A J Amass,M S Beevers, T R Farren Polymer 30 195 (1989) 295. N M Galashina, P M Nedorezova, V I Tsvetkova, F S D'yachkovskii, N S Enikolopyan Dokl. Akad. Nauk SSSR 278 620 (1984) b 296. Yu A Berlin, S I Beshenko, V A Zhorin, N M Galashina, N S Enikolopyan Dokl. Akad. Nauk SSSR 267 1390 (1982) b 297. Y Fujikura, M Kawarai Polymer J. 21 609 (1989) 298. S Wu Polymer Interface and Adhesion (New York; Basel: Marcel Dekker, 1982) 299. Yu S Lipatov, A E Fainerman Dokl. Akad. Nauk SSSR 256 624 (1981) b 300. B S Kolupaev Vysokomol. Soedin., Ser. B 21 223 (1979) a 301. A E Chalykh, G S Golovkin, V P Dmitrenko, A E Rubtsov, in Tezisy Dokladov V Vsesoyuz. Konf. po Kompozitsionnym Materi- alam (Abstracts of Reports of the Fifth All-Union Conference on Composite Materials) (Moscow: Moscow State University, 1981) Pt. 2, p. 104 302. I I Nazarova, A D Pomogailo, V B Nazarov, S M Baturin Vysokomol. Soedin., Ser. A 29 714 (1987) a 303. S Wu Polymer 26 1855 (1985) 304. O Roetting, G Hinrichen Adv. Polym. Technol. 13 57 (1994) 305. A G Machiels, K F J Denys, J Van Dam, A Posthuma de Boer Polym. Eng. Sci. 36 2451 (1996) 306. M B Nir, A Ram, J Miltz Polym. Eng. Sci. 35 1878 (1995) 307. W P Gergen, R Lutz, S Davison, in Thermoplastic Elastomers. A Comprehensive Review (Eds N R Legge, G Holden, S Davison) (MuÈ nich: Hanser, 1987) p. 507 308. R C Willemse, A Posthuma de Boer, J Van Dam, A D Gotsis Polymer 39 5879 (1998) 309. T Tang, Z L Lei, B T Huang Polymer 37 3219 (1996)31 Molecular polymer ± polymer compositions. Synthetic aspects 310. P L Beltrame, A Gastelli, M Canetti, A Seves Macromol. Chem., Phys. 196 2751 (1995) 311. J D Lee, S M Yang Polym. Eng. Sci. 35 1821 (1995) 312. A A Torosyan, V G Shteinberg, A D Pomogailo, G P Belov, S L Saratovskikh, F S D'yachkovskii, in Kompleksnye Metallo- organicheskie Katalizatory Polimerizatsii Olefinov (Complex Organometallic Catalysts of Polymerisation of Alkenes) (Chernogolovka: Institute of Chemical Physics, Academy of Sciences of the USSR, 1986) Vol. 10, p. 129 313. V A Sarkisyan, G A Airapetyan, A K Dadivanyan Vysokomol. Soedin., Ser. B 26 860 (1984) a 314. K Keynosuke, T Tosisada Kobunshi Kogaku 28 178 (1971) 315. M Iguchi, H Kanetsuna, T Kawai Br. Polym. J. 3 177 (1971) 316. A A Donatelli, H L Sperling, D A Thomas J. Appl. Polym. Sci. 21 1189 (1977) 317. S C Kim, D Klempner, K C Frisch,W Rodigan, H L Frisch Macromolecules 9 258 (1976) 318. A D Pomogailo Polimery 43 161 (1998) 319. J P Kennedy,M Nakao J. Macromol. Sci. A 12 197 (1978) 320. Yu A Glagoleva, I V Kurbatova, V M Listkov, V I Bukhgalter, T A Ivanova Plast. Massy (9) 22 (1980) 321. I V Grigorov, S I Sagalaeva, N Yu Golubeva Plast. Massy (9) 33 (1988) 322. S S Glazkov, E N Levykin, M V Enyutina Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 44 142 (2001) 323. V I Terent'ev Plast. Massy (12) 50 (1991) 324. D Darras, R Sequela, A Rietsch J. Polym. Sci., Polym. Phys Ed. 30 349 (1992) 325. J T Yeh, S S Chang, M S Yen J. Appl. Polym. Sci. 70 149 (1998) 326. J J Laverty, R L Bullach, T S Ellis, T E McMinn Polym. Rec. 2 159 (1996) 327. G I Dzhardimalieva, B S Selenova, T I Ponomareva, V K Skachkova, A D Pomogailo, in Kompleksnye Metalloorgani- cheskie Katalizatory Polimerizatsii Olefinov (Complex Organo- metallic Catalysts of Polymerisation of Alkenes) (Chernogolovka: Institute of Chemical Physics, Academy of Sciences of the USSR, 1986) Vol. 10, p. 63 328. F Avalos,M A Lopez-Manchado,M Arroyo Polymer 39 6173 (1998) 329. Appl. BRD 3 526 234; Ref. Zh. Khim. 15 S 375 (1987) 330. N M Livanova, E S Popova, O A Ledneva, A A Popov Vysokomol. Soedin., Ser. A 40 51 (1998) a 331. L S Shibryaeva, A A Popov Khim. Fiz. 20 47 (2001) d 332. C David, F Zabeau Eur. Polym. J. 21 343 (1985) 333. A Daro, F Zabeau, C David Eur. Polym. J. 25 71 (1989) 334. A Drago,M Trojan,R Jacobs,C David Eur. Polym. J. 26 47 (1990) 335. P Galli, J C Haylock Prog. Polym. Sci. 16 443 (1991) a�Polym. Sci. (Engl. Transl.) b�Dokl. Chem. (Engl. Transl.) c�Kinet. Catal. (Engl. Transl.) d�Chem. Phys. Rep. (Engl.

ISSN:0036-021X    

出版商:RSC    

年代:2002

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Russian Chemical Reviews 71 (1) 33 ± 48 (2002) Mechanisms of asymmetric induction in catalytic hydrogenation, hydrosilylation and cross-coupling on metal complexes V A Pavlov Contents I. Introduction II. Asymmetric induction prior to the catalytic reaction III. Asymmetric induction in the stage of substrate coordination by catalytic complexes IV. Asymmetric induction in the course of transformations of the first or subsequent intermediates of the catalytic reaction V. Asymmetric induction caused by nonbonded diastereomeric interactions VI. Conclusion Abstract. of mechanisms the on studies of state current The The current state of studies on the mechanisms of asymmetric and hydrosilylation hydrogenation, asymmetric hydrogenation, hydrosilylation and cross-coupling cross-coupling induced by metal complexes is considered.The possibilities for the induced by metal complexes is considered. The possibilities for the identification involving reactions catalytic of stages of identification of stages of catalytic reactions involving asymmetric asymmetric induction of types the of classification The analysed. are induction are analysed. The classification of the types of asym- asym- metric stages these in differences the to according induction metric induction according to the differences in these stages is is proposed. the in occurs induction asymmetric cases, most In proposed. In most cases, asymmetric induction occurs in the stage stage of ± key (the complexes catalytic by coordination substrate of substrate coordination by catalytic complexes (the key ± lock lock interaction).the of favour in presented is Evidence interaction). Evidence is presented in favour of the assumption assumption that in product the in formed is atom carbon asymmetric the that the asymmetric carbon atom is formed in the product in the the stages intermediate, asymmetric the of transformations of stages of transformations of the asymmetric intermediate, even even though the starting chiral catalytic complex is The though the starting chiral catalytic complex is C2-symmetric. -symmetric. The bibliography includes 118 references bibliography includes 118 references. I. Introduction Asymmetric catalytic reactions involving prochiral substrates on chiral metal-complex catalysts proceed through the interaction of these species to form diastereomeric intermediate complexes.The rational description of the phenomenon of asymmetric induction (or diastereoselectivity) { in asymmetric synthesis can be given using empirical stereochemical rules and models 1 analogous to those proposed by Prelog, Cram, Cornforth, et al. Asymmetric induction can take place either in the stage of interaction (coor- dination) of a metal-complex catalyst with a substrate to form an intermediate complex or in the course of subsequent transforma- tions of the resulting complex. The first alternative is sometimes called the steric control or the `key ± lock' interaction (the terms `chiral recognition' and `chiral discrimination' are also used).For the reaction product, [AB]2... [AB]1 A+B where A is the substrate and B is the reagent (catalyst), the following general classification of the types of asymmetric induc- tion is proposed: (1) prior to the reaction (due to a chiral medium); V A Pavlov N D Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp. 47, 119991 Moscow, Russian Federation. Fax (7-095) 135 53 28. Tel. (7-095) 938 35 02. E-mail: pvlv@cacr.ioc.ac.ru Received 25 April 2001 Uspekhi Khimii 71 (1) 39 ± 56 (2002); translated by T N Safonova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n01ABEH000677 33 33 35 40 44 46 (2) the stage involving the interaction of A with B (coordina- tion of the substrate by the catalyst); (3) transformations of the first ([AB]1), second ([AB]2) and subsequent intermediates already involving the coordinated sub- strate; (4) nonbonded diastereomeric interactions analogous to those existing between enantiomers in racemates.Asymmetric induction of the first type is observed in reactions performed in a chiral matrix [for example, in cholesteric liquid crystals (CLC) in the mesophase temperature range, where a chiral helical structure is formed] or in a chiral solvent and, probably, under the combined action of the electric and magnetic fields.2, 3 Asymmetric induction of the second type takes place in asymmet- ric metal-complex catalysis, in particular, on hydrogenation and hydrosilylation of aromatic ketones.Asymmetric induction occurring in the course of transformations of the first and second intermediates (the third type) has been examined in detail for catalytic allylic alkylation.4 The nature of asymmetric induction, which is caused by interactions between enantiomers in racemates (the fourth type) and due to which the racemates differ from the constituent individual enantiomers in some chemical and physical properties, is poorly known. Asymmetric induction, if considered as a way of communicat- ing stereochemical information, involves two components, viz., static discrimination due to steric hindrance and dynamic dis- crimination resulting from kinetic or thermodynamic factors.5 Hydrogenation, hydrosilylation and cross-coupling are of importance from the practical standpoint because they comprise a minimal set of reactions, which provide the preparation of chiral products bearing many functional groups involved in pharma- ceuticals.6 Because of this, the present review deals with the mechanisms of asymmetric induction occurring in these three reactions.II. Asymmetric induction prior to the catalytic reaction Asymmetric induction prior to the reaction should be considered as a manifestation of the chiral action of a medium both on the catalyst, which in this case can be achiral, and the prochiral substrate. Asymmetric reactions can proceed in a CLC medium { The enantioselectivity can be considered as a measure of asymmetric induction in asymmetric catalysis where the enantiomeric product is isolated from a diastereomeric intermediate complex.34 in the mesophase temperature range where a chiral helical structure is formed (for more detailed arguments in favour of asymmetric induction prior to the reaction, see below).However, the published data on asymmetric synthesis in a CLC medium as a chiral matrix are contradictory. The enantioselective Claisen rearrangement performed in a CLC mesophase has been reported:7 OH O * CNB 200 8C, 6 h CNB is cholesteryl p-nitrobenzoate. The authors judged the enantioselectivity of the reaction from the CD spectra of the product. Ethyl(phenyl)malonic acid was decarboxylated in a CLC mesophase in an optical yield (p) of 18%:8H CO2H Et CO2H Et CO2H cholesteryl benzoate 160 8C, 2 h (R) In the photochemical synthesis of hexahelicene performed in CLC within the mesophase temperature range of 20 ± 25 8C, the enantioselectivity of the process was 1%, which was confirmed by the presence of a CD maximum in the absorption band of the hexahelicene obtained.9cholesteryl nonanoate ± cholesteryl chloride (3:1), I2, hn 23 8C, 1.5 h [a]D=+40 (CHCl3) However, photocyclisation of a-(N-methylanilino)styrene into N-methyl-2-phenylindoline in various CLC mesophases afforded a racemic product.10 When studying thermochemical interconversions of sulfoxides in a cholesteryl p-nitrobenzoate mesophase, a small enantiomeric excess (ee) of the product was observed.11 O O CNB, 150 ± 200 8C S S Me Me (S) (R) ee=9.2% O O S S CNB, 150 ± 200 8C Me Me (S) (R) ee=2.4% It was concluded 10, 11 that asymmetric transformations in cholesteric mesophases, like those in standard chiral solvents, proceed in low optical yields.However, the authors of the cited papers noted that exceptions are possible in the case of strong specific interactions between the dissolved compound and CLC. In another study,12 even the theoretical possibility of the manifestation of the asymmetric effect in a CLC mesophase was V A Pavlov denied because the helical step of CLC (300 ± 400 nm) is incom- patible with the molecular sizes of standard reagents. These molecules do not `notice' the helical structure of CLC, much as people do not notice the curvature of the earth's surface.Never- theless, CLC, owing to the chirality of their molecules, can exert a slight asymmetric effect on chemical reactions, if only as optically active solvents. The asymmetric influence of the magnetic field is poorly known.{ Pasteur was the first to take an interest in this problem. He believed that if the magnetic field can induce rotation of the polarisation plane of light in the case of achiral compounds (the Faraday effect), it can generate the disymmetry analogous to that possessed by chiral molecules. Kelvin reasoned that this idea is in error because the magnetic rotation does not possess the right- or left-handed quality, i.e., it is not chiral. However, P Curie noted that the magnetic field parallel to the electric field can induce chirality because the mirror operation (which provides interchan- geability of components) is applicable to the parallel and antipar- allel molecular arrangements in this combined field.14 The above- considered enantiomorphism and the enantiomorphism of chiral molecules are distinguished as the `false' and 'true' chirality, respectively.2, 3 This raises the question of whether the `false' chiral effect can cause asymmetric induction.It is known that the symmetry is broken at the level of elementary particles. It is assumed that the `false' chiral effect can induce analogous symmetry distortions in chemical reactions. Changes in the symmetric (in the absence of this effect) energy barriers in the path of formation of enantiomers of the product can lead to differences between the rate constants of formation of enantio- mers in the chemical reaction away from the equilibrium.2, 3 This theoretical reasoning has been supported experimentally.Thus Gerike 15 discovered the enantiomeric enrichment (although small and irregular) of the product (ee<1%) in alkylation of prochiral ketones with Grignard reagents, the addition of bromine at the double bond and some other reactions using various combinations (parallel, antiparallel and orthogonal mutual arrangements) of the electric (15 ± 8100 V cm71) and magnetic (constant and alternating, 86103 ± 1.176105 A m71, 0 ± 50 Hz) fields. It was reported that reduction of ketones with Grignard reagents and lithium aluminium hydride in a constant magnetic field proceeded with high enantioselectivity.16 However, this conclusion was disproved by one of the authors.17 It is agreed 2, 3 that the concept of the asymmetric influence of the constant magnetic field is reasonable, but this influence can serve only as a `starter' and must be combined with a chiral autocatalytic process. Apparently, this problem calls for further careful exper- imental investigation. We also examined asymmetric induction in catalytic reactions proceeding in a liquid-crystal chiral matrix 18, 19 by studying hydrogenation of a-acetamidocinnamic acid (ACA) in the pres- ence of achiral Wilkinson's catalyst RhCl(PPh3)3 possessing a substantial molecular volume (this fact is of importance in imparting the helical ordering of the CLC molecules to the molecules of the catalyst).Since RhCl(PPh3)3 and ACA are insoluble in a CLC melt, viz., in cholesteryl tridecanoate (CTD), the reaction was carried out in a CTD±BunOH±C6H6 mixture. In this solution, the molecules of CTD, the substrate (ACA) and the catalyst RhCl(PPh3)3 have helical ordering at the mesophase temperature of 60 ± 70 8C. This is evidenced by the fact that the CD spectra show maxima of induced circular dichroism of the same sign in the absorption bands of the substrate and the catalyst. Probably, the same sense of helical ordering of the substrate and catalyst molecules facilitates discrimination between the Re- and Si-sides of the C=C bond in the substrate coordinated to the catalyst in an intermediate complex.Actually, a moderate temperature-dependent enantioselectivity was observed in the reaction under consideration (Fig. 1). { The non-asymmetric influence of the magnetic field on chemical reac- tions has long been known (see, for example, the review 13).Mechanisms of asymmetric induction in catalytic hydrogenation, hydrosilylation and cross-coupling on metal complexes p (%) 15 105 58 60 62 T /8C Figure 1. Temperature dependence of the optical yield of N-acetylphe- nylalanine in hydrogenation of ACA. The dependence has a maximum in the middle of the temper- ature range of the CTD mesophase where the cholesteric liquid crystal is helically ordered. This effect could also be attributed to the effect of CLC as a chiral solvent.However, the fact that the maximum of enantioselectivity coincides with the middle of the temperature range of the liquid crystal mesophase counts in favour of the former explanation. With the aim of investigating the effect of a combination of the electric and constant magnetic fields on chemical reactions, we } used a solenoid.20 Acid- and base-catalysed mutarotation of D-glucose in water was examined. In the mutarotation of mono- saccharides, the equilibrium was established between the a and b forms, which are diastereomers. In the initial stage, the reaction was far from the equilibrium (this is a necessary condition for the magnetic field to exert the asymmetric effect 2, 3).The reaction was carried out in a temperature-controlled polarimetric tube placed in a solenoid and its course was followed from the change in the optical rotation. The rate constant of mutarotation k was calcu- lated according to the first-order equation. The magnetic field strength (H) was calculated from the magnetic rotation of the polarisation plane of light under the action of water at l=589 nm. The direction of magnetic field was judged from the direction of the observed rotation of the plane of polarisation induced by water. To obtain a more accurate estimate of the dispersion of the rate constants, some experiments were repeated several times at the same temperature. The results are given in Table 1. Presently, one cannot give an unambiguous answer to the question of how the rate constant is changed depending on the direction of magnetic field.However, the reaction at low temper- atures in the field with (+)-aD H2O proceeds somewhat more rapidly than that in the field with (7)-aD H2O. At the same time, Table 1. Rate constants of mutarotation of D-glucose at different temper- atures depending on the direction of magnetic field in a solenoid.20 H /G 103 k /min71 103Dk /min71 T /8C (+)-aD H2O (7)-aD H2O 1266 6.2 1265 10.2 3.69 0.02 3.42 0.08 5.65 0.03 5.59 0.02 3.73 0.04 3.74 0.03 6.01 0.02 5.81 0.04 1262 20 1259 30 5.50 0.04 16.72 0.05 16.82 0.06 41.57 0.08 6.02 0.01 16.68 0.05 16.80 0.03 43.57 0.29 +0.04 +0.32 +0.36 +0.22 +0.52 70.04 70.02 +2.00 71.60 +7.29 1256 38.3 43.10 0.20 83.58 0.62 42.50 0.39 90.87 0.49 } The experiment was carried out in collaboration with E I Klabunovskii.35 it can be said with reasonable confidence that the effect of a constant magnetic field (without combining with the electric field) is absent. The results of the investigation of D-glucose mutarota- tion in water in a constant magnetic field demonstrated that the changes in the rate constants depending on the direction of the magnetic field are comparable in value to the errors of the measurements (Table 2). Table 2. Rate constants of mutarotation of D-glucose at 200.1 8C depending on the direction of constant magnetic field (H=7490 G).103 k /min71 103Dk /min71 (+)-aD H2O (7)-aD H2O 70.94 0.85 70.19 0.26 71.02 0.41 +0.29 0.56 70.07 0.42 14.67 0.29 15.03 0.07 15.19 0.23 14.85 0.11 15.07 0.11 13.73 0.85 14.84 0.26 14.17 0.41 15.14 0.56 15.00 0.42 III. Asymmetric induction in the stage of substrate coordination by catalytic complexes The elucidation of the nature of asymmetric induction and the identification of the stage where it takes place are closely related to the investigation of the general mechanism of asymmetric reac- tions. For the latter problem to be solved, it is necessary to reveal individual elementary stages, determine their rates, detect and characterise intermediates (including the establishment of their absolute configurations) and, finally, identify the key stage of asymmetric induction.If asymmetric induction occurs in the stage of substrate coordination by a chiral catalytic complex, this fact can be established using rather simple logical approaches consid- ered below. The mechanism of hydrogenation of the C=C bond in substrates was studied using rhodium phosphine catalysts, involv- ing such ligands as dipamp,21 ± 25 chiraphos,26 ± 29 diop,21, 29 bppm,30, 31 dioxop 32, 33 and BisP*.34 Me Me OMe Ph O O P P Me Me Ph PPh2 Ph2P MeO PPh2 Ph2P (S,S)-( ± )-chiraphos (R,R)-( ± )-dipamp (R,R)-( ± )-diop Me P R Ph2P R P O Me PPh2 BisP* O N Ph2P PPh2 CO2But dioxop bppm R=But, Et3C, cyclo-C5H9, cyclo-C9H11, 1-adamantyl In studies on hydrogenation of itaconic, citraconic and mesaconic acids in the presence of [Rh(bppm)]+ giving rise to 2-methylsuccinic acid, an attempt was made to reveal the stage involving asymmetric induction (the optical yields of the products are given below the formulae of the acids).35 CO2H HO2C HO2C HO2C CH2CO2H CO2 Me Me CH2 50.9% (S ) 22.0%(S ) 85.3% (S )36 In the event that the enantioselectivity of the reaction is controlled by intermediate 1 common to all the three substrates (which follows from Halpern's theory discussed below), the optical yields must be close, which is contradictory to the experimental data considered above.Hence, it was concluded that asymmetric induction occurred in the stage of substrate coordination (the `key ± lock' interaction).OHO S P Rh HO2C H P Me 1 S is the solvent It should be noted that the chiral catalytic complex with the bppm ligand does not possess symmetry elements (except for C1) and, hence, asymmetric induction in the stage of substrate coordination is performed by the asymmetric complex. The hydrogen pressure influences the enantioselectivity of hydrogenation of acetyl- and benzoylaminocinnamic acids and their esters in the presence of rhodium complexes with the bppm, diop or dipamp ligands (with NEt3 and without it). To explain this fact, Ojima et al.35 offered the hypothesis that two reaction pathways compete with each other: Path A H2 * Rh+(C=C) * Rh(C=C) * Rh(C=C)H2 Path B (C=C) * RhH2 * Rh+H2 * RhH2(C=C) Paths A and B are favourable at low and high hydrogen pressures, respectively.Path B has been found previously in the studies of hydrogenation of alkenes on Wilkinson's catalyst. The authors of the cited study postulated 35 that paths A and B afforded different enantiomers. Hence, an elevation of the pres- sure, which facilitates path B, would be expected to decrease the enantioselectivity up to the inversion of the configuration. It is this situation that was experimentally observed. Interestingly, the addition of triethylamine weakened the influence of the hydrogen pressure on the enantioselectivity. The anion, which is generated upon the addition of NEt3 to acid (the substrate), has a much higher tendency to form an adduct with the rhodium phosphine complex than the corresponding acid.Consequently, the contri- a p (%) p (%) 60 60 40 40 20 200 0 pH /atm 25 2 H Figure 2. The optical yield of the product (S)-AcPhe in hydrogenation of ACA vs. the hydrogen pressure (a), the reaction temperature (b), the ratio of the concentrations of ACA and the catalytic complex (c) and the dielectric constant of the solvent (d ). The reaction conditions: (a) � T=20 8C, [ACA] : [Ph]=100 : 1, C6H6 ±MeOH (1 : 1); (b) � p 2 =1 atm, [ACA] : [Ph]=100 : 1, C6H6 ±MeOH (1 : 1); (c) � T=50 8C, p 2 =1 atm, PriOH; (d) � T=25 8C, pH2 =1 atm, [ACA] : [Ph]=100 :1, the solvent: (1 ) DMF; (2) dioxane; (3) THF (4) EtOH ± C6H6; (5) MeOH±C6H6 ; (6) EtOH; (7 ) MeOH.(S/R)-product (R/S)-product b 50 25 T /8C V A Pavlov bution of path B in the presence of a base is insignificant, which agrees with the experimental evidence that the enantioselectivity remained virtually unchanged at different hydrogen pressures. Hence, the hypothesis under consideration provides an adequate explanation for the effect of the hydrogen pressure and the influence of the addition of a base on the enantioselectivity. However, this scheme, as applied to the hydrogenation of derivatives of amidocinnamic acid, does not allow one to reveal the stage responsible for asymmetric induction due to a lack of detailed stereochemical information on two competitive reaction pathways. Nevertheless, the hypothesis for two competitive reaction pathways is fruitful.It was found 36 ± 39 that the enantio- selectivity of hydrogenation of ACA in the presence of the [Rh((R)-PheNOP)(cod)]ClO4 complex H CH2Ph OPPh2 N Ph2P Me (R)-PheNOP depends on the hydrogen pressure, the temperature, the ratio of the concentrations of ACA and the complex and the dielectric constant of the solvent (Fig. 2). These dependences can readily be explained within the framework of the above-considered concept. The contribution of the reaction path B becomes more significant and, correspondingly, the higher enantiomeric excess of the product adopting a certain configuration, viz., (S )-AcPhe, is achieved as the hydrogen pressure is elevated, the concentration of the substrate with respect to the catalyst is decreased, the temperature is lowered and the dielectric constant of the solvent decreases.The role of the first two factors follows directly from the schemes of the reaction paths A and B. The influence of the two last-mentioned factors is in agreement with the observations made in the study.35 The proposed explanation is also confirmed by the fact that rhodium hydride was observed in the reaction mixture at low temperature (720 8C).37 Upon hydrogenation of the enamide substrate on complexes with bppm and PheNOP, asymmetric induction occurs during the transformation of an asymmetric intermediate. The pure dihydride mechanism (path B), which is realised even at low temperature and hydrogen pressure, was considered in the study 34 devoted to hydrogenation of methyl a-acetamidocinna- mate (AcDPheOMe) on a rhodium complex containing (S,S )-1,2- bis(tert-butylmethylphosphino)ethane (Scheme 1).Asymmetric induction takes place during substrate coordina- tion by the dihydride complex (major intermediate) present in an p (%) c d p (%) 30 60 1 20 3 40 2 4 10 20 5 7 6 0 0 300 [ACA] : [Rh] 20 10 e 710 HMechanisms of asymmetric induction in catalytic hydrogenation, hydrosilylation and cross-coupling on metal complexes But Me P +Rh But P Me H2, MeOH(S) But Me S (R)-AcPheOMe P +Rh But S P Me 2 But Me P +Rh But P NHCOMe Me CH2CH CO2Me (R)-AcPheOMe 2 Me ButH Me O P Rh + NH But P O CH2Ph Me OMe Me S P + ButHRh CH2Ph But P O X Me NH Me excess.As can be seen from Scheme 1, this intermediate is devoid of symmetry elements, i.e., the asymmetric carbon atom is generated in the product in the stage of the transformation of the asymmetric intermediate although the starting chiral catalytic complex 2 possessed the symmetry C2. The mechanism of asymmetric induction was investigated for hydrogen-transfer hydrogenation of aromatic ketones R1COR2 O O In all cases, the configuration of the product was determined by the configuration of the ligand and was independent of the nature of the metal atom. If ketones as substrates of this reaction are arranged in order of decreasing difference between the molecular volumes of two substituents (Fig. 3), it can be seen that ee tends to decrease in the series FlCOMe>2-NpCOMe> PhCOMe> PhCOEt>PhCOPri.In the case of rhodium com- plexes with the ligand 3a, the corresponding series is as follows: FlCOMe>2-NpCOMe&PhCOMe&PhCOEt&PhCOPri. Me Me The most pronounced difference in ee is observed for FlCOMe and PhCOMe. This fact can be interpreted using the quadrant rule. 2-NpCOMe FlCOMe O O O Pri Et Me Ar on rhodium and iridium complexes, which were generated in situ and contained Schiff bases, viz., derivatives of (S,S)- (3a ± c) and (R,R)-1,2-diaminocyclohexane (4).40 N N N N Ar Ar Ph Ph 3a ± c (S,S ) 4 (R,R) 3: Ar=Ph (a), C6H4OMe-o (b), Np-2 (cetal atom in the C2-symmetric chelate chiral complex (the catalyst) is oriented toward the intersection point of the Me Me ButH S P P + ButHRh Rh + But But H P P S Me Me 1 : 10 AcDPheOMe Me P + ButH H Rh But H P X O Ph Me NH Me X=CO2Me N C N C M Ar 37 Scheme 1 H + S S Blocked quadrants M R2 R1 C R2 >R1 O38 ee, % 60 40 60 40 60 40 60 40 70 50 II I Figure 3.Influence of the difference between the molecular volumes of the substituents R1 and R2 in ketone R2COR1 (R2>R1) on the enantio- selectivity of hydrogenation. (The catalytic system: (a) Rh/3a, Rh/4; (b) Rh/3b; (c) Rh/3c; (d ) Ir/4; (e) Ir/3b. Ketone: (I) FlCOMe; (II) 2-NpCOMe; (III) PhCOMe; (IV) PhCOEt; (V) PhCOPri.quadrants and the C2-symmetrically arranged aryl substituents fall in the quadrants related by the symmetry C2 (left upper and right lower), thus `blocking' them. The larger the difference between the molecular volumes of the substituents in the ketone involved in the intermediate complex (the average of two possible positions, which can occupy the ketone in the intermediate, is indicated by an empty arrow), the higher the probability of the bulky substituent of the carbonyl group of the ketone being located in the free quadrant and, correspondingly, the larger the chiral discrimination and, consequently, the higher the enantiose- lectivity. It is this situation that was experimentally observed and, hence, the data obtained can be considered as an argument in favour of the fact that asymmetric induction (chiral recognition) takes place in the stage of substrate coordination in an intermedi- ate complex.Other authors also support the idea that asymmetric induction in hydrogenation with the hydrogen transfer catalysed by rho- dium and iridium complexes occurs in the stage of ketone coordination. Thus the following catalytic cycle (Scheme 2) was proposed 41 for hydrogen-transfer hydrogenation of acetophe- none catalysed by (S)-(+)-3-sec-butyl-, (S)-(7)-3-(1,2,2)-trime- thylpropyl- or (S,S)-(+)-3,8-di-sec-butyl-1,10-phenanthroline complexes of rhodium(I) in a PriOH±KOH medium. a (R) product (S) product bcde IV III V MeCHOH Me Me Ph O H N Rh N N N 9 As can be seen from Scheme 2, asymmetric induction occurs upon substrate coordination by asymmetric intermediate 7.How- ever, this mechanism of hydrogenation needs additional evidence. For the reactions catalysed by iridium complexes with Schiff bases, which were prepared from pyridine-2-carbaldehyde and a-phenylethylamine, a mechanism was proposed according to which asymmetric induction takes place in the stage of the hydro- gen transfer from the donor (isopropyl) group to the acceptor molecule (ketone subjected to hydrogenation) in the cyclic tran- sition state (Scheme 3).42, 43 Products DH2 N N Ir O X O H C OH7 Ph H2O N N Ir O X O CPh DH2 is propan-2-ol; X=DH2, OH7; R=Me, Et, Prn, Pri. H Me N Me N Rh O N H N 5 Ph Me HO7 HO H H2O N N N Rh NH O Me 6 Me Me Ph H O N Rh N N N 8N N Ir DH2 X DH2 Me Me C HR Me Me C HR N C N Ir O X O C H R Ph V A Pavlov Scheme 2 + O Me Me H N Rh N N N 7O Ph Me Scheme 3 PhCOR DH2 N H Me C N Ir O X Me O C Ph H ROH7 H2O N Me C N Ir O X Me O C H R Ph Me MeMechanisms of asymmetric induction in catalytic hydrogenation, hydrosilylation and cross-coupling on metal complexes Me Me O N P Me Ir P H H MeN P Ir O P Me Me OH H Me H MeN Me P HO Ir O P Me H P=PPh2. The catalytic cycle proposed 44 for hydrogenation of benzylideneacetone in the presence of the HIr[(R)-Ph..CH(Me)N(CH2CH2PPh2)2] complex is shown in Scheme 4. The structure of the intermediate was established by X-ray diffraction analysis. According to this scheme, asymmetric induction occurs in the stage of coordination of the ketone by the asymmetric complex. Scheme 3 involves direct hydrogen transfer, whereas Scheme 4 implies that the transfer proceeds with the participation of metal hydride.44 Both schemes are to a large extent conven- tional, but the latter scheme seems to be better justified } because the hydride intermediate was found experimentally.44 Therefore, there is some evidence that asymmetric induction takes place in the stage of ketone coordination by an asymmetric intermediate complex, which lost the symmetry C2 (if it was present in the starting catalytic complex). The complex, which is formed in situ based on CoCl2, semi- corrin ligand 10 and NaBH4 in alcohol, serves as an efficient catalytic system for asymmetric reduction.46, 47 CNN N H OSiMe2But ButMe2SiO 10 Amides and esters of a,b-unsaturated acids are reduced by this system with high enantiomeric excess (ee=90% ± 99%).O O NaBH4 (1 equiv.), CoCl2 (0.1 mol.%),Me Me 10 (0.12 mol.%) Me Me R R EtOH ± diglyme, 23 8C HN HN ee=98.7% (R=(CH2)2Ph), 98.9% (R=C6H11), 92.4% (R=Ph); O Me Me O Me Me OEt Me OEt Me ee=96% } At the same time, Scheme 4 ignores the reversibility of the reaction involving the hydrogen transfer from isopropanol 45 and, hence, Scheme 3 seems to be preferable.H O Me MeN PP H Me The investigation of the reaction mechanism followed with the use of NaBH4 in an EtOD ±DMF medium or using NaBD4 in an EtOH ±DMFmedium demonstrated that borohydride and EtOH serve as sources of hydrogen atoms at the b- and a-carbon atoms of the product, respectively.47 Hydrogenation of substrates bear- ing a prochiral carbon atom in the a-position proceeds non- enantioselectively with respect to this atom.47, 48 The assumed structure of the intermediate complex is shown in Fig. 4. R4 EtOOCN N R1 R3 R2 A Figure 4. Possible structures of the intermediate complex in reduction reactions on a cobalt-containing catalyst (the cobalt ± hydride bond is omitted for clarity).47 The involvement of the coordinated alkene in structure B is sterically hindered so that the transition state A is more favour- able.Unfortunately, the authors did not indicate the Co7H bond, whereas the latter, probably, provides an explanation for the fact that the asymmetric carbon atom is generated only in the b-position of the alkene and hydrogen is generated from NaBH4. Actually, the experimental observations mentioned above can readily be explained by assuming that the alkene containing the non-equivalent a- and b-carbon atoms is coordinated to the cobalt atom (whose hydride ion is, undoubtedly, generated from NaBH4) as shown belowHOEt EtOH a N C Co N H Cb 39 Scheme 4 H Ir OMe R1 R4 COOEt R1 N N R1 R2 R3 B40 H C O * Si Rh(P2)Cl Si H * Si Rh(P2)Cl * Rh(P2)(S)Cl+ ±S *Pis chiral phosphine, S is the solvent.Reduction of the C=C bond in substrates of the types O Me Me O Me Me or OEt Me Ph NH Me Me involves the enantioselective addition of the hydride ion at the Cb atom, which is controlled by an asymmetric Co hydride complex, whereupon the intact chiral C2-symmetric metal complex frag- ment, apparently, cannot ensure the enantioselective attack on the a-carbon atom. This explanation agrees well with the conclusion made previously that asymmetric induction is realised in the stage of the transformation of an asymmetric intermediate. Hence, turning back to Fig. 4, it can be said that asymmetric induction takes place in the stage of coordination of alkene by the asym- metric monohydride complex of cobalt.The mechanism of asymmetric hydrosilylation has been studied less thoroughly than the mechanism of asymmetric hydro- genation. It was suggested 49 that the enantioselectivity results from the competition between diastereomeric silyloxyalkyl(aryl)- rhodium intermediates (Scheme 5). According to another assumption, asymmetric induction takes place in the stage of the addition of ketone to an intermedi- ate complex 50, 51 (Scheme 6). Scheme 6 O O H H H R1 R2 R1 R2 L* Rh Si L* Rh Si L* Rh Si R1 R2 O O R2 R1 HSi H H L* L* L* Rh Rh Rh Si Si R1 R2 O O H H R2 R1 Si Si R2 R1 O O R1 R2 L* is (S,S)-(+)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenyl- phosphino)butane [(S,S)-(+)-diop].This scheme is similar to that proposed in the study 52 where the following structure of the five-membered transition state was assumed d7 d+ O Si Rh d7 d+ H C R1 R2 V A Pavlov H Scheme 5 S C *C ** H+Rh(P2)(S)Cl * Rh(P2)Cl H SiO SiO H O C * Si Rh(P2)Cl O *CH In all the above-considered cases, asymmetric induction is realised in the stage of substrate coordination by an asymmetric intermediate. IV. Asymmetric induction in the course of transformations of the first or subsequent intermediates of the catalytic reaction Research on the mechanism of catalytic hydrogenation of sub- strates like (Z)-a-acetamidocinnamic acid in the presence of rhodium phosphine complexes was started with the detailed investigation of the kinetics of hydrogenation of methyl (Z)-a- acetamidocinnamate catalysed by a rhodium complex containing achiral diphosphine, viz., 1,2-bis(diphenylphosphino)ethane (diphos).27, 53 ± 55 The catalytic cycle of this reaction is shown in Scheme 7.The precursor of the catalytic complex, viz., an ionic adduct of diene with a catalyst (11), is involved in the catalytic cycle after reduction of the diene. The structure of complex 12 was determined by 31P NMR spectroscopy 53 and the structure of complex 13 was established by 31P, 13C and 1H NMR spectro- scopy and X-ray diffraction analysis.24 At room temperature, the oxidative addition of hydrogen to the complex 13 is the rate- limiting step of the reaction, whereas reductive elimination of the product becomes the rate-limiting step at 740 8C.As a result, alkyl hydride complex 15 at 775 8C was accumulated in a concentration sufficiently high for analysis by 31P, 13C and 1H NMR spectroscopy.29, 54 Of compounds presented in Scheme 7, only intermediate 14 was not identified and its structure was proposed `by anal- ogy'.56, 57 Later on, the dihydride intermediate was found exper- imentally.58 The intermediates 13, 14 and 15 as well as the reaction product are racemates (both enantiomers of these intermediates are shown in Scheme 7). 2 2 2 2 2 2 The catalytic cycle of hydrogenation of the same substrate catalysed by {Rh[(R,R)-dipamp]}+ is shown in Scheme 8.In this catalytic cycle, the corresponding intermediates are diastereomers and it is reasonable that the reactions involving these intermedi- ates proceed at different rates.59 An analogous catalytic cycle was proposed 26, 27 for hydrogenation in the presence of {Rh[(S,S)- chiraphos]}+. In both cases, 31P NMR spectroscopic studies demonstrated 26, 29, 55, 59 that one of the diastereomers (16maj) was present in a high excess with respect to the other diastereomer (16min). The structure of the major diastereomer (16maj) obtained in the reaction in the presence of {Rh[(S,S)-chiraphos]}+ was established by X-ray diffraction analysis 26 and circular dichroism spectroscopy.27 However, the configuration of the resulting product was opposite to that, which would be expected to be realised if the reaction proceeds through this intermedi- ate.23, 26, 60, 61 Hence, it was suggested 26, 55 that the enantioselec- tivity is associated with the difference between the constants kmin and kmaj (kmin kmaj, see Scheme 8) due to which the reaction proceeded predominantly through the intermediate 16min whose concentration was small compared with that of 16maj.In the case of hydrogenation of methyl (Z)-a-acetamidocinnamate in the presence of {Rh[(R,R)-dipamp]}+ at 0 8C, the ratio of the constants kmin : kmaj&1000.59 Halpern 55 assumed that this sit- uation follows the Curtin ± Hammet principle according to whichMechanisms of asymmetric induction in catalytic hydrogenation, hydrosilylation and cross-coupling on metal complexes (R,S)-PhCH2CH(NHAc)CO2Me Me O P RhIII H P Me HN S MeO2C Ph 15MeO2C * H2 kmaj 2 + NH HMeO2C P * Rh Me P Ph O H17maj S kmaj 3 H + NHSCO2Me Ph + O H P RhIII P HMe + NH P Rh Me P Ph O 16maj major cycle + Me O P Rh NHSCO2Me P * Ph 18maj Me H H N O minor product, (R) + P RhI P 11 2H2, MeOH(S) + S P k1 RhI P S k71 12 + NH MeO2C PRhIII Me P Ph O H + HN H CO2Me PRhIIIP PhO H 14 H CO2Me Ph NHCOMe Me kmin 1 kmaj 1 P * S Rh(I)S P kmin 4 kmaj 4 HN S MeO2C Ph H HN CO2Me MeO2C O Ph Ph major product, (S) CO2Me NHAc Ph + NH MeO2C P RhI Me P Ph O + HN CO2Me P RhI Me P PhO 13 H2 + HN CO2Me P Rh * P PhO 16min HN minor cycle Me PhO + Me kmin 3 O P Rh H P * 18min Me 41 Scheme 7 Scheme 8 H2 kmin 2 + CO2Me P H Rh * P H 17min S42 1 1 1 1 the ratio of the products depends only on the difference between the free energies of the transition states and is independent of the ratio of the conformers (intermediates).According to Halpern's theory, the chiral recognition (asymmetric induction) takes place in the transformation 16!17 involved in the catalytic cycle. Unfortunately, no explanation was offered for the differences in stability and reactivity of the intermediates 16maj and 16min, which could allow an understanding of the reason for the substantial difference in the rates of their transformations.Halpern's theory provides an explanation for the effects of hydrogen pressure and temperature on the enantioselectivity of the reaction. It was demonstrated 35 that the enantioselectivity decreased and even the configuration of the product was changed as the hydrogen pressure was increased. According to Halpern's mechanism (see Scheme 8), the formation of the intermediates 17 is accelerated as the hydrogen pressure is increased. At some time, this step ceases to be rate-limiting and the process is controlled by the reversible formation of the diastereomeric adduct 16, i.e., the enantioselectivity is determined by the ratio kmin : kmaj.Since kmaj>kmin, another catalytic cycle, which affords an opposite isomer of the product, becomes more favourable and the enantio- selectivity is decreased. It is this situation that was experimentally observed. The formation step of adduct 16 is characterised by a substantially higher enthalpy of activation than the next step of the addition of hydrogen (in the presence of [Rh-dipamp]+ as the catalyst, 18.3 and 6.3 kcal mol71, respectively). What this means is the rate of the first step is decreased much more rapidly than that of the second step as the temperature is decreased and, conse- quently, the first step can become rate-limiting. Hence, the enantioselectivity would be expected to decrease with decreasing temperature. This situation was observed in reactions involving some phosphine catalysts.32, 35 Since Halpern's theory provides an explanation for many experimental observations, it was commonly accepted in the 1980s.62 ± 65 However, this theory has a number of drawbacks.1. According to Scheme 8, the coordination mode of enamide is not changed (Re ± Si ) and, consequently, the balance between the major and minor catalytic cycles remains unchanged on going from the planar-square complex 16 containing the C2-symmetric chiral (diphosphine)Rh(I) fragment to the octahedral complex 17 containing the asymmetric (diphosphine)Rh(III)H2 fragment. S +Rh X X P *P S R R X +Rh X R Rh + P *P P *P R However, the real situation may be different.Moreover, intra- molecular interconversions of the complex 16 are quite possible and even (see Ref. 66) the energy barrier for these interconver- sions is low. It is conceivable that the interconversion Re ± Si occurs in the step 16!17, in which case the major enantiomer is formed in the major catalytic cycle. 2. Most likely, Halpern's theory is inapplicable in the case of rhodium complexes serving as catalysts of hydrogenation of enamides with ee=99.0% ± 99.9%. In this case, with the gener- ally observed ratio between the major and minor intermediates (10 : 1), the ratio of the rate constants kmin : kmaj must be even several orders of magnitude larger than that in the case of the rhodium complex with the chiraphos ligand (ee=88%, V A Pavlov kmin : kmaj&1000).This difference in the rate constants of the transformations of diastereomeric intermediates seems to be unlikely. For the reaction involving the rhodium complex with the BisP* ligand, another mechanism was proposed 34 (see above). 3. Halpern's scheme provides no explanation for some exper- imental evidence, for example, for the difference in enantioselec- tivity of hydrogenation of the E and Z isomers.67, 68 The enantioselectivities of hydrogenation of (Z)- and (E )-a-benzoyl- aminocinnamic acids using the Rh/(S,S)-diop system differ not only in magnitude but also in sign.69 This may be associated with the different modes of coordination of the above-mentioned substrates by the rhodium atom (the E isomer is coordinated through the carboxy carbonyl group, whereas the Z isomer is coordinated through the amide carbonyl fragment).70 Appa- rently, the Re- or Si-side of the coordinated double bond of the substrate may also be changed. Halpern's theory provides no explanation for the influence of bases on the value of ee in the reactions performed in the presence of catalysts for which this theory has been initially proposed.35 Thus according to Halpern's hypothesis, the enantioselectivity of hydrogenation in the pres- ence of diphosphine catalysts decreases as the temperature is decreased; however, this is not necessarily the case.Thus in hydrogenation of dehydroamino acids in the presence of rhodium complexes with the prophos, cycphos or diop ligands, the optical yields decreased as the temperature was raised.42, 71 Apparently, Halpern's mechanism is only a special case.The mechanism of hydrogenation of substrates, such as tiglic acid, catalysed by the Ru(OAc)2[binap] complex has been studied in sufficient detail. It was demonstrated that hydrogenation proceeded through the formation of a ruthenium monohydride complex.72 The mechanism of hydrogenation of tiglic acid on this catalyst was detailed in the study.73 Me Me H+ H2 O O P P O O Ru Ru O O *P *P O H O CO2H * CO2H Me Me O O P P O O Ru Ru O O *P *P O O H+ * * The bis(carboxylato)ruthenium complex involving a substrate performs the heterolytic splitting of H2 giving rise to a hydride.The hydride ion reduces the C=C bond in the coordinated substrate to form the chelate which contains the chiral product in the half-hydrogenated state. Protonolysis of the newly formed Ru7Cbond affords the target product. According to this scheme, asymmetric induction takes place in the formation of the chelate complex with the substrate hydrogenated, i.e., in the step of the transformation of the intermediate devoid of symmetry elements (except for C1). An analogous mechanism for hydrogenation of tiglic acid on the catalytic methallyl complex [binap]Ru. .[Z3-(CH2)2CHMe]2 was proposed in the study. 74 In the cited study, it was also assumed that asymmetric induction occurs in the step of the transformation of the asymmetric intermediate, which already involves the substrate in the coordination sphere.Asymmetric cross-coupling of Grignard reagents with alkenes has been less well studied. According to the presently accepted mechanism, the reaction proceeds through the formation of the intermediate p-allyl(aryl)ML2 (Scheme 9).75 ± 79Mechanisms of asymmetric induction in catalytic hydrogenation, hydrosilylation and cross-coupling on metal complexes Scheme 9 Y Me Y 2ArMgX R=H, Ph, CO2Et, SiMe3; X2ML2 Ar2ML2 A Ar2 2MgX2 Cat= ArMgX MgXY + Ar Y L L L Y7 M M M L L L C Y Ar B Y Ar Among the intermediates which confirmed the general reac- tion mechanism proposed previously, species involving p-coordi- nated alkene (the substrate) were identified by 31P NMR and CD spectroscopy.Probably, these intermediates, which exist in the equilibrium with intermediates 20, are involved in the overall reaction (Scheme 10). X=Hal;M=Ni, Pd; L2=P2, P P; Y=Br, OH, OPh, COMe, OSiMe3, CO2Me. The stages A!B, B!C or C!products may be responsible for asymmetric induction. With the aim of elucidating this problem, we studied the reaction of arylmagnesium bromides with derivatives of crotyl alcohol under the action of a nickel catalyst by increasing successively the molecular volume of the leaving group (OR):80 The chiral arrangement of the phosphine phenyl groups of the coordinated dpcp ligand presented in Scheme 10 is based on the results of X-ray diffraction analysis of the complex 81 (the cyclo- pentane ring of the dpcp ligand is omitted). The crotyl ligands in the intermediates 20a ± 20d involve the stereogenic 82 carbon atom whose configurations are symbolised as (R) and (S ).The p-co- ordinated crotyl ligand of this type can be subjected both to the (R) ± (S )-inversion of the p-allylic chirality and the syn ± anti transformation according to the p ± d ± p mechanism.83 ± 85 Hence, it can be assumed that all possible forms of the intermedi- ate 20 (a ± d) exist in the equilibrium. In turn, each intermediate can exist in equilibrium with the intermediates 21a ± 21d. The interconversion 20d NiBr2[(R,R)-dpcp] 2 PhMgBr 2 MgBr2 Ph2Ni[(R,R)-dpcp] 19 Ph7Ph Ni0[(R,R)-dpcp] Y Y Me Ph Ph Ph Ph Ph P P P P Ni P Ni Ph (R) (S) (R) Ph Ph Ph Me MePh 20a 20b Me Ph Ph Ph Ph Ph P Ni P P Ni P P Me Ph Ph Y Ph Me Y Ph Ph 21a 21b Me Ph H Y (S) [(R,R)-dpcp]Ni0 Cat Me OR+ArMgBr THF PPh2 NiBr2 {( ± )-NiBr2[(R,R)-dpcp]}.PPh2 21d experiences the smallest of steric Y Y Ph Ph P Ni P Ph Ph 20c Ph Ph P Ni P Y Ph Ph Y 21c (allyl)NiY[(R,R)-dpcp] Ph Me Ph Ph Ni P P Ph (S ) Ph 43 Ar* Ar + Me Scheme 10 Y Me Ph P Ni Ph (S ) 20d Me Ph P Ni Ph 21d PhMgBr MgBrY44 hindrances. The reaction proceeding through these intermediates affords a product whose configuration is identical with that observed experimentally. The larger the molecular volume of the leaving group, the more favourable this path and the higher the expected enantioselectivity. This fact was confirmed experimen- tally.Hence, asymmetric induction in the reaction under consid- eration takes place in the stage of the interconversion of the asymmetric intermediates 20(a ± d) This mechanism allows one to explain rather readily the characteristic features of the reaction observed experimentally, viz., the dependence of the enantioselectivity on the molecular volume of the leaving group (the larger the molecular volume, the more favourable the path through the intermediate 21d and the higher the enantioselectivity) and the fact that the enantiomeric excess of the product is independent of the temperature (because the equilibrium can be established even at low temperatures).Both intermediates responsible for asymmetric induction already involve the coordinated substrate and are asymmetric. Another type of cross-coupling, for example, the reaction of a racemic mixture of 1-phenylethylmagnesium chloride with (Z)-b- bromostyrene catalysed by nickel complexes (the dynamic kinetic resolution) can be represented by Scheme 11.86 MgCl Ph H Me MgBrCl AS L Ni L2BrNi L Ph Ph H Me BR Ph Br Ph (R) Ph H Me Asymmetric induction can take place both in steps A and B (in both cases, the intermediates are asymmetric). V. Asymmetric induction caused by nonbonded diastereomeric interactions Solutions containing one enantiomer or a mixture of enantiomers (a racemate) may differ in physical properties.Thus the NMR spectra of the racemate differ from those of the optically active compounds comprising this racemate.87, 88 Analogous effects were also observed in calorimetric studies. These effects are attributable to diastereomeric interactions between enantiomers in solu- tions.89, 90 The enthalpies of interactions of amide derivatives of L- and D-alanine as well as of dipeptides containing the terminal L- and D-alanine residues in aqueous solutions are given in Table 3. R2 R1 C MeC(O)HN CONH2 (S )-22: R1=Me, R2=H; (R)-22: R1=H,R2=Me. 21(a ± d). Scheme 11 MgCl Me Ph H MgBrCl AR L Ni L Ph Ph Me Ph H BS Ph Br Ph (S) Me Ph H HNCONH2 C N R2 R1 O C O Me (S,S )-23: R1=Me, R2=H; (S,R)-23: R1=H,R2=Me.V A Pavlov Table 3. Enthalpies of interactions of peptide fragments containing the terminal L- and D-alanine residues.91 Solution B Solution A HAB /J kg mol72 (see a) (S )-22 (R)-22 (S )-22 (S )-22 (R)-22 (S )-22 (R)-22 (S )-22 (S,S )-23 (S,R)-23 (S,R)-23 (S )-22 (R)-22 (R)-22 (S,S )-24 (S,S )-24 (S,S )-23 (S,S )-23 (S,R)-23 (S,S )-23 (S,R)-23 (S,S )-23 2695 2775 33710 48817 5789 76949 74816 80322 162924 13998 151543 a This dimension was used by the authors of the cited study.91 Me H HN CONH2 C MeC(O)HN H Me O (S,S )-24 The observed differences in H are indicative of the presence of nonbonded interactions between enantiomers in solutions, such as monomer ± monomer, monomer ± dimer and dimer ± dimer con- taining the terminal L- and D-alanine residues.91 An impressive example of the separation of an enantiomer and a racemate by fractional distillation was reported.92 It appeared that the boiling point of the racemate of trifluorolactic acid differs from that of its enantiomers.This is indicative of the presence of strong nonbonded diastereomeric interactions. However, this effect was not observed for the ester of lactic acid. The differences in the physical properties of enantiomers and the racemate in solutions are generally rather small,{ but they are sufficiently large for these substrates to exhibit different chemical activities.94 The following general principle was stated: the reaction rate for a chiral compound and the ratio of the resulting products depend, among other things, on the enantiomeric composition of the starting compound.95 The following reactions can be distin- guished: R+R RR(enantiomeric recognition), S+S SS(enantiomeric recognition), R+S RS(antipode interaction).Below examples are given of reactions where one of the nonbonded interactions (or all these interactions taken together) leads to different compositions of the products. K (1 equiv.) 1. NH4Cl + NH3 (liq.) 2. H2O OH OH O (R)-borneol (R)-camphor (R)-isoborneol 1. NH4Cl K (1 equiv.) OH O NH3 (liq.) 2. H2O + OH (S)-isoborneol (S)-camphor (S)-borneol { Differences in the physical properties of the enantiomers and the race- mate in crystals have long been known and were described in handbooks (for example, see Ref.93). These differences are due primarily to the differences in the molecular packings in the crystals.Mechanisms of asymmetric induction in catalytic hydrogenation, hydrosilylation and cross-coupling on metal complexes If (R)- or (S )-camphor was used as the starting substrate, the excess of borneol in the resulting mixture was insignificant. However, the reaction starting from racemic camphor afforded borneol as the major product.96 In contrast, reduction of (R)-cam- phor with LiAlH4 gave rise predominantly to isoborneol, whereas borneol was obtained as the minor product (90.2% and 9.8%, respectively).95, 97 A somewhat different composition of the mix- ture was obtained in the reaction starting from (R,S)-camphor (88.7% and 11.3%, respectively).Reductive dimerisation of camphor LiAlH4 TiCl3, THF O afforded a mixture of isomers (R,R)-cis (R,R)-trans The reaction involving the (R)-monomers of camphor gave rise to the (R,R)-cis and (R,R)-trans dimers in a ratio of 34.8 : 65.2. In the product obtained from the racemate of camphor, (R,S)-cis [(R,R,S,S)-cis] and (R,S)-trans [(R,R,S,S)-trans] isomers were present in a ratio of 37.4 : 62.6.95 Oxidative dimerisation of the (S)-monomer of a phenol derivative Me * K3[Fe(CN)6] MeMe OH afforded the (P,S,S) and (M,S,S) diastereomers in a yield greater than 97.5%.95 The racemic monomer can, in principle, give eight diastereomers: Me Me Me MeMe OH Me HO (P,S,S) Me Me Me MeMe OH Me HO (P,R,R) + (R,S)-cis (R,S)-trans * * Me Me Me MeMe OH Me HO Me Me Me MeMe OH Me HO (M,S,S)Me Me Me MeMe OH Me HO (M,R,R) 45 Me Me Me Me Me Me Me Me Me Me OH Me OH Me HO HO (M,R,S) (P,R,S) Me Me Me Me Me Me Me Me Me Me OH Me HO OH Me HO (M,S,R) (P,S,R) However, dimerisation of this monomer produced a mixture of the (P,S,S ), (M,S,S), (P,R,R) and (M,R,R) diastereomers (trans dimers) in 66% yield, whereas the theoretically possible yield is 50%.This inconsistency can be considered as an argument in favour of active interactions between the enantiomers. The reaction CHO ZnPri2 20% DBNE, C7H8, 0 8C AcO OH * AcO HO NBu2 , ( ± )-DBNE = Me Ph catalysed by chiral aminoalcohol yielded a chiral compound bearing an asymmetric centre in the terminal fragment.98 (7)-Aminoalcohol catalysed the formation of (R)-alcohol with the diastereomeric excess de=97% independently of its enantio- meric purity (100% and 21%), whereas the (S) product with de=87% was obtained in the presence of (+)-aminoalcohol (ee=100%).This result is attributable to nonbonded diastereo- meric interactions between the chiral substrate and the amino- alcohol. The non-linearity of the changes in the optical and enantio- meric purities, which was observed in some cases, is accounted for by similar reasons. The effect of the non-linear dependence of the enantiomeric excess of the product on the enantiomeric (or diastereomeric in the case of a diastereomer) purity of the catalyst (the reagent) can be manifested in asymmetric catalysis 99 (in asymmetric synthesis and in stereoselective reactions 100) if a chiral catalyst (or a reagent) can form a dimeric diastereomeric complex with one of the reagents of the catalytic (or non-catalytic) reaction.Possible dependences of the enantiomeric excess of the product on the enantiomeric excess of the catalyst (the reagent) are shown in Fig. 5. A wide range of such reactions is presently known.99, 100 These effects are attributed to the differences in the reactivity of homochiral and heterochiral dimeric complexes.46 ee (of the product) (%) 100 2 80 60 1 40 200 60 40 20ee (of the catalyst) (%) Figure 5. Possible deviations from the linear dependence of ee of the product on ee of the catalyst.(1) the linear dependence, (2) the positive non-linear effect, (3 ) the negative non-linear effect. For example, in the reaction O R2Zn H NMe2 OH the homochiral (7) . (7) dimer R N Zn O O Zn N R ( ± ) . ( ± ) homochiral Table 4. Ligands involved in catalytic complexes, which are used in various reactions. ee (%) Ligand, complex Hydrogenation of enamides catalysed by rhodium complexes 98 ± 100 Ph2P PPh2 Bz N 99 Ph2P PPh2 100 Ph2P PPh2 Me But P P CH2 >99.9 But Me 3 100 80 OH * R R N Zn O O Zn N R (+) .(+) Ref. 20, 81 104 105 106 is 1200 times more active than the heterochiral dimer, R N Zn O O Zn N R(+) . ( ± ) heterochiral which gives rise to the positive non-linear effect.101 ± 103 VI. Conclusion Recent investigations demonstrated that the enantioselectivity of some metal-complex catalysts approaches 100%. Ligands for complexes, which ensure the record-breaking enantiomeric purity of the products in various reactions, are given in Table 4. All the listed ligands are conformationally rigid and possess a pro- nounced helical component bearing bulky groups at the ends of an incomplete helical turn.6 The investigation into the mechanism of hydrogenation of enamides in the presence of one of such complexes (the octahedral asymmetric rhodium dihydride complex containing the BisP* ligand) demonstrated that asymmetric induction takes place in the stage of substrate coordination by the complex. The high enantioselectivity of this catalytic reaction (99.9%) is determined by the 99.95% probability of the coordination of enamide by one side {the Re-side in the case of the Rh[(S,S)-BisP*] complex}.106 This is indicative of a large difference between the free activation energies of substrate coordination through the favourable and unfavourable sides.The probability of the substrate being coor- dinated through a particular side is, evidently, determined by the helical component of the conformationally rigid ligand of the complex bearing bulky groups at the ends of the incomplete turn of the helix.As an alternative mechanism for improving the diastereomeric purity of an adduct of a dihydride complex with enamide, intra- molecular isomerisation is less probable due to the high reactivity (short lifetime) of this intermediate (such an isomerisation is more likely in the case of an adduct of the starting complex with the substrate,66 which is not involved in the catalytic cycle 34). Ligand, complex Hydrogenation of enamides catalysed by rhodium complexes Me 1-Adamantyl P P Me Adamantyl-1 Et Et P P Et Et Pr Pr P P Pr Pr V A Pavlov Ref. ee (%) 107 99.9 108 98.1 108 99.8Mechanisms of asymmetric induction in catalytic hydrogenation, hydrosilylation and cross-coupling on metal complexes Table 4 (continued).ee (%) Ligand, complex Hydrogenation of ketones, ketoesters and aminoketones catalysed by ruthenium complexes 99 ± 100 Ph2P PPh2 MeO OMe 98 Ph2P PPh2 MeO OMe 97.8 2,4-But2H3C6 C6H3But2-2,4 Reduction of enamides and imines catalysed by cobalt complexes CN 98.9 N N Co ButMe2SiO OSiMe2But 98 N N Co O O O O The relationship between the sense of the helical component of a ligand in a rhodium complex involved in hydrogenation of enamides and the configuration of the product 6 either results from asymmetric induction taking place in the stage of substrate coordination or is a consequence of an alternative mechanism, which is most likely common to the overall series of reactions in which this regularity is observed.The available experimental data indicate that the reactions proceeding under the action of chiral C2-symmetric metal-com- plex catalysts generate the asymmetric carbon atom in the product in a step involving an asymmetric intermediate. Apparently, the Curie ± Pasteur principle, which states that there is `no asymmetry without asymmetry', has a literal sense. References 1. Y Izumi, A Tai Stereo-Differentiating Reactions (New York: Academic Press, 1977) 2. L D Barron Science 266 1491 (1994) 3. M Avalos, R Babiano, P Cintas, J L Jimenez, J C Palacios, L D Barron Chem. Rev. 98 2391 (1998) 4. B M Trost Acc. Chem. Res. 29 355 (1996) 5. H Brunner Acc. Chem. Res. 12 250 (1979) 6. V A Pavlov Usp.Khim. 70 1175 (2001) [Russ. Chem. Rev. 70 (2001)] 7. F D Saeva, P E Sharpe, G R Olin J. Am. Chem. Soc. 97 204 (1975) 8. L Verbit, T R Halbert, R B Patterson J. Org. Chem. 40 1649 (1975) 9. M Nakazaki, K Yamamoto, K Fujiwara Chem. Lett. 8 863 (1978) 10. C Eskenazi, J F Nicoud, H B Kagan J. Org. Chem. 44 995 (1979) 47 Ref. ee (%) Ligand, complex Ref. Hydrogen-transfer hydrogenation of ketones catalysed by ruthenium complexes Ph N PPh2 O 114 97 109, 110 FeH H 111 115 97 N Cl N Ru P Cl P Ph2 Ph2 Hydrosilylation of ketones catalysed by rhodium complexes 112 S 116 98 N CO2Et NH O O 117, 118 95 N 40, 47 N N Pri Pri 113 11. W H Pirkle, P L Rinaldi J. Am. Chem. Soc. 99 3510 (1977) 12. A Dondoni, A Medici, S Colonna, G Gottarelli, B Samorui Mol.Cryst. Liq. Cryst. 55 47 (1979) 13. F F Roca Ann. Chim. 2 255 (1967) 14. P Curie J. Phys. (Paris) 3 393 (1894) 15. P Gerike Naturwissenschaften 62 38 (1975) 16. G Zadel, C Eisenbraun, G-J Wolff, E Breitmaier Angew. Chem., Int. Ed. Engl. 33 454 (1994) 17. E Breitmaier Angew. Chem., Int. Ed. Engl. 33 1461 (1994) 18. V A Pavlov, N I Spitsyna, E I Klabunovsky Izv. Akad. Nauk SSSR, Ser. Khim. 12 2838 (1982) 19. V A Pavlov, N I Spitsyna, E I Klabunovsky Izv. Akad. Nauk SSSR, Ser Khim. 1653 (1983) a 20. V A Pavlov, Doctoral Thesis in Chemical Sciences, Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 2000 21. J M Brown, D Parker J. Chem. Soc., Chem. Commun. 342 (1980) 22. W C Christophel, B D Vineyard J.Am. Chem. Soc. 101 4406 (1979) 23. J M Brown, P A Chaloner Tetrahedron Lett. 21 1877 (1978) 24. A S C Chan, J J Pluth, J Halpern Inorg. Chim. Acta 37 L477 (1979) 25. J M Brown, P A Chaloner J. Am. Chem. Soc. 102 3040 (1980) 26. A S C Chan, J J Pluth, J Halpern J. Am. Chem. Soc. 102 5952 (1980) 27. P S Chua, N K Roberts, B Bosnich, S J Okrasinski, J Halpern J. Chem. Soc., Chem. Commun. 1278 (1981) 28. J M Brown, P A Chaloner J. Chem. Soc., Chem. Commun. 321 (1978) 29. J M Brown, P A Chaloner J. Chem. Soc., Chem. Commun. 344 (1980) 30. K Achiwa, P A Chaloner, D Parker J. Organomet. Chem. 218 249 (1981) 31. KAchiwa,Y Ohga,Y Itaka,HSaito Tetrahedron Lett. 47 4683 (1978) 32. D Sinou Tetrahedron Lett. 22 2987 (1981)48 33.J M Brown, P A Chaloner, G Descotes, R Glaser, D Lapont, D Sinou J. Chem. Soc., Chem. Commun. 611 (1979) 34. I D Gridnev, N Higashi, K Asakura, T Imamoto J. Am. Chem. Soc. 122 7183 (2000) 35. I Ojima, T Kogure, N Yoda J. Org. Chem. 45 4728 (1980) 36. V A Pavlov, A A Voloboev, L S Gorshkova, E I Klabunovsky, in The 4th International Symposium on Homogeneous Catalysis (Abstracts of Reports), Leningrad, 1984 Vol. I, p. 290 37. V A Pavlov, E I Klabunovsky, Yu T Struchkov, A A Voloboev, A I Yanovsky J. Mol. Catal. 44 217 (1988) 38. V A Pavlov, A A Voloboev, L S Gorshkova, E I Klabunovsky Dokl. Akad. Nauk SSSR 283 1206 (1985) b 39. V A Pavlov, A A Voloboev, L S Gorshkova, E I Karpeiskaya, E I Klabunovsky Izv. Akad. Nauk SSSR, Ser. Khim. 513 (1987) a 40.V A Pavlov,M G Vinogradov, E V Starodubtseva, G V Chel'tsova, V A Ferapontov, O R Malyshev, G L Kheis Izv. Akad. Nauk, Ser. Khim. 704 (2001) a 41. S Gladiali, L Pinna, G Delogu, S De Marton, G Zassinovich, G Mestroni Tetrahedron Asymmetry 1 635 (1990) 42. G Zassinovich, G Mestroni J. Mol. Catal. 42 81 (1987) 43. G Zassinovich, G Mestroni, S Gladiali Chem. Rev. 92 1050 (1992) 44. C Bianchini, E Farnetti, L Glendenning, M Graziani, G Nardin, M Peruzzini, E Roccini, F Zanobini Organometallics 14 1489 (1995) 45. R Noyori, S Hashiguchi Acc. Chem. Res. 30 97 (1997) 46. U Leutenegger, A Madin, A Pfaltz Angew. Chem., Int. Ed. Engl. 28 60 (1989) 47. A Pfaltz Acc. Chem. Res. 26 339 (1993) 48. P von Matt, A Pfaltz Tetrahedron: Asymmetry 2 691 (1991) 49.I Ojima, K Hirai, in Asymmetric Synthesis (Ed. J D Morrison) (New York: Academic Press, 1985) Vol. 5, p. 103 50. R Glaser Tetrahedron Lett. 25 2127 (1975) 51. D Haag, J Runsink, H D Scharf Organometallics 17 398 (1998) 52. M Akita, O Mitani,Y Moro-oka J. Chem. Soc., Chem. Commun. 527 (1989) 53. J Halpern, D P Riley, A S C Chan, J J Pluth J. Am. Chem. Soc. 99 8055 (1977) 54. A S C Chan, J Halpern J. Am. Chem. Soc. 102 838 (1980) 55. J Halpern Pure Appl. Chem. 55 99 (1983) 56. J Halpern Inorg. Chim. Acta 50 11 (1981) 57. J Halpern Acc. Chem. Res. 3 386 (1970) 58. A Harthun, R Kadyrov, R Selke, J Bargon Angew. Chem., Int. Ed. Engl. 36 1103 (1997) 59. C R Landis, J Halpern J. Am. Chem. Soc. 109 1746 (1987) 60. M D Fryzuk, B Bosnish J.Am. Chem. Soc. 99 6262 (1977) 61. P A MacNeil, N K Roberts, B Bosnich J. Am. Chem. Soc. 103 2273 (1981) 62. J M Brown, in Hydrogenation of Functionalized Carbon ± Carbon Double Bond Vol. 1 (Eds E N Jacobsen, A Pfaltz, H Yamamoto) (Berlin: Springer, 1999) p. 119 63. C R Landis, S Feldgus Angew Chem., Int. Ed. Engl. 39 2863 (2000) 64. H Bircher, B R Bender,W van Philipsborn Magn. Reson. Chem. 31 293 (1993) 65. J M Brown, P A Chaloner J. Chem. Soc., Perkin Trans. 2 1583 (1987) 66. R Kadyrov, T Freier, D Heller,M Michalik, R Selke J. Chem. Soc., Chem. Commun. 1745 (1995) 67. G Gelbard, H B Kagan, R Stern Tetrahedron 32 233 (1976) 68. J D Morrison,W F Masler, S Nathaway, in Catalysis in Organic Synthesis (Eds P N Rylander, H Garfield) (New York: Academic Press, 1976) p.203 69. C Detellier, G Gelbard, H B Kagan J. Am. Chem. Soc. 100 7556 (1978) 70. J M Brown, P A Chaloner J. Chem. Soc., Chem. Commun. 613 (1979) 71. W R Cullen, Y Sugi Tetrahedron Lett. 19 1635 (1978) 72. T Ohta, H Takaya, R Noyori Tetrahedron Lett. 31 7189 (1990) 73. M T Ashby, J Halpern J. Am. Chem. Soc. 113 589 (1991) 74. J P Genet, C Pinel, V Ratovelomanana-Vidal, S Mallart, X Pfister, L Bischof,M C Cano de Andrade, S Darses, C Galopin, J A Laffitte Tetrahedron: Asymmetry 5 675 (1994) 75. G Consiglio, O Piccolo, L Roncetti, F Morandini Tetrahedron 42 2043 (1986) 76. T Hayashi, M Konishi, K I Yokota, M Kumada J. Organomet. Chem. 285 359 (1985) V A Pavlov 77. K Tamao, K Sumitani, Y Kiso,M Zembayashi, A Fujioka, S Kodama, I Nakajima, A Minato,M Kumada Bull.Chem. Soc. Jpn. 49 1958 (1976) 78. H Felkin, M Joly-Goudket, S G Davies Tetrahedron Lett. 22 1157 (1981) 79. H Felkin, G Swierczewski Tetrahedron 31 2735 (1975) 80. V A Pavlov, E A Misrtyukov, H Duddeck,M G Vinogradov, G Snatzke J. Mol. Catal. 79 55 (1993) 81. D L Allen, V C Gibson,M L H Green, J F Skinner, J Bashkin, P D Grebenik J. Chem. Soc., Chem. Commun. 895 (1983) 82. V Prelog, G Helmchen Angew. Chem. 94 614 (1982) 83. J W Faller,M E Thomsen, M J Mattina J. Am. Chem. Soc. 93 2642 (1971) 84. J W Faller,M T Tully J. Am. Chem. Soc. 94 2676 (1972) 85. T Corradini, G Maglio, A Musco, G Paiaro J. Chem. Soc., Chem. Commun. 618 (1966) 86. G C Lloyd-Jones, C P Butts Tetrahedron 54 901 (1998) 87. M J P Harger J. Chem. Soc., Chem. Commun. 555 (1976) 88. M I Kabachnik, T A Mastryukova, E I Fedin, M S Vaisberg, L I Morozov, P V Petrovsky, A E Shipov Tetrahedron 32 1719 (1976) 89. A Horeau Tetrahedron Lett. 3121 (1969) 90. A Horeau, J P Guette Tetrahedron 1923 (1974) 91. G M Blackburn, T H Lilley, P J Milburn J. Chem. Soc., Chem. Commun. 299 (1985) 92. T Katagiri, C Yoda,K Furuhashi, K Ueki, T Kubota Chem. Lett. 115 (1996) 93. E l Eliel Stereochemistry of Carbon Compounds (New York: McGraw-Hill, 1962) 94. D P Craig, D P Mellor Fortschr. Chem. Forsch. 63 1 (1976) 95. H Wynberg, B Feringa Tetrahedron 32 2831 (1976) 96. V Rautenstrauch, P Megard, B Bourdin, A Furrer J. Am. Chem. Soc. 114 1418 (1992) 97. H C Brown, H R Deck J. Am. Chem. Soc. 87 5620 (1965) 98. M Okamoto,M Tabe, T Fujii, T Tanaka Tetrahedron: Asymmetry 6 767 (1995) 99. H B Kagan, C Girard, D Guillaneux, D Rainford, O Samuel, S Y Zhang, S H Zhao Acta Chem. Scand. 50 345 (1996) 100. C Girard, H B Kagan Angew. Chem., Int. Ed. 37 2923 (1998) 101. M Kitamura, S Okada, R Noyori J. Am. Chem. Soc. 111 4028 (1989) 102. M Yamakawa, R Noyori J. Am. Chem. Soc. 117 6327 (1995) 103. R Noyori,M Kitamura Angew. Chem., Int. Ed. Engl. 30 49 (1990) 104. U Nagel Angew. Chem. 96 425 (1984) 105. H Takaya, T Souchi, R Noyori Tetrahedron 40 1245 (1984) 106. Y Yamanoi, T Imamoto J. Org. Chem. 64 2988 (1999) 107. T Imamoto, J Watanabe, Y Wada, H Masuda, H Yamada, H Tsuruta, S Matsukawa, K Yamaguchi J. Am. Chem. Soc. 120 1635 (1998) 108. M J Burk, J E Feaster, W A Nugent, R L Harlow J. Am. Chem. Soc. 115 10125 (1993) 109. R Noyori Acta Chem. Scand. 50 380 (1996) 110. V A Pavlov, E V Starodubtseva, M G Vinogradov, V A Ferapontov, O R Malyshev, G L Kheis Izv. Akad. Nauk, Ser. Khim. 725 (2000) a 111. B Heiser, E A Broger, Y Crameri Tetrahedron: Asymmetry 2 51 (1991) 112. M J Fehr, G Consiglio, M Scalone, R Schmid J. Org. Chem. 64 5768 (1999) 113. K D Sugi, T Nagata, T Yamada, T Mukaiyama Chem. Lett. 493 (1997) 114. T Sammakia, E L Stangeland J. Org. Chem. 62 6104 (1997) 115. J X Gao, T Ikariya, R Noyori Organometallics 15 1087 (1996) 116. H Brunner, R Becker, G Riepl Organometallics 3 1354 (1984) 117. H Nishiyama,M Kondo, T Nakamura,K Hoh Organometallics 10 500 (1991) 118. H Nishiyama, S B Park, K Itoh Tetrahedron: Asymmetry 3 1029 (1992) a�Russ. Chem. Bull., Int. Ed. (Engl. Transl.) b�Dokl. Chem. (Engl. Tran

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年代:2002

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Russian Chemical Reviews 71 (1) 49 ± 70 (2002) Controlled synthesis of stereoblock polypropylene. New trends in the development of elastomers NMBravaya, PMNedorezova, V I Tsvetkova Contents I. Introduction II. Catalyst systems for the synthesis of elastomeric stereoblock polypropylene III. Characteristic features of the structure of elastomeric stereoblock polypropylene and the nature of relaxation processes IV. Conclusion Abstract. of synthesis the on data published generalises review The The review generalises published data on the synthesis of elastomeric stereoblock polypropylene, a representative of ther- elastomeric stereoblock polypropylene, a representative of ther- moelastoplastics, block or random by prepared are which moelastoplastics, which are prepared by random or block copoly- copoly- merisation in used widely are and co-monomers various of merisation of various co-monomers and are widely used in the the manufacture of diverse mechanical rubber goods.New unique manufacture of diverse mechanical rubber goods. New unique applications in catalysts post-metallocene and metallocene of applications of metallocene and post-metallocene catalysts in the the design of polyolefin elastomers with a broad spectrum of phys- design of polyolefin elastomers with a broad spectrum of phys- icomechanical attention Particular discussed. are characteristics icomechanical characteristics are discussed. Particular attention is is given on based systems catalyst homogeneous modern to given to modern homogeneous catalyst systems based on Group Group IVB highly ensure which complexes, metallocene element IVB element metallocene complexes, which ensure highly efficient efficient synthesis various with elastomers polypropylene of synthesis of polypropylene elastomers with various stereoblock stereoblock structures. Data on the specific features of the structure and structures.Data on the specific features of the structure and properties are polypropylene stereoblock elastomeric the of properties of the elastomeric stereoblock polypropylene are ana- ana- lysed. references 160 includes bibliography The lysed. The bibliography includes 160 references. I. Introduction Homo- and copolymers of a-olefins currently rank as one of the most important industrial polymeric materials. The world manu- facture of polyolefins such as high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), and isotactic polypro- pylene (IPP), estimated as tens million tons a year, constantly increases and now it accounts for more than 50% of the output of all manufactured plastics.1 Expansion of the production of these materials was promoted by the discovery of metal complex Ziegler ± Natta catalysts with high activity and regio- and stereo- specificity.2± 4 At present, industrial processes, mainly, the syn- thesis of IPP, make use of fourth-generation heterogeneous Ziegler ± Natta catalysts containing TiCl4 , d-MgCl2, Et3Al, and modifying additives.5 ±7 Due to replication (the reproduction of the shape of a catalyst particle by the growing polymer particle), the catalysts used ensure the formation of spherical polymer particles with specified size and porosity.The range and the fields of application of homo- and copoly- mers of a-olefins are constantly extending. The consumption of NMBravaya Institute of Problems of Chemical Physics, Russian Academy of Sciences,142432 Chernogolovka Moscow Region, Russian Federation. Fax (7-096) 514 32 44. Tel. (7-096)522 11 31. E-mail: nbravaya@cat.icp.ac.ru PMNedorezova, V I Tsvetkova N N Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, 119991 Moscow, Russian Federation. Fax (7-095) 137 82 84. Tel. (7-095) 939 73 71. E-mail: pned@chph.ras.ru Received 9 October 2001 Uspekhi Khimii 71 (1) 57 ± 80 (2002); translated by Z P Bobkova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v07n01ABEN00698 49 50 65 67 binary ethylene ± propylene and ternary ethylene ± propylene ± diene copolymers permanently increases.1, 8, 9 The production volume of these materials exceeds a million tons a year.The industrial synthesis is performed most often using vanadium metal complex systems.10, 11 Highly effective two-stage processes for the production of heterophase copolymers based on IPP (1st stage) and ethylene ± propylene copolymers (2nd stage) 1 are widely used in industry. By controlling the fraction and the composition of the copolymer and the molecular masses of polypropylene (PP) and the copolymer, one can obtain materials with enhanced elasticity and impact strength, thermoplastic polyolefins (TPO) and ther- moplastic elastomers (TPE).2 ±6 A special position among polyolefins is held by elastomers.They are usually prepared by statistic or block copolymerisation of various co-monomers. Elastomers are capable of being rapidly stretched and contracted; they exhibit high strength and high modulus of elasticity in the stretched state and return to the initial size as the load has been removed. The elastic properties of these materials are due to the formation of chemical or physical networks. The physical network junctions can be represented by crystalline regions joined by the transient chains of the amorphous polymer (Fig.1). The fractions of amorphous and crystalline phase, the average size, the size distribution, the shape of crystallites, the network density, the possibility of changing the network structure under the action of load and several other factors determine the properties of elastomeric materials. In the last two decades, the discovery of new highly effective homogeneous systems based on metallocene complexes of Group 12 Figure 1. Schematic representation of the microstructure of elastomeric stereoblock polypropylene. (1) Crystallites formed by isotactic blocks; (2) amorphous sections consisting of irregular blocks.50 IVB elements activated by poly(methylaluminoxane) (MAO) or by other compounds opened up new opportunities for the devel- opment of polymer technologies and for extension of the range and applications of homo- and copolymers of a-olefins. Various aspects of metallocene and post-metallocene catalysis of olefin polymerisation have been covered in monographs, special editions and reviews.12 ±20 Due to the uniformity of active centres, these catalysts make it possible to produce polymers with a narrow molecular mass distribution (MMD) and copolymers with a homogeneous composition.The narrow molecular mass distribu- tion and structural and compositional uniformity result in enhanced mechanical strength, impact strength and clarity of materials and determine specific properties of homo- and copoly- mers. The successive or simultaneous conduction of polymer- isation in the presence of a mixture of complexes having different structures ensures the synthesis of polymers with bi- or polymodal MMD and with a controllable composition.When metallocene catalysts are used, the differences between the reactivities of ethylene, propylene, higher a-olefins and a number of other monomers are much less pronounced 21 than in the case of heterogeneous or vanadium systems.22 By varying the composition, structure and the type of symmetry of metallocenes, one can control the stereospecificity of metallocene systems and prepare PP and higher poly-a-olefins with various microstruc- tures. The use of metallocene systems not only ensures the formation of PP having different stereochemical compositions (iso-, syndio-, hemiiso- and atactic polymers) at satisfactory rates but also extends the range of applicable monomers (for example, cycloolefins and polar monomers can be involved in reactions).A large number of new materials are currently produced in industry by metallocene-catalysed processes.23, 24 Metallocene catalysts also open up the way for the prepara- tion of new polymers with diverse stereo- and regiochemical structures, in particular, synthetic blends of various PP, or for the selective synthesis of stereoblock PP (SBPP). The SBPP macromolecules either consist of alternating stereoblocks with different structures or have a uniform structure separated into blocks by regularly arising `errors'. If the length of the co-crystallised stereoregular sequences (i ) is relatively short, for example, i410 ± 12 (Fig.2 a ± d ), and the molecular mass of the polymer is high, the polymer exhibits elastomeric properties. The interest in the production of elasto- meric stereoblock polypropylene (elSBPP) is caused by the fact that this is a way of one-pot synthesis of TPE from a single monomer, which has obvious advantages over the processes that require the use of at least two monomers. Due to the specific properties of metallocene catalyst systems and the possibilities they provide, the number of papers and patents devoted to various processes of elSBPP synthesis has substantially increased during the last five years. Quite recently, the range of PP-based elasto- meric materials, presented in Fig.2 and considered in this review, was extended by elastomeric PP (elPP), which is branched atactic PP with short regular, ordered (isotactic or syndiotactic) side chains.25 These branches are organised in well dispersed crystal- a b ... ... ... ... s i i a c d ... ... ... ... i k i i i Figure 2. Schematic views of various types of SBPP. (a) Alternation of isotactic and syndiotactic blocks; (b) alternation of isotactic and atactic blocks; (c) isotactic blocks with a correcting `error'; (d ) isotactic blocks with an extending `error'. NMBravaya, PMNedorezova, V I Tsvetkova line domains connected by amorphous sections of the backbone, their presence preventing `sliding' of the atactic chains under load, which is typical of atactic PP.Investigations by a number of companies aimed at the development of elSBPP-based materials which could replace plasticised PVC, and some other expensive elastomers are in progress, and this is largely related to the possibility of designing macromolecules in the presence of metal- locene catalyst systems.25, 26 The present review surveys data on the influence of the structure of catalysts and conditions of synthesis of polypropylene on the activity of catalyst systems and the structure of elSBPP; the relationship between the structure and properties (strain, relaxa- tion, thermal, etc.) of the SBPP produced. The attention is focused on the homogeneous catalyst systems based on metallocene complexes of Group IVB elements. II.Catalyst systems for the synthesis of elastomeric stereoblock polypropylene This section considers views on the mechanism of catalytic and stereospecific action of metal-complex catalysts and on the devel- opment of methods for the analysis of polymer microstructure and its relationship with the properties of catalysts; it describes the history of the discovery of elSBPP and the development of heterogeneous catalysts and highly effective homogeneous cata- lyst systems for the synthesis of elSBPP. 1. Characteristics of the mechanisms of catalytic and stereospecific action of metal complex catalysts Elucidation of the mechanisms of catalytic and stereocontrolling action of catalysts is very significant for the target-directed and controlled synthesis of polyolefins.Research into the mechanism of action of Ziegler ± Natta metal complex catalysts of polymer- isation is the subject of a number of monographs and reviews.12, 13 ± 17, 20, 27 The currently accepted mechanism of formation of an active centre (AC) based on Shilov's and Dyachkovskii's concept 28 includes the steps of coordination of catalyst components, alky- lation and ionisation of a transition metal compound to give a coordinatively unsaturated cation containing an active metal7 carbon bond. These views have been further developed after the discovery of highly effective homogeneous catalyst systems based on metallocenes in combination with MAO or weakly coordinat- ing anions such as B7(C6F5)4.13 ± 20, 24 The mechanism of action of the specific activating agents for metallocene systems is exten- sively studied. Several alternative mechanisms of chain growth are described in the literature, namely, the coordinate-anionic mechanism, direct insertion of the monomer into the active metal7carbon bond and migration mechanism.On the basis of the results of recent investigations into propylene polymerisation catalysed by homogeneous syndio- and hemiisospecific metallocene catalysts, preference is given to the migration mechanism. According to this mechanism, growth of the polymer chain occurs through its migration to the coordinated monomer and insertion of the monomer. The next monomer molecule is coordinated to the newly appeared vacancy, and migration of the polymer chain and insertion are repeated; thus, after each monomer insertion event, the growing polymer chain and the coordination vacancy exchange places (the Cossee mechanism). This mechanism also assumes the possibility of chain migration to the initial position without monomer insertion with retention of the AC geometry (the Cossee ± Arlman mechanism).The symmetry of the AC and the ratio between the rates of the `monomer insertion ± chain migration' and `chain migration without monomer insertion' processes largely determine the polymer microstructure. In terms of the two-step mechanism of chain growth, the stereocontrol can take place both at the coordination step and during monomer insertion into the metal7carbon active bond.29 ± 33 The insertion of the first monomer is usually notControlled synthesis of stereoblock polypropylene. New trends in the development of elastomers stereospecific.31 Stereocontrol is enhanced as a result of coopera- tive action of the AC, the ligand and the growing polymer chain.The views on the mechanism of stereocontrol by metal complex heterogeneous catalysts is based on the detailed investigation of the polymer microstructure and the properties of separate poly- mer fractions. In conformity with the Bovey statistics,34, 35 a polymer chain is regarded as a sequence of isotactic dyads characterised by identical arrangements of methyl groups relative to the plane of the carbon chain of the macromolecule (m-dyads) and syndiotac- tic dyads in which the methyl groups of propylene are located on different sides of this plane (r-dyads).Methods for determination of the contents of m- and r-dyads, triads (mm, rr, mr), tetrads (mmm, rrr, mmr, rrm), various pentads (iso � mmmm; syndio � rrrr, and hetero � rmmr, mmrr, mrrr, rmrr, rrmr, mrmm, mrrm, mmrm) and higher sequences (hexads, heptads, etc.) based on the use of high-resolution 13C NMR spectroscopy have been devel- oped.12, 13, 17, 29, 33, 36 ± 38 Analysis of the configuration statistics allows one to identify the stereocontrol mechanism. Two key mechanisms are possible according to which stereocontrol is determined either by the chirality of the catalytic site or by the last inserted monomeric unit in the growing polymer chain.Correspondingly, two simple statistical models have been developed to interpret the distribu- tion of the stereosequences in the polymer, known as the enantio- morphic model and the Bernoullian model. According to the enantiomorphic model, high stereospecificity is attained when an accidental `error' arising during the polymer chain growth is corrected in the next step. The following ratios hold in this case: rr : mr=1:2 or mmmr : mmrr : mrrm=2 : 2 : 1. When the stereo- control is accomplished by the last incorporated unit of the polymer chain, 4[mm][rr][mr]72=1.37 Methods for estimating the length of continuous isotactic sequences and the regiospecificity of catalyst systems have been developed. The degree of regiospecificity is normally determined from the 13C NMR spectra of polymer solutions by analysing the proportion of abnormal patterns of monomer addition (2,1- or 1,3- instead of the 1,2-addition).Depending on the type of system and polymerisation conditions, the frequency of abnormal addi- tion patterns can vary from fractions of a percent to tens of percent.29, 33 The fraction of isotactic sequences with different numbers of monomer units and the pentad composition of the polymer can var a broad range, depending on the stereocontrol capacity of catalyst systems, while in the case of stereoblock polymers, it is determined by the content of monomer units in regular and atactic sequences. The formation of stereoblock fractions with the use of heterogeneous catalyst systems is usually explained by the pres- ence of highly coordinatively unsaturated AC the structure of which changes repeatedly over the period of polymer chain growth, for example, due to different ways of coordination of the cocatalyst and modifying additives.The microstructure of these materials is described using parametric models, in particular, the enantiomorphic model, the Bernoullian statistic model and the first- and second-order Markov chains. In analysis of the mechanism of the stereocontrolling action of catalyst systems, the method of polymer fractionation by extrac- tion with solvents having increasing boiling points is often used. The fractionation at rather high molecular masses (>50 000 g mol71) is mainly determined by the stereoregular structure of polymer chains, while at lower molecular masses, it is determined simultaneously by the molecular mass and the microstructure.39 When polymerisation takes place under the action of modern highly active titanium magnesium catalysts (TMC), the content of irregular fractions in the product reaches *50%.However, the introduction of electron-donating modifiers, for example, dibutyl phthalate as an internal donor or various dialkoxysilanes as external donors, markedly increases the isospecificity of the systems. High isospecificity of TMC is also attained by using 51 2,2-substituted dialkoxypropanes as modifying additives.6 The isotactic fractions of polymers obtained with these catalysts are characterised by high contents ofmmmmpentads (594%± 98%).The discovery of highly effective stereospecific homogeneous catalysts made it possible to establish the relationship between the metallocene structure, the nature of the ligands, the type of symmetry, the mechanism of polymerisation and the microstruc- ture of the resulting PP.37, 40, 41 An advantage of homogeneous metal complex systems is the possibility of direct investigation of the AC nature by various physical methods. Ewen 42 has formu- lated the symmetry rule which establishes the relationship between the structure of the initial catalyst and the microstructure of PP synthesised using this catalyst. Thus ansa-metallocenes of C2 symmetry can ensure the synthesis of IPP with a content of mmmm pentads of up to 99%, those of Cs symmetry provide the synthesis of syndiotactic PP with up to 94% of rrrr-pentads, C2u symmetry leads to atactic PP, and C1 symmetry provides hemi- isotactic, isotactic or stereoblock PP depending on the type of p-bonded ligands and substituents in them.Catalysts with C2 symmetry are characterised by homotopic, Cs, by enantiotopic, and C1, by diastereotopic coordination positions of the olefin and the growing polymer chain. The knowledge of the mechanistic details of the stereo- and regiocontrol of the polymer gives a key to the rational design of homogeneous catalysts. By changing the metallocene structure, the type of symmetry, the ligands and substituents in the ligands and in bridges and by varying the conditions of polymerisation, one can control the microstructure, the molecular mass, and the terminal groups of polymers in the desired direction and produce materials with necessary properties.In the case of metallocene catalysts, unlike heterogeneous catalyst systems, polymerisation conditions (reaction temperature and monomer concentration) influence appreciably the stereo and regio structure of the resulting polymers. 2. Heterogeneous Ziegler ± Natta catalyst systems for selective synthesis of elastomeric stereoblock polypropylene It was found in the first of Natta's studies that PP formed in the presence of isospecific catalyst systems based on the layered violet modifications of a- or d-TiCl3 and Et3Al (or Et2AlCl) as a cocatalyst contains*80 mass %of the major substance, isotactic crystalline PP insoluble in boiling n-heptane,43, 44 *5% of amor- phous low-molecular-mass atactic PP soluble in ether, and*15% of an intermediate fraction soluble in boiling n-heptane.It was shown that fractions with degrees of crystallinity of 15% ±30% and an intrinsic viscosity of*1 dl g71, unlike isotactic fractions, exhibit `stress ± strain' curves typical of elastomers, namely, a low initial modulus of elasticity, its substantial decrease upon stretch- ing to 200% ± 300% and an increase in the modulus and strength upon further stretching (Fig. 3). A number of other properties typical of elastomers have been noted.A stereoblock structure has been proposed for this polymer.43, 44 In 1982, the synthesis of high-molecular-mass elSBPP induced by tetraalkyl or tetraaryl derivatives of titanium, zirconium and s /kg mm72 4 1 32 2 10 600 400 200 e (%) Figure 3.Stress (s) ± strain (e) curves for highly isotactic (1 ) and stereo- block (2) fractions of PP.4352 Table 1. Polymerisation of propylene catalysed by Collette catalyst systems (liquid propylene, 50 8C, 1 h).45 ± 47 Catalyst Row A(see b) [Cat.] (see a) 0.08 0.17 0.15 0.53 0.12 0.59 0.23 0.12 0.18 0.29 0.3 0.4 0.5 0.2 0.6 0.2 0.2 0.6 0.2 0.2 ZrBn4 ZrBn4 (see h) Zr(C5H5CMe2)4 Zr(C5H5CMe2)4 (see h) Zr(C5H5CMe2)4 (see h) Zr(C5H5CMe2)4 (see h, i) Zr(ButCH2)4 Zr(ButCH2)4 TiBn4 (see h) TiBn4 (see j) 123456789 10 2 450 520 1 430 376 456 395 2 737 2 016 1 163 1 287 a Catalyst loading expressed in mmol (g Al2O3)71.b Catalyst activity expressed in kg of PP (mmol M)71 h71, whereMis a transition metal. c Viscosity- average molecular mass (in g mol71) calculated from the intrinsic viscosities (taken from Ref. 46) using the Mark ± Houwink equation with the parameters K=1.5861074, a=0.77. d Macrotacticity index of polymer samples determined from the intensities of the absorption bands at 993 and 975 cm71 in the IR spectra. e Melting temperature. f Degree of crystallinity calculated from the heats of melting (taken from Ref. 46): C= (DHm/209)6100, where DHm is the heats of melting (in J g71), 209 J g71 is the specific heat of melting of crystalline polypropylene.g Content of the fraction soluble in diethyl ether. h During polymerisation, hydrogen was introduced (*3 atm). i Polymerisation was carried out with the addition of triisobutylaluminium (TIBA), Al : Zr=1. j The catalyst was preliminarily hydrogenated for 20 min at 25 8C. hafnium immobilised on Al2O3 [with a specific surface area (Ssp) of 100 ± 150 m2 g71] has been patented.45 These catalyst systems allow the preparation of high-molecular-mass PP (with an intrin- sic viscosity of 1.5 ± 13.0 dl g71) with low or moderate degrees of crystallinity (5% ± 30%) and high melting points (135 ± 155 8C).46, 47 The glass transition temperature (Tg) of the polymers varies from 77 to 710 8C. The content of fractions soluble in ether and heptane ranges from 30% to 50%, depending on the catalyst and polymerisation conditions. The polydispersity coefficients of the polymer samples also vary over wide limits, Mw :Mn=6 ± 23 (Mw andMn are the weight average and number average molecular masses of the polymer, respectively).The influence of the nature of the transition metal, the ligands attached to it and polymerisation conditions on the activity and properties of the resulting polymer is illustrated by the data of Table 1.45 ± 47 Zirconium complexes with bulky ligands (neopentyl or neophyl) lead to the formation of PP with a higher degree of isotacticity than ZrBn4 (see Table 1, rows 8, 3, 1). The stereo- specificity of tetraneophyl and tetraneopentyl complexes of zirco- nium depends appreciably on the concentration of the supported catalyst.An increase in the content of the complex on the support (correspondingly, an increase in the Zr :Al ratio) is accompanied by an increase in the isotacticity of the resulting polymer and simultaneous decrease in the catalyst activity (see, for example, rows 4, 5 and 7, 8). When the zirconium complex is pre-hydrogenated, or poly- merisation is conducted in the presence of hydrogen (to control the molecular mass of polymers), the catalyst activity substantially increases and the crystallinity of the resulting polymer decreases due to the increase in the content of the elastomer fraction. It should be noted that upon the addition of stoichiometric amounts of triisobutylaluminium or diisobutylaluminium hydride, the crystallinity of polymers markedly increases (see Table 1, rows 6, 4).Titanium complexes are more active than zirconium com- plexes. In the presence of hydrogen, the activity of titanium complexes decreases by about 20% and becomes comparable with the activity of zirconium complexes under similar conditions. The molecular mass of PP obtained under the action of titanium complexes is much higher. The crystallinity of polymers synthes- ised using titanium complexes in the presence of hydrogen is relatively high (see Table 1, rows 9, 10). To characterise the elastomeric properties of polymers, most researchers use either the recovery or the residual elongation, which are determined by the following relations NMBravaya, PMNedorezova, V I Tsvetkova D993/D975 C (%) (see f) Tm /8C (see e) 1073MZ (see c) FEt2O (%) (see g) (see d) 7 23 27 11 24 279 20 29 20 148 155 153 149 155 156 144 153 150 153 0.40 0.53 0.57 0.46 0.55 0.56 0.30 0.52 0.53 0.56 741 754 42 77737 7 lmax ¡ l1 100% or l1 ¡ l0 100%, l0 lmax ¡ l0 where lmax is the maximum elongation, l1 is the sample length after relaxation, l0 is the length of the initial sample.The results of mechanical tests of samples of elastomeric PP obtained in the studies by Collette are presented in Fig. 4. It can be seen that the polymers synthesised possess good elastomeric properties.Upon stretching by 300%, the residual elongation is 93%. However, it was found that mechanical compositions obtained by blending the high-molecular-mass fraction soluble in ether with the fraction insoluble in hexane exhibit similar properties (see Fig. 4, dashed lines). After blending for 10 min at 180 8C, the percentage of the ether-soluble fraction decreased from 69% to 37%.46 It was concluded 46 that the key role is played by the high-molecular-mass atactic fraction, which can be co- crystallised with a more stereoregular fraction to give cross-linked structures, and the following criterion was introduced: a catalyst effective for the formation of elSBPP should possess relatively low s 4 3 0 2 0 3 1 0 2 1 500 300 700 e (%) Figure 4.Hysteresis tests of elastomeric SBPP (1 ± 4) and mechanical mixtures (1 0 ± 3 0 ) of ether-soluble (MZ=284 000 g mol71) and hexane- insoluble (MZ=1 478 000 g mol71) fractions.46 The rate of sample drawing and unloading is 51 cm min71. Draw ratio (%): (1, 10 ) 300; (2, 2 0 ) 500; (3, 3 0 ) 700; (4) 820.Controlled synthesis of stereoblock polypropylene. New trends in the development of elastomers stereospecificity and ensure the production of a high-molecular- weight polymer. Later, Job 48 developed modified heterogeneous titanium magnesium catalysts for the synthesis of stereoblock elPP. The catalysts were prepared by the reaction of TiCl4 with Mg(OAlk)2 in the presence of electron-donating substances (dialkyl phtha- lates) and activated by an organoaluminium compound with addition of a reagent to control stereoselectivity (heterocyclic aromatic nitrogen-containing compounds: 2,6-lutidine or pyra- zine).In the absence of a nitrogen-containing modifying agent, these catalyst systems allow the synthesis of isotactic PP, while in the presence of this agent, stereoblock PP with short syndiotactic blocks is obtained. The specific activity (per g of the catalyst) of the TMC is 50 ± 100 times as high as that of the Collette catalysts: *9 kg PP (g Cat.)71 h71 and *200 g PP (g Cat.)71 h71, respectively. The PP samples contained *50% of isotactic pen- tads, the elongation at rupture reached 1000%, and the residual elongation after stretching to 300% was*70%± 85%.It was shown 49, 50 that the use of heterogeneous vanadium systems, for example, VCl3 together with TIBA, results in the formation of PP with an increased proportion of abnormal additions of the monomer (2,1- or 1,3- instead of the typical 1,2- addition) due to the lower stereo- and regioselectivity of vanadium systems. The resulting polymer possesses a number of properties typical of elastomeric PP, namely, low Young's modulus, the absence of yield point and high impact strength. In addition, it has a high frost resistance. The elastomeric SBPP synthesised on heterogeneous catalyst systems has a broad MMD (Fig. 5) and can be separated into fractions by treatment with various solvents. The broadestMMD is observed for the most stereoregular fractions (see. Fig.5). This property, common to heterogeneous catalyst systems, is a con- sequence of the multi-site nature of the AC formed in heteroge- neous catalysts; it hampers the target-directed synthesis of poly- mers with specified characteristics. [Pi ] 1.0 0.8 1 2 0.6 4 0.4 3 0.20 logM 7 6 5 4 Figure 5. The molecular-mass distributions of the fractions of elSBPP prepared with the Collette catalysts (1,3,5-trichlorobenzene, 135 8C).46 (1) elSBPP soluble in ether; (2) non-fractionated elSBPP; (3) elSBPP soluble in heptane; (4) elSBPP insoluble in hexane, [Pi ] is the proportion of the polymer fraction. 3. Metallocene catalysts for the synthesis of elastomeric polypropylene The next generation of catalysts used for the synthesis of elSBPP are homogeneous catalyst systems based on metallocene com- plexes of Group IVB elements activated by MAO.Advantages of these systems include high activity and the possibility of obtaining 53 elSBPP as the only reaction product or as the predominant fraction. In addition, by selecting metallocenes and polymer- isation conditions, one can control the microstructure of the growing polymer chain and change the molecular-mass character- istics of PP over wide limits.19 There are several types of homogeneous metallocene catalyst systems efficient in the synthesis of elSBPP, which differ in the mechanism of action. Since the AC structure, the stereocontrol mechanism and the properties of the resulting polymer are interrelated for these catalysts, it is expedient to consider various types of catalyst systems and the proposed mechanisms of polymerisation and to analyse the properties of the resulting elSBPP.It should be noted, however, that most of the mecha- nisms, although based on modern views and confirmed by some experimental results and model calculations, are still under discussion and require more detailed experimental evidence. The main types of homogeneous catalyst systems for the synthesis of elSBPP include ansa-metallocenes with C1 symmetry in the presence of the MAO cocatalyst; bisindenyl 2-aryl-substi- tuted metallocenes in the presence of MAO; `hybrid' metallocene systems in which metallocenes of several types are used simulta- neously; alkylated bisindenyl 2-aryl-substituted metallocenes acti- vated by TIBA, and post-metallocene catalyst systems. a.ansa-Metallocenes with C1 symmetry activated by polymethylaluminoxane The synthesis of elSBPP with macromolecules consisting of alternating short stereoregular crystallisable and atactic sequences catalysed by MAO-activated asymmetric titanocenes 1 and 2 was first reported by Chien and coworkers.51 ± 55 Under certain con- ditions, the reaction gave PP completely soluble in ether. For the polymer obtained at 25 8C, analysis of the density of amorphous and crystalline PP blocks 52 made it possible to estimate the lengths of atactic and isotactic blocks; they were 50 and 20 monomer units, respectively. Propylene polymerisation catalysed by a series of structurally homogeneous metallocenes 3 ± 7 in the presence of the MAO cocatalyst has also been studied.56 ± 58 The catalytic properties of these systems, their dependence on the nature of the metallocene and on the polymerisation conditions and some characteristics of the resulting elSBPP samples are presented in Table 2.Me Me Me Me Me MeH Si MCl2 MCl2 TiX2 Me Me Me 3 ± 5 6, 7 1, 2 M=Zr (6), Hf (7). X=Cl (1), Me (2). M=Ti (3), Zr (4), Hf (5). ZrCl2 ZrCl2 ZrCl2 Me Me Me 10 9 8NMBravaya, PMNedorezova, V I Tsvetkova 54 Table 2. Polymerisation of propylene induced by ansa-metallocenes with C1 symmetry (MAO cocatalyst, Al :M=2000).51 ± 53, 56 ± 59 Ab C (%) Complex Row Mw :Mn C p 3H6 /atm [mmmm] (%) (see c) 1073Mw /g mol71 Tm /8C (see d ) Tp /8C (see a) 51/66 3.0 127 77719 15 17 77 7 7 7 7 7 7 7 7 47/61 see f see f 54/93 50/79 53/74 7 7 507 7 7 7 7 7 7 7 40 7 7 72 42 15 30 38 32 30 54 45 52 43 37 49 62 44 67 25 725 25 25 25 25 250 25 250 30 30 50 30 30 50 1.0 1.0 see e 1.0 1.0 1.0 3.0 3.0 1.0 4.0 3.0 2.0 6.5 7.5 2.0 8.5 7.5 70.001 0.002 1.0 1.1 1.9 1.6 0.01 2.4 772.0 2.3 4.6 see g see g see g 2.3 2.0 1.8 2.1 2.1 2.1 1.7 1.9 1.9 1.8 2.0 1.9 1.8 1.9 1.9 123456789 10 11 12 13 14 15 16 17 13346555777 10 10 10 13 13 13 70 229 30 43 75 49 140 380 55 171 46 103 158 85 a Polymerisation temperature.b Here and in Tables. 3, 7 and 8 the activity is expressed in kg PP (mmol M)71 [C3H6]71 h71, where M is a transition metal. c Content of isotactic pentads determined from the 13C NMR spectra. d Here and in Table 6, two Tm values are presented corresponding to two melting peaks observed in the DSC curves. e Polymerisation was carried out in the liquid monomer. f Amorphous polymer. g The sample is unstable. ZrCl2 ZrCl2 ZrCl2 Me Si Ph Me Me Me 12 11 13 ZrCl2 ZrCl2 Me Me Si Si Me Me Me Ph15 14 accompanied by a decrease in the isotacticity of the polymer for both hafnium and zirconium catalysts (see Table 2, rows 6, 7 and 9, 10), whereas an increase in the reaction temperature from 0 to 25 8C markedly decreases the isotacticity of the PP formed with the hafnium complex (see Table 2, rows 11, 10).In a study of a stereoblock polymer prepared by anionic polymerisation of acrylates, an idea was put forward that two different states of the AC, aspecific (non-selective) and isospecific ones, may exist.60 Subsequently, these views have been often used as the basis of the mechanisms of formation of stereoblock PP, in particular for the catalysts of C1 symmetry.51 ± 58 The key feature of asymmetric ansa-metallocenes is the presence of two inequiva- lent vacancies in the AC accessible for the monomer coordination and growth of the polymer chain.Usually, one vacancy is aspecific (non-selective) and the other one is isospecific. Chien and Collins explained the formation of a sequence of atactic and isotactic blocks in the presence of these catalysts in terms of models which take into account polymer chain growth in the active centre (the small square shows a coordination vacancy), whose stereo- specific action changes as a result of slow (see Refs 51 ± 55) or fast (see Refs 56 ± 58) inversion (on the time scale of olefin coordina- tion and addition), for example, due to migration of the polymer chain (P). Me Me Me Me Me Me Me Me + + M M P PIsospecific state Aspecific state Of the series of catalysts 1 ± 7, the hafnium complexes 5 and 7 are the most effective.They combine rather high activity and stability; they catalyse the formation of PP with fairly high molecular mass and good elastomeric and strength characteristics. An increase in the monomer concentration, all other conditions being the same, induces an increase in the molecular mass and some decrease in the content of isotactic pentads (see Table 2, rows 6, 8 and 9, 10). Note that in the presence of titanium metallocenes of this type, elSBPP is formed only in a narrow temperature range, 25 to 30 8C.55 At 725 8C, the content of isotactic pentads reaches 72% (see Table 2, row 2). In addition, titanium complexes are much less stable than zirconium and hafnium analogs. Hafnium complexes give rise to PP with higher molecular masses than zirconocenes (see Table 2, rows 4, 6 and 5, 9); this is typical of metallocene catalysis.An unusual feature is that the activities of zirconocenes and hafnocenes are comparable. In the case of hafnium complexes, all other factors being the same, PP samples with higher degrees of isotacticity are formed (see Table 2, rows 4, 6 and 5, 9). The replacement of the propylidene bridging groups by dimethylsilylene groups results in an increase in the molecular mass and isotacticity of the PP (see Table 2, rows 6, 9; 7, 10 and 8, 11). An increase in the monomer concentration is According to the Chien model, several steps of olefin coordi- nation and insertion occur in turn in either of the active sites until its inversion takes place.However, within the framework of this model, it is difficult to explain the decrease in the content of the mmmm pentads following an increase in the monomer concen- tration on structurally similar Collins complexes. In conformity with the Collins model, the monomer is randomly coordinatedControlled synthesis of stereoblock polypropylene. New trends in the development of elastomers and inserted at the first or second accessible reactive active site. When two competing reactions are involved, namely, olefin insertion and chain isomerisation (migration without insertion), short isotactic sequences would regularly arise in the macro- molecule; this would determine the elastomeric properties of the resulting PP; the length of these sequences would depend on the type of the coordination vacancies and the ratio of the chain growth rates in each site to the rate of active centre isomerisation.This model is also supported by the fact that hafnium complexes exhibit more pronounced stereocontrol action than the zirconium analogues, all other conditions being the same. The rate of polymer chain growth for hafnocenes is much lower than that for zirconocenes. Within the framework of this model, the replace- ment of zirconium by hafnium can result in a decrease in the ratio of the chain growth rate to the inversion rate. A similar mechanism of the action of complexes with C1 symmetry assuming the presence of two sites with different stereospecificities and competition between chain growth and migration without insertion (which also gives rise to elSBPP) has been proposed recently 59, 61 for a series of structurally rigid zirconocenes 8 ± 15.A characteristic feature of these zirconocenes, as noted above, is `stereoduality', i.e., the existence of two different coordination vacancies a and b (Scheme 1, small squares) in the cationic metal alkyl active site formed under the action of MAO. Scheme 1 Zr + Zr + 1 P P Me Me a a...mmmm... 2 5 ...mrrm... 3 Migration without insertion + P P Zr + Zr 4 Me Me b b The non-selective coordination vacancy b is sterically less crowded than the isospecific vacancy a, and the processes of monomer coordination and chain migration to coordination vacancy b without insertion compete in the active centre.When the monomer concentration is low, chain migration without insertion can kinetically predominate. In this case, the reactions `monomer insertion at vacancy a with chain migration (a?b) and subsequent chain migration without insertion (b? a)' will result in the formation of ...mmmm... sequences (see Scheme 1, cycle 1? 2?3). When the monomer concentration increases, the proba- bility of coordination and insertion at the sterically less hindered vacancy b becomes higher. The chain migration to the monomer that has added non-selectively gives rise to `errors,' which are `corrected' at the next step of monomer addition at vacancy a, and the...mrrm.... sequences are thus formed (see Scheme 1, cycle 4? 5? 1).This type of polymer chain growth virtually excludes the propagation of `errors' and, depending on the conditions, it can result in alternation of short isotactic sequences separated by single stereodefects. Polypropylene formed by this mechanism is an isotactic macromolecule divided into short continuous isotac- 55 tic segments by regularly arising `errors' (see Fig. 2 c, elSBPP with k=1). An increase in the polymerisation temperature and a decrease in the monomer concentration lead to a relative increase in the rate of chain migration without insertion and, correspondingly, to a higher content of isotactic pentads (see Table 2, rows 13, 14; 16, 17; 12, 13 and 15, 16). Conversely, an increase in the monomer concentration increases the probability of monomer insertion at coordination vacancy b, resulting in a decrease in isotacticity.It will be shown below that, in conformity with the proposed mechanism, the PP samples prepared with these catalyst systems are characterised by either the absence or low content of pentads of the ...rmrm... type with double `errors'.61 It was also found for these types of catalyst systems that no `errors' caused by isomerisation of the growing chain unit attached to the catalyst via b-hydride shift or the formation of allylic intermediates, typical of catalysts with C2 symmetry,62 ± 66 arise in this case. The absence of these reactions was confirmed experimentally by analysis of the pentad composition of the PP synthesised by polymerisation of CH2=CDMe.59 The catalysts of C1 symmetry substituted at the 2-position make it possible to prepare PP with a very broad range of properties.Some parameters of the resulting polymers are pre- sented in Table 2. An increase in the monomer concentration entails an increase in the molecular mass and a decrease in the content of isotactic pentads and in the degree of crystallinity of the polymer, while an increase in the reaction temperature is accom- panied by lowering of the polymer molecular mass and an increase in the crystallinity. The best catalysts are 2-methyl-substituted complexes 8 ± 10 with ethylene bridges because they exhibit high activity and ensure the formation of elSBPP with a relatively high molecular mass and a content of isotactic pentads at a level of 30%± 50%. These catalyst systems allow effective preparation of PP the properties of which vary over very broad limits, from those typical of highly crystalline rigid thermoplastics to those peculiar to low- crystallinity elastomers, which behave as viscous liquids under the action of load (Fig.6). The samples combining high molecular mass (*200 000 g mol71) and moderate isotacticity ([mmmm]=30%± 40%) display good elastomeric and strength properties. The elongation at rupture for some samples reaches 1700% ± 2000%. However, a characteristic feature of elSBPP prepared on catalysts with C1 symmetry is a relatively low melting s /MPa 6 10 5 86 2 4 4 3 2 1 0 900 500 100 1300 e (%) Figure 6.Drawing curves of PP samples prepared on the 10 ±MAO catalyst system and exhibiting properties of viscoelastic (1), elastomeric (2 ± 4) and stiff thermoplastics (5, 6).59 Mw (g mol71): (1) 134 000; (2) 170 000; (3) 96 000; (4) 63 000; (5) 48 000; (6) 48 000. [mmmm] (%): (1) 20; (2) 37; (3) 38; (4) 37; (5) 60; (6) 57.56 point.59 This indicates the presence of a substantial number of regiodefects in the polymer chain. New metallocene complexes of C1 symmetry for the synthesis of elastomeric PP have been described.26 These ansa-metallocenes with strap-type 7(CH)27 or 7(CH2)27 substituents at the 4- and 5-positions of the fluorenyl ligand possess high activity, 20 ± 70 kg PP (g Cat.)71 h71 or 1 ± 5 kg PP (mmol M)71 [C3H6]71 h71 (polymerisation in the liquid monomer medium); they provide the synthesis of PP with a molecular mass of 86104 to 1.46106 g mol71 and ensure very good elastomeric properties.Indeed, the residual elongation after stretching the sample by 300% is only 2.5% ± 10.0%. The greatest observed elongation of the sample reaches 5200%, the tensile strength is 0.8 ± 2.5 MPa, and Young's modulus is 2.1 ± 9.0 MPa. It has been assumed 26 that the unique properties of elSBPP prepared with these catalysts are determined by ultrasmall (nano-sized) crystallites which form the junctions of the polymer network. A criterion of a `good catalyst' of this new type for the synthesis of elastomeric PP has been proposed,26 determined by the ratio between the parameters of the catalyst asymmetry (ratio of the van der Waals areas of the larger ligand and the smaller one) and parameters of stereorgularity of the synthesised PP (the ratio of mm- to rr-triads).The proportion of the mmmm pentads in the resulting polymers does not exceed 31%; however, unlike elSBPP formed in the presence of C1-symmetric catalysts considered above, these polymers contain substantial numbers of syndiotac- tic pentads ([rrrr]&28%). b. The `bis(2-substituted indenyl) metallocene ± polymethylaluminoxane' catalyst systems Bisindenyl metallocenes with aryl (16 ± 25) and alkyl (26 ± 29) substituents at the 2-position of the indenyl ring represent the most extensive and thoroughly studied class of metallocenes effective in the synthesis of elSBPP.In recent years, this series has been extended by zirconocenes with mixed hapto-bonded (30 ± 35) and heterocyclic (36 ± 38) ligands. Due to the intensive research, one can expect that new catalyst systems of this class effective in the synthesis of stereoblock PP would appear in the near future. The 2-aryl-substituted dichloride complexes 16, 19 were the first of the series of `oscillating' metallocenes.67 ±72 Upon activa- tion with MAO, these catalysts ensure the formation of stereo- block PP with high molecular masses. R R R MCl2 16 ± 19 RM=Zr: R=H (16), Me (17), CF3 (18);M=Hf, R = H (19). R ZrCl2 R26 ± 29 R=Me (26), Bun (27), cyclo-C6H11 (28), Bn (29). R ZrCl2 R 20 ± 25 R=Me (20), Et (21), Bun (22), SiMe3 (23), CF3 (24), Cl (25).Me Me Me Me ZrCl2 Me 30 NMBravaya, PMNedorezova, V I Tsvetkova R1 X Me R3 R1 ZrCl2 ZrCl2 X Me R3 R2 36 ± 38 31 ± 35 X = O (36), S (37), NMe (38). R1=R2=H, R3=CF3 (31); R1=R3=H, R2=Me (32); R1=H, R2=Me, R3=CF3 (33); R1=CF3, R2=H, R3=Me (34); R1=R3=CF3, R2=Me (35). X-Ray diffraction studies 67, 73 ± 75 showed that both the zirco- nocene 16 and the hafnocene 19 exist as two rotation isomers relative to the metal7hapto-ligand bond in the crystal unit cell. This attests to the energetic equivalence of both forms. This fact served as a pre-requisite for the proposed 67 `oscillation' mecha- nism of the formation of stereoblock PP on catalysts of this type (Scheme 2, where the I and A stand for the isospecific and aspecific centres, respectively, kpi and kpa are the rate constants for polymerisation at the I and A centres, Ci and Ca are the concentrations of the I and A centres; and wi and wa are the rates of chain growth at the I and A centres, respectively).Scheme 2 k1 Zr Zr k2 P P the A centre the I centre wi=kpaCa [C3H6] wi=kpiCi [C3H6]m n It was assumed that during polymerisation, the active cationic alkyl metal complex retains the ability to undergo barrier-free rotation. During the growth of the polymer chain, reversible isomerisation of the complex from the isospecific rac-form to the aspecific meso-form takes place repeatedly. The insertion of propylene during the period of existence of the rac-form of the active centre (the I centre) gives rise to isotactic sequences.On the meso-form of AC (the A centre), propylene insertion yields atactic blocks. To obtain the stereoblock structure of the polymer, it is necessary that the isomerisation rate be lower than the rate of olefin insertion but higher than the chain growth rate. Thus, chain growth on the `oscillating' type AC gives macromolecules of the stereoblock structure. The length of the isotactic sequences is usually rather small and the stereoblock PP synthesised under the action of these metallocenes is distinguished by a low degree of crystallinity with a rather high molecular mass. For a large number of complexes of this type, the influence of polymerisation conditions on the catalyst activity and character- istics of the resulting polymers (microstructure and fractional composition, molecular mass, MMD, elasticity) has been studied.67, 70, 72, 73, 76 ± 78 Typical properties of catalyst systems and elSBPP prepared on these catalysts, and their dependence on the reaction conditions are presented in Table 3.Controlled synthesis of stereoblock polypropylene.New trends in the development of elastomers Table 3. Polymerisation of propylene induced by `oscillating' catalysts (MAO cocatalyst). Al : Zr Complex Row 16 16 16 16 20 20 16 16 16 19 16 19 16 18 16 16 16 26 27 1000 2200 1000 1000 1840 1550 4800 10 000 31 600 1000 1000 1000 1000 1000 1000 1000 1000 3500 3500 123456789 10 11 12 13 14 15 16 17 18 19 Note. The catalyst concentration was varied in the range of 1075 ±1076 mol litre71. a Polymerisation was carried out in the liquid monomer.The catalyst systems in question are characterised by a slight increase in the activity and a sharp decrease in the molecular masses with an increase in polymerisation tempera- ture 67, 70, 72, 73, 76 ± 78 (see, for example, Table 3, rows 1, 4 and 2, 7). The temperature at which a high-molecular-mass elSBPP can be prepared usually does not exceed 30 ± 40 8C. It has been noted 70 that the yield of the solid polymer formed upon polymer- isation in the liquid monomer medium at 50 8C equals 30% based on the monomer consumed; the rest falls to low-molecular-mass products.The same sharp decrease in the molecular mass of the polymer upon an increase in the reaction temperature has been observed 72 for the 20 ±MAO catalyst system, which induces the formation of elSBPP with a higher molecular mass and a higher degree of crystallinity (see Table 3, rows 5, 6). From the tem- perature dependence of the activity of the complex 16 (liquid propylene, 0 ± 40 8C) measured in several studies,70 ± 72 the effective activation energy of polymerisation was found to be Ea&4 kcal mol71. Unlike a number of other metallocene catalyst systems, for these complexes, a marked decrease in the polymerisation rate with time was observed, even for low reaction temperatures (Fig.7).72 The activity of catalyst systems and the molecular- kef litre (mol Zr)71 min71 150 100 50 1 0 80 40 Figure 7. Effect of conditions of propylene polymerisation on the variation of the catalytic activity of the 16 ±MAO system with time.70 Polymerisation was carried out in the liquid monomer medium; kef=wp[C3H6]71 [M]71, where wp is the rate of polymerisation. Tp (8C): (1) 2; (2) 8; (3) 30. Tp /8C pC3H6 /atm 08 20 40 30 50 30 20 20 20 20 20 20 25 25 25 25 720 720 see a see a see a see a see a see a see a see a see a 2.5 2.5 7.0 7.0 6.3 1.8 3.5 6.3 1.0 1.0 3 2 Time /min A 1073Mw /g mol71 556 411 539 163 360 37 145 549 236 345 232 486 435 332 179 241 369 600 260 0.20 0.10 0.19 0.35 0.18 0.40 0.54 1.02 1.77 0.54 0.50 0.21 0.17 0.37 0.32 0.36 0.54 0.03 0.004 mass characteristics of elSBPP largely depend on the method used to prepare the catalyst.70, 73, 80 In many cases, the resulting polymers are characterised by a broad MMD (Mw :Mn=3±6) and can be separated into several fractions by extraction with boiling ether and n-heptane.It is of interest that the differences in molecular masses and stereoisomeric compositions of the frac- tions prepared in the presence of these catalyst systems are much smaller than those obtained with the use of isospecific heteroge- neous catalysts.82, 83 Comparison of the data obtained by different researchers 70, 79, 80 allows the conclusion that the activity of complexes of this type increases with an increase in the Al : Zr ratio (see Table 3, rows 3, 7 ± 9), which is typical of catalyst systems based on metallocenes.An interesting feature of these catalysts is that the activities of the zirconium and hafnium complexes 16 and 19 are compara- ble.69, 72, 73 As noted above, hafnocenes with C1 symmetry behave in a similar way.56 ± 58 Normally, hafnium metallocenes of other types are much less active than zirconium analogues, all other factors being the same, and give polymers with much higher molecular masses. The 2-phenyl-substituted complexes 16, 19 have approximately equal activities; in the case of elSBPP pre- pared on the hafnium complex, the molecular mass is higher and the polydispersity and the degree of isotacticity are much lower than those for elSBPP synthesised under the action of zirconium analogue (see, for example, Table 3, rows 11, 10 and 13, 12).Bruce et al.73 believe that the comparable levels of activity of the complexes 16 and 19 are due to the more effective formation of AC in the case of the hafnium catalyst. A number of publications have been devoted to the analysis of the influence of substituents on the catalytic properties of 2-aryl- substituted complexes and characteristics of the resulting elSBPP. The introduction of methyl substituents at the 3- and 5-positions of the benzene ring (complex 17) causes some decrease in the catalyst activity and the polymer molecular mass; in addition, the isotacticity of the polymer substantially decreases.68, 69 The com- plex 18 containing trifluoromethyl substituents provides the formation of a crystalline PP (see Table 3, row 14).It is of interest that polymers prepared under the action of the complexes 16 ± 18, even with different contents of isotactic pentads, have rather high melting points (140 ± 150 8C), which implies a small number of regiodefects in isotactic blocks. The introduction of various substituents into the para-position of the benzene ring (complexes Mw :Mn 2.8 4.0 4.7 3.4 4.5 2.6 3.0 3.5 2.4 2.3 3.8 2.1 3.0 3.7 3.0 3.5 3.9 3.5 2.1 57 Ref.[mmmm] (%) 76 70 76 76 72 72 70 79 80 73 73 73 73 68 68 68 68 81 81 38 29 39 30 730 31 7168 239 31 73 20 26 32 11 1658 20 ± 25) barely affects the activity of the system and the properties of the elSBPP formed.72, 78 Both rac- and meso-forms of the analogue of the complex 16 with the methyl substituent at the 1-position give rise to an amorphous low-molecular-mass PP with very high polydispersity coefficients, Mw :Mn=4 ± 15.84 The elSBPP formed under the action of the complex 30 is close in stereochemical composition to the polymer formed with the 16 ±MAO catalyst system; however, regarding the melting point and the macrotacticity index, it is close to the elSBPP prepared on catalysts with C1 symmetry.71 Complexes 31 ± 35 with mixed ligands and different substituents are less active catalysts than the bisindenyl complexes with identical ligands.85 The main channel of chain transfer by the catalyst systems considered is the transfer of the b-hydrogen atom to the metal and the coordinated monomer.80 An interesting feature of 2-arylin- denyl systems is the clear-cut effect of the co-monomer, which was identified in a study of propylene copolymerisation with ethylene induced by the complexes 16 and 18.79, 86 The activity of the catalyst system increases 2- to 8-fold even at a low ethylene : pro- pylene molar ratio (0.05 ± 0.08) in the monomer mixture.The increase in the activity is accompanied by a substantial (2 ± 3-fold) increase in the molecular mass of the copolymer.The ethylene content in the copolymer varies in the range of 15 mol %± 50 mol %. The system activity also increases appreciably upon the introduction of hydrogen. It has been assumed 86, 87 that the co-monomer effect shows itself as the ability of ethylene to be inserted and to `wake up' the AC after the formation of a regiodefect of the polymer chain as a result of the monomer 2,1-addition. Studies of the complexes 16 and 18 showed 87 that they are more regiospecific than the isospecific EtInd2ZrCl2 catalyst (Ind is indenyl). The non-bridged complex 16 produces PP containing 0.1 mol %± 0.3 mol % of 2,1-added units. It is of interest that `regioerrors' are detected only in isotactic sequences of the polymer, i.e., they arise only in those cases where the `oscil- lating' metallocene occurs in the isospecific state.This is confirmed by the fact that no `errors' are found in the PP synthesised by the polymerisation of propylene in the presence of meso-Me2Si(2-PhInd)2ZrCl2, whereas the rac-analogue of this complex produces PP with `errors' caused by the 3,1-addition.88 A specific feature of the `oscillating' catalysts is an increase in the content of the mmmm pentads following an increase in the monomer concentration (see, for example, Table 3, rows 15 ± 17). Quantitative analysis of this phenomenon is based on the fact that the rate of polymerisation is proportional to the monomer concentration, while the rate of isomerisation does not depend on it and, since the content of isotactic pentads is determined by the ratio of the rates of these reactions, the regularity of the polymer increases with an increase in the monomer concentration.This has been observed in numerous experiments.67 ± 69, 73, 78 As discussed above, an opposite dependence of the content of isotactic pentads on the monomer concentration was found for metallocenes with C1 symmetry. A mathematical model has been proposed to describe the oscillation mechanism of polymer chain growth.80, 89, 90 Computer generation of PP macromolecules has been carried out with variation of three parameters, namely, the stereoselectivity of the isospecific active centre (a), the selectivity of the aspecific centre (b) and the g/K parameter, which specifies the ratio of the relative reactivity to the stability for iso- and aspecific states (g=kpa/kpi , K=k1/k2).Thus g/K>1 corresponds to the predominant exis- tence of the isospecific AC. The dependence on the pentad composition on the monomer concentration for definite a, b and g/K values is determined by the parameter D=Ökpi=k1 á kpa=k2ÜâC3H6ä . 2 The best agreement between experimental and calculated pentad compositions was found for a=0.97, b=0.56 and NMBravaya, PMNedorezova, V I Tsvetkova g/K=0.6. The D parameter varies in the 1 ± 10 range as the propylene concentration changes from 1.2 to 11 mol litre71. However, the authors of the model admit 80 that the same kinetic dependence of the pentad composition on the monomer concen- tration can be described qualitatively and quantitatively by the Busico model,63, 64, 91 which takes into account the competition between the chain growth and epimerisation processes with the assumption that the AC have two active vacancies accessible for monomer coordination and the polymer chain growth. Since the model of the `oscillating' complex is widely recog- nised, several factors deserve attention, both those confirming the proposed mechanism and those conflicting with it.The mecha- nism in question is supported by the increase in the content of the mmmm pentads with an increase in the monomer concentration, which was observed experimentally for these complexes. The mathematical model describing this mechanism provides a quan- titative correspondence between the polymer microstructures calculated and found experimentally depending on the polymer- isation conditions (monomer concentration, temperatures) for reliable values of model parameters.The X-ray diffraction data for the complexes 16, 19 are also a forcible argument in favour of the `oscillating' complex. Theoretical studies 92, 93 demonstrated the presence of two energy minima for the transition state corresponding to the rac- and meso-forms of the complex 16 and stabilised by the p-interactions between the phenyl substituents and the aromatic system of the indenyl ligands (p-stacking interactions); the two minima are energetically equivalent (DDH=0.6 kcal mol71).Indeed, on passing to hydrogenated analogues of the complex 16, (2-CyInd)2ZrCl2 (Cy is cyclohexyl), (2-Ph-H4Ind)(2-PhInd)ZrCl2 and (2-Cy-H4Ind)2ZrCl2, all other factors being the same, a substantial decrease in the PP isotacticity has been noted: [mmmm] = 32%, 15%, 13% and 5%, respec- tively.77 Direct investigation of isomerisation processes for `oscillating' catalysts is of interest. The data of dynamic NMR for (2-PhInd)2HfBn2 (see Ref. 73) and structurally related (2-phenyl- cyclopenta[l ]phenanthryl)2ZrCl2 (see Ref. 94) indicate a very high frequency of rotation of hapto-bonded ligands: 6800 s71 for the former complex and *1610 8 s71 for the latter one. The latter zirconocene activated by MAO ensures (although with a low activity) the formation of a low-molecular-mass PP similar in stereoisomeric composition to the elSBPP prepared under the action of the complex 16.The frequencies of ligand rotation about the axis passing through the M atom and the centroid of the Cp ring are several orders of magnitude greater than the frequency of olefin insertion estimated from the maximum rate of monomer absorption, which equals 0.1 ± 10.0 insertions of C3H6 per second even with allowance for the low efficiency of the formation of AC. Evidently, such a fast rotation should average the stereospecific action of the AC and ensure the formation of PP with a narrow MMD. No data on the rate of isomerisation in real catalyst systems (MAO-activated metallocenes) are available.To explain the observed contradiction, it has been suggested 94 that the chain growth actually proceeds at a very high rate, which exceeds the rate of rotation of the hapto-bonded ligands but during the period of growth of the macromolecules, theACpass many times into the `dormant' state, perhaps, due to the formation of a contact ion pair with the counter-ion. Note that only for the complexes 16 and 19, was the existence of two energetically equivalent stereoisomers identified. Data from X-ray diffraction analyses of a number of other complexes which catalyse the formation of stereoblock PP indicate the presence of only one stereoisomer, for example, the meso-form of the complex 17,68 the rac-form of the complex 18 68, 73 and its hafnium analogue 73 and the meso-form of the complex 21,78 which does not differ from the complex 16 in catalytic properties.Although 13C NMR spectroscopy is a unique tool for ana- lysing the structure of the polymer chain and, correspondingly, the catalytic properties of the AC, which allows one to evaluate its stereo- and regiospecificity, the stereocontrol mechanisms and soControlled synthesis of stereoblock polypropylene. New trends in the development of elastomers on,34 ± 37, 39, 40, 41, 95 this method still does not provide sufficient information for the analysis of the contents of sequences compris- ing 15 ± 16 monomer units capable of crystallisation. In the opinion of Gauthier and Collins,56, 58 the known data on the microstructure of elSBPP do not allow one to decide unambiguously in favour of one or another statistic model.For example, elSBPP formed under the action of the 19 ±MAO catalyst system 72 has a pentad composition that can be obtained by mathematical generation of the polymer chain in terms of the Bernoullian model with a probability of m-addition (pm) equal to 0.58. However, no correspondence was attained between the calculated and experimental compositions for polymers with higher contents of isotactic pentads, which also exhibit elasto- meric properties; this indicates that the given model is inapplicable to the description of the real mechanism of formation of stereo- block PP. Nevertheless, the possibility of existence of catalyst systems which form stereoblocks also by this mechanism cannot be ruled out completely.A finding quite unusual for metallocene catalysis is that PP samples prepared by virtue of `oscillating' catalyst systems are characterised in some cases by high polydispersity valuesMw :Mn (Tables 3, 4). The correlation between the degree of isotacticity and the polydispersity of PP was first pointed out by Bruce et al.73 This can be illustrated by the plots shown in Fig. 8 for the PP samples prepared using the [2-(4-RC6H4)Ind]2MCl2 and [2- (3,5-R2C6H3)Ind]2MCl2 complexes with the MAO cocatalyst (Al :Zr=1000) at 20 ± 25 8C.68, 69, 73, 78 An increase in the poly- dispersity with an increase in the isotacticity can be followed in Fig. 8 for each type of complex. The hafnium analogues catalyse the formation of PP with a much lower crystallinity and poly- dispersity.Since the content of isotactic pentads is determined by the isospecific state of complexes of C2u symmetry and can be regarded as a measure of activity, it is reasonable to suggest that this state is realised upon reversible coordination of a component (or components) of the reaction mixture (MAO, AlMe3 , solvent, monomer) by the cationic alkyl metal complex. These interactions might be responsible for broadening of the MMD of the elSBPP samples prepared in the presence of the catalyst systems. In this connection, the following example is of interest. It has been reported 82 that the elastomeric PP prepared in the presence of hydrogen (0.044 mmol of H2 per mole of C3H6) in order to decrease the molecular mass differs appreciably from the elSBPP produced in the absence of hydrogen, all other conditions being the same (liquid propylene, 23 8C). The molecular mass of the product decreases and the polymer synthesised in the presence of hydrogen has the following composition: 72% of ether-soluble fraction, 13% of heptane-soluble fraction and 15% of a fraction C Table 4.Strain and elastic pproperties of elSBPP prepared under the action of bis(2-alkylindenyl)zirconium dichlorides in the presence of the MAO cocatalyst (Tp=720 8C, toluene, p 3H6 =1 atm, Al : Zr= 3500).96, 97 36 27 26 29 Parameter Mw /g mol71 Mw :Mn [mmmm] (%) Tensile strength /MPa 90 000 2.6 26 0.55 1085 0.04 0.24 260 000 2.1 16 6.85 1109 1.04 0.86 600 000 3.5 10 2.4 Elongation at rupture (%) 1224 Modulus of elasticity /MPa 0.98 0.81 720 000 2.8 19 7839 70.77 77.3 95.3 93 94 0.25 1.13 1.07 0.76 70.2 95.7 95.1 81 Stress at 100% elongation /MPa Recovery after 100% elongation (%) Stress at 300% elongation /MPa Recovery after 300% elongation (%) 59 Mw :Mn 123456789 65432 [mmmm] (%) 60 40 20 0 Figure 8.Polydispersity vs. isotacticity for PP samples prepared on `oscillating' catalysts activated by MAO at 20 ± 25 8C.68, 69, 73, 78 Catalyst: (1) 16; (2) 20; (3) 21; (4) 22; (5) [2-(4-ButC6H4)Ind]2ZrCl2; (6) 19; (7) 17; (8) 18; (9) hafnium analogue of the complex 18.insoluble in heptane, whereas the fractional composition of the polymer obtained without hydrogen is completely different: 36%, 43% and 21%, respectively. The microstructures of the fractions are also markedly different, the isotacticity of all fractions being much higher in the case of the polymer prepared in the presence of hydrogen (Table 5, rows 11 ± 14, 15 ± 18). In a recent study,80 it was reported that the synthesis of PP induced by the complex 16 can yield elSBPP with a narrowMMD (Mw :Mn=2.1 ± 2.7). However, the polymerisation procedure employed in this study 80 results, with other conditions remaining the same, the formation of a PP in which the molecular mass is 2 ± 4 times lower than that reported previously. The observed extension of the MMD was attributed 80 to the increase in the polymerisation rate following an increase in the monomer con- centration and by a decrease in the rate of mass transfer and propylene solubility during polymerisation at high rates.The catalytic properties of 2-alkyl-substituted complexes 26 ± 29 in the propylene polymerisation in toluene have been studied.81, 96 At a propylene pressure equal to 1 atm, elSBPP with a rather high molecular mass can be formed only at low temperatures (720, 0 8C). Thus at 720 8C, elSBPP with a molecular mass of 600 000 g mol71 is formed under the action of the complex 26. When more active catalysts are used, PP with a higher molecular mass is produced. The maximum activity is exhibited by the complexes 26 and 27 (see Table 3, rows 18, 19).The specific activity of the complex 26 is four times lower than that of the complex 16 at 0 8C. The activity of catalysts in propylene polymerisation increases 1.3 ± 2.3-fold as the reaction temperature decreases from 0 to 720 8C. All 2-alkyl-substituted complexes, except for compound 26, are inactive in ethylene polymerisation at low temperatures. Similar properties are found for complexes 36 and 37 with heterocyclic substituents at the 2-position in the presence of the MAO cocatalyst:97 high-molecular-mass elasto- meric SBPP is formed on these catalysts only at low temperatures (0,720 8C); the catalyst activity increases as the reaction temper- ature decreases; and more active catalysts give rise to elSBPP with a higher molecular mass.Thus the activity of complex 38 at 720 8Cequals 59 kg PP (mol Zr)71 h71 atm71, and the weight- average molecular mass of the resulting PP is 3600 g mol71. Meanwhile, under similar conditions, complex 36 exhibits an activity comparable to that of the complex 16 and leads to the formation of PP with a molecular mass of 720 000 g mol71. Dynamic NMR analysis of 36 and 37 identified fast rotation around the axis that passes through the Zr atom and the centroid of the Cp ring and the presence of a conformational equilibrium, fast on the NMR time scale, although an X-ray diffraction investigation of the complex 36 indicated that only meso-36 exists in the solid state,97 unlike the situation with the complex 16.It can be demonstrated in relation to the elSBPP prepared with the complexes 26, 27, 29 and 36 that the isotacticity ± molecular mass combination of properties is important for the strength and elastomeric characteristics of the polymer (see Table 4).60 Table 5. Pentad composition of the elSBPP samples prepared using homogeneous catalyst systems. Row 123456789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25C a Effective length of isotactic sequences estimated from the content of the corresponding pentads: niso=4+2[mmmm]/[mmmr] (see Ref. 47). b Polymerisation was carried out in the liquid monomer. c Non-fractionated elSBPP prepared on the 16 ±MAO catalyst system (Al : Zr=1000). d Ether-soluble fraction (36%). e Heptane-soluble fraction (43%).f Fraction insoluble in boiling heptane (21%). g Non-fractionated elSBPP prepared on the 16 ±MAO catalyst system (Al : Zr=1000) in the presence of hydrogen (0.044 mol H2 per mol C3H6). h Ether-soluble fraction (72%). i Heptane- soluble fraction (13%). j Fraction insoluble in boiling heptane (15%). kR=2-PhInd, Al : Zr=120. l Polymerisartion was carried out in toluene, p 3H6 =4 atm, R=2-PhInd, Al : Zr=100. m Polymerisation was carried out in toluene, pC3H6 =4 atm, R=2-PhInd, Al : Zr=300. n Polymerisation was carried out in methylene chloride, p 3H6 =6 atm, Al : Zr=480. Analysis of the microstructure of the PP samples prepared with the 26 ±MAOcatalyst system led Naga and Mizunuma 100 to the conclusion that the formation of isotactic sequences during the growth of the polymer chain is dictated by the terminal unit (4[mm][rr][mr]72&1) in the range of polymerisation temper- atures of 720 to 40 8C.These results indicate that this stereo- control mechanism may be involved in the propylene polymer- isation under the action of 2-aryl-substituted metallocenes. The synthesis of the high-molecular-mass elSBPP on catalysts with C2 symmetry,101 namely, rac-Me2C(3-PriInd)2ZrCl2 and rac-H2C(3-PriInd)2ZrCl2, should also be mentioned. In the pres- ence of MAO, at 50 8C in liquid propylene, these complexes (Al :Zr = 1000 ± 4000) actively {A&2.5 kg PP (mmol Zr)71 [C3H6]71 h71} form amorphous polypropylene with a molecular mass of 100 000 ± 160 000 g mol71 and with a content of isotactic pentads of 15.6% for the former catalyst and 25.5% for the latter catalyst.The formation of stereo blocks on these catalysts is determined by the presence of two enantiomers with different isospecificity and a noticeable contribution of the stereocontrol- ling influence of the terminal unit of the polymer chain to the enantiomorphic stereocontrol. An increase in the reaction temper- ature is accompanied by a decrease in the content of isotactic pentads typical of metallocene complexes of C2 symmetry (unlike catalysts of C1 symmetry).59, 61, 102 An increase in the Al : Zr ratio accompanied by an increase in the activity of the 16 ±MAO system reduces the content of Catalyst Tp /8C pC3H6 /atm 113142 108 see b see b see b see b see b see b see b see b see b see b see b see b 1 see b 0 25 25 25 25 30 30 508 30 23 23 23 23 23 23 23 23 30 30 720 30 5 ±MAO 5 ±MAO 5 ±MAO 7 ±MAO 7 ±MAO 10 ±MAO 10 ±MAO 10 ±MAO 16 ±MAO 16 ±MAO 16 ±MAOc 16 ±MAOd 16 ±MAOe 16 ±MAOf 16 ±MAOg 16 ±MAOh 16 ±MAOi 16 ±MAOj 19 ±MAO 20 ±MAO 27 ±MAO R2ZrMe2 ± AlBui3 (see k) 4 30 R2ZrMe2 ± AlBui3 (see l) 4 30 R2ZrMe2 ± AlBui3 (see m) 6 0 (acac)2ZrCl2 ± MAOn C [mmmm] [mmmr] [rmmr] [mmrr] [mrmm+ [rmrm] [rrrr] [rrrm] [mrrm] niso +rmrr] 0.03 0.03 0.03 0.02 0.03 0.02 0.03 0.02 0.05 0.06 0.04 0.06 0.05 0.03 0.04 0.05 0.02 0.01 0.07 0.06 0.08 0.07 0.17 0.19 0.18 0.16 0.16 0.18 0.17 0.16 0.15 0.15 0.15 0.16 0.16 0.13 0.14 0.17 0.13 0.07 0.15 0.14 0.17 0.14 0.39 0.37 0.32 0.53 0.50 0.43 0.25 0.49 0.29 0.31 0.32 0.18 0.33 0.51 0.34 0.23 0.53 0.81 0.12 0.36 0.16 0.27 0.03 0.14 0.49 0.07 0.18 0.18 0.12 0.28 7 NMBravaya, PMNedorezova, V I Tsvetkova 0.02 0.02 0.02 0.02 0.03 7770.08 0.08 0.09 0.11 0.08 0.06 0.08 0.09 0.05 0.01 0.13 0.08 0.11 0.08 0.06 0.07 0.08 0.04 0.06 0.03 0.09 0.03 0.16 0.16 0.18 0.22 0.18 0.13 0.16 0.20 0.11 0.04 0.25 0.16 0.20 0.16 0.19 0.19 0.20 0.14 0.13 0.20 0.22 0.18 0.14 0.13 0.10 0.12 0.11 0.07 0.09 0.12 0.09 0.04 0.12 0.10 0.12 0.14 0.05 0.11 0.10 0.09 0.18 0.15 0.08 0.15 0.12 3 isotactic pentads in the resulting polymer (see Table 3, rows 3, 7, 9).In this connection, it is of interest to turn to the data of Petoff et al.103 Analysis of the effect of AlR3 additives (R=Me, Bui ) introduced inMAOdemonstrated that aluminium alkyls decrease the activity of the rac- and, more appreciably, meso-form of Me2Si(2-PhInd)2ZrCl2 but do not change substantially the micro- structure of PP prepared on each complex alone. A pronounced increase in the polymer isotacticity upon the introduction of aluminium alkyls, accompanied by a decrease in the activity, was observed in the polymerisation guided by a mixture of rac- and meso-forms of the complex as the cationic meso-form became passive after coordination to TIBA.The addition of aluminium alkyls to the 16 ±MAO system (AlMAO : Zr=1000) brought about a decrease in the activity by factors of 1.4 and 1.7 for R=Bui (AlAlBui : Zr=1000) and Me (AlAlMe3 :Zr=100), respectively, but induced an increase in the content of the mmmm pentads from 39% to 47% in the former case and a drop of this value to 19% in the latter case (similarly to polymerisation with a large excess of MAO). Thus, one cannot rule out the possibility of MAO participation in the stereocontrol through chain transfer between aspecific and isospecific sites by the AlMe3 contained in MAO, as in the case of `hybrid' metallocene catalysts considered below. Apparently, parameters of MAO (degree of oligomerisa- tion, structural characteristics, Lewis acidity), which depend on the temperature and the presence and concentration of AlMe3, are also important factors influencing the state of the active centre Ref.(see a) 58 58 58 56 58 59 59 59 70 70 82 82 82 82 82 82 82 82 72 72 96 98 8.6 8.1 7.5 10.7 10.2 8.9 6.9 10.3 7.9 8.2 8.4 6.3 8.2 12.1 8.8 6.7 12.5 28.9 5.6 9.3 5.9 7.9 0.09 0.09 0.09 0.06 0.06 0.10 0.10 0.09 0.03 0.04 0.05 0.05 0.04 0.04 0.06 0.06 0.04 0.02 0.06 0.03 0.06 0.04 0.02 0.04 0.02 0.03 0.04 0.05 0.01 0.02 0.01 0.02 0.02 0.03 0.06 0.09 0.02 0.02 0.03 0.07 0.03 0.05 0.02 0.05 0.03 0.08 0.02 0.05 0.01 0.04 0.02 0.06 0.03 0.05 0.01 0.03 0.01 0.03 0.08 0.02 0.05 0.04 0.06 0.04 0.08 98 11.0 0.03 0.01 0.03 98 6.0 0.07 0.04 0.07 99 8.7 0.09 0.06 0.11Controlled synthesis of stereoblock polypropylene.New trends in the development of elastomers a s /MPa 4 1 2 2 1000 600 200 c si /MPa 40 200 1 2 eH (polarisation of the M7C bond, capacity for dissociation upon coordination and insertion of olefin) .104, 105 The microstructure of stereoblock PP obtained in the presence of `oscillating' catalysts differs from the microstructure of PP formed on catalysts with C1 symmetry by a much higher content of heterotactic pentads, the average length of isotactic sequences being the same (see Table 5).It is also noteworthy that PP synthesised using `oscillating' catalysts has a stereoblock structure even in samples with low contents of isotactic pentads. Thus a sample of elSBPP with good elastomeric properties formed on the 19 ±MAO catalyst system contains only twice as many isotactic mmmm pentads as the ideal atactic PP and, correspondingly, half the syndiotactic rrrr pentads. In addition, the content of boundary mmmr pentads is higher, while that for rrrm is lower for stereo- block PP. The proportion of other pentads corresponds to atactic PP.Alot of data on the strain, strength and elastomeric properties of elSBPP have now been accumulated.70, 72, 80, 106 ± 111 Figure 9 a ± c shows the data on the relaxation dependences for stereoblock PP.It can be seen that polymer samples exhibit good elastomeric properties. The stretching curves follow a pattern typical of classical elastomers,112 i.e., they show uniform defor- Table 6. Properties of the elSBPP prepared under the action of various metallocene catalyst systems. Catalyst Row 1073Mw /g mol71 1577710 16 16 16 20 20 20 19 16 16 b 127 30 49 140 380 171 160 145 410 37 210 360 190 455 257 123456789 10 11 12 13 14 15 138 140 a The index shows the elongation (in %) after which the load was removed. b The sample was prepared in the presence of hydrogen.c Recovery. b s /MPa 1062 200 400 600 e (%) e (%) Figure 9. Engineering (a, b) and true (c) stretching diagrams and hys- teresis tests of the elSBPP samples obtained on 16 ±MAO [a (curve 1), c], 19 ±MAO [a (curve 2)], 20 ±MAO (b) catalyst systems.70, 72 si is true stress, eH is the measure of relative deformation equal to ln(l/l0). [mmmm] (%) C (%) Tp /8C 51/66 47/61 54/93 50/79 53/74 77 19 15 17 140 150 141 151 157 151 4 123 11 14 10 525 200 500 750 800 7 7 1700 1070 1015 1170 790 750 854 7 7 1930 878 753 40 38 54 45 52 37 31 33 26 30 36 712 32 34 11 18 61 ep (%) 1 2000 1500 2 1000 500 [mmmm] (%) 50 40 30 20 10 Figure 10.Elongation at rupture (ep) vs. content of isotactic pentads in elSBPP samples obtained on metallocenes of C1 symmetry (1 )60 and `oscillating' catalysts 72 (2). mation upon stretching to great degrees of elongation and high reversibility of deformation. Some properties of the elSBPP samples obtained on treatment with various types of catalyst are compared in Table 6. The elastomeric SBPP with macromolecules containing more uniform isotactic but also longer atactic sequences have higher melting temperatures but exhibit smaller elongation at rupture, while the molecular masses and the contents of the mmmm pentads are the same. Figure 10 illustrates the dependence of the elongation at rupture on the content of isotactic pentads for the PP samples prepared on metallocenes of various types.Figure 11 a ± c presents the variation of the modulus of elasticity, tensile strength and the residual elongation vs. the molecular mass and the degree of crystallinity for the elSBPP samples synthesised in the presence of the metallocenes 16, 19 and 20. As can be seen from the data of Table 6 and Figs 10, 11, an increase in the molecular mass of polymers with invariant stereoregularity and degree of crystal- linity results in a higher modulus of elasticity, tensile strength and a smaller residual elongation. A decrease in the stereoregularity for similar molecular masses and crystallinity degrees reduces the modulus of elasticity, the tensile strength and the residual elonga- tion.An increase in the sample degree of crystallinity with invariant molecular masses and stereoregularity brings about an increase in the modulus of elasticity, the strength and the residual elongation. It can be seen that the strength and elastomeric properties are determined by several characteristics including the molecular mass, the degree of crystallinity and the stereoregularity of the polymer. Presumably, the size and size distribution of the crystallites and the dynamic changes in the structure of the network junctions under the action of load at various temperatures are also signifi- Ref. Residual elongation (%) (see a) Tensile strength /MPa Elongation at rupture (%) 57300 51 56 790200 (see c) 56 97200 (see c) 58 93200 (see c) 58 769300 101300 31300 175300 100300 100300 47300 4.0 3.0 16.0 16.0 39.0 6.0 4.8 6.8 7.9 2.0 11.4 14.3 1.5 9.3 7.0 60 72 72 72 72 72 72 72 79 79 30 ± 40300 50 ± 9030062 a E /MPa 60 40 200 b sp /MPa 15 1050 c l1 ¡ l0 100 (%) l0 200 1000 200 100 1073Mw /g mol71 Figure 11.Modulus of elasticity, E (a), tensile strength, sp (b), and residual elongation, (l17l0)/l0 (c) vs. molecular mass of elSBPP samples with different degrees of crystallinity:70, 72 (1) 10%± 14%, (2) 0%± 4%. cant for strain and elastomeric parameters of elSBPP. However, at present, it is impossible to perform a comparative analysis of the effects of these factors on the mechanical properties of elSBPP samples obtained using different catalyst systems as no informa- tion needed for this purpose is available.Thus, by varying the type of the catalyst or polymerisation conditions, one can prepare elSBPP possessing a required set of properties. 4. Catalyst systems based on metallocenes with activators other than polymethylaluminoxane In the preceding sections, we considered metallocene catalysts for the synthesis of elastomeric PP activated by MAO. In these catalyst systems, MAO functions as an alkylating and a cation- generating reagent without being an effective controller of the microstructure or chain transfer agent.80 The main factors that control the microstructure of the macromolecule in the catalyst systems considered above are the composition and structure of the initial catalyst and the monomer concentration.The catalyst systems containing MAO as a cocatalyst suffer from some draw- backs. The maximum activity of metallocene catalysts, in partic- ular those considered in this review, is observed at great cocatalyst : catalyst ratios (see, for example, Table 3, rows 4 and 9). Due to the high cost of MAO, the catalysts are expensive. In this section, we give examples of metallocene catalyst systems in which other types of activating additives are used and also catalyst systems that provide additional possibilities of controlling the microstructure of the macromolecules formed. a. The (2-PhInd)2ZrMe2 ± TIBA catalyst systems Recently, we found a new class of homogenous catalyst systems in which TIBA serves as the activating additive.In terms of activity in ethylene and propylene polymerisation, these catalysts are commensurable with systems containing MAO as the cocatalyst; they also retain the stereospecific action.98, 113 This group of complexes includes the dimethyl derivatives of 2-substituted 1 2 1 2 1 2 400 300 NMBravaya, PMNedorezova, V I Tsvetkova Table 7. Polymerisation of propylene in toluene or in the liquid monomer induced by the (2-PhInd)2ZrMe2 ± TIBA catalyst system.98, 113 A Al : Zr Row D998/D973 pC3H6 [mmmm] (%) /atm Tp /8C 120 100 200 300 300 120 120 see a 6.3 6.3 6.3 2.3 6.3 6.3 28 39 718 21 77 0.10 0.13 0.27 0.42 0.18 0.15 0.76 30 30 30 30 30 10 50 1234567 0.26 0.32 0.17 0.18 0.18 0.25 0.17 a Polymerisation was carried out in the liquid monomer; Mw= 230 000 g mol71,Mw :Mn=2.8.bisindenyl zirconocenes, in particular, the dimethylated analogue of the complex 16. Study of the catalytic properties of the (2-PhInd)2ZrMe2 ± TIBA system in toluene and in liquid monomer showed that its activity is similar to that of 16 ±MAO, the molecular mass of the resulting elSBPP being the same or higher (Table 7). Unlike the 16 ±MAO catalyst, for this system, no increase in the content of isotactic pentads following an increase in the monomer concen- tration was observed (see Table 7, rows 4, 5 and 1, 2).The concentration of TIBA or, with a constant metallocene concen- tration, the AlTIBA :Zr ratio is the factor which allows variation over broad ranges of the system activity, the kinetic characteristics of the polymerisation process and the properties of the PP formed. Thus an increase in the AlTIBA :Zr molar ratio from 100 to 300 entails a 4-fold increase in the activity, a growth of the molecular mass from 80 000 to 220 000 g mol71 and a decrease in the content of isotactic pentads (Fig. 12). Yet another feature of this catalyst system is the fact that it permits polymerisation to be carried out at relatively high temperatures (see Table 7, row 7). The increase in the activity of the given system with an increase in the reaction temperature is not accompanied by a sharp decrease in the molecular mass, as was observed for the MAO-activated [mmmm] (%) D998/D973 40 40 30 30 20 20 21 10 10 50 100 150 200 250 Al : Zr Figure 12.Pentad composition (1) and macrotacticity index (2) of elSBPP obtained with the (2-PhInd)2ZrMe2 ± TIBA catalyst system vs. Al : Zr molar ratio (Tp=30 8C).113 system, and makes it possible to change the isotacticity of elSBPP (see Table 7, rows 6, 7). Thus, by varying the concentration of the activating agent and the reaction temperature, one can prepare elSBPP with a specified set of properties. Panin et al. 98 proposed a model which takes into account the participation of TIBA and the monomer in the formation of the AC (Scheme 3, initiation); this allows one to explain why the rate of polymerisation is first order with respect to TIBA and second order with respect to the monomer.Another model explains the role of TIBA in the stereocon- trolling action of the AC (Scheme 3, stereocontrol).113 This modelControlled synthesis of stereoblock polypropylene. New trends in the development of elastomers Scheme 3 Initiation Me AlBui (2-PhInd)2ZrMe2+AlBui (2-PhInd)2Zr 3 3 Me Me + MeAl7Bui (2-PhInd)2Zr (2-PhInd)2Zr 3 AlBui3 +C3H6 Me Me Stereocontrol the I block the A block pm,r=0.5 pm Bui Al Bui Bui + + RAl7Bui RAl7Bui (2-PhInd)2Zr (2-PhInd)2Zr 3 3 P the I centre P the A centre Bui exchange or the P q 17q implies repeated inversion of the AC (fast dynamic equilibrium) from the isospecific to aspecific state during chain growth as a result of coordination of an additional TIBA molecule or the generation of `errors' due to fast exchange between the alkyl groups of TIBA and the growing polymer chain.Computer simulation of the growing chain of stereoblock PP (*300 000 units) was carried out with the optimised parameters of probability of monomer addition to the active site in the isospe- cific (q) or aspecific (17q) state and the probability of monomer addition with a definite orientation in the isospecific centre (pm). In all model experiments, the last-mentioned value was taken to be unity. The probability ofm- or r-addition (pm,r) in the aspecificAC was taken to be 0.5. The pentad composition of experimentally obtained elSBPP samples is consistent with the value calculated in terms of this model; in addition, a logical dependence of the q parameter on the TIBA concentration is observed.It should be noted that elSBPP formed under the action of the (2-PhInd)2ZrMe2 ± TIBA catalyst system is similar in pentad composition to the PP produced by the 16 ±MAO system (see Table 5, 6) and exhibits similar elastomeric properties. The attraction of this catalyst system for the synthesis of elSBPP is in the use of cheap trialkylaluminium instead of expensive MAO as the activating agent and in the possibil- ity of changing the molecular-mass characteristics and the microstructure of the resulting polymer by varying the Al : Zr ratio and the polymerisation temperature.Apparently, the (2-PhInd)2ZrMe2 ± TIBA system should be sensitive to the action of hydrogen and ethylene, like the 16 ±MAO system described above.79, 82, 85, 86 This provides additional opportunities for the control of activity, molecular mass and stereo and fractional composition of the polymer. b. `Hybrid' metallocene catalyst systems The mechanisms of action of the above-described homogeneous metallocene catalyst systems for the synthesis of elSBPP have a common feature, i.e., the possibility of the existence of a cationic AC in the isospecific or aspecific state. In recent years, publica- tions appeared on the synthesis of stereoblock PP by polymer- isation induced by mixtures of metallocenes with different stereo- specificity, `hybrid' homogeneous 114 ± 116 or supported 117 catalyst systems.In studies on homogeneous catalyst systems, a combined zirconocene dichlorides, for namely, activator 63 TIBA ± CPh3B(C6F5)4 114 ± 116 orMAO 116 was used. Zirconocene dichlorides on a MAO-treated support were activated by TIBA.117 Presumably, in the homogeneous system, TIBA acts as the alkylating agent in the initiation step, and the subsequent reaction of the alkylated catalyst with the borate gives rise to a cationic alkyl metal AC. The mechanism of activation of metal- locene catalyst systems involving trialkylaluminium is currently open to question and its detailed discussion is beyond the scope of this review.Only some facts will be presented. Monoalkylation of a large series of zirconocene dichlorides induced by AlMe3 has been confirmed experimentally.118 In the case of the Cp2ZrCl2 ± TIBA system (Al :Zr=10), a 13CNMRstudy showed that the Cp2Zr(Bui)Cl . TIBA complex is the only identifiable product (19%), whereas in the (Me5C5)2ZrCl2 ± TIBA system, the (Me5C5)2Zr+Cl_ClAl7Bui3 complex is formed selectively.119 Preliminary reaction of zirconocene dichlorides with MAO at relatively low Al :Zr ratios (*102) results in complexes that are effectively activated by TIBA in the polymerisation of propylene.120 ± 122 The Me2Si(Me4C5)NButZrCl2 ±MAO, Me2Si(Me4C5)NButZrCl2 ± TIBA ± CPh3B(C6F5)4 and Me2Si. .(Me4C5)NButZrMe2 ± TIBA ± CPh3B(C6F5)4 systems exhibit dif- ferent catalytic properties in ethylene polymerisation under iden- tical conditions.101, 123 ± 125 Despite the fact that TIBA is widely used as an activator of immobilised metallocene complexes supported on MAO-treated materials,126 no data on the mecha- nism of activation or the role of TIBA have been published.In a number of publications,123, 127 ± 130 it is noted that TIBA is not a chain transfer agent in homogeneous catalyst systems for ethylene and propylene polymerisation but functions as an activator. Propylene polymerisation using a mixture of dichloride metal- locene complexes with different stereospecificity, namely, isotac- tic ± atactic complexes 39 ± 40 or 41 ± 40 114 and isotactic ± syndio- tactic complexes 41 ± 42 115 activated by TIBA ± CPh3B(C6F5)4 gave 114 high-molecular-mass products, which were mixtures of homopolymers with the composition corresponding to the stereo- specific action of the initial catalysts and a stereoblock fraction.The catalyst systems studied by Chien et al.114 exhibit activities of up to 50 kg PP (mol Zr)71 h71 [C3H6]71. The content of the stereoblock fraction depends on the molar ratio of the complexes in the hybrid mixture and the TIBA concentration. The stereo- block fraction ensures the compatibility of the resulting polymers. An increase in the concentration of TIBA is accompanied by an increase in the activity of the systems and a decrease in the crystallinity of the polymeric product. At a definite composition, the resulting polymers exhibit good elastomeric properties: the elongation at rupture reaches 900% and the recovery is 97%± 98%.The formation of the stereoblock fraction was attributed 114 to the fast exchange of growing polymeric chain Me Si ZrCl2 ZrCl2 ZrCl2 Me 41 40 39 Bui MeMe Me Ph Si ZrCl2 ZrCl2 ZrCl2 Me Me Ph Me Bui 44 43 4264 fragments between the AC with different stereospecificities with participation of TIBA. Recently, experimental data have been published confirming the possible formation of the stereoblock PP fraction by the `hybrid' mechanism.116 Analysis of the terminal groups of the PP macromolecules formed under the action of zirconocenes with different structures, the isospecific complexes rac-Me2Si..(2-MeInd)2ZrCl2 (45) and 43, the aspecific complex 40 and the syndiospecific zirconocene 42 activated by MAO or by the TIBA ± CPh3B(C6F5)4 system showed that chain transfer to the organoaluminium compound proceeds efficiently for some cata- lysts. This is indicated by the high content of the isopropyl groups in the polymer. + + Al L2Zr Me+P L2Zr P+ Me Al + ... L2Zr L2Zr Me H+ (hydrolysis) P Al P �end of the polymeric chain. L�ligand, The efficiency of chain transfer during propylene polymer- isation onMAO (or AlMe3 present in MAO) is governed by steric parameters of metallocene and is maximum for the sterically most hindered highly stereo- and regiospecific zirconocene 43. The introduction of an additional amount of AlMe3 (Al : Zr= 500 ± 5000) results in a considerable increase in the activity and a decrease in the molecular mass of isotactic PP.The efficiency of the chain transfer to the organoaluminium compound reaches 95%; evidently, this process takes place through alkyl ± polymer chain reversible exchange reactions in heteronuclear cationic intermediates.116 + P P + L2Zr +Me Al A7 Al L2Zr A7 Me Me + P + L2Zr Al A7 The aspecific complex 40 is less capable of chain transfer by this mechanism. The introduction of an additional amount of AlMe3 results in a lower molecular mass of the polymer and a lower system activity, apparently, due to the formation of coor- dinatively saturated `dormant' active centres.131, 132 The chain transfer during polymerisation is less pronounced in the case of the syndiospecific complex 42 and does not take place when sterically open isospecific zirconocene 45 is used.On none of the zirconocenes studied, was transfer of the polymer chain to TIBA observed in the presence of a combined activator. By combining complexes with different stereoselectivity (40 ± 43) exhibiting the maximum capacity for chain transfer to the organoaluminium compound, it was possible to prepare PP with the highest (up to 30%) content of the stereoblock fraction (atactic ± isotactic blocks) soluble in boiling hexane.116 The pro- portion of the stereoblock fraction increases upon an increase in the content of the catalyst more prone to chain transfer to the organoaluminium compound.Unfortunately, the researchers cited 116 did not analyse the dependence of the content of the stereoblock fraction on the concentration of the complexes 40 and 43 and TIBA.Adecrease in the concentrations of these components should decrease the probability of exchange reactions and thus reduce the amount of the stereoblock fraction. However, in a recent study,133 doubt is cast upon the widely recognised opinion regarding the nature of the AC (the cationic metal alkyl site stabilised by the counter-ion). Analysis by NMR spectroscopy of anion exchange reactions NMBravaya, PMNedorezova, V I Tsvetkova (dynamic symmetrisation of ion pairs of zirconocene) which are accelerated in the presence of Li+_MeB7(C6F5)3 and upon an increase in the zirconocene concentration, mainly due to the entropy factor, provides grounds for belief that quadrupole ion ± counter-ion dimers (or polynuclear complexes) act as AC for the ion-coordinate polymerisation of olefins.Proof of the existence of such intermediates would provide a new interpreta- tion of exchange reactions involving zirconocenes with different stereospecificities. Apparently, stereoblock PP is formed by a chain transfer mechanism between the AC with different specificity through TIBA or the monomer during propylene polymerisation induced by binary supported catalysts which have been reported recently.117 These catalysts were synthesised by co-precipitation of the isospecific zirconocene 41 and the syndiospecific complex 44 from a toluene solution onto silica gel treated with MAO (PQ-SiO2 ± MAO).This method of deposition ensures homoge- neous distribution of complexes on the surface of the support and creates the possibility for the arrangement of two AC possessing different stereospecific action in the close vicinity of each other. Comparison of the molecular-mass characteristics and the xxxx/xxxy ratios of the pentad contents (x, y=m or r) for the PP samples prepared on the PQ-SiO2 ±MAO± 41, PQ-SiO2 ± MAO± 44 and PQ-SiO2 ±MAO± (41+43) catalyst systems with all other factors remaining the same showed that the polymer formed under the action of the binary supported catalyst con- tained a stereoblock fraction in addition to isotactic and syndio- tactic fractions.On the basis of the analysis of 13C NMR spectra, it was concluded 117 that the chain transfer to TIBA or to the monomer takes place, on average, after 75 steps of insertion. Apparently, due to the low activity of catalyst and relatively low molecular mass of PP samples (40 000 ± 50 000 g mol71), data on the properties of polymers are not presented in the publication. The melting temperatures of PP formed on the binary supported catalyst at 30 ± 60 8C are close to those of IPP ([mmmm]&90%) prepared using the supported complex 41. It should be noted that the use of `hybridisation' with utilisation of procedures giving rise to highly effective immobilised catalysts 134 after optimisation of the conditions of their synthesis and polymerisation and with appropriate selection of metallocenes, can prove highly promising for the synthesis of elSBPP or polypropylene composites.5. Catalyst systems of the post-metallocene type Since the first publications 135, 136 and to the present day, the interest in the catalyst systems for olefin polymerisation based on chelates formed by Group VIII elements, so called post-metal- locene catalyst systems, has continually increased. Most publica- tions are devoted to the study of catalytic activity, the mechanisms of formation and the properties of polyethylene (PE) formed under the action of these systems (see, for example, reviews 137, 138). In recent years, approaches to the design of chiral catalyst systems of this type effective for the synthesis of stereo- regular PP have taken shape.139 ± 143 The chelate complexes of Group IVB metals can also be regarded as post-metallocene catalysts.In a number of publications,144 ± 146 high catalytic activity of these complexes activated by MAO in ethylene poly- merisation has been reported.Anumber of publications have been devoted to the catalyst systems based on Group IVB metal chelate complexes efficient in the synthesis of elSBPP.99, 147, 148 The catalytic activity of MAO-activated zirconium dichloride rac-bisacetylacetonate has been studied.99 This catalyst system in toluene produces fractions of highly isotactic PP (76%) and elSBPP (24%), while in methylene chloride, elSBPP is formed as the only polymerisation product.It has been suggested 99 that the cationic metal alkyl complex with a coordinated toluene molecule exhibits an isospecific action. In methylene chloride, which is more polar than toluene, the electrophilicity of the cationic species is manifested as agostic interactions with the growing polymer chain, which result, however, in chain isomerisation rather than chain transfer.Controlled synthesis of stereoblock polypropylene. New trends in the development of elastomers Me Me H C P H C P Zr Zr CH2 CH2 P H Me CH2 H C Me Zr Zr C P C Zr CH2 P Me Me Under certain conditions, this process competes with chain growth and acts a source of `errors', which deteriorate the growth of isotactic sequences. Unlike the mechanism realised on C1-symmetric catalysts and giving rise to single ...mrrm...stereodefects, this mechanism does not rule out the appearance of double `errors' with the formation of short atactic sequences, which is indicated by the presence of intense signals corresponding to the mrmm+rrmr, rmrm and rrrm pentads in the 13CNMR spectrum (see Table 5, row 25). An increase in the reaction temperature promotes more efficient isomerisation. To confirm the proposed mechanism, Shmulinson et al.99 demonstrated that this system effectively catalyses isomer- isation of oct-1-ene to oct-2-, -3- and -4-enes. High-molecular-mass elastomeric SBPP (Mw=120 000 ± 160000 g mol71, Mw :Mn=1.7 ± 2.4) is formed on chiral M(Z2-NPhPPh2)2(Z1-NPhPPh2)2 complexes (M=Ti, Zr) upon polymerisation in the liquid monomer at 25 8C.147 Apparently, a reason for the formation of alternating isotactic ± atactic sequen- ces during polymerisation of propylene induced by these catalysts is reversible coordination of the phosphine ligand resulting in a dynamic equilibrium between the tetrahedral and octahedral isomers.The tetrahedral isomer is aspecific, while the octahedral one is isospecific. The 31P NMR spectra recorded at room temperature point to the predominant formation of the octahedral isomer under the action of MAO, although the tetrahedral complex has also been identified. R P R R +Zr PZr + P Me Me R P the I centre the A centre The dynamic equilibrium between the tetrahedral and octa- hedral isomers during polymerisation can result in the formation of isotactic and atactic sequences whose relative lengths are determined by the activity of the complexes.However, in the case of this catalyst system, this is not the only factor deteriorating the growth of isotactic sequences. As in the previous case, Kuhl et al. 147 observed isomerisation of alk-1-enes into internal olefins. The possibility of formation of stereoblock PP as a result of the change in the AC geometry in a dynamic equilibrium during the growth of the polymer chain has been studied 148 in propylene polymerisation in the presence of Ti(III) and Ti(IV) bisallyl chelate complexes, [(ButMe2SiCH)2CH]2Ti(m-Cl)2Li .TMEDA (where TMEDA is N,N,N0,N0-tetramethylethylenediamine) and [(ButMe2SiCH)2CH]2TiCl2 .Similar complexes of zirconium induce the formation of isotactic PP under the same conditions. Several allylic complexes of Group IVB metals active in olefin polymerisation have been described.149 ± 154 As a rule, these are half-sandwich zwitterion Ti(IV) and Zr(IV) complexes in which the allylic ligand stabilises the electronic state of the active site.149 ± 151 The allylic intermediates are described most often as products of deactivation (or temporary deactivation) of the AC.152 ± 154 The Ti(III), Ti(IV) and Zr(III), Zr(IV) bisallylic complexes 148 present unusual examples of comprehensively characterised cata- lysts active in the polymerisation of ethylene and propylene, in which the M7allyl bond mimicks the type of binding in cene or chelating ligands, and the lability of the allylic bond (s, Z1, Z3)155 ± 157 determines the stereocontrolling effect.An unusual 65 finding is that paramagnetic bisallyl Zr(III) (meff=1.5 mB) and Ti(III) complexes (meff=1.7 mB) are active in the polymerisation of ethylene and propylene upon activation by a slight excess of MAO. Polymers formed in the presence of MAO-activated M(III) complexes possess the same properties as the polymers obtained with M(IV) ±MAO catalyst systems (Table 8, rows 4, 5 and 10, 12). It was suggested that, under these conditions, the olefin undergoes oxidative addition to the M(III) bisallyl complex with subsequent formation of the cationic M(IV) alkyl AC upon treatment with MAO.However, participation of the monomer at the active centre initiation step is debatable because the specific activities of M(III) ±MAO systems normalised to the monomer concentration are similar (see Table 8, rows 1, 2 and 4, 6). Table 8. Polymerisation of propylene in toluene induced by the [(ButMe2SiCH)2CH]2M(m-Cl)2Li .TMEDA±MAO catalyst system, where M=Zr(III), Ti(III), and the [(ButMe2SiCH)2CH]2MCl2 ±MAO catalyst system, whereM=Zr(IV), Ti(IV).148 Row Cata- Al :M Tp lyst 1073Mw Mw :Mn [mmmm] /g mol71 (%) pC3H6 A /8C /atm 10.1 0.04 Ti(III) Ti(III) Ti(III) Ti(III) 7.2 0.03 61 56 96 74 71 33 43 45 27 20 15 22 2.6 5.3 2.4 2.9 2.9 3.6 2.9 5.5 2.5 2.7 2.8 2.7 7.2 0.001 16 7.2 0.003 16 10.1 0.005 12 7.2 0.003 16 7.2 0.002 15 5.1 0.01 2030 7.2 0.03 336 78 7.2 0.04 140 7.2 0.03 115 86 7.2 0.03 115 Zr(III) 400 Zr(III) 800 Zr(III) 800 Zr(III) 1000 Zr(IV) 1000 400 400 400 600 Ti(III) 1000 Ti(III) 1200 Ti(IV) 1000 123456789 10 11 12 25 25 50 25 250 25 50 25 25 25 25 These catalyst systems induce the formation of polyethylene with a very high molecular mass and a quite substantial poly- dispersity coefficient, especially in the case of zirconium com- plexes.Under the action of zirconium complexes, IPP is formed. An increase in the reaction temperature from 25 to 50 8C is accompanied by an increase in the isotacticity from 56% to 96% (see Table 8, rows 2, 3).Catalyst systems based on titanium complexes give rise to elSBPP. Ray et al.148 believe that the dynamic change in the geometry of the AC, C2u > C2, caused by reversible Z1 > Z3 isomerisation of the allylic ligands is respon- sible for the formation of stereoblocks. Meanwhile, an increase in the Al/Ti ratio is accompanied by a decrease in the molecular mass of the polymer and the content of isotactic pentads and by some decrease in the system activity. Apparently, these facts indicate that the chain transfer by titanium complexes to the organo- aluminium compound and the back reaction can account for the appearance of regular `errors' in isotactic sequences, similar to the situation observed for the `hybrid' catalyst systems.Unfortu- nately, the pentad composition of elSBPP prepared in the presence of Ti(III) and Ti(IV) bisallyl complexes was not reported in the study under discussion.148 III. Characteristic features of the structure of elastomeric stereoblock polypropylene and the nature of relaxation processes The possibility of preparing elastomeric materials by homopoly- merisation of propylene has triggered considerable interest in these processes. For many elastomers, the influence of the micro- structure and the molecular-mass characteristics on the strain, elastic and thermal properties of polymers has been studied. A number of publications have been devoted to the investigation of the rates and mechanisms of the relaxation processes using66 differential scanning calorimetry (DSC),82, 110, 114 DSC performed in the quasi-isothermal stepwise mode,158 dynamic mechanical analysis (DMA),26, 158, 159 dynamic thermomechanical analysis (DMTA),107 birefringence,107 IR polarimetry 108 etc.The struc- ture of the materials has been studied by X-ray diffraction analysis,107 solid-phase 13C NMR spectroscopy, scanning elec- tron microscopy and atomic-force microscopy (AFM).159 The elSBPP samples subjected to analyses were synthesised using different catalyst systems, both heterogeneous and homogeneous ones based on bridged and non-bridged metallocenes. The materials prepared using heterogeneous catalysts, namely, non-modified TMC and supported catalysts based on tetraneophylzirconium have been studied.110 The contents of the isotactic pentads in the PP were 60 mass % and 40 mass %, respectively.The samples studied had similar values of melt flow index (0.5 g min71 at 230 8C under a load of 21.6 kg) and impact strength (280 ± 300 kJ m72). The polymer prepared using TMC was characterised by a higher modulus of elasticity and tensile strength. The DSC curves for homopolymers exhibited clear-cut peaks for melting (from 155 to 140 8C), crystallisation (from 110 to 808C) and glass transition (from 4 to 75 8C). The peaks observed at 50 8C correspond, evidently, to relaxation processes in the disordered phase. For the PP samples studied, the DMTA spectra were also measured; they indicated an increase in the material stiffness following an increase in the degree of isotactic- ity. The DSC curves for compositionally homogeneous elSBPP samples completely soluble in ether (synthesised using ansa- metallocenes) display feebly pronounced melting and crystallisa- tion peaks.55, 57, 114 In a study of the properties of elSBPP produced under the action of homogeneous systems based on non-bridged metal- locenes, both the initial samples and separate fractions were analysed, first of all, the fraction soluble in boiling ether, the fraction soluble in boiling n-heptane and that insoluble in boiling n-heptane. For the polymer synthesised using the 16 ±MAO catalyst system,80 two melting peaks were detected (60 and 140 8C), whereas the ether-soluble fraction {36 mass %, [mmmm]=0.19} showed no peaks in the DSC curves.In the case of the heptane-soluble fraction {43 mass %, [mmmm]=0.33}, a broadened endothermic peak in the temper- ature range from 40 to 160 8C was found, pointing to broad size and stability distributions of the crystallites. According to 13C NMR spectroscopy, all fractions had a stereoblock structure. For the fraction insoluble in n-heptane {57 mass %, [mmmm]=0.38}, a melting peak at 150 8C is clearly defined. It can be seen from Fig. 13 that non-fractionated elSBPP exhibits small melting peaks. The pattern of the melting curves depends not only on the method used to prepare the samples but also on their thermal pre-history. X-Ray diffraction analysis, DSC, and solid-phase 13C NMR spectroscopy have been used to compare the structure of the crystallising and amorphous phases of non-fractionated samples and separate fractions of elSBPP prepared by polymerisation under different conditions and differing in the content of mmmm 25 20 120 80 40 T /8C Figure 13.Comparison of the DSC curves of elSBPP samples (16 ±MAO catalyst system) with different contents of isotactic pentads.80 (1) 30% (crystallinity, 11%), (2) 19% (crystallinity, 2%). Thermal flux /mW30 2 1 NMBravaya, PMNedorezova, V I Tsvetkova pentads and in the molecular mass.107 Unlike the ether smectic fractions, fractions soluble and insoluble in n-heptane crystallise to give a- and g-forms of PP. It is of interest that only formation of the a-form is noted for the initial polymer.To characterise the degree of ordering of amorphous phases, the influence of the temperature in the range of 20 ± 80 8C on the peak intensity in the 13C NMR spectra was studied. The content of the crystalline phase in the elSBPP samples determined from the 13C NMR spectra in the solid phase is much higher than the values found by DSC or X-ray diffraction analysis. The results obtained attest to involvement of a synergistic effect in the crystallisation of polymers which contain easily crystallisable chains with high contents of isotactic sequences. The properties of polymers synthesised using the `oscillating' catalyst and the catalyst con- taining rac- and meso-forms of ansa-zirconocene were compared as well as those of mixtures of isotactic and atactic PP.The materials were characterised by virtually equal contents of the mmmm pentads. The researchers noted 107 that the thermoplastic elastomers prepared by polymerisation, unlike blend composi- tions, possess excellent elastic properties even at elevated temper- atures. In their opinion, the capacity for relaxation can be regarded as evidence for a multiblock structure of elSBPP. It has been shown by X-ray diffraction analysis, solid-phase NMR spectroscopy and birefringence that samples prepared in the presence of the 16 ±MAO catalyst behave as elastomers up to a temperature of 80 8C at which they are subject to shear deforma- tion, unlike mixtures of isotactic and atactic PP, which have similar stereoisomer compositions and degrees of crystallinity.In order to gain a better understanding of the domain structure of elSBPP, Kravchenko et al.158 studied the thermal and morphological properties of PP obtained using the 16 ±MAO catalyst by the DSC method with temperature fractionation. The PP samples contained 25% and 30% isotactic mmmm pentad; their molecular masses were 240 000 ± 260 000 g mol71. Prelimi- narily, the temperature of crystallisation for the studied samples varied from 160 to 50 8C with a step of 10 8C, and the samples were annealed at the appropriate temperatures for 12 h and then heated in the usual way. The resulting curves (Fig. 14) exhibit multiple peaks (Table 9) and differ substantially from the curves observed with fast scanning; this points to the influence of kinetic factors on the processes of crystallisation. Interesting results have been obtained by treating polymeric materials with hot nitric acid:158 only *10% of the polymer was insoluble (this corre- sponds to the content of perfect insoluble crystals).This method is widely used in the investigation of the structures of various polymeric materials. To characterise the domain structure of elSBPP samples, the AFM method was used.158 Regions consist- ing of ribbon crystals distributed in the amorphous phase were found in the samples. It was also found 158 that the thickness of the lamellae in elSBPP equals 124 nm, which corresponds to 54 monomer units. The elongation at rupture for the material 2 Specific thermal flux /Wg71 1 70.35 70.40 70.45 70.50 70.55 160 120 80 40 0 T /8C Figure 14.Temperature fractionation of DSC curves for two elSBPP samples with contents of isotactic mmmm pentads of 25% (1 ) and 30% (2).158Controlled synthesis of stereoblock polypropylene. New trends in the development of elastomers Table 9. Melting temperatures and heats for elSBPP samples subjected to stepwise annealing.158 [mmmm] (%) DHm /J g71 Tm /8C 46.32 68.71 83.19 100.12 109.11 119.34 129.72 150.07 151.63 1.059 2.242 3.091 1.274 0.938 0.963 1.399 5.520 2.210 25 25 25 25 25 25 25 25 25 P=19.39 a 44.07 68.86 93.45 99.98 109.18 119.38 131.21 145.65 1.883 2.364 2.860 0.896 0.768 0.635 1.644 3.699 30 30 30 30 30 30 30 30 a Total heat of melting. P=14.93 a studied reaches 1000% and the tensile strength was 15 MPa.When deformations do not exceed 300%, the hysteresis is high and no destruction of crystals in the samples is observed. In the case of great deformation, the initial lamellar crystallites are destroyed and small blocks are produced, which are mainly contained in fibrils. These processes are similar in mechanism to those taking place in other segmented elastomers such as aromatic polyesters and polyamides. The nanofibrils formed upon drawing contain alternant structures of rigid crystallites and, possibly, initially amorphous segments crystallising under the action of stress with chains oriented in the direction of drawing for both types of segments.The length of these domains is several hundreds of nanometers and the width is *12 nm. After relief of the stress and relaxation, the initial structure of the sample is not restored. The results obtained in the studies of various samples of elSBPP and their fractions by DSC, DSC with temperature fractionation, by treatment with nitric acid and by AFM point to the cooperative nature of crystallisation processes and to the presence of diffusion and kinetic restrictions. The rates of relaxation processes for compositionally inho- mogeneous elSBPP and separate fractions have been studied by the birefringence method.108 It was shown that the relaxation time and the size of the relaxation plateau depend on the temperature and shear deformation.In the case of 100% deformation, the relaxation time decreases as the temperature rises (2 h at 25 8C, 900 s at 80 8C, and 440 s at 115 8C), and the size of the relaxation plateau changes from 0.62 at 25 8C to 0.43 at 85 8C.107 The effect of temperature on the rate of the relaxation processes for non- fractionated samples and for separate fractions of PP at different shear deformations (3% to 100%) was studied in detail. In the temperature range of 200 ± 105 8C, complete isotropisation occurs rapidly. At 105 8C and at lower temperatures, the rates and the degrees of isotropisation for the initial samples and for separate fractions are markedly different.It is of interest that repeated application of shear strain resulted under some conditions in an increase in the rate and the degree of relaxation. The results of studies 107, 110 indicate that great shear defor- mations (550%) are able to destroy the physical junctions at weak points; this gives smaller crystals. An increase in the deformation results in irreversible anisotropy of non-equilibrium crystals. The fractions insoluble in heptane exhibit properties typical of solid elastomers; they are able to induce co-crystallisa- 67 tion of fractions with lower isotacticity. This results in a higher cross-linking density. It was concluded that the composition and the cooperative role of different fractions determine the elastic properties of materials.Madkour and Mark 111, 160 arrived at analogous conclusions. The effect of the molecular mass,MMDand the length and length distribution of isotactic blocks on the moduli of elasticity of the materials have been studied by mathematical modelling. Model- ling with allowance for co-crystallisation processes gave the dependences of the modulus of elasticity on the content of isotactic sequences similar to those established experimentally. The results of an investigation of the fracture mechanism and examination of the fracture surface of elSBPP by scanning electron microscopy are rather interesting.110 The dependence of the fracture energy on the stereochemical composition was elucidated. When the content of the mmmm pentads decreases from 60% to 40%, the cohesion energy of fracture for the samples studied changes from 8 to 20.6 kJ m72, and the plastic energy, from 53.8 to 9.4 kJ m72.In the examination of the fracture surface, the formation of sinusoidal structures parallel to the sample surface and tto he load axis has been observed for some samples (Fig. 15). Structures of this type have been described only for some thermoplastic elastomers. 50 mm Figure 15. Photomicrograph of the fracture surface of the elSBPP sample.110 Thus, it was shown that the regularities observed for elSBPP are similar to those typical of crystalline elastomeric materials. The structure and the properties of the elastomeric PP, as well as of a number of other elastomers are due to the formation of crystallites from thin lamellae or nanofibrils joined by straight- ened or bent transient chains, which form an amorphous phase in addition to the end chains.The number, volume and the structure of junctions in the physical networks and the degree of ordering of the amorphous phase, the surface free energy of lamellae (end faces), and the values of cohesion and plastic energy determine the sub- and microstructure, the elastomeric properties of these materials, and the patterns of the observed dependences. IV. Conclusion The information considered above shows that the discovery of highly effective homogeneous metallocene and post-metallocene catalyst systems has stimulated work on the synthesis of new materials based on polyolefins. Among these, thermoplastic elastomeric stereoblock PP occupies a special place.The interest in its synthesis is great because these polymers can be used as adhesives, binders or elastic films. Numerous catalyst systems and their mixtures have been developed, which allow targeted control of the microstructure and molecular-mass characteristics of elastomeric stereoblock polymers. The mechanism of catalytic68 and stereospecific action of these catalysts has been studied. Data on the influence of the microstructure on the mechanical, thermal and relaxation properties of elastomers have been obtained. This review surveys the state-of-the-art of the research along this line. The necessity of this generalisation is dictated by the vigorous development of the work dealing with the design of new polymeric and structural materials.This review was financially supported by the Russian Foun- dation for Basic Research (Projects No. 99-03-32948a and No. 01- 03-97002�regional, Moscow region). References 1. L L Bohm, J Berthold, H-F Enderle, M Fleissner, in Metalorganic Catalysts for Synthesis and Polymerisation: Recent Results by Zie- gler ± Natta and Metallocene Investigations (Ed.W Kaminsky) (Ber- lin: Springer, 1999) p. 3 2. P Galli, G Ceccin, J C Chadwick, D Del Duca, G Vecellio, in Met- alorganic Catalysts for Synthesis and Polymerisation: Recent Results by Ziegler ± Natta and Metallocene Investigations (Ed. W Kaminsky) (Berlin: Springer, 1999) p. 14 3. P Galli, G Vecellio, in Organometallic Catalysts and Olefin Polymerization.Catalysts for a New Millennium (Eds R Blom, A Follestad, E Rytter,M Tilset,M Ystenes) (Berlin: Springer, 2001) p. 169 4. Polypropylene. Past, Present and Future: The Challenge Continues (Ferrara: Montell, 1998) 5. D A Walker, J D McCullough, F D Hussein ANTEC'89 851 1989 6. G Penzo, in Polypropylene. Past, Present and Future: The Challenge Continues (Ferrara: Montell, 1998) p. 53 7. G D Bukatov, V S Goncharov, V A Zakharov Macromol. Chem. Phys. 196 1751 (1995) 8. U Zucchini, T Dell'Occo, L Resconi Indian J. Technol. 31 247 (1993) 9. A L Rice, D R Parikh, J J Gathers, in Proceedings of MetCon'99 `Polymer in Transition', Houston, TX, 1999 10. E Albizzati, U Giannini, G Morini, M Galimberti, L Barino, R Scordamaglia Macromol.Symp. 89 73 (1995) 11. V A Zakharov, L G Echevskaya, G D Bukatov Polimery 34 277 (1989) 12. G Fink, R MuÈ lhaupt, H H Brintzinger (Eds) Ziegler Catalysts. Recent Innovaons and Technological Improvements (Berlin: Springer, 1995) 13. H-H Brintzinger, D Fischer, R MuÈ lhaupt, B Rieger, R Waymouth Angew. Chem., Int. Ed. Engl. 34 1143 (1995) 14. K Mashima, Y Nakayama, A Nakamura Adv. Polym. Sci. 133 1 (1997) 15. W Kaminsky J. Chem. Soc., Dalton Trans. 9 1413 (1998) 16. J. Mol. Catal. A, Chem. 128 (Spec. Issue) (1998) 17. W Kaminsky (Ed.) Metalorganic Catalysts for Synthesis and Poly- merisation: Recent Results by Ziegler ± Natta and Metallocene Inves- tigations (Berlin: Springer, 1999) 18.Top. Catal. 7 (Spec. Issue) (1999) 19. Chem. Rev. 100 (Spec. Issue) (2000) 20. R Blom, A Follestad, E Rytter,M Tilset, M Ystenes (Eds) Organometallic Catalysts and Olefin Polymerization. Catalysts for a New Millennium (Berlin: Springer, 2001) 21. W Kaminsky, M Arndt Adv. Polym. Sci. 127 143 (1997) 22. B A Krenstel, Y V Kissin, V I Kleiner, L L Stotskaya Polymers and Copolymers of Higher Olefins (Munich; Vienna; New York: Carl Hanser Verlag, 1997) 23. V T Ponomareva, N N Likhacheva Plast. Massy 4 8 (2001) 24. V I Tsvetkova Vysokomol. Soedin., Ser. C 42 1954 (2000) a 25. US P. 6 184 327 (2001) 26. US P. 6 265 512 (2001) 27. F S Dyachkovskii, V I Tsvetkova Kinet. Katal. 35 534 (1994) b 28. F S Dyachkovskii, A K Shilova, A E Shilov J.Polym. Sci., Part C, Polym. Symp. 16 2333 (1967) 29. Y V Kissin Isospecific Polymerization of Olefins (New York: Springer, 1985) 30. E I Vizen, L A Rishina, L N Sosnovskaya Kinet. Katal. 35 743 (1994) b NMBravaya, PMNedorezova, V I Tsvetkova 31. A Zambelli P Lokatelli, M C Sacchi, E Rigamonti Macromolecules 13 798 (1980) 32. M C Sacchi, E Barsties, I Tritto, P Locatelli, H-H Brintzinger, U Stehling Macromolecules 30 1267 (1997) 33. L Cavallo, G Guerra P Corradini J. Am. Chem. Soc. 120 2428 (1998) 34. F A Bovey, G V D Tiers J. Polym. Sci. 44 173 (1960) 35. F A Bovey Chain Structure and Conformation of Macromolecules Ch. 3 (New York: Academic Press, 1982) 36. A Zambelli, P Locatelli, A Provasoli, D R Ferro Macromolecules 13 267 (1980) 37.J A Ewen J. Am. Chem. Soc. 106 6355 (1984) 38. V Busico, R Cipullo, G Monaco,M Vacatello, A L Segre Macromolecules 30 6251 (1997) 39. I Pasquon Pure Appl. Chem. 15 465 (1967) 40. W Kaminsky, K Kulper, H Brintzinger, F Wild Angew. Chem., Int. Ed. Engl. 24 507 (1985) 41. J A Ewen, R L Jones, A J Razavi J. Am. Chem. Soc. 110 6255 (1988) 42. J A Ewen J. Mol. Catal. A., Chem. 128 103 (1998) 43. G Natta, G Mazzanti, G Crespi, G Moraglio Chim. Ind. (Milan) 39 275 (1957) 44. US P. 3 175 999 (1965) 45. US P. 4 335 225 (1982) 46. J W Collette, C W Tullock, R N MacDonald,W H Buck, A C L Su, J R Harell, R Mulhaupt, B C Anderson Macromolecules 22 3851 (1989) 47. J W Collette, D W Overnall,W H Buck, R C Ferguson Macromolecules 22 3858 (1989) 48.US P. 5 270 410 (1993) 49. V I Tsvetkova, D M Lisitsyn, B A Uvarov, L A Novokshonova, L A Rishina Stereospetsificheskaya Polimerizatsiya Propilena (Preprint) [Stereospecific Polymerisation of Propylene (Preprint)] (Chernogolovka: Institute of Chemical Physics in Chernogolovka, Russian Academy of Sciences, 1983) 50. USSR P. 682 262; Byull. Izobret. (32) 27 (1979) 51. D T Mallin, M D Raush, J C W Chien J. Am. Chem. Soc. 112 2030 (1990) 52. J C W Chien, in Ziegler Catalyst (Eds G Fink, R MuÈ lhaupt, H H Brintzinger) (Berlin: Springer, 1995) p. 200 53. H N Cheng, G N Babu, R A Newmark, J C W Chien Macromolecules 25 6980 (1992) 54. G N Babu, H N Cheng, G H Llinas, J C W Chien Macromolecules 25 7400 (1992) 55. US P. 575 6614 (1998) 56.W J Gauthier, S Collins Macromol. Symp. 98 223 (1995) 57. W J Gauthier, S Collins Macromolecules 28 3771 (1995) 58. W J Gauthier, S Collins Macromolecules 28 3779 (1995) 59. U Dietrich, M Hackmann, B Rieger, M Klinga, M LeskelaÈ J. Am. Chem. Soc. 121 4348 (1999) 60. B D Coleman, T G Fox J. Chem. Phys. 38 1065 (1963) 61. J Kukral, P Lehmus, T Feifel, C Troll, B Rieger Organometallics 19 3767 (2000) 62. M K Leclerc, H H Brintzinger J. Am. Chem. Soc. 118 9024 (1996) 63. V Busico, L Caporaso, R Cipullo, L Landriani, G Angelini, A Margonelli, A L Segre J. Am. Chem. Soc. 18 2105 (1996) 64. V Busico, R Cipullo, L Caporaso, G Angelini, A L Segre J. Mol. Catal. A, Chem. 128 53 (1998) 65. L Resconi J. Mol. Catal. A, Chem. 146 167 (1999) 66.L Resconi, L Cavallo, A Fait, F Piemontesi Chem. Rev. 100 1253 (2000) 67. G W Coates, R M Waymouth Science 267 217 (1995) 68. E Hauptmann, R M Waymouth, J W Ziller J. Am. Chem. Soc. 117 11586 (1995) 69. US P. 5 594 080 (1997) 70. V I Tsvetkova, P M Nedorezova, N M Bravaya, D V Savinov, I L Dubnikova, V A Optov Vysokomol. Soedin., Ser. A 39 389 (1997) a 71. R Kravchenko, A Masood, R M Waymouth, C L Myers J. Am. Chem. Soc. 120 2039 (1998) 72. P M Nedorezova, V I Tsvetkova, N M Bravaya, D V Savinov, V A Optov Vysokomol. Soedin., Ser. A 42 901 (2000) a 73. M D Bruce, G W Coates, E Hauptmann, R M Waymouth, J W Ziller J. Am. Chem. Soc. 119 11174 (1997)Controlled synthesis of stereoblock polypropylene. New trends in the development of elastomers 74.O N Babkina, T A Bazhenova, N M Bravaya, V V Strelets, M Yu Antipin, K A Lysenko Izv. Akad. Nauk, Ser. Khim. 1529 (1996) c 75. T A Bazhenova, M Yu Antipin, O N Babkina, N M Bravaya, K A Lysenko, V V Strelets Izv. Akad. Nauk, Ser. Khim. 2161 (1997) c 76. P Witte, T K Lal, R M Waymouth Organometallics 18 4147 (1999) 77. J L M Petoff,M D Bruce, R M Waymouth, A Masood, T K Lal, R W Quan, S J Behrend Organometallics 16 5909 (1997) 78. S Lin, E Hauptman, T K Lal, R M Waymouth, R W Quan, A B Ernst J. Mol. Catal. A, Chem. 136 23 (1998) 79. R Kravchenko, R M Waymouth Macromolecules 31 1 (1998) 80. S Lin, C D Tagge, R M Waymouth,M Nele, S Collins, J C Pinto J. Am. Chem. Soc. 122 11275 (2000) 81. G Y Lee,M Xue,M S Kang, O C Kwon, J-S Yoon, Y-S Lee, H S Kim, H Lee, I-M Lee J.Organomet. Chem. 558 11 (1998) 82. Y Hu,M T Krejchi, C D Shan, C L Myers, R M Waymouth Macromolecules 31 6908 (1998) 83. M Kakugo, T Miyatake, Y Naito, K Mizunuma Macromolecules 21 314 (1988) 84. R Kravchenko, A Masood, R M Waymouth Organometallics 16 3635 (1997) 85. C D Tagge, R L Kravchenko, T K Lal, R M Waymouth Organometallics 18 380 (1999) 86. S Lin, R Kravchenko, R M Waymouth J. Mol. Catal. A, Chem. 158 423 (2000) 87. S Lin, R M Waymouth Macromolecules 32 8283 (1999) 88. J Petoff, T Ageston, T Lai, R Waymouth J. Am. Chem. Soc. 120 11316 (1998) 89. M D Bruce, R M Waymouth Macromolecules 31 2707 (1998) 90. M Nele, S Collins, M L Dias, J C Pinto, S Lin, R M Waymouth Macromolecules 33 7249 (2000) 91.V Busico, D Brita, L Caporaso, R Cipullo, M Vacatello Macromolecules 30 3971 (1997) 92. L Cavallo, G Guerra P Corradini Gaz. Chim. Ital. 126 463 (1996) 93. M A Pietch, A K Rappe' J. Am. Chem. Soc. 118 10908 (1996) 94. N Schneider, F Schaper, K Schmidt, R Kirsten, A Geyer, H H Brintzinger Organometallics 19 3597 (2000) 95. J C Randall Polymer Sequence Determination: Carbon 13 NMR Method (New York: Academic Press, 1977) 96. J-S Yoon, Y-S Lee, E-S Park, I-M Lee, D-K Park, S-O Jung Eur. Polym. J. 36 1271 (2000) 97. T Dreier, G Erker, R FroÈ hlich, B Wibbeling Organometallics 19 4095 (2000) 98. A N Panin, Z M Dzhabieva P M Nedorezova, V I Tsvetkova, S L Saratovskikh, O N Babkina, N M Bravaya J. Polym. Sci., Part A, Polym.Chem. 39 1915 (2001) 99. M Shmulinson, M Galan-Fereres, A Lisovskii, E Nelkenbaum, R Semiat,M S Eisen Organometallics 19 1208 (2000) 100. N Naga, K Mizunuma Polymer 2703 (1998) 101. D Baldoni, G Moscardi, G Baruzzi, V Braga, I Camurati, F Piemontesi, L Resconi, I E Nifant'ev, V Venditto, S Antinucci Macromol. Chem. Phys. 202 1780 (2001) 102. E J Thomas, J C W Chein, M D Rausch Macromolecules 33 1546 (2000) 103. J L M Petoff, C L Myers, R M Waymouth Macromolecules 32 7984 (1999) 104. V A Zakharov, E P Talzi, I I Zakharov Kinet. Katal. 40 926 (1999) b 105. D E Babushkin, Candidate Thesis in Chemical Sciences, Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, Novosibirsk, 1999 106. E D Carlson,M T Krejchi, C D Shan, T Terekawa, R M Waymouth, G G Fuller Macromolecules 31 5343 (1998) 107. Y Hu, E D Carlson, G G Fuller, R M Waymouth Macromolecules 32 3334 (1999) 108. E D Carlson, G G Fuller, R M Waymouth Macromolecules 32 8094 (1999) 109. E D Carlson, G G Fuller, R M Waymouth Macromolecules 32 8100 (1999) 110. D E Mouzakis,M Gahleitner, J Karger-Kocsis J. Appl. Polym. Sci. 70 873 (1998) 111. T M Madkour, J E Mark Polymer 39 6085 (1998) 69 112. Yu K Godovskii, in Trudy 4-i Sessii `Inzhenerno-Khimicheskaya Nauka dlya Peredovykh Tekhnologii', Moskva, 1998 (Proceedings of the Fourth Session `Engineering Chemical Science for Advanced Technologies', Moscow, 1998 p. 115 113. O N Babkina, N M Bravaya, P M Nedorezova, S L Saratov- skikh, V I Tsvetkova Kinet. Katal. 43 (2002) (in the press) b 114. J C W Chien, Y Iwamoto,M D Rausch,W Wedler, H H Winter Macromolecules 30 3447 (1997) 115. J C W Chien, Y Iwamoto,M D Rausch J. Polym. Sci., Part A, Polym. Chem. 37 2439 (1999) 116. S Lieber, H-H Brintzinger Macromolecules 33 9192 (2000) 117. C Przybyla, G Fink Acta Polym. 50 77 (1999) 118. S Beck, H H Brintzinger Inorg. Chim. Acta 270 376 (1998) 119. I Tritto, D Zucchi, M Destro,M C Sacchi, T Dall'Occo, M Galimberti J. Mol. Catal. A, Chem. 160 107 (2000) 120. O M Khukanova, O N Babkina, L A Rishina P MNedorezova, N M Bravaya Polimery 45 328 (2000) 121. P M Nedorezova, V I Tsvetkova, A M Aladyshev, D V Savinov, I L Dubnikova, V A Optov, D A Lemenovskii Polimery 45 333 (2000) 122. P M Nedorezova, V I Tsvetkova, A M Aladyshev, D V Savinov, A N Klyamkina, V A Optov, D A Lemenovskii Vysokomol. Soedin., Ser. A 43 605 (2001) a 123. N M Bravaya, A N Panin, O N Babkina, N D Karpova Polimery 46 397 (2001) 124. A N Panin, T A Sukhova, N M Bravaya J. Polym. Sci., Part A, Polym. Chem. 39 1901 (2001) 125. A N Panin, I L Dubnikova, N M Bravaya Izv. Akad. Nauk, Ser. Khim. 1189 (2000) c 126. G G Hlatky Chem. Rev. 100 1347 (2000) 127. R Kleinschmidt, Y Leek, M Reffke, G Fink J. Mol. Catal. A, Chem. 148 29 (1999) 128. G Luft, R A Dyroff, C Gltz, S Schmitz, T Wieczorek, R Klimesch, A Gonioukh, in Metalorganic Catalysts for Synthesis and Polymerisation: Recent Results by Ziegler ± Natta and Metal- locene Investigations (Ed. W Kaminsky) (Berlin: Springer, 1999) p. 651 129. I Kim, J M Choi J. Polym. Sci., Part A, Polym. Chem. 37 1523 (1999) 130. I Kim, J M Zhou J. Polym. Sci., Part A, Polym. Chem. 37 2093 (1999) 131. M Bochmann, S Lancaster Angew. Chem., Int. Ed. Engl. 33 1634 (1994) 132. K Vanka, T Ziegler Organometallics 20 905 (2001) 133. S Beck, S Lieber, F Schaper, A Geyer, H-H Brintzinger J. Am. Chem. Soc. 123 1483 (2001) 134. D Harrison, I M Coulter, S Wang, S Nistala, B A Kuntz, M Pigeon, J Tian, S Collins J. Mol. Catal. A, Chem. 128 65 (1998) 135. L K Johnson, C M Killian,M S Brookhart J. Am. Chem. Soc. 117 6414 (1995) 136. C M Killian, D J Tempel, L K Johnson,M Brookhart J. Am. Chem. Soc. 118 11664 (1996) 137. G J P Britovsek, V C Gibson,D F Wass Angew. Chem., Int. Ed. 38 428 (1999) 138. S D Ittel, L K Johnson,M Brookhart Chem. Rev. 100 1169 (2000) 139. C Pellecchia, A Zambelli Macromol. Rapid Commun. 17 333 (1996) 140. C Pellecchia, A Zambelli, L Oliva, D Pappalardo Macromolecules 29 6990 (1996) 141. C Pellecchia, A Zambelli,M Mazzeo, D Pappalardo J. Mol. Catal. A, Chem. 128 229 (1998) 142. B L Small,M Brookhart Macromolecules 32 2120 (1999) 143. E F McCord, S J McLain, L T J Nelson, S D Arthur, E B Coughlin, S D Ittel, L K Johnson, D Tempel, C M Killian, 144. T Fujita,M Mitani,Y Tohi,H Makio,N Kihara, in Proceedings of 145. C Wang, S Friedrich, T R Younkin, R T Li, R H Grubbs, 146. T R Younkin, E F Connor, J I Henderson, S K Friedrich, 147. O Kuhl, T Koch, F B Somoza P C Junk, E Hey-Hawkins, D Plat, 148. B Ray, T G Neyroud,M Kapon, Y Eichen, M S Eisen M Brookhart Macromolecules 34 362 (2001) MetCon'99 `Polymer in Transition',. Houston, TX, 1999 D A Bansleben, M W Day Organometallics 17 3149 (1998) R H Grubbs, D A Bansleben Science 287 460 (2000) M S Eisen J. Organomet. Chem. 604 116 (2000) Organometallics 20 3044 (2001)NMBravaya, PMNedorezova, V I Tsvetkova 70 149. M Bochmann Top. Catal. 7 9 (1999) 150. G Erker Acc. Chem. Res. 34 309 (2001) 151. B Hessen, H van der Heijden J. Organomet. Chem. 534 237 (1997) 152. L Resconi, I Camurati, O Sudmeijer Top. Catal. 7 145 (1999) 153. G Moscardi, L Resconi, L Cavollo Organometallics 20 1918 (2001) 154. V K Dioumaev, J F Harrod Organometallics 16 1452 (1997) 155. G Erker,K Berg,K Angermund, C KruÈ ger Organometallics 6 2620 (1987) 156. M B Abrams, J C Yoder, C Loeber,M W Day, J E Bercaw Organometallics 18 1389 (1999) 157. J Karl,M Dahlmann, G Erker, K Bergander J. Am. Chem. Soc. 120 5643 (1998) 158. R L Kravchenko, B B Sauer, R Scott McLean,M Y Keating, P M Cotts, Y H Kim Macromolecules 33 11 (2000) 159. E E Al'yanova, N M Bravaya, T I Ponomareva, V I Tsvetkova, P M Nedorezova, V I Irzhak Vysokomol. Soedin., Ser. B 40 1691 (1998) a 160. T M Madkour, J E Mark Polym. Bull. 39 385 (1997) a�Polym. Sci. (Engl. Transl.) b�Kinet. Catal. (Engl. Transl.) c�Russ. Chem. Bull., Int. Ed. (Engl. Tra

ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC

摘要:

Russian Chemical Reviews 71 (1) 71 ± 83 (2002) Peptide nucleic acids: structure, properties, applications, strategies and practice of chemical synthesis S I Antsypovitch Contents I. Introduction II. Properties of peptide nucleic acids III. Applications of peptide nucleic acids IV. Basic principles of chemical synthesis of peptide nucleic acids V. Factors determining the efficiency of condensation in the synthesis of peptide nucleic acids VI. The main strategies of peptide nucleic acid synthesis VII. Some regularities of condensation reactions in the synthesis of peptide nucleic acids VIII. Conclusion Abstract. of properties and structure the on information The The information on the structure and properties of peptide nucleic acids (PNA) is generalised. The use of PNA peptide nucleic acids (PNA) is generalised.The use of PNA oligomers is biotechnology and studies biomolecular in oligomers in biomolecular studies and biotechnology is exempli- exempli- fied. for methods important most the on data published The fied. The published data on the most important methods for the the chemical emphasis main the with oligomers PNA of synthesis chemical synthesis of PNA oligomers with the main emphasis on on the considered. are reactions condensation of efficiency the efficiency of condensation reactions are considered. The The methods their systematised; are synthesis PNA for methods for PNA synthesis are systematised; their advantages advantages and disadvantages are discussed. Some recommendations for and disadvantages are discussed.Some recommendations for optimisation of the condensation procedure and synthesis of optimisation of the condensation procedure and synthesis of PNA are presented. The bibliography includes 153 references PNA are presented. The bibliography includes 153 references. I. Introduction Peptide nucleic acids (PNA, 1) represent analogues of nucleic acids (NA, 2),1±4 but, in contrast to the latter, contain neither carbohydrate nor phosphate residues and have uncharged pseu- dopeptide backbones.1, 5 ± 8 The monomeric unit of classical PNA comprises N-(2-aminoethyl)glycine and a heterocyclic base (purine or pyrimidine) bound through an acetyl linker. The monomers are linked by amide bonds. The geometry of the achiral backbone and its relative flexibility 3, 9 confer onPNA an ability to mimic, with striking exactness, the spatial structure of carbohy- drate ± phosphate backbones of NA.4 From the chemical standpoint, PNA represent a hybrid of an oligonucleotide wherefrom the nucleases have been adopted and a peptide, the structure of which gave birth to the structural principle of the PNA backbone.Thus, PNA possess properties of both these classes of compounds.8, 10 This structural-and-func- tional duality of PNA determines their unique property.4 Indeed, these molecules combine uniquely the strict recognising ability inherent in NA with the flexibility and stability of proteins. It should be noted that the term `peptide nucleic acids' is used to point to the structural similarity of these compounds toNAand to reflect the similarity of the PNA oligomeric backbone to that of S I Antsypovitch Department of Chemistry, M V Lomonosov Moscow State University, Leninskie Gory, 119992 Moscow, Russian Federation. Fax (7-095) 939 31 81.Tel. (7-095) 939 31 48. E-mail: antsypov@genebee.msu.su Received 11 July 2001 Uspekhi Khimii 71 (1) 81 ± 96 (2002); translated by R L Birnova #2002 Russian Academy of Sciences and Turpion Ltd H2N B N O O HN 50 10 40 B 20 N30 O n O HN B N O O H2N 1 B=Ade, Gua, Thy, Cyt. peptides, although neither the term `acid' nor `peptide' are applicable to PNA, for in contrast to nucleic acids PNA are not polymeric acids and in contrast to peptides they do not contain amino acids.Nevertheless, the abbreviation `PNA' has now come into general use, although it would be more correct to refer to these compounds as polyamide analogues of oligonucleotides.10 II. Properties of peptide nucleic acids Complementary PNA molecules form specific antiparallel PNA±PNA duplexes having helical structures 4, 11 similar to those of DNA and RNA duplexes but, which is even more important, they form highly stable specific (antiparallel and parallel) duplexes with complementary DNA and RNA sequen- ces 1, 5±7, 9, 12 ±14 containing Watson ± Crick base pairs.2, 15, 16 In all cases, antiparallel PNA±DNA duplexes are more stable than the parallel ones, viz., their melting temperatures differ by*1 8C for each base pair.2 The circular dichroism spectra of PNA±DNA and DNA±DNA duplexes are similar,2, 17 which points to the formation of right-hand helices during the formation of DOI 10.1070/RC2002v071n01ABEH000691 71 71 72 73 73 74 75 81 HO O B O O O7 PO 50 O B 10 40 20 30 O n OP O7 O O B OP O7 O OH 272 PNA±DNA duplexes despite the fact that base pairs in PNA±DNA and DNA±DNA duplexes have slightly different geometries.17 NMR and X-ray crystallography studies of PNA±PNA and PNA±DNA complexes revealed that PNA±PNA duplexes possess broad, deep major grooves and narrow, shallow minor grooves; noteworthy, one complete turn of a helix in PNA±PNA duplexes corresponds to 18 base pairs, whereas that in PNA±DNA duplexes, to 13 base pairs.17 ± 20 PNA± PNA, PNA±DNA and PNA±RNA duplexes are considerably more stable than DNA± DNA, DNA±RNA and RNA±RNA duplexes of the same compositions.1, 2, 11 ± 17, 21 ± 23 Even four-membered PNA sequences produce highly stable duplexes with complementary DNAs.17 In contrast to NA±NA duplexes, the stabilities ofPNA±NAduplexes are little dependent on the solution ionic strength.2, 24 ± 27 PNA±DNA duplexes are formed faster than the corresponding DNA±DNA duplexes,14, 22 but high specificity of hybridisation is preserved.28 The stabilities ofPNA±DNAduplexes can be predicted based on a model which takes into account the interactions between only the nearest adjacent bases;17, 29, 30 however, this model describes adequately the stabilities of short duplexes comprising no more than eight units.17 The most essential property of PNA is their unique sensitivity to mismatches in the structure of NA targets.7 The difference in melting temperatures of a perfect PNA±DNA complex and a duplex containing one mismatch amounts to 20 8C and even more.3 Peptide nucleic acids form PNA± (DNA)2 triplexes with double-stranded DNA.31 ± 34 These triplexes are less stable than the classical (DNA)3 triplexes.34 On the other hand, PNA forms stable (PNA)2 ±DNA and (PNA)2 ±RNA triplexes with single- stranded DNA and RNA targets, respectively.5, 12, 14, 16, 33, 35 ± 39 Such triplexes are often formed in the interaction of PNA with double-stranded NA.37, 40 ± 44 In the latter case, the formation of triplexes leads to the displacement of one of the DNA strands resulting in complete separation or P-loop formation 38 with subsequent incorporation of PNA chains.5, 33, 37, 40 ± 44 PNA form Watson ± Crick pairs with DNA.The attachment of the second PNA chain is accompanied by the formation of Hoogsteen pairs.35, 39 It was found that Watson ± Crick chains of PNA are antiparallel to DNA chains, while Hoogsteen strands are parallel to them.37, 39 It is noteworthy that Hoogsteen strands stabilise Watson ± Crick PNA±DNA duplexes which are formed first; the latter can be relatively unstable.38 Homopyrimidine PNA molecules and PNA enriched with pyrimidine residues are especially prone to triplex forma- tion.9, 35 ± 39 The respective triplexes are extremely stable, e.g., ten-membered (PNA)2 ±DNA triplexes have the melting temper- atures of *70 8C.9, 39 The ability of cytosine-containing PNA to form triplexes is pH-dependent, since cytosine can form Hoogs- teen pairs with guanine residues only in the protonated state.9 No triplexes with the composition (PNA)3 have been found.45 Recently, a new type of PNA-containing triplexes has been discovered.37 Both PNA chains in the (PNA)2 ±DNA triplex formed upon binding of the PNA oligomer 50-T4G2(TG)2-30 to the oligonucleotide 50-A4C2(AC)2-30 are in the antiparallel orien- tation relative to the DNA strand.The binding potential of PNA with respect to NA is far from being exhausted. Studies of NA-binding properties of PNA aimed at the synthesis of novel PNA possessing improved structures and able to enhance the specific binding of PNA to complementary NA are currently under way.37 The melting of short-chain (PNA)2 ±DNA triplexes is a non- equilibrium process, viz., the melting temperature depends on both the concentrations of components and heating velocity.38 Thus, the stabilities of (PNA)2 ±DNAcomplexes were found to be kinetic.38 The dissociation of such triplexes occurs in two steps, viz., the first (limiting) step includes separation of the Hoogsteen S I Antsypovitch strand of PNA, while the second step includes rapid separation of the Watson ± Crick strands of PNA.38 Peptide nucleic acids manifest high chemical and biological stabilities.25, 46 They are highly resistant against cell nucleases, proteases and peptidases.25, 27, 47 PNA oligomers undergo very slow enzymatic hydrolysis in both cell extracts and in vivo.25, 46 PNA molecules are distinguished by generally low toxicity and are not prone to non-specific binding to cellular proteins.Being immobilised on solid supports, PNA molecules preserve their hybridisation properties.28 III. Applications of peptide nucleic acids It is known that modifications of NA backbones by replacing phosphodiester or carbohydrate units by non-charged or cationic structures may confer useful properties on NA, e.g., enhanced resistance against nucleases and effective penetration through cell membranes and more specific and stronger binding to comple- mentary target NA.48 Attempts are being made to improve the PNA structure in order to increase the ability of PNA for nonspecific binding to NA and their transport across cellular membranes.2, 3, 8 ± 10, 36 Thus the addition of certain peptides,49 ± 53 e.g., the 16-membered peptide `transportan',50, 51 to PNA increases considerably the rate of intracellular transport of PNA.9, 23, 49 ± 53 a-Helical PNA (aPNA) have been obtained in which the role of the backbones is played by a-helical peptide structures.54 ± 56 Such PNA analogues form highly specific stable Watson ± Crick duplexes with complementary NA54, 55 and man- ifest very high biological stabilities.56 PNA analogues with chiral backbones with positively and negatively charged groups, PNA±DNA chimeras, etc., have been synthesised.36 Comparative studies of structurally different PNA analogues have shown that classical PNA with N-(2-aminoethyl)glycine backbones first synthesised by Peter E Nielsen ten years ago 1 manifest optimum NA-binding properties.4 Therefore, the main attention in this review will be given to methods of synthesis of classical PNA molecules based on N-(2-aminoethyl)glycine resi- dues.1 By virtue of their unique properties,13, 37 PNA have found wide use in molecular-biological, biochemical, genetic engineering and clinical investigations.2, 3, 14, 26, 27, 36, 57 ± 65 They represent attrac- tive candidates for new-generation genetic therapeutic drugs which interfere selectively with gene expression.9, 14, 26, 36, 57, 62 ± 71 The use of PNA oligomers in antisense 9, 26, 27, 36, 57, 67, 69, 72 ± 74 and antigen 27, 36, 57, 67, 68, 72, 75, 76 biotechnologies is of considerable promise.Antisense PNA selectively inhibit the expression of brain proteins.47 The design of anticancer and antiviral drugs based on PNA seems to be a very promising approach.26, 57, 65 Some PNA derivatives manifest antibacterial 36 and antisense activities 9, 47 towards eukaryotic cells and animal organisms.9, 47 There is evidence that PNA interfere with all the key stages of gene expression.9, 46, 57, 77 Used in nanomolar concentrations, PNA cause a practically complete specific arrest of transcription ofDNAtemplates.46, 78 The usefulness of PNA oligomers in gene- oriented technologies has been demonstrated with a PNA- dependent arrest of transcription elongation of RNA polymerase as an example.27, 46, 67, 68, 72 By virtue of their ability to induce effective blocking of transcription, PNA oligomers represent potential inhibitors of cell growth, which makes them a useful tool in the design of antitumour drugs.78 PNA duplexes and especially triplexes with mRNAs, e.g., PNA±RNA and (PNA)2 ± RNA, effectively inhibit the trans- lation of mRNA.67, 73, 74 Their potent antisense effects in vitro are due to high specificities and stabilities of triplex (PNA)2 ±RNA.38 The arrest of translation elongation of mRNA occurs even upon addition of six-membered complementary homopyrimidine PNA.73 The antisense efficiencies of duplex-forming PNA are lower than those of triplex-forming ones; in this case, no less than 20-Peptide nucleic acids: structure, properties, applications, strategies and practice of chemical synthesis membered PNA are required for the inhibition of translation elongation of mRNA (however, the translation initiation can be arrested due to formation of even short-chain PNA±RNA duplexes).73, 74 Nevertheless, RNA molecules in hybrid PNA±RNA duplexes are not cleaved by RNase H.67, 73, 79 ± 81 The antisense effect of duplex-forming PNA is largely due to steric hindrances upon formation of stable PNA±RNA complexes 9 which hinder the translation of mRNA.PNA-induced degrada- tion of mRNA, which is unrelated to the effect of RNase H, should not be ruled out either.9 Duplex-forming PNA can inhibit translation in vitro, being specifically directed against the binding sites of ribosomes, whereas triplex-forming PNA are more specific against polypur- ine sites located `below' the translation initiation point.9 Peptide nucleic acids are used for mapping of RNA molecules in molecular biological studies, particularly for detection of RNA domains responsible for binding to other RNAs and peptides.80 Their applications open up new possibilities for elaboration of novel approaches to the study ofRNA±RNAandRNA± protein interactions and such processes involving non-translatable RNA molecules as splicing.There is evidence that PNA molecules behave as effective `traps' for some DNA-binding proteins.81 PNA±DNA chimeras are convenient primers for DNA polymer- ases.82 In recent years, PNA have extensively been used as biomolecular tools for the studies of various intracellular proc- esses.2, 3, 8, 10, 27, 57, 61 ± 64, 80 ± 82 The use of PNA in the design of efficient procedures for the detection of hybridisation, which are extremely sensitive to mis- matches in NA targets, is a promising approach.28, 83 Fluores- cently labelled PNA are used as diagnostic probes for detecting specific NA sequences and for the study of penetration of PNA oligomers through cellular membranes and their intracellular distribution.84 ± 87 The use of PNA in combination with ion- exchange HPLC (the detection limit is 150 pmol),88 MALDI TOF mass spectrometry,89 capillary electrophoresis 90 and other advanced analytical techniques 88 allows reliable identification of specific genetic sequences in various test samples.The use of PNA in the design of electrochemical biosen- sors 28, 91 ± 93 opens up new possibilities for fast screening of primary NA structures and helps overcome many problems of modern biotechnology.2 ± 4, 8, 10, 61 These compounds can be used as an outstanding basis for the construction of new generations of highly efficient diagnostic tools, e.g., biochips.83 In this context, the development and optimisation of versatile techniques for the synthesis of PNA oligomers are becoming currently central tasks. The condensation of PNA monomers and oligomers with formation of amide bonds induced by various activating reagents is the key step in PNA synthesis.The combi- nation of protective groups, deprotection and capping conditions as well as post-synthetic work-up of synthetic PNA oligomers strongly depend on the condensation method used. IV. Basic principles of chemical synthesis of peptide nucleic acids The chemical synthesis of PNA molecules consists essentially in the oligomerisation of the monomers 3 ± 6 comprising N-(2- aminoethyl)glycine backbones and acetic acid residues (acetyl linkers), each containing one of four nucleobases as a substitu- ent.14 At present, a broad range of methods for the chemical O NH2 Me N HN N N O N N O O COOH COOH N N H2N H2N 4 3 73 NH2 O N N HN N O N N H2N O O COOH COOH N N H2N H2N 6 5 synthesis of PNA are available; the most efficient of them have become especially popular in the past decade.14, 94 ± 97 First of all, a solid-phase methodology is applied for the synthesis of PNA.The synthesis of oligomeric molecules on the surface of polymeric supports was first developed by Merrifield for the synthesis of peptides and proteins in 1962. The strikingly simple idea to immobilise growing oligomeric chains on solid supports has brought biooligomer synthesis to a qualitatively new level. At present, the principle proposed by Merrifield 98 is widely used in the synthesis of peptides and proteins as well as of DNA and RNA fragments (oligonucleotides). In the overwhelming majority of cases, PNA oligomers are also synthesised on solid polymeric supports.In this review, the main emphasis will be laid on the problems related to the efficiency of solid-phase synthesis of PNA. Although PNA oligomers can be synthesised by classical methods commonly employed in peptide synthesis,14 specially designed condensation procedures should be preferred, taking into account peculiarities of chemical structures of PNA mono- mers. In fact, of the different methods for the activation of carboxy groups based on the use of activated esters, symmetrical anhydrides, acid halides and in situ activating reagents, the in situ activation has become the most promising approach, which is currently especially popular. This method envisages the use of reagents based on uronium and phosphonium salts which effect fast (within several seconds) activation of carboxy groups of PNA monomers.Owing to high activation rates, mixing of PNA monomers with an activating reagent can be performed directly in a column with a polymeric support to which the growing oligomeric chain is attached (it is this procedure that represents in situ activation) or immediately before the addition of the monomer to the reaction column. This makes it possible to conduct PNA synthesis in an automated regime. The solid-phase procedure for PNA synthesis involving in situ activation will be considered below; its advantages have been corroborated by chemical practice. The data on the efficiencies of other activation techniques can also be useful and proper consid- eration will be given to them.V. Factors determining the efficiency of condensation in the synthesis of peptide nucleic acids The yields of condensation products in the synthesis of PNA using the in situ activation procedures depend critically on a number of factors. The most essential of them are as follows: � the nature and concentration of the activating reagent; � the nature and concentration of the PNA monomer; � the nature of a nucleobase component of the PNA mono- mer and the nature of the nucleobase incorporated into the PNA oligomer in the preceding step; � the nature and the concentration of a base (as a rule, tertiary amines); � the nature of the solvent(s); � the presence or absence of catalysts, e.g., 1-hydroxybenzo- triazole; �the experimental procedure (e.g., preactivation of the PNA monomer or mixing of the PNA monomer with the condensation reagent in situ);74 �condensation conditions (reaction time and temperature); � other conditions (e.g., the quality of reagents, dryness of solvents, inertness of the reaction atmosphere, etc.).It should be noted that information concerning the depend- ence of the yields of the condensation products on the nature of the heterocyclic bases of PNA monomers is practically absent. This problem demands special investigation. The majority of literature sources cite average yields of PNA condensation prod- ucts calculated per coupling cycle of a hypothetical monomeric fragment (irrespective of the nature of the monomer) or the total yields of the PNA oligomer.It seems reasonable to consider all the factors mentioned above. Hence, it is expedient to discuss in detail the main aspects related to the efficiencies of PNA condensations inherent in particular synthetic strategies. VI. The main strategies of peptide nucleic acid synthesis Usually, PNA synthesis utilises conventional solid-phase peptide synthesis protocols.14 Three synthetic strategies are currently especially popular which produce PNA in high yields and purity. These strategies differ in the nature of protective groups blocking 50-terminal primary aliphatic amino groups in PNA monomers. tert-Butoxycarbonyl (tBoc or Boc), 9-fluorenylmethoxycarbonyl (Fmoc) and 4-methoxyphenyldiphenylmethyl (monomethoxytri- tyl, MMT) groups are generally used as protective groups, and Boc,12, 94, 99, 100 Fmoc,77, 97, 101 ± 106 and MMT strategies 96, 107 ± 112 of PNA synthesis are distinguished, correspondingly.But O O MeO O O MMT Fmoc Boc The chemical nature of these groups and the differences in the conditions for their removal determine the choice of optimum condensation reagents and condensation conditions for each particular strategy, although the main principles of PNA synthesis are the same. The Boc strategy 12, 94, 99, 100 was the first to be used for the PNA synthesis. In this case, exocyclic amino groups of hetero- cyclic bases are usually protected by the benzyloxycarbonyl (Z, Cbz) group (the Boc/Z version),94 and O-benzyl groups are sometimes used as additional protective groups for guanine.113 The Boc/acyl version of the Boc strategy, where exocyclic amino groups of nucleobases are protected by acyl groups, has also been described.95 One of the well-known disadvantages of the Boc strategy is the necessity to use strong acids for the removal of Boc groups (trifluoroacetic acid) and the cleavage of PNA oligomers from the polymeric supports (hydrofluoric acid, trifluoro- methanesulfonic acid, etc.). Such drastic conditions limit the range of PNA synthesised according to the Boc protocol.A search for milder conditions has led to the development of the Fmoc strategy of PNA synthesis.14, 77, 96, 97, 101 ± 106, 114 Here, the Fmoc groups are removed by mild treatment with piperi- dine.14 The benzyloxycarbonyl (Z),97 benzhydryloxycarbonyl (Bhoc) 106 or MMT groups 12, 96, 114 are used to protect amino groups of nucleobases.The advantage of a Fmoc/acyl version of this strategy 77, 102 ± 105 is the possibility of selective removal of the Fmoc groups without affecting the acyl protective groups of the heterocyclic bases.102, 103 The use of this strategy ensures higher yields of PNA in comparison with the Fmoc/Z strategy, while the S I Antsypovitch conditions forPNA synthesis are compatible with those of peptide and oligonucleotide syntheses.104, 115 This opens up new oppor- tunities for the synthesis of hybrid PNA±DNA and PNA± pep- tide molecules.In addition to Boc and Fmoc protection, it was proposed to use MMT groups for protection of 50-terminal primary amino groups.96, 107 ± 112, 116 Although the MMT group, like the Boc group, is acid-labile, this can be cleaved under considerably milder conditions than the Boc groups (the MMT groups are split off by treatment with 3% trichloroacetic acid).107 In the MMT strategy, the amino groups of heterocyclic bases of the PNA monomers are usually protected by acyl groups (e.g., acetyl, isobutyryl, anisoyl, benzoyl, tert-butylbenzoyl) (MMT/acyl version of the MMT strategy).96, 107, 110, 116 The reaction conditions are mild, which makes it possible to perform automated synthesis of PNA oligomers using the oligonucleotide synthesisers, while their compatibility with oligonucleotide synthesis protocols allows one to obtain PNA±DNA mn; 109 The efficient condensation requires that the 50-terminal amino groups were not protonated, since in the form of cations they do not manifest nucleophilic properties.However, under basic con- ditions where the amino group is not protonated and hence is active, the undesirable transfer of the N-terminal acetyl group with the attached heterocyclic base to 50-terminal primary ali- phatic amino group may take place.94, 97, 115, 117 B O HN B H2N pH>7 O N NH O O NH NH PNA PNA PNA�C-terminal fragment of the PNA oligomer. This reaction can also occur under neutral conditions 94 resulting in the break of growing PNA chains and accumulation of short-chain oligomers. This affords a mixture of products and the isolation of target PNA presents a serious problem.The formation of isomeric structures can be avoided provided the condensation is very fast. In this case, side products cannot be formed, since the N-acyl transfer is a rather slow proc- ess.94, 97, 100, 117 In the presence of reagents based on uronium and phosphonium salts, the condensation occurs so fast that even in situ activation of carboxy groups ofPNAmonomers is possible. Splitting of theN-terminal monomeric unit 12 under the action of piperidine used for removal of Fmoc groups is yet another side reaction. O B O N B HN N O NH2 O NH + H2N HN O O B B N N O O N N PNA PNAPeptide nucleic acids: structure, properties, applications, strategies and practice of chemical synthesis It should be stressed that the problem of side reactions and the efficiency of PNA synthesis on the whole depend critically on the combination of protective groups used. Indeed, the Boc and Fmoc strategies differ essentially in the conditions of deprotection of 50-amino groups of the last added monomer.As mentioned above, the removal of Boc groups requires rather drastic acidic treatment, which leads to the protonation of 50-amino groups. Therefore, this group should be neutralised before the addition of the next monomeric fragment, which is not required in the case of the Fmoc strategy. On the other hand, the removal of Boc groups is always quantitative and rather fast, whereas splitting of Fmoc groups by basic treatment proceeds slowly and not always completely,118, 119 which negatively affects the efficiency of condensation.Aggregation of growing oligomeric chains is an additional obstacle to efficient condensation. Interchain aggregation makes the deprotected terminal amino group only partly accessible for subsequent condensation, which decreases the efficiency of the process on the whole.120, 121 It should be noted that aggregation is only possible in the case of the non-protonated amino group.120, 121 Apparently, repulsion of positively charged amino groups prevents the aggregation of oligomers. Thus, the inter- chain aggregation never takes place in acidic media and terminal amino groups are more accessible to condensation. However, the amino group cannot efficiently react with the activated monomer, since its nucleophilicity is suppressed.Deprotonation of terminal amino groups of PNA with simul- taneous condensation (neutralisation in situ) is the most elegant approach to solving this problem. This methodology was first employed for peptide synthesis in 1987.122 The use of neutralisa- tion in situ in the synthesis of peptides using Boc and Fmoc protocols results in a significant increase in the condensation rate.123 ± 125 This effect was especially spectacular in the synthesis of `problematic' sequences which are especially prone to undergo interchain aggregation. Their syntheses by conventional methods involving preliminary neutralisation of terminal amino groups face the most serious problems.126, 127 At present, neutralisation in situ has become very popular for the synthesis of PNA along with conventional methods where deprotonation of 50-amino groups of PNA oligomers precedes condensation.In some cases, the use of neutralisation in situ helps solve the problem of side reactions of PNA isomerisation and increases the yields of condensation products. VII. Some regularities of condensation reactions in the synthesis of peptide nucleic acids A detailed knowledge of condensation reactions associated with PNA synthesis and a search for efficient procedures for its optimisation demand that the data available should be interpreted with due regard to the nature of reagents used for the activation of PNA monomers.The methods for the synthesis of PNA±DNA chimeras are described separately, since in this case condensation has a number of specific features. 1. Activated esters and carbodiimide activation For the first time, the method of activated esters has been successfully employed for the synthesis of thymidine PNA oligo- mers.1, 5 The synthesis of thymidine PNA using Boc-protected pentafluorophenyl (PFP) ester of a thymidine PNA monomer as a monomer was carried out in 1992.5 When the monomer concen- tration was 0.1 mol litre71, the yield of the condensation product was >99%. However, an attempt to apply this method to the synthesis of cytidinePNAoligomers was without success: the yield of the target product did not exceed 50% under identical con- ditions.6 There are some examples of successful applications of the method of activated esters for the synthesis of heterogeneous PNA.Thus the synthesis of PNA oligomers from Fmoc/Z- protected monomers by ester activation has been described.97 75 This was one of the first examples of the implementation of the Fmoc protocol to the synthesis of PNA. In this case, the use of the activated esters strategy ensured high efficiency of condensation. The yields of condensation products in the synthesis of PNA oligomers with the chain lengths of up to 20 residues varied from 95% to 99%.97 The average yields in the synthesis of PNA containing all the four types of nucleobases were 97% over each step, which corresponds to the 70% yield of target oligomers.With allowance for subsequent isolation, the yield of PNA oligomers was 43%. It is of note that the choice of strategies for the synthesis of PNA is often determined by the necessity to obtain high yields of condensation products at low expenditures of expensive PNA monomers where the use of manyfold (fourfold and higher) monomer excess is undesirable. A combined use of the Fmoc protocol and PFP activation allows the use of as little as a twofold excess of PNA in a single condensation.97 The use of threefold (or greater) excesses of PNA monomers did not result in further increase in the yields of condensation products.The dimethyl sulfoxide (DMSO) ±N-methyl-2-pyrrolidinone (MP) mixture (1 : 4) appeared to be the solvent system of choice.97 Apparently, this system favours rapid access of the reagents to the growing PNA oligomers.128 However, for other activation procedures, other solvent systems were more efficient. The use of the activated esters approach to PNA synthesis sometimes gives reasonable results;12 however, in the majority of cases, higher (94% ± 99%) yields of condensation products were obtained using the in situ activating reagents.1, 5 In principle, the PNA synthesis can successfully be performed using the so-called carbodiimide activation involving, e.g., dicyclohexylcarbodiimide (DCC) or N,N0-diisopropylcarbodiimide,12 which sometimes affords high (up to 98%± 99%) yields of condensation prod- ucts.6, 7 Low rates of PNA condensation is the main disadvantage of this activation procedure.99 Carbodiimide activation by DCC made it possible to obtain addition products of thymidine and cytidine PNA monomers in quantitative yields.Purine monomers are only partly incorpo- rated into PNA oligomers; repeated condensation does not result in quantitative yields of the addition products. The use of N,N0-diisopropylcarbodiimide as the activating reagent has made it possible to obtain nearly quantitative yields even for purine monomers.99 However, this required the use of a fourfold excess of the monomers and the activating reagent, and the condensation lasted no less than 60 min.99 In addition, the introduction of the adenine and the guanine monomers into PNA oligomers requiredwo and three condensation cycles, respec- tively.The carbodiimide activation is a convenient procedure for obtaining PNA adducts with other molecules. Thus the synthesis of hybrid PNA peptides containing biotin residues, which confer on PNA the ability to penetrate cell membranes efficiently, has been described.129 The peptide fragment of the chimeric molecule was prepared using a standard peptide synthesis protocol,130 while the synthesis of the PNA fragment was carried out manually, using the Boc strategy based on the methods described in the classical work by Nielsen.94 The activation with DCC was performed in the presence of 1-hydroxybenzotriazole, using a fivefold excess of a PNA monomer.The condensation was performed at an elevated (37 8C) temperature to increase the yield of the target product.129 In other examples of the synthesis of hybrid PNA± peptide molecules,49 the Boc protocol was combined with carbodiimide activation.94, 129 Such an approach to the preparation of chimeras is justified, since it ensures complete compatibility of syntheses of both the peptide and PNA fragments of the hybrid molecules 49 and overall yields of target products of no less than 50%.49 A combined use of carbodiimide activation with the Boc/acyl protocol common in peptide synthesis,130 allows one to obtain both classical PNA molecules withN-(2-aminoethyl)glycine back- bones and molecules with non-canonical backbones containing76 optically active monomeric fragments where the glycine residues are substituted by other amino acids, e.g., lysine, serine, isoleucine and glutamic acid.131 Incorporation of D-lysine-based monomers into PNA oligomers increases the stabilities of PNA±DNA and PNA±RNA duplexes.With other amino acids, the stabilities of these types of duplexes are usually low.131 In some cases, such as in the synthesis of PNA± peptide chimeras and thymidine PNA oligomers, the activated esters and carbodiimide methodologies are employed. Sometimes, the yields of condensation products prepared by the activated esters (PFP) method even exceed those obtained by in situ activation.97 Nonetheless, it is generally acknowledged 12 that PNA syn- thesis based on the use of in situ activation reagents, viz., uronium and phosphonium salts, is the most reliable approach, since it ensures higher yields of the condensation products.This approach has considerably been developed in the recent years and it is this procedure that offers the broadest opportunities for the synthesis of PNA and their derivatives. 2. In situ activation by reagents based on uronium and phosphonium salts The most popular activating reagents for PNA synthesis are O-(benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluoro- phosphate (HBTU), O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetra- methyluronium hexafluorophosphate (HATU), (benzotriazol-1- yloxy)tris(pyrrolidino)phosphonium hexafluorophosphate (PyBOP), O-(benzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU), O-[(ethoxycarbonyl)cyanomethyl- ideneamino]-1,1,3,3-tetramethyluronium tetrafluoroborate (TOTU) and (benzotriazol-1-yloxy)-tris(dimethylamino)phos- phonium hexafluorophosphate (BOP), although other reagents are also known.N N N N PF¡ PF¡ N 6 6 N N + + O O NMe2 NMe2 C C NMe2 NMe2 HBTU HATUN N N N N N BF¡4 PF¡6+ O NMe2 O +P N C 3 NMe2 TBTU PyBOP O N CN N EtO N BF¡4 PF¡6N + O NMe2 O +P (NMe2)3 CNMe2 BOP TOTU Since condensation strongly depends on the strategy used for PNA synthesis, in the first place, on the combination of protective groups, it seems expedient to classify the data on the in situ activation into three groups corresponding to Boc, Fmoc and MMT strategies.a.The Boc strategy The Boc/Z-modification of the Boc strategy is a classical approach to PNA synthesis. The dependence of the condensation efficiency of the Boc/Z strategy on different factors has been studied in sufficiently great detail.94 This strategy makes it possible to obtain high yields of PNA containing>15 units with all the four types of S I Antsypovitch heterocyclic bases. The use of the carbodiimide activation proce- dure for the synthesis of such oligomers is usually less efficient.94 Studies of the effects of various factors, such as the nature of activating reagents, solvents, monomer concentrations, the nature of the organic base (tertiary amine), catalysts, etc., on the efficiency of condensation in the in situ activation revealed that all of them are important for the optimisation of condensation conditions.94 Comparison of the efficiency of condensations under the action of some uronium activating reagents, viz., the most popular reagents HBTU and TBTU and the relatively new reagents HATU and O-(1,2-dihydro-2-oxo-1-pyridyl)-N,N,N0,N0-bis- (tetramethylene)uronium hexafluorophosphate (HDPU), revealed that the condensation was efficient in all the cases under study, but the yields of addition products were the highest with HBTU.94, 100 It should be noted that these results are valid exclusively for a DMF± pyridine solvent system and for the concentrations of the PNA monomer and the base [diethyl(cyclohexyl)amine, DECHA] of 0.05 and 0.1 mol litre71, respectively.Under these conditions, the average yields of the condensation products were 92.2% ± 97.1% irrespective of the nature of PNA monomers. However, conditions can be selected where other activating reagents will be more efficient than HBTU, e.g., in other solvent systems or in the presence of other bases. This suggests that condensation conditions are to be chosen for each activating reagent. Comparison of the efficiencies of HBTU and PyBOP has demonstrated the former to be superior under identical condi- tions. The overall yields of PNA oligomers using HBTU and PyBOP activation were 61% and 40%, respectively, which corre- sponds to average yields of 97.1% and 94.7% in each step.94, 100 It is of note that this comparative study was carried out under conditions optimum for HBTU.An important role in efficient condensation is played by the solvent system used. Thus in a DMF± pyridine mixture, the overall yield of the condensation product was 61%, whereas those in DMF or DMF±DMSO were 55% and 22%, respec- tively.94 However, it was not indicated which monomers deter- mined the lowest yields of PNA. Noteworthy, virtually none of the works cited in this review provide these data. Studies of the dependence of the yield of a PNA oligomer on the nature of an organic base (tertiary amine) have shown that 4-dimethylaminopyridine (DMAP), DECHA, (dicyclohexyl)me- thylamine and (dicyclohexyl) ethylamine were as efficient as (diisopropyl)ethylamine (DIPEA) which is widely used in peptide synthesis.94 The nature of the tertiary amine only slightly affected the yields of the condensation product (93.7% ± 95.3%).The con- densation in the presence of DIPEA in theDMF± pyridine system is not optimum, since this amine reacts with the monomers to give insoluble salts. Better conditions for this reaction can be found. The condensation in the presence of DIPEA is efficient in MP±DMSO. Studies of relationships between the yields of condensation products and concentrations of PNA monomers revealed that acceptable yields can be obtained when the monomers are used at concentrations no less than 0.1 mol litre71. At lower concentra- tions (0.05 mol litre71), the condensation proceeds as a rule too slowly resulting in the accumulation of side products from competing reactions.94 Addition of catalytic amounts of DMAP and 1-hydr- oxybenzotriazole to the reaction mixture may have a negative effect on the efficiency of condensation, although it is known that their addition sometimes favours the formation of amide bonds.94 The loading { of the polymeric support should not exceed 0.1 ± 0.2 mol-equiv.g71, which is essential for the maximum yield { Here, loading is expressed as the number of functional groups per unit surface area.Peptide nucleic acids: structure, properties, applications, strategies and practice of chemical synthesis of the PNA oligomer. With a higher degree of loading, the efficiency of PNA synthesis is lower.94, 100 The use of the Boc/Z strategy ensures high (99.4%) yields of target products with in situ activation (HATU) in the presence of DIPEA.95 In this case, the reaction mixture must contain a large (e.g., sevenfold) excess of the PNA monomer with respect to loading of the polymeric carrier.95 The amount of the activating reagent is usually reduced by 10%, while the tertiary amine is taken in a twofold excess with respect to the PNA monomer.88 Acceptable yields are obtained with a fourfold excess of the PNA monomer and by activation with HATU (the amount of HATU is 0.9 mol-equiv.with respect to a monomer) in the presence of DIPEA and lutidine.47 The use of a fivefold excess of PNA monomers and a tenfold excess of DIPEA makes it possible to increase the overall yields of PNA oligomers to 92%.17 Very often, the yields can be increased upon preactivation of PNA monomers.To this end, the PNA monomer is mixed and incubated with the activating reagent for several seconds before being loaded onto the column.95, 96, 109, 110, 113, 115, 116 Strictly speaking, here we do not deal with the in situ activation, however, the term `in situ activation' is conventionally related to the nature of the activating reagent rather than to the order of mixing of the reagents. It is of note that the methodology of preactivation does not imply the isolation of activated monomers. Usually, the amount of the activating reagent is 5%± 10% smaller than that of the monomer.Preactivation of PNA monomers by incubating them with the activating reagent for 2 min and condensation in an MP± pyr- idine mixture make it possible to obtain PNA oligomers in overall yields of 90%. Depending on the length and composition of sequences, the yields of PNA vary from 68% to 90%.88 In a search for the most rational technique for PNA synthesis, an investigator has to select between a rapid and cheap synthesis with the use of a small excess of PNA monomers and low (but acceptable) yields and lengthy syntheses with large expenditures of expensive monomers and activating reagents, but giving nearly quantitative yields. Attempts to specify the conditions for effective synthesis of PNAhave been undertaken time and again.95 Primary attention in the optimisation of PNA synthesis is usually given to such factors as the low cost and ease of synthesis of PNA monomers, nearly quantitative yields of condensation products, the simplicity and efficiency of the procedure for isolation of PNA oligomers from the reaction mixture after completion of the synthesis and the possibility to obtain chimeric DNA±PNA duplexes and ligand- containing PNA molecules.Such a choice of optimisation parameters seems to be justified, however, other factors, such as reaction rate and economy, are no less important. Thus the rate of condensation should be high enough to minimise side reactions. The synthetic strategy should first of all be efficient and allow for economic expenditure of expensive PNA monomers and activating reagents.The latter factor is of crucial importance in large-scale syntheses of PNA. While comparing the Boc/Z and the Boc/acyl strategies of PNA synthesis, preference is given to the latter, since it is thought to be more promising. Although the average yields of oligomers calculated per one condensation cycle are high in both cases, viz., no less than 98% (Boc/acyl) and about 99% (Boc/Z), the yields of PNA oligomers in the case of the Boc/acyl synthetic strategy and conventional isolation procedure do not exceed 65% (relative to the support loading), and are as low as 20% according to the Boc/Z protocol.95 In addition, the Boc/acyl strategy allows one to synthesise PNA±DNA chimeras and to prepare addition prod- ucts of acid-labile ligands to PNA, which is inattainable in the case of the Boc/Z strategy.95 The synthesis of thymidine PNA oligomers makes use exclu- sively of Boc-protection of 5 0-terminal amino groups, since the thymidine PNA monomer does not require protection of the heterocyclic base.77, 79 Such oligomers are conveniently synthes- ised by manual techniques.79 In this case, by analogy with 77 previously developed procedures,94, 131, 132 the Boc strategy is combined with HBTU activation in the presence of DECHA.This approach affords high yields of condensation products, but requires the use of a fourfold excess of PNA monomers.79 The Boc strategy is also used in the synthesis of PNA oligomers containing modified units.131, 133 ± 135 Thus PNA mole- cules may incorporate units containing anthraquinone and acri- dine residues.133 Such oligomers are used to study the melting behaviour of hairpin-shaped PNA of high-molecular-mass PNA±PNA duplexes by fluorescence quenching (the so-called `molecular beacon' method).High yields of condensation prod- ucts are obtained. Its repeated condensation is used for the attachment of a modified PNA monomer followed by a non- modified monomer to PNA oligomers.133 The Boc/Z method combined with HBTU activation 94, 134 allows synthesis of PNA molecules with fluorescent labels at their N-termini.134 Such PNA derivatives can be used for detecting specific NA sequences. The fluorescence of the label may increase more than 50-fold upon hybridisation of PNA with the comple- mentary NA target in comparison with that of the free PNA.134 The classical Boc/Z strategy is also used for the synthesis of PNA chimeras containing clusters of modified fragments, e.g., those with positively charged, chiral backbones built up of D-lysine residues.135 The presence of such fragments in PNA confers useful properties on the latter.131, 135 In contrast with ordinary PNA, chimeric PNA form exclusively antiparallel duplexes with DNA; their stability depends critically on the presence of mismatches.Even one mismatch decreases sharply the stability of hybrid duplexes. If a mismatch is located in the middle of clusters containing three modified residues, ten-mem- bered PNA±DNA duplexes are unstable even at 15 8C (DTm=28 8C).Such PNA can serve as the basis for the develop- ment of genetic diagnostic tools, particularly, for the detection of point mutations. Synthesis of these PNA derivatives by the Boc strategy combined with HBTU activation in the presence of DECHA,131, 136 affords chiral oligomers in overall yields of 80%± 90%,135 and optical purity of no less than 90%. In the majority of cases, the Boc protocol includes the use of standard polymeric supports containing (4-methylbenzhydryl)- amino groups (MBHA).11, 131, 132, 135, 136 Cl7 NHá3 MBHA PNA oligomers are cleaved from the solid phase by treatment with CF3COOH±CF3SO3H.11, 131, 136 The Boc strategy allows the use of various activating reagents including BOP.137 The use of BOP and DIPEA as a base affords high yields of condensation products in various solvent systems, e.g., DMF±CH2Cl2 and DMF±DMSO.In this case, the use of a twofold excess of PNA monomers is sufficient, but each conden- sation reaction should be repeated in order to provide more efficient binding of the monomers.137 PNA can also be synthesised using the Boc/Z strategy com- bined with the in situ activation with TBTU inDMF± pyridine 113 as described in the classical work of Nielsen.94 However, it is more expedient to carry out the condensation inDMFin the presence of a twofold excess of DECHA as a base with respect to the monomer. This is associated with the good solubility of PNA monomer salts formed in DMF.113 The use of a threefold excess of PNA monomers is optimum.This approach allows one to use 2-amino-6-benzyloxypurine with the non-protected amino group as a guanine precursor.113 The side reaction (capping) of the oligomeric chain can be avoided if the amount of TBTU is 10%78 less than that of the PNA monomer 113 or if the PNA monomer is subjected to 5-min preactivation.96 Capping of oligomeric chains occurs both in PNA and peptide synthesis 123 provided the free activating reagent is present in the reaction mixture. This is possible owing to the fact that the terminal amino group of the peptide following deprotection reacts with both the activated amino acid derivative and the activating reagent.123 Thus in the presence of an excess of HBTU, the tetramethylguanidine derivative was formed, which did not undergo subsequent elongation of the peptide chain.138 This side reaction can be avoided if the amount of the activating reagent is smaller (by 5%± 20%) with respect to the monomer.113 With TBTU as the activating reagent, another side reaction, viz., the N-acyl transfer, can take place.94, 113, 123 To avoid this, neutralisation in situ is used.123 In this case, condensation is carried out in the presence of a base without preliminary neutral- isation of the 5 0-terminal primary amino group of the oligomer.113 The efficiency of condensation is monitored by HPLC analysis of aliquots obtained by appropriate treatment of a small portion of a polymeric carrier (3 ± 5 mg) following attachment of the next monomeric unit.113 In the synthesis of thymidine PNA oligomers based on the use of the Boc strategy and TBTU activation, a decrease in the yields of condensation products is observed sometimes after addition of the first 3 ± 4 monomeric fragments.113 This leads to the accumu- lation of short chains, whereas the overall yield of PNA does not exceed 30% (in the case of a 10-membered oligomer).The same problem sometimes arises with HATU activation, presumably, due to aggregation of growing thymidine PNA oligomers. This can partly be overcome through the attachment of lysine residues at the C-ends of PNA chains, which prevents the interchain aggregation of the oligomers formed. It is noteworthy that the synthesis of heterogeneous PNA oligomers containing monomeric fragments of all the four types is not accompanied by significant reduction of product yields in the PNA synthesis.113 In this case, the yield over each condensation step reaches 97%, which corresponds to 66% overall yield of the target 16-membered oligomer.Similar problems, viz., aggregation of PNA chains, may arise during the removal of protective Z groups after completion of PNA synthesis resulting in a significant decrease in the yields of PNA.11, 113 A crucial role in this process belongs to complemen- tary interchain and intramolecular coupling of PNA oligomers. Even four-membered PNA±PNA duplexes significantly hinder post-synthetic work-up of PNA oligomers.113 Therefore, oligo- meric PNA sequences should be analysed for the possibility of intramolecular hairpin and intermolecular cluster formation prior to the synthesis. It was found that the overall yields of PNA can be increased to 75% and even more through incorporation of a lysine residue into PNA heterooligomer, since the charged e-amino group of lysine partly prevents the aggregation of PNA chains.These data suggest that the efficiency of synthesis of PNA oligomers strongly depends on the properties of PNA sequences to be prepared irrespective of the synthetic procedure used.139 Theo- retically, PNA may contain any combination of monomers, while the synthesis of certain sequences in quantitative yields faces difficulties. Thus the attachment of a purine monomer to an oligomeric PNA sequence containing a 5 0-terminal purine base is problematic.Therefore, ifPNAcontaining purine clusters are to be synthesised, the synthetic protocol should include repeated con- densations.139 The lowest yields were obtained in the synthesis of PNA oligomers containing several consecutive guanine residues. In conclusion, it may be said that the Boc strategy can be implemented in both manual and automated variants.139 Although the former seems to be rather efficient and inexpen- sive,140 automated synthesis using peptide and oligonucleotide synthesisers holds especially great promise.141 The reason is that the manual synthesis of PNA is not only lengthy and laborious, but also needs large-scale synthesis (not less than 5 mmol) in order to obtain acceptable yields of condensation products.S I Antsypovitch b. The Fmoc strategy It is generally recognised that the Fmoc strategy of PNA syn- thesis14, 77, 96, 100 ± 106, 114 requires milder conditions than the Boc strategy.12 In particular, no treatment of PNA oligomers with strong acids before and after synthesis is necessary. This allows the use of other protective groups for exocyclic amino groups of heterocyclic bases of the PNA monomers. Correspondingly, the Fmoc strategy affords higher degrees of purity of reaction mixtures and higher overall yields of PNA oligomers. The Fmoc/ acyl strategy of PNA synthesis is especially promising,77, 104, 105 since it can also be used for the synthesis of PNA±DNA and PNA± peptide chimeras.Various versions of the Fmoc strategy are currently known. Thus Bhoc protection of heterocyclic bases of the PNA monomers and HATU activation allows automated synthesis on oligonu- cleotide synthesisers.106 The use of a DIPEA ± lutidine mixture seems to be more effective than the use of only one base, since it affords higher yields of the condensation products.106 DMF is a suitable solvent for the Fmoc condensation, and HATU is one of the most potent activating reagents.141 However, in this case, too, the synthesis of individual PNA oligomers may face problems related to non-efficient condensation.141 Thus the synthesis of PNA oligomers with sequences containing several identical consecutive heterocyclic bases (not necessarily purines) yields short-chain products.This problem can partly be overcome through the use of repeated condensations.141 Two condensation cycles are carried out in the following cases: in the synthesis of sequences containing four or more identical consecutive residues after attachment of two or three identical PNA monomers to the oligomer, in the synthesis of PNA oligomers containing purine clusters and in the synthesis of 18-membered and more extended PNA oligomers independent of the composition of the sequence after the attachment of the 17th residue.141 The yields of PNA obtained using the Fmoc strategy and HATU activation vary from 26% to 38%.141 In some cases, the overall yields of PNA oligomers do not depend on the sequence lengths but are determined by the efficiency of the attachment of the first PNA monomer to the polymeric support, since the yield of the first condensation product can be lower than the yields of the products obtained in subsequent condensations.141 In the majority of cases, reversed phase HPLC is used for the analysis of reaction mixtures and isolation of target products under conditions for the separation of peptides rather than oligonucleotides.141 The condensation efficiency of the Fmoc strategy can also be estimated spectrophotometrically by measur- ing UV absorption of a product formed upon removal of Fmoc groups by piperidine in the range of 256 ± 301 nm.This method is especially convenient for stepwise monitoring of PNA oligomer- isation.97 Thus, the Fmoc strategy can be used for the synthesis of PNA oligomers unattainable by other methods.This procedure allows modifications to the synthetic protocol aimed at increasing the yields of condensation products in separate synthetic cycles.141 The low efficiency of the condensation encountered in the Fmoc strategy of PNA synthesis seems to have the same reasons and solutions as those in peptide synthesis.142 It is known that many problems of automated solid-phase peptide synthesis are related to the nature of the sequence to be synthesised.142 Low condensation yields are due to the formation of bulky spatial peptide structures, which may interfere with the formation of amide bonds. Those peptide chain fragments which are susceptible to interchain aggregation, can also decrease the accessibility of the amino group and thus prevent further elongation of the chain.120 Spatial hindrances appear in the course of PNA synthesis which decelerate the removal of protective Fmoc groups from the 5 0-termini of PNA oligomers and decrease the efficiency of subsequent condensation.143Peptide nucleic acids: structure, properties, applications, strategies and practice of chemical synthesis The synthesis of purine-rich PNA presents a special problem.If a PNA sequence contains more than two consecutive purine residues, the efficient attachment of the next purine monomer requires a longer reaction time and repetition of the condensation procedure three or four times.The attachment of the guanine monomer to the 5 0-terminal guanine unit proceeds at a very slow rate.Aggregation of PNA chains is the main reason for low condensation efficiency inherent in both Fmoc and Boc strategies. The synthesis of partly or fully self-complementary sequences may produce problems relevant to intrachain association of PNA oligomers. In addition, the solid-phase synthesis conditions favour intermolecular aggregation of growing oligomers. Thus purine-rich PNA and thymidine PNA oligomers (both four- membered and more extended ones) are prone to aggregation. The condensation efficiency strongly depends on such factors as the loading of the polymeric support and the composition of the solvents. Thus, low densities of oligomers `growing' on solid supports favour solution of the problem of intermolecular aggre- gation of PNA chains, whereas certain solvents prevent inter- and intramolecular aggregation of PNA molecules (e.g., the formation of hairpin structures) and ensure effective diffusion of reagents to the N-termini of the growing PNA chains.Studies aimed at optimisation of conditions forPNA synthesis are currently under way, since no ideal method for PNA synthesis has been developed so far. The Fmoc/acyl strategy of PNA synthesis seems to be the most advanced one, since it permits one to obtain oligomers of virtually any composition in high yields.104, 115 Thus the synthesis of thymidine PNA oligomers by the Fmoc/ acyl method combined with HATU activation requires only small (twofold) excess of the monomer 115 and a 20% deficiency of the activating reagent.It is more expedient to use MP as a solvent and the DIPEA ± lutidine mixture as a base. Preactivation of PNA monomers for 2 min is yet beneficial. The yield of the condensa- tion product in each step is 85%± 90%, the condensation time is 30 min and the overall yields of the 7-membered oligomer is no less than 50%.115 The efficiency of the reaction is most conven- iently monitored spectrophotometrically. Attempts have been made to optimise condensation condi- tions in solution using mixtures of PNA monomers and amino acid esters as model systems.115 Direct application of the data obtained in these model systems to solid-phase PNA synthesis is questionable, since the optimum conditions for the synthesis in solution and on polymeric supports may differ in principle. This circumstance should be taken into consideration when selecting model systems.On the other hand, these studies sometimes give valuable results. The yields of condensation products in the reaction of PNA monomers with L-phenylalanine tert-butyl ester were not lower than 95% irrespective of the nature of the activating reagent (HATU or HBTU in the presence of 1-hydroxybenzotriazole), of the tertiary amine (N-methylmorpholine, lutidine or the DIPEA ± lutidine mixture) and of the solvent (DMF or MP). The nature of the base and the activating reagent did not influence the efficiency of the reaction. The best result was obtained in the case of HATU activation in the presence of lutidine in DMF.115 As mentioned above, the overall yields of PNA oligomers may depend on the efficiency of attachment of the first monomer to the polymeric support.Thus the yield of the attachment product of the first cytosine monomer to the polymeric support (Tentagel) in the synthesis of heterogeneous PNA by the Fmoc/acyl strategy combined with conventional HBTU activation in the presence of 1-hydroxybenzotriazole and lutidine did not exceed 50%,115 whereas those obtained in subsequent condensation steps were no less than 80%. The efficiency of attachment of the first PNA monomer can be increased to 80% and even higher using repeated condensation. In this case, the time for each condensation can be reduced.115 Low yields of the attachment products of the first 79 monomeric unit to a polymeric support have also been reported.141 The best conditions for the PNA synthesis by the Fmoc/acyl strategy include the use of a twofold monomer excess (0.125 mol litre71), preactivation with HATU (0.8 mol-equiv.), DIPEA and lutidine (1 ± 2 min), condensation (20 min) and repetition of the condensation procedure in each (with the exception of the first) reaction cycle.This provides an overall yield of heterogeneous PNA of 43% (for 16-membered oligom- ers), which corresponds to the average yield of the condensation product of 95% (in this case, the attachment of the first mono- meric fragment occurs with high yield).The main disadvantage of this method is the necessity to repeat the condensation procedure in each cycle.115 The expedience of this approach is doubtful despite its obvious advantages, viz., good reproducibility of experimental results. c. The MMT strategy This strategy of PNA synthesis is rather promis- ing 38, 96, 107 ± 112, 116, 144, 145 particularly its MMT/acyl ver- sion,96, 107, 110, 116 which is fully compatible with oligonucleotide synthesis protocols. It allows the use of automatic DNA synthes- isers without modification of their design or software, which is especially convenient for conducting the syntheses ofPNA±DNA chimeras. The MMT strategy allows the use of a broad range of activating reagents,38, 112, 144, 145 including mesitylenesulfonyloxy- benzotriazole (TMSOBt) 112 and 3,4,6-triisopropylbenzenesulfo- nyloxybenzotriazole (TPSOBt).144 The latter gives higher yields of the condensation products (the average yield in one cycle reaches 96%) than the commercially available activating reagent PyBOP.112, 144 Depending on the nature of the PNA monomer and the 5 0-terminal oligomeric fragment, the yield in each condensation step varies from 91% to 99%.144 The efficiency of condensation carried out by the MMT strategy can be estimated spectrophotometrically by measuring UV absorption spectra of the MMT+ cation formed upon removal of the protective MMT group from the 5 0-terminal amino group of a PNA oligomer.The MMT strategy combined with TPSOBt activation is compatible with oligonucleotide syn- thesisers.112 This approach was first used in the synthesis of PNA molecules containing uracil residues.112 The `manual' variant of the MMT strategy is also effective, e.g., in the synthesis of thymidine PNA oligomers.38 The use of PyBOP as an activating reagent allows one to reach 95%± 99% yields in each step of the monomeric fragment coupling.96 However, in this case, the use of concentrated (0.3 M) solutions of PNA monomers and PyBOP is necessary.It is more expedient to use N-ethylmorpholine as a base and to perform preactivation of PNA monomers.96 The MMT strategy is used for the synthesis of phosphonate analogues of PNA (pPNA).110, 116, 146, 147 The presence of nega- tively charged groups in the pPNA backbone makes PNA analogues readily soluble in aqueous solutions in comparison with classical PNA oligomers.These molecules bind specifically to complementary fragments in DNA and RNA, although the melting temperatures of pPNA±NA complexes are somewhat lower than those of the corresponding PNA±NA complexes. A combination of the MMT/acyl protocol with triisopropyl- benzenesulfonylnitrotriazole activation is efficient in the synthesis of pPNA.110, 116, 146, 147 The condensation in the presence of N-methylimidazole as a nucleophilic catalyst permits one to obtain the average yields of 95% in the condensation step with a reaction time of 10 min.110, 116 However, the quantitative over- all yields of condensation products are not achieved, although the use of dilute solutions of PNA monomers (0.05 M) and the activating reagent (0.06 M) together with preactivation (mixing of the PNA monomer with the activating reagent and N-methyl- imidazole), makes this procedure attractive.110, 11680 3.Some peculiarities of the synthesis of PNA±DNA chimeras The interest in PNA±DNA chimeras has arisen in the past decade, which gave a strong impetus to the development of methods for their synthesis.77, 109, 111, 144, 148 ± 150 The use of classical PNA in biochemical studies is limited due to their poor solubilities in aqueous solutions, proneness to self-aggrega- tion 4, 12, 34 and low penetrability through cell mem- branes.4, 9, 84, 151 The latter is the main obstacle for the use of canonical PNA oligomers as antisense agents in vivo.9 Chimeric PNA±DNA molecules are devoid of most of these drawbacks.They possess all the advantages of PNA together with valuable properties inherent in NA. Indeed, the PNA±DNA chimeras synthesised so far combine high biological stabilities, high affinities and selectivities of binding to NA targets typical of PNA with perfect solubilities and the ability to activate hydrolysis of RNA targets by RNAse H, which are characteristic of DNA.9, 34 Chimeric PNA±NA molecules have various applications, viz., they are promising therapeutic (including antisense) drugs 9 and can be used as a basis for highly effective diagnostic tools and highly sensitive biomolecular probes.77, 96, 107 ± 109, 144, 148, 149 Synthesis of hybrid PNA±NA molecules manifests specific features, which necessitates a consid- eration in a separate section.The correct choice of linkers between PNA and DNA frag- ments of the chimeric molecules is one of the most important problems. Depending on whether the 5 0-terminal fragment of the hybrid molecule belongs to PNA or DNA, the linker used is represented either by modified nucleosides (e.g., 5 0-amino-2 0,5 0- dideoxynucleosides 77, 149) or modified PNA monomers [e.g., N-(2-hydroxyethyl)glycine derivatives].148 HO H2N O B B N O O OH HO Syntheses of both types of PNA±DNA hybrids, viz., 5 0-PNA ± DNA-3 0 and 5 0-DNA± PNA-3 0, have been described. Owing to the charged backbones of their DNA fragments, PNA±DNA chimeras are perfectly soluble in aqueous solutions, which makes possible their isolation and analysis by standard methods, such as polyacrylamide gel electrophoresis and ion- exchange reversed phase HPLC.108 In the case of 5 0-terminal PNA fragments, PNA and DNA fragments are linked by the amide bond and 5 0-amino-2 0,5 0- dideoxynucleosides are used as linkers.77 Such hybrid molecules are synthesised by various methods.Thus 5 0-PNA ± DNA-3 0 hybrids are prepared according to Boc/Z protocols commonly used in the synthesis of PNA-peptide conjugates.152 However, the use of this technique for the synthesis of PNA±DNA chimeras containing purine nucleotide residues may result in acid-catalysed apurinisation of the DNA fragment during deprotection of heterocyclic bases of PNA.77 Therefore, this approach is used exclusively for the synthesis of PNA±DNA hybrids the DNA fragments of which contain more stable pyrimidine nucleotides, whereas 5 0-PNA ±DNA-3 0 chimeras are more efficiently synthes- ised using the Fmoc/acyl strategy.77 If hybrid molecules contain 5 0-terminal DNA fragments, the PNA and DNA parts can be linked by phosphoramide bonds without any linkers.In this case, the synthesis ofPNAfragments is carried out using the Fmoc/acyl protocol. With the thymidine monomer, Boc-protection of the 5 0-terminal amino group is possible. The activation is performed with HATU in the presence of DIPEA and DMAP; the condensation is carried out in DMF. In this case, the use of acid-labile groups for protection of heterocyclic bases of PNA monomers is inadmissible because of easy acid hydrolysis of phosphoramide bonds.77 S I Antsypovitch The synthesis of PNA fragments of such chimeric oligomers usually employs the MMT/acyl strategy.In this case, the con- ditions of PNA synthesis are compatible with those of oligonu- cleotide synthesis;36, 149 PNA fragments can be synthesised by manual techniques. The condensation is performed in the DMF± pyridine mixture with 2-[2-oxo-1(2H)-pyridyl]-1,1,3,3- bis(pentamethylene)uronium tetrafluoroborate (TOPPipU) as the activating reagent and DECHA as the base.153 Under these conditions, the condensation of thymidine and cytidine PNA monomers proceeds smoothly, purine monomers are attached inefficiently to the growing PNA chain.149 If HATU is used as the activating reagent, DIPEA as the base and acetonitrile as the solvent in the presence of a fivefold excess of PNA monomers (necessary to attain high yields of condensation products), the reaction time is no less than 15 min.34 This method was used for the synthesis of chimeric molecules containing 5-bromouracil and 5-methylcytosine residues.34 The incorporation of 5-methylcyto- sine residues into the PNA chains of chimeric molecules increases the stabilities of their duplexes and triplexes with complementary DNA.34 The modified thymidine PNA monomer based on N-(2- hydroxyethyl)glycine 34, 107, 148 is often used as a linker between DNA and PNA fragments of chimeric molecules of the 5 0-DNA± PNA-3 0 type; the latter can be synthesised using both Boc 148 and MMT strategies.34, 107 If the Boc/Z strategy is used, the PNA synthesis is carried out on a solid phase; the linkers are attached under the same conditions.148 Subsequent synthesis of DNA fragments of hybrid molecules should also be performed on the solid phase. It is inadmissible to perform the synthesis ofDNA fragments in solution, since the solubility of PNA oligomers devoid of protective groups in organic solvents is insufficient to provide efficient condensation with phosphoroamidite derivatives of nucleosides.148 The solid-phase MMT method is more suitable for the syn- thesis of the 3 0-PNA fragments; after completion of PNA syn- thesis, solid-phase synthesis of the DNA fragment is continued without detachment of the oligomer from the support.This prevents the use of PNA monomers the heterocyclic bases of which are protected by acid-labile groups, since the DNA frag- ments will not withstand acid treatment used to remove protective groups. This synthetic procedure is inapplicable to chimeric molecules containing PNA monomers of all the four types, but can be used for the synthesis of hybrid molecules with the pyrimidine monomers constituting the PNA fragments.148 The MMT/acyl modification of this method is devoid of these disadvantages and allows the synthesis of PNA±DNA hybrids containing all the four types of nucleobases in both DNA and PNA fragments.108, 109, 150 The MMT/acyl strategy allows the application of the fully automated protocol on oligonucleotide synthesisers.The DNA fragments of hybrid molecules are usually synthesised according to a conventional phosphoroamidite protocol, which makes use of commercial 2 0-deoxynucleoside phosphoroamidites.108 This method of synthesis of PNA±DNA chimeras has practically no drawbacks.108, 150 The synthesis of PNA fragments of 5 0-PNA ± DNA-3 0 oligomers may involve HBTU activation in the presence of DIPEA in DMF± acetonitrile.150 Solutions of PNA monomers and activating reagents should be used at concentrations of no less than 0.1 M (preferably, 0.2 M); preactivation is also desirable. A commercially available aminohexanol phosphoroamidite deriva- tive can be used as a linker between the PNA andDNAfragments; this is attached to the 5 0-end of a DNA fragment by a standard oligonucleotide synthesis protocol. Then the synthesis of a 5 0-ter- minal PNA fragment of a chimeric molecule is followed.150 The MMT/acyl strategy is used in the synthesis of chimeric molecules with the composition 5 0-PNA ±DNA± PNA-3 0.109 In this case, both PNA fragments are synthesised in an automated regime using HBTU activation in the presence of DIPEA in DMF± acetonitrile mixture; this may require an eightfold excessPeptide nucleic acids: structure, properties, applications, strategies and practice of chemical synthesis of the reagents with respect to the carrier loading.Preactivation of PNA monomers makes it possible to increase the condensation efficiency and the average yields in the attachment of monomeric fragments up to 96%.109 Obviously, the problem of efficiency of each individual approach to the synthesis of PNA molecules has no unambiguous solution.The choice of the most adequate strategy for the PNA synthesis depends on the goal and facilities as well as on the number ofPNAoligomers to be synthesised, the scale of synthesis, composition and purity of PNA oligomers. VIII. Conclusion The design of the most efficient method for the synthesis of PNA oligomers requires that a rational compromise between the efficiency and economy of the synthetic process be found. On the one hand, one has to reach the maximum yields of condensation products and the choice of synthetic strategy must take into account both the nature of the activating reagent and other factors discussed in this review.On the other hand, the synthesis of PNA should be rational. This implies that the synthetic procedure should not only be efficient, but also fast and as cheap as possible. Examples of both the approaches to PNA synthesis have been presented in this review. In low-budget laboratories, where the primary goal is eco- nomic PNA synthesis, it is the `slow' synthesis that is most commonly used. Although this procedure is rather laborious, it gives excellent yields in the condensation step. `Fast' processes are utilised in the majority of large biotechno- logical companies which manufacture PNA oligomers for com- mercial purposes.Here, large excesses of PNA monomers and the most potent activating reagents are employed in order to ensure high yields of the condensation products. However, quantitative yields cannot be attained due to a reduction of the condensation time; therefore, pure PNA oligomers can be obtained by virtue of complex isolation procedures. The most rational synthetic strategies combine the best features of both approaches, viz., the `fast' and the `slow' syntheses of PNA. This work has been written within the framework of the State Programme for Support of Leading Scientific Schools of the Russian Federation (Grant No. 00-15-97944). References 1. P E Nielsen, M Egholm, R H Berg, O Buchardt Science 254 1497 (1991) 2.M Egholm, O Buchardt, L Christensen, C Behrens, S M Freier, D A Driver, R H Berg, S K Kim, B Norden, P E Nielsen Nature (London) 365 566 (1993) 3. B Hyrup,M Egholm, P E Nielsen, P Wittung, B Norden, O Buchardt J. Am. Chem. Soc. 116 7964 (1994) 4. P E Nielsen Acc. Chem. Res. 32 624 (1999) 5. M Egholm, O Buchardt, P E Nielsen, R H Berg J. Am. Chem. Soc. 114 1895 (1992) 6. M Egholm, P E Nielsen, O Buchardt, R H Berg J. Am. Chem. Soc. 114 9677 (1992) 7. M Egholm, C Behrens, L Christensen, R H Berg, P E Nielsen, O Buchardt J. Chem. Soc., Chem. Commun. 800 (1993) 8. P E Nielsen, G Haaima Chem. Soc. Rev. 26 73 (1997) 9. H J Larsen, T Bentin, P E Nielsen Biochim. Biophys. Acta 1489 159 (1999) 10. K L Dueholm, P E Nielsen New J.Chem., 21 19 (1997) 11. P Wittung, P E Nielsen, O Buchardt, M Egholm Nature (London) 368 561 (1994) 12. E Uhlmann, A Peyman, G Breipohl, D W Will Angew. Chem., Int. Ed. Engl. 37 2796 (1998) 13. P E Nielsen, M Egholm, O Buchardt Bioconj. Chem. 5 3 (1994) 14. B Hyrup, P E Nielsen Bioorg. Med. Chem. 4 5 (1996) 81 15. S C Brown, S A Thompson, J M Veal, D G Davis Science 265 777 (1994) 16. M Eriksson, P E Nielsen Q. Rev. Biophys. 29 369 (1996) 17. N Sugimoto, N Satoh, K Yasuda, S-I Nakano Biochemistry 40 8444 (2001) 18. M Leijon, A Graslund, P E Nielsen, O Buchardt, B Norden, S M Kristensen,M Eriksson Biochemistry 33 9820 (1994) 19. M Eriksson, P E Nielsen Nat. Struct. Biol. 3 410 (1996) 20. R Rasmussen, J S Kastrup, J N Nielsen, J M Nielsen, P E Nielsen Nat.Struct. Biol. 4 98 (1997) 21. C Meier, J Engels Angew. Chem., Int. Ed. Engl. 31 1008 (1992) 22. D J Rose J. Anal. Chem. 65 3545 (1993) 23. W M Pardridge, R J Boado, Y-S Kang Proc. Natl. Acad. Sci. USA 92 5592 (1995) 24. J C Norton, M A Piatyszek, W E Wright, J W Shay, D R Corey Nat. Biotechnol. 14 615 (1996) 25. V V Demidov, V N Potaman, M D Frank-Kamenetskii, M Egholm, O Buchardt, S H Sonnichsen, P E Nielsen Biochem. Pharmacol. 48 1310 (1994) 26. L Good, P E Nielsen Antisense Nucl. Acids Drug Devel. 7 431 (1997) 27. P E Nielsen, M Egholm, R H Berg, O Buchardt Anti-Cancer Drug Design 8 53 (1993) 28. J Wang, E Palecek, P E Nielsen, G Rivas, X Cai, H Shiraishi, N Dontha, D Luo, P A M Farias J.Am. Chem. Soc. 118 7667 (1996) 29. T J Griffin, L M Smith Anal. Biochem. 260 56 (1998) 30. U Giesen,W Kleider, C Berding, A Geiger, H Orum, P E Nielsen Nucl. Acids Res. 26 5004 (1998) 31. D Y Cherny, B P Belotserkovskii, M D Frank-Kamenetskii, M Egholm, O Buchardt, R H Berg, P E Nielsen Proc. Natl. Acad. Sci. USA 90 1667 (1993) 32. P E Nielsen, M Egholm, O Buchardt J. Mol. Recognit. 7 165 (1994) 33. P E Nielsen Methods Enzymol. 340 329 (2001) 34. E Ferrer, A Shevchenko, R Eritja Bioorg. Med. Chem. 8 291 (2000) 35. L Betts, J A Josey, J M Veal, S R Jordan Science 270 1838 (1995) 36. R Gambari Curr. Pharm. Des. 7 1839 (2001) 37. P E Nielsen, M Egholm Bioorg. Med. Chem. 9 2429 (2001) 38. Yu N Kosaganov, D A Stetsenko, E N Lubyako, N P Kvitko, Yu S Lazurkin Mol.Biol. 32 121 (1998) a 39. M Egholm, L Christensen, K Dueholm, O Buchardt, J Coull, P E Nielsen Nucl. Acids Res. 23 217 (1995) 40. V V Demidov, M V Yavnolovich, B P Belotserkovskii, M D Frank-Kamenetskii, P E Nielsen Proc. Natl. Acad. Sci. USA 92 2637 (1995) 41. P Wittung, P E Nielsen, B Norden J. Am. Chem. Soc. 118 7049 (1996) 42. V V Demidov, M V Yavolovich, M D Frank-Kamenetskii Biophys. J. 72 2763 (1997) 43. K N Ganesh Curr. Sci. 75 1346 (1998) 44. H Kuhn, V V Demidov, P E Nielsen, M D Frank-Kamenetskii J. Mol. Biol. 286 1337 (1999) 45. M C Griffith, L S Risen,M J Greig, E A Lesnik, K G Sprankle, R H Griffey, J S Kiely, S M Freier J. Am. Chem. Soc. 117 831 (1995) 46. S E Hamilton,M Iyer, J C Norton, D R Corey Bioorg. Med.Chem. Lett. 6 2897 (1996) 47. B M Tyler-McMahon, J A Stewart, J Jackson,M D Bitner, A Fauq, D J McCormick, E Richelson Biochem. Pharmacol. 62 929 (2001) 48. J Micklefield Curr. Med. Chem. 8 1157 (2001) 49. T Koch,M Naesby, P Wittung, M Jùrgensen, C Larsson, O Buchardt, C J Stanley, B Norden, P E Nielsen, H érum Tetrahedron Lett. 36 6933 (1995) 50. C G Simmons, A E Pitts, L D Mayfield, J W Shay, D R Corey Bioorg. Med. Chem. Lett. 7 3001 (1997) 51. M Pooga, U Sommets,M Hallbrink, A Valkna, K Saar, K Rezaei, U Kahl, J-X Hao, Z Wiesenfeld-Hallin, T Hokfelt, T Bartfai, U Langel Nat. Biotechnol. 16 857 (1998) 52. G Aldrian-Herrada,M G Desarmenien, H Orcel, L Boissin-Agasse, J Mery, J Brigidou, A Rabie Nucl. Acids Res. 26 4910 (1998) 53. S Basu, E Wickstrom Bioconj.Chem. 8 481 (1997) 54. P Garner, S Dey, Y Huang, X Zhang Org. Lett. 1 403 (1999) 55. P Garner, S Dey, Y Huang J. Am. Chem. Soc. 122 2405 (2000) 56. P Garner, B Sherry, S Moilanen, Y Huang Bioorg. Med. Chem. Lett. 11 2315 (2001)82 57. P E Nielsen, M Egholm, in Peptide Nucleic Acids: Protocols and Applications. Synthesis of PNA Oligomers by Fmoc Chemistry (Eds P E Nielsen,M Egholm) (Wymondham: Horizon Scientific Press, 1999) p. 1 58. P E Nielsen Antiviral News 1 37 (1993) 59. P E Nielsen, H Orum, in Molecular Biology: Current Innovations and Future Trends (Eds A M Griffin, H G Griffin) (Wymondham: Horizon Scientific Press, 1995) p. 73 60. M Egholm, P E Nielsen, O Buchardt, R H Berg, in Innovations and Perspectives in Solid Phase Synthesis.Peptides, Proteins and Nucleic Acids. Biological and Biomedical Applications (Ed. R Epton) (Birmingham: Mayflower Worldwide, 1994) p. 145 61. P E Nielsen, in Perspectives in Drug Discovery and Design Vol. 4 (ESCOM Science Publishers, 1995) p. 76 62. A Ray, B Norden FASEB J. 14 1041 (2000) 63. P E Nielsen Methods Enzymol. 313 156 (2000) 64. P E Nielsen Pharmacol. Toxicol. 86 3 (2000) 65. H Knudsen, P E Nielsen Anti-Cancer Drugs 8 113 (1997) 66. P E Nielsen, M Egholm, R H Berg, O Buchardt, in Antisense Research and Applications (Eds S Crook, B Lebleu) (Boca Raton, FL: CRC Press, 1993) p. 363 67. J C Hanvey, N J Peffer, J E Bisi, S A Thomson, R Cadilla, J A Josey, D J Ricca, C F Hassman, M A Bonham, K G Au, S G Karter, D A Bruckenstein, A L Boyd, S A Noble, L E Babiss Science 258 1481 (1992) 68.P E Nielsen, M Egholm, O Buchardt Gene 149 139 (1994) 69. M A Bonham, S Brown, A L Boyd, P H Brown, D A Bruckenstein, J C Hanvey, S A Thomson, A Pipe, C F Hassman, J E Bisi, B C Froehler,M D Matteucci, R W Wagner, S A Noble, L E Babiss Nucl. Acids Res. 23 1197 (1995) 70. P E Nielsen Annu. Rev. Biophys. Biomol. Struct. 24 167 (1995) 71. A De Mesmaeker, K-M Altman, A Waldner, S Wendeborn Curr. Opin. Struct. Biol. 5 343 (1995) 72. H J Larsen, P E Nielsen Nucl. Acids Res. 24 458 (1996) 73. H Knudsen, P E Nielsen Nucl. Acids Res. 24 494 (1996) 74. C Gambacorti-Passerini, L Mologni, C Bertazolli, E Marchesi, F Grignani, P E Nielsen Blood 88 1411 (1996) 75.T A Vickers, M C Griffith, K Ramasamy, L M Risen, S M Freier Nucl. Acids Res. 23 3003 (1995) 76. B P Casey, P M Glazer Prog. Nucl. Acid Res. 67 163 (2001) 77. F Bergmann,W Bannwarth, S Tam Tetrahedron Lett. 36 6823 (1995) 78. P E Nielsen Curr. Opin. Biotechnol. 10 71 (1999) 79. G Dieci, R Corradini, S Sforza, R Marchelli, S Ottonello J. Biol. Chem. 276 5720 (2001) 80. A Beletskii, Y-K Hong, J Pehrson,M Egholm,W M Strauss Proc. Natl. Acad. Sci. USA 98 9215 (2001) 81. C Mischiati,M Borgatti, N Bianchi, C Rutigliano, M Tomassetti, G Feriotto, R Gambari J. Biol. Chem. 274 33 114 (1999) 82. H S Misra, P K Pandey,M J Modak, R Vinayak, V N Pandey Biochemistry 37 1917 (1998) 83. J Weiler, H Gausepohl, N Hauser, O N Jensen, J D Hoheisel Nucl.Acids Res. 25 2792 (1997) 84. X Liu, S Balasubramanian Tetrahedron Lett. 41 6153 (2000) 85. S E Hamilton, C G Simmons, I S Kathiriya, D R Corey Chem. Biol. 6 343 (1999) 86. J Lohse, P E Nielsen, N Harrit, O Dahl Bioconj. Chem. 8 503 (1997) 87. O Seitz, F Bergmann, D Heindl Angew. Chem., Int. Ed. Engl. 38 2203 (1999) 88. F Lesignoli, A Germini, R Corradini, S Sforza, G Galaverna, A Dossena, R Marchelli J. Chrom. A 922 177 (2001) 89. T J Griffin, W Tang, L M Smith Nat. Biotechnol. 15 1368 (1997) 90. C Carlsson, M Jonsson, B Norden, M T Dulay, R N Zare, J Noolandi, P E Nielsen, L C Tsui, J Zielenski Nature (London) 380 260 (1996) 91. E Palecek, M Fojta,M Tomschik, J Wang J. Biosens. Bioelectron. 13 621 (1998) 92.J Wang J. Biosens. Bioelectron. 13 757 (1998) 93. J Wang, G Rivas, X Cai,M Chicharro, C Parrado, N Dontha, A Begleiter,M Mowat, E Palecek, P E Nielsen Anal. Chim. Acta 344 111 (1997) 94. L Christensen, R Fitzpatrick, B Gildea, K H Petersen, H F Hansen, T Koch,M Egholm, O Buchardt, P E Nielsen, J Coull,R H Berg J. Pept. Sci. 3 175 (1995) S I Antsypovitch 95. T Koch, H F Hansen, P Andersen, T Larsen, H G Batz, K Ottesen, H Orum. J. Pept. Res. 49 80 (1997) 96. D W Will, G Breipohl, D Langner, J Knolle, E Uhlmann Tetrahedron 51 12069 (1995) 97. S A Thomson, J A Josey, R Cadilla, M D Gaul, C F Hassman, M J Luzzio, A J Pipe, K L Reed, D J Ricca, R W Wiethe, S A Noble Tetrahedron 51 6179 (1995) 98. R B Merrifield J. Am. Chem. Soc. 85 2149 (1963) 99.K L Dueholm, M Egholm, C Behrens, L Christensen, H F Hansen, T Vulpius, K H Petersen, R H Berg, P E Nielsen, O Buchardt J. Org. Chem. 59 5767 (1994) 100. L Christensen, R Fitzpatrick, B Gildea, B Warren, J Coull, in Innovations and Perspectives in Solid Phase Synthesis. Peptides, Proteins and Nucleic Acids. Biological and Biomedical Applications (Ed. R Epton) (Birmingham: Mayflower Worldwide, 1994) p. 149 101. L A Carpino Acc. Chem. Res. 20 401 (1987) 102. P Kocienski Protecting Groups (Stuttgart: Georg Thieme, 1994) 103. E Sonveaux, in Protocols for Oligonucleotides Conjugates, Methods in Molecular Biology Vol. 26 (Ed. S Agrawal) (Totowa: Humana Press, 1994) p. 1 104. Z Timar, L Kovacs, G Kovacs, Z Schmel J. Chem. Soc., Perkin.Trans. 1 19 (2000) 105. M Kuwahara,M Arimitsu, M Sisido J. Am. Chem. Soc. 121 256 (1999) 106. R Casale, I S Jensen, M Egholm, in Peptide Nucleic Acids: Proto- cols and Applications. Synthesis of PNA Oligomers by Fmoc Chemistry (Eds P E Nielsen, M Egholm) (Wymondham: Horizon Scientific Press, 1999) p. 39 107. G Breipohl, D W Will, A Peyman, E Uhlmann Tetrahedron 53 14 671 (1997) 108. E Uhlmann, D W Will, G Breipohl, D Langner, A Ryte Angew. Chem., Int. Ed. Engl. 35 2632 (1996) 109. A C van der Laan, R Brill, R G Kuimelis, E Kuyl-Yeheskiely, J H van Boom, A Andrus, R Vinayak Tetrahedron Lett. 38 2249 (1997) 110. V A Efimov, M V Choob, A A Buryakova, O G Chakhmakhcheva Nucleosides Nucleotides 17 1671 (1998) 111. A C van der Laan, N J Meeuwenoord, E Kuyl-Yeheskiely, R S Oosting, R Brands, J H van Boom Recl.Trav. Chim. Pays-Bas 114 295 (1995) 112. D A Stetsenko, S V Veselovskaya, E N Lubyako, V K Potapov, T L Azhikina, E D Sverdlov Dokl. Akad. Nauk 338 695 (1994) b 113. G Aldrian-Herrada, A Rabie, R Winersteiger, J Brugidou J. Pept. Sci. 4 266 (1998) 114. G Breipohl, J Knolle, D Langner, G Omalley, E Uhlmann Bioorg. Med. Chem. Lett. 6 665 (1996) 115. G Kovacs, Z Timar, Z Kele, L Kovacs, in The Fourth International Electronic Conference on Synthetic Organic Chemistry (ECSOC-4), 2000 p. B0003. www.mdpi.org/ecsoc-4.htm 116. V A Efimov, M V Choob, A A Buryakova, A L Kalinkina, O G Chakhmakhcheva Nucl. Acids Res. 26 566 (1998) 117. M Eriksson, L Christensen, J Schmidt, G Haaima, L Orgel, P E Nielsen New J.Chem. 22 1055 (1998) 118. B Due Larsen, C Larsen, A Holm, in Peptides 1990, Proceedings of the 21st European Peptide Symposium (Eds E Giralt, D Andreu) (Leiden: ESCOM, 1991) p. 183 119. E Bayer, C Goldammer, in Peptides, Proceedings of the 12th American Peptide Symposium (Eds J A Smith, J E Rivier) (Leiden: ESCOM, 1992) p. 589 120. S M Meister, S B H Kent, in Peptides�Structure and Function: Proceedings of the Eighth American Peptide Symposium (Eds V J Hruby, D H Rich) (Rockford, IL: Pierce Chem., 1984) p. 103 121. C Mapelli,M D Sverdloff, in Peptides 1990, Proceedings of the 21st European Peptide Symposium (Eds E Giralt, D Andreu) (Leiden: ESCOM, 1991) p. 316 122. D Le-Nguyen, A Heitz, B Castro J. Chem. Soc., Perkin. Trans. 1 1915 (1987) 123. M Schnolzer, P Alewood, A Jones, D Alewood, S B H Kent Int. J. Pept. Protein Res. 40 180 (1992) 124. J P Briand, J Coste, A Van Dorsselaer, B Raboy, J Neimark, B Castro, S Muller, in Peptides 1990, Proceedings of the 21st Euro- pean Peptide Symposium (Eds E Giralt, D Andreu) (Leiden: ESCOM, 1991) p. 80Peptide nucleic acids: structure, properties, applications, strategies and practice of chemical synthesis 125. M Schnolzer, P Alewood, A Jones, S B H Kent, in Peptides, Proceedings of the 12th American Peptide Symposium (Eds J A Smith, J E Rivier) (Leiden: ESCOM, 1992) p. 623 126. J Jezek, R A Houghten, in Peptides 1990, Proceedings of the 21st European Peptide Symposium (Eds E Giralt, D Andreu) (Leiden: ESCOM, 1991) p. 74 127. G E Reid, R J Simpson Anal. Biochem. 200 301 (1992) 128. G B Fields J. Am. Chem. Soc. 113 4202 (1991) 129. S Scarfi, A Gasparini, G Damonte, U Benatti Biochem. Biophys. Res. Commun. 236 323 (1997) 130. J P Tam,W F Heath, R B Merrifield J. Am. Chem. Soc. 108 5242 (1986) 131. G Haaima, A Lohse, O Buchardt, P E Nielsen Angew. Chem., Int. Ed. Engl. 35 1939 (1996) 132. A PuÈ schl, S Sforza, G Haaima, O Dahl, P E Nielsen Tetrahedron Lett. 39 4707 (1998) 133. B Armitage, D Ly, T Koch, H Frydenlund, H Orum, G B Schuster Biochemistry 37 9417 (1998) 134. N Svanvik, G Westman, D Wang,M Kubista Anal. Biochem. 281 26 (2000) 135. S Sforza, R Corradini, S Ghirardi, A Dossena, R Marchelli Eur. J. Org. Chem. 2905 (2000) 136. S Sforza, G Haaima, R Marchelli, P E Nielsen Eur. J. Org. Chem. 197 (1999) 137. A Lenzi, G Reginato, M Taddei, E Trifilieff Tetrahedron Lett. 36 1717 (1995) 138. H Gausepohl, U Pieles, R W Frank, in Peptides, Proceedings of the 12th American Peptide Symposium (Eds J A Smith, J E Rivier) (Leiden: ESCOM, 1992) p. 523 139. D R Corey Trends Biotechnol. 15 224 (1997) 140. J Norton, J H Waggenspack, E Varnum, D R Corey Bioorg. Med. Chem. 3 437 (1995) 141. L D Mayfild, D R Corey Anal. Biochem. 268 401 (1999) 142. M Quibell, T Johnson,W G Turnell Biomed. Pep. Protein Nucl. Acids 1 (1994-1995) 143. E Atherton, in Solid Phase Peptide Synthesis: A Practical Approach (Practical Approach Series) (Eds E Atherton, R C Sheppard) (Oxford: Oxford University Press, 1989) p. 117 144. D A Stetsenko, E N Lubyako, V K Potapov, T L Azhikina, E D Sverdlov Tetrahedron Lett. 37 3571 (1996) 145. D A Stetsenko, E N Lubyako, V K Potapov, T L Azhikina, E D Sverdlov Dokl. Akad. Nauk 343 834 (1995) b 146. A C van der an, R StroÈ mberg, J H van Boom, E Kuyl-Yeheskiely, V A Efimov, O G Chakhmakhcheva Tetrahedron Lett. 37 7857 (1996) 147. V A Efimov, M V Choob, A L Kalinkina, O G Chakhmakhcheva, R StroÈ mberg, A C van der Laan, N J Meeuwenoord, E Kuyl- Yeheskiely, J H van Boom Collect. Czech. Chem. Commun. (Spec. Issue) 61 262 (1996) 148. K H Petersen, D K Jensen, M Egholm, P E Nielsen, O Buchardt Bioorg. Med. Chem. Lett. 5 1119 (1995) 149. P J Finn, N L Gibson, R Fallon, A Hamilton, T Brown Nucl. Acids Res. 24 3357 (1996) 150. R Vinayak, A C van der Laan, R Brill, K Otteson, A Andrus, E Kuyl-Yeheskiely, J H van Boom Nucleosides Nucleotides 16 1653 (1997) 151. L D Mayfield, D R Corey Bioorg. Med. Chem. Lett. 9 1419 (1999) 152. F Bergmann,W Bannwarth Tetrahedron Lett. 36 1839 (1995) 153. J Coste, M-N Dufour, A Pantaloni, B Castro Tetrahedron Lett. 31 669 (1990) a�Mol. Biol. (Engl. Transl.) b�Dokl. Chem. (Engl. Transl.

ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC