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). 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