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QUARTERLY REVIEWS - AN OUTLINE OF TECHNETIUM CHEMISTRY By R. COLTON (CHEMISTRY DIVISION ATOMIC ENERGY RESEARCH ESTABLISHMENT HARWELL DIDCOT BERKS) AND R. D. PEACOCK (DEPARTMENT OF CHEMISTRY UNIVERSITY OF BIRMINGHAM EDGBASTON BIRMINGHAM 15) THE first synthetic element was discovered in 1937 when Perrier and Segr2 showed that a radioactive material produced by irradiating molyb- denum sheet with deuterons behaved in such a way that it could only have been element 43. They have since named the element “technetium” from the Greek word meaning artificial.2 Much of the work on technetium has been carried out at the Oak Ridge National Laboratory by Boyd and his collaborators who have recently produced technetium in kilogramme quantities. It is reasonable to assume that the element will soon be as readily available and at no greater cost than the less-common platinum metals.The subject has been reviewed by Tribalat3 and more recently by Anders4 and by Boyd.5 A considerable amount of new information has appeared meanwhile and is incorporated in the present Review. The reader may find it useful to refer to the companion Review on rhenium6 to ap- preciate the close resemblance between the two elements. The Natural Occurrence of Technetium.-There have been several claims to the discovery of element 43 in nature the best known being that of Noddack et al. who purported to have found it at the same time as they discovered rhenium.’ Subsequently as knowledge of rhenium and its chemistry developed rapidly no more was heard of element 43 which had been named “masurium” by the claimants and its existence remained in doubt until its synthesis by Perrier and Segrk.It was quickly realised that with the exception of technetium-98 the isotopes were all too short lived for any primordial technetium to be Perrier and Segre J. Chem. Phys. 1937 5 712. Perrier and Segre Nature 1947 159 24. Tribalat “Rhenium et Technetium,” 1957 Gauthier-Villars Paris. Boyd J. Cheni. Edw. 1959 36 7. Woolf Quart. Rev. 1961 15 372. * Anders Ann. Rev. Nuclear Sci. 1959 9 203. ’ Noddack Tacke and Berg Naturwiss. 1925 13 567. 299 1 300 QUARTERLY REVIEWS detectable today. The half life of technetium-98 was not known and several investigator^^*^ claimed to have detected it in Nature by using neutron activation analysis techniques and observing an activity with a 6-hr. half life which was taken to be technetium-99m.A later search for natural technetium failed,l* and since the half life of technetium-98 determined at a later date is about lo6 yr. the occurrence of primordial technetium is very unlikely. Natural uranium undergoes spontaneous fission to a small extent and the technetium-99 formed as a fission product has been detected.ll It has also been reported that technetium lines can be observed in the spectra of certain stars.12 Isotopes of Technetium.-Perrier and SegrC discovered the isotopes technetium-95 and -97 which have half lives of 60 days and 90 days respectively. Many other isotopes have been made since that time the more important of which are listed in Table 1. TABLE 1 . Isotopes of technetium. Isotope Preparation Half-life Decay* Ref. 92Mo ( p y) 93Tc 93mTc 92Mo (a n) 93T~ "3gTc 94Mo (d 2n) 9 4 T ~ 94Mo ( p n) g 4 T ~ 95Mo ( p n) 9 5 T ~ 92Mo (a p ) 95T~ 95Mo ( p y) 96T~ 93Nb (a n) 9 6 T ~ 9 6 R ~ (n y ) 9 7 R ~ 97Tc 97Mo (d 2n) 9 7 T ~ 98Mo ( p n) 98Tc 98Mo (n y) 99Mo 99Tc fission of U (6.2%) looMo ( p n) looTc 99Tc (n y) loOTc looMo (a n) lolTc 43.5 min.2.8 hr. 52.5 min. 60 days 20 hr. 51.5 min. 4.3 days 90 days 2.6 x lo6 yr. 2 x lo6 yr. 5-9 hr. 2.12 x lo5 yr. 15.8 sec. 15 min. i.t. 13 14 K 15 pS K y 14 16 17 K 3% i.t. 17 i.t. 17 K 18 i.t. 18 19 K 20 ? 21 22 P- K Y 18 23-27 28 29 ? 18 Y 30 * K = K electron capture. i.t. = internal rearrangement. m = metastable state. * Herr 2. Naturforsch. 1954 9A 907. Alperovitch and Miller Nature 1955 176 299. lo Boyd and Larson J. Chem. Phys. 1956,24 222. l1 Kenna and Kuroda J. Inorg.Nuclear Chem. 1962 23 143. l2 Merrill Astrophys. J. 1952 116 21. l3 Easterday and Medicus Phys. Rev. 1953 89 752. l4 Motta and Boyd Phys. Rev. 1948 74 220. l5 Levi and Papineau Compt. rend. 1954 239 2313. l6 Marmier Phys. Rev. 1948 73 1208. g = ground state. Medicus Prieswerk and Scherrer Helv. Phys. Acta 1950 23 299. Motta and Boyd Phys. Rev. 1948 74 344. COLTON AND PEACOCK TECHNETIUM CHEMISTRY 30 1 The Laboratory Handling of Technetium.-The handling of technetium (99Tc) on a small scale (<& g.) does not present a serious health hazard provided elementary precautions are taken. This isotope emits only weak /3 radiation (/3,, = 0.3 Mev) which is effectively stopped by ordinary laboratory glassware and there is no associated y radiation The essential requirements for safe handling are a well-ventilated box or cupboard a simple means of monitoring the working area and the rigorous avoidance of all operations which might allow fumes or dust to escape to the atmo- sphere.Standard quantitative techniques on the semimicro-scale are easily adapted for work with technetium for example it is convenient to evapor- ate solutions under infrared lamps; to safeguard against loss by spraying a beaker is inverted over the vessel. These remarks should suffice to show that handling a weak /3 emitter is a very much easier matter than handling an 01 emitter such as polonium which requires the full resources of a radiochemical lab~ratory.~~ The Isolation and Purification of Technetium.-Early samples of techne- tium were prepared by deuteron or neutron irradiation of molybdenum and separations were worked out by using rhenium and platinum as carriers.Technetium is now almost wholly obtained from fission product wastes but many of the early methods of separation are still used. The first weighable amounts of technetium were produced by the neutron irradiation of 5.7 kg. of molybdenum powder in a nuclear reactor.32 The technetium could be readily separated from the bulk of the molybdenum by dissolving the metal in concentrated sulphuric acid and distilling over the volatile heptaoxide. Technetium heptasulphide was precipitated from the distillate platinum disulphide being used as a carrier. After the precipitate had been dissolved in ammoniacal hydrogen peroxide the technetium heptaoxide was again distilled from sulphuric acid. A separation from the rhenium present as an impurity in the molybdenum powder was obtained by repeating the sulphide precipitation step this time in hydrochloric acid stronger than 8 ~ .Technetium heptasulphide is not precipitated under these conditions but rhenium heptasulphide separates from the solution. l9 Motta and Boyd Phys. Rev. 1948 74 344. 2o Boyd Phys. Rev. 1954,95 113. 21 Boyd Siles Larson and Baldock Phys. Rev. 1955 99 1030. 22 Katcoff Phys. Rev. 1955 99 1618. 23 Seaborg and Segre Phys. Rev. 1939 55 808. 24 Lincoln and Sullivan Nut. Nuclear Energy Series W 1951 9 228. 25 Motta Boyd and Larson Phys. Rev. 1947 72 1270. 26 Sullivan Nat. Nuclear Energy Series ZV 1951 9 783. 27 SegrC and Wu Phys. Rev. 1940 57 552. 28 House Colligan Kundu and Pool Phys. Rev. 1952 86 654. Boyd Larson and Parker Phys.Rev. 1952 86 1051. 30 Boyd and Kettle Phys. Rev. 1951 83 216. 31 Bagnall Quart. Rev. 1957 11 30. 32 Boyd Larson and Motta Unclassified Document 1948 A.E.C.D. 2151. 302 QUARTERLY REVIEWS Irradiation of molybdenum is unsuitable for the production of large amounts of technetium and accordingly attention was soon turned to the recovery of technetium from fission products. Parker Reed and R a ~ c h ~ ~ isolated milligramme amounts of technetium from several kilogrammes of uranium metal which had been irradiated in a nuclear reactor. The fuel elements were dissolved in 37 % hydrochloric acid and the resulting slurry of uranium tetrachloride was slowly oxidised by hydrogen peroxide and bromine water to uranyl chloride. The element was isolated by a series of sulphide precipitations and distillation of the heptaoxide as described above.The first gram of technetium was isolated from fission-product waste solutions by precipitating tetraphenylarsonium pertechnetate with tetra- phenylarsonium perchlorate as a carrier dissolving the mixed salts in concentrated sulphuric acid and electrolysing the solution at platinum electrode~.~~ The black deposit obtained was dissolved in 18~-sulphuric acid technetium lieptaoxide was distilled out of the solution and the element finally isolated by precipitation of the heptasulphide. In a modi- fication the mixed precipitate of tetraphenylarsonium pertechnetate and perchlorate was dissolved in alcohol and passed through an anion ex- change resin in the chloride form. The pertechnetate and perchlorate ions were strongly absorbed and the expensive tetraphenylarsonium chloride was recovered in the eluent.The technetium was eluted quantitatively from the resin by 2~-sodium hydroxide. More recently solvent extraction has been used to recover the fission product technetium from the residue remaining after the fluorination of irradiated uranium fuel elements.35 It was extracted by 0.3M-trilaurylamine in a hydrocarbon diluent and after removal of uranium neptunium and nitrate ion it was back-extracted into 4~-sodium hydroxide solution. Boyd and Larson have made an extensive study of the extraction of the pertechnetate ion into about 70 organic solvent^^^-^^ including alcohols esters ketones ethers amines and hydrocarbons. They found that the presence of an electron-donating oxygen or nitrogen atom in the solvent was essential for extraction.The partitioning species was shown to be the pertechneta te ion and with technetium concentrations below 1 0 - 3 ~ the extraction coefficient was independent of the concentration of technetium. Boyd and Larson have also studied the solvent extraction of technetium in valency states other than seven.39 They found that chloride complexes of technetium(v) in 12~-hydrochloric acid were not extracted into ether but partial extraction (E,"-3) was obtained with 0-1M-trioctyl- 33 Parker Reed and Rauch Unclassified Document 1948 A.E.C.D. 2043. 34 Cobble Nelson Parker Smith and Boyd J . Amer. Chem. Soc. 1952,74 1852. 35 Coleman Kappelman and Weaver Nuclear Sci. and Eng. 1960 8 507. 36 Boyd and Larson Unclassified Document 1957 ORNL 2386 26. 37 Boyd and Larson Unclassified Document 1958 ORNL 2384 5.38 Boyd and Larson J. Phys. Chem. 1960,64 988. 39 Boyd and Larson Unclassified Document 1959 ORNL 2782 12. COLTON AND PEACOCK TECHNETIUM CHEMISTRY 303 phosphine oxide in cyclohexane. The hexachlorotechnetate(1v) ion TcClG2- was extracted efficiently from G~-hydrochloric acid by trioctylphosphine oxide in cyclohexane. Radiochemical techniques have been used to follow the separation of technetium from other elements on the small scale and at tracer level particularly from molybdenum and rhenium. The coprecipitation of tetraphenylarsonium pertechnetate with perchlorate and perrhenate was studied soon after the war.4o Tribalat and Beydon41 extracted tetra- phenylarsonium pertechnetate with chloroform from alkaline solutions. On the tracer scale it has been shown that pertechnetate ion is extracted by pyridine from sodium hydroxide solutions with an extraction coefficient of about 800.42 Later work showed that the extraction coefficient increased with increasing concentrations of sodium hydroxide.43 It was also shown that technetium in potassium carbonate solutions could be extracted efficiently with ethyl methyl ketone.43.Ion-exchange techniques have been used to separate technetium from both rhenium and molybdenum. The methods are all similar in principle and the results are tabulated in Table 2. Technetium has been separated from molybdenum and rhenium by electrical methods. Polarographic results had shown that pertechnetate TABLE 2. Ion exchange separations of technetium from molybdenum and rhenium. Separation Resin TcO from Anion Dowex- 1 Tc04- from Amberlite IRA- (weakly alkaline) 400 (C104-) Tc04- from Re04- Dowex-1 Tc04- from Re0,- Dowex-2 (and form) (O.5M-HCI) (Cl-) neutral (C104-1 neutral (SO,? TcO from Re04- Amberlite ( 1 M-HCl) IRA400 K1-1 Eluent Effici- Ref.(a) Mo in IM-HCI Quanti- 44 (6) Tc in ~M-HNO tative (a) Mo in 10% NaOH Quanti- 45 (6) Tc in O*SM-NH,.CNS tative O.2M-HCIO4 Poor 46 ency (a) Re in 0.1 M-(NH,),SO +NH,CNS in Moderate 47 NaOH; pH 8.3 (b) Tc by increasing salts to 1~ (a) Re with 5 % NH,*CNS Quanti- 48 (b) Tc with ~M-HNO tative 40 Parker and Martin Unclassified Document 1950 ORNL 870,47. 41 Tribalat and Beydon Analyt. Chim. Acta 1953 8 22. 4p Goishi and Libby J. Arner. Chem. Suc. 1952 74 6109. 43 Rimshaw and Malling Analyt. Chem. 1961 33 751. 44 Huffman Oswalt and Williams J.Inorg. Nuclear Chem. 1956 3 49. 45 Hall and Johns J. Amer. Chem. Soc. 1953 75 5787. 46 Sen Sarma Anders and Miller J. Phys. Chem. 1959 63 559. 47 Atterbury and Boyd J . Amer. Chem. SOC. 1950 72 4805. Pirs and Magee Talanta 1961 8 395. 304 QUARTERLY REVIEWS ion could be reduced in sodium hydroxide solution but that perrhenate and molybdate ions were not. Electrolysis of an alkaline solution at a controlled potential produced a deposit of technetium dioxide whilst the rhenium and molybdenum remained in solution.49 Carvallo has shown that technetium molybdenum and rhenium can be separated by electrophoresis techniques after reduction to lower valency states.50 The best results were obtained when the electrolyte consisted of hydrazine sulphate and hydrazine hydrate at pH 9.Separation of the elements was obtained after 90 minutes at a potential of 200 volts. Chemistry of Technetium Introduction.-Technetium exhibits valencies ranging from 7 to 0. The commonest states encountered are 7 represented by the heptaoxide and the pertechnetates and 4 represented by the dioxide the tetra- chloride and the halogenotechnetates. At the present time each of the other valency states is represented by at least one compound. In this multiplicity of valency states technetium closely resembles its congener rhenium and its neighbour ruthenium. Corresponding technetium and rhenium compounds are often isostructural and are frequently similar in appearance and in physical and chemical properties. Like most of its neighbours in the Periodic Table technetium exhibits co-ordination numbers commonly of 4 or 6 but a co-ordination number of 8 has been recognised in the complex [Tc(diars),Cl,]+ClO,-.That technetium is reluctant to attain high co- ordination numbers is strikingly shown by the absence of a heptafluoride. The oxidation-reduction potentials in acid solution of the three Group VII elements confirm (Fig. 1) the intermediate position of technetium although the difference between rhenium and technetium is not very great. In practice it seems that only in low oxidation states such as +3 will the chemistry of technetium prove to more closely resemble that of manganese and the little that is known of technetium(r1r) agrees with this. Some calculated heats of formation and free energies of technetium compounds have been reported.51 Elementary Technetium.-The element has been prepared by hydrogen reduction of the heptasulphide T C ~ S ~ ~ at 1100" and of ammonium pertechnetate NH4Tc04 at 500-600°.53 The latter gives very pure metal.From solution the element has been obtained as a bright cathodic deposit on platinum by the electrolysis of ammonium pertechnetate in 2N- sulphuric acid in the presence of a trace of fluoride It is likely that other laboratory preparations of the element will become available in due 49 Rogers J. Amer. Chem. SOC. 1949 71 1507. 50 Carvallo Proc. Internat. Conf. Peaceful Uses of Atomic Energy 1956,28,97. 51 King and Cobble J. Amer. Chem. SOC. 1957 79 1559. 52 Fried J. Amer. Chem. SOC. 1948 70 442. 53 Cobble J. Amer. Chem. SOC. 1952 74 1852. 54 Motta Larson and Boyd Unclassified Document 1947 Mon C-99 22.COLTON AND PEACOCK TECHNETIUM CHEMISTRY 305 I 0.477 course; for example by analogy with rhenium the hydrogen reduction of ammonium hexachlorotechnetate(rv) (NH4),TcCls should give a quite pure preparation at comparatively low temperatures (-300"). Physical Properties.-The atomic weight (based on natural 0 = 16) of the element ("Tc) is 98.8 & 0.1 determined by the ratio TC/TC~O,~ compared with the mass spectrographic value of 98~913.~~ Technetium FIG. 1. Oxidation-reduction potentials of the manganese groupS1 (acid solution) 0-115 1.695 I i r- 1 -1.18 1.229 1 1 2.26 0.5 64 Mn-Mn2+ Mn0,- Mn042--- Mn04- I 0.78 1 0.738 (-0.5) 0.28 1 0.65 I Tc- Tc TcO TcO TcO (-0.4) 0-510 r 0.260 I 0.386 0-768 1 Re- Re ReO Re03-- Re04- I 0.367 I melts at 2150"56*57 (rhenium at 3180").Technetium crystallises in a close- packed hexagonal structure similar to rhenium ruthenium and osmium and the density calculated5* from the unit cell size is 1 1.50 g./cm.,. Values59 for the twelve co-ordinate metallic radii are shown in Table 3. 65 Ingram Hess and Hayden Phys. Rev. 1947 72 1269. 58 Parker Unclassified Document 1952 ORNL 1260 29. 67 Anderson Buckley Helliwell and Hume-Rothery Nature 1960 188 48. 58 Mooney Acta Cryst. 1948 1 161. Pauling "The Nature of the Chemical Bond," 3rd Edn. Oxford University Press 1960,403. 306 QUARTERLY REVIEWS TABLE 3. Metallic radii of technetium and its neighbours (in A)52 Cr 1 *276 Mo 1.386 W 1.394 Mn 1 ~268 Tc 1.361 Re 1 -393 Fe 1 -260 Ru 1.336 0 s 1.350 Chemical Properties.-The spongy metal tarnishes slowly in air and burns in oxygen.With fluorine the hexafluoride is fortned at 400" and with chlorine at the same temperature a mixture of the hexachloride and tetrachloride is obtained. Technetium dissolves in oxidising acids such as nitric (dilute or concentrated) sulphuric and aqua regia but not in hydrochloric Contrary to earlier it dissolves in aqueous hydrogen peroxide solutions the course of this reaction as with rhenium is undoubtedly influenced by the state of division of the metal. Oxides.-Technetium is known to form two oxides the volatile hepta- oxide Tc,O and the comparatively involatile dioxide TcO,. Technetium heptaoxide is the only product when the metal is burned in excess of oxygen at 500".g1*62 The yellow crystalline solid melts at 119.5" to a viscous yellow liquid which is presumably polymeric and could therefore contain six-co-ordinate technetium.The solid is not isostructural with rhenium heptaoxide and is apparently of lower ~ymrnetry.~ Unfortunately only the unit cell size of rhenium heptaoxide has been so it is not possible to determine whether the different symmetry of the technetium compound really indicates a fundamental change in structure. However it is interesting to note that although solid technetium heptaoxide conducts electricity and liquid technetium heptaoxide is non-conducting the reverse holds for rhenium heptaoxide; interesting too is the large liquid range of technetium heptaoxide compared with that of the rhenium compound. Technetium heptaoxide has a boiling point estimated from the vapour pressure curve of 311" & 2°,62 a value which has recently been con- firmed by direct meas~rernent.~~ It is stable up to the boiling point.64 The compound is weakly paramagnetks5 Chemically technetium heptaoxide is a stronger oxidising agent than rhenium heptaoxide.It dissolves readily in water and is also soluble in dioxan presumably forming a complex. Several workers have tried to make the trioxide TcO, by reaction between the metal and the heptaoxide66 (cf. rhenium trioxide) but none 6o Colton Unpublished observation 1961. 61 Boyd Cobble Nelson and Smith J. Amer. Chenz. SOC. 1952 74 556. 62 Smith Line and Bell J. Amer. Chem. Soc. 1952 74 4964. 63 Wilhelmi Acta Chem. Scand. 1955 9 1378. 64 Peacock Unpublished observation 1962. 65 Nelson Boyd and Smith J. Amer. Chem. SOC. 1954 76 348. 66 Cobble Smith and Boyd J. Amer. Chem.SOC. 1953 75 5777. COLTON AND PEACOCK TECHNETIUM CHEMISTRY 307 has been completely successful although Fried and Hall67* 68 described a volatile purple oxide TcO~.,,~ from the action of oxygen on technetium metal at 400-1000" (see also refs. 66 and 92). The black anhydrous dioxide like rhenium dioxide has the molyb- denum dioxide s t r ~ c t u r e ~ ~ . It sublimes unchanged at temperatures be- tween 900" and 1 and it is the ultimate product of the decomposition of the heptaoxide. Technetium dioxide has been prepared in a number of ways. The dihydrate can be readily precipitated by the reduction of a pertechnetate solution with zinc and hydrochloric (cf. rhenium dioxide) by the hydrolysis of a warm solution of a halogenotechnetate'with sodium carbonate or hydrogen carbonate or by the electrolysis of neutral or alkaline pertechnetate solutions at platinum electrode^.^^ The anhydrous compound made by dehydrating the hydrated dioxide in a vacuum at 250-500" has also been prepared directly by the thermal decomposition of ammonium pertechnetate NH4*T~04.67 The anhydrous compound is stable to the atmosphere at the ordinary temperature but it is easily oxidised to the heptaoxide by oxygen and it also interacts with chlorine at 300" to give a technetium(v) oxide chloride.60 The hydrated oxide is converted into pertechnetate by hydrogen peroxide in alkaline solution or in neutral solution by cerium(1v) or by bromine water.70 Oxyacids and their Salts.-Only one oxyacid of technetium and the corresponding series of salts are known-pertechnetic acid and the pertechnetates.Technetium heptaoxide dissolves in water to form a colourless solution which on evaporation gives dark red crystals of anhydrous pertechnetic acid H T c O ~ . ~ ~ Concentrated solutions of the acid are also red. Pertechnetic acid is a strong acid and like perrhenic acid it can be titrated by using indicators such as methyl red. The colourless pertechnetates closely resemble the perrhenates though they are more soluble in water e.g. the solubility of potassium pertechnet- ate KTcO (21.3 g./lOOO g.),71 is intermediate between that of potassium perrhenate KReO (1 1-8 g./lOOO g.) and permanganate KMnO (64.0 g./lOOO g.). The alkali salts are very stable and potassium pertechnetate which undergoes a reversible colour change to yellow at 500" can be fused at about 540" and sublimed at about 1000" without decomp0sition.7~ The pertechnetate ion like the perrhenate ion is stable in solution over a very wide range of pH and so far no direct evidence has been found for the existence of an ion corresponding to the mesoperrhenate ReOS3- nor for lower states such as TCO,~- and TC043- although these are reasonably 67 Fried and Hall "Chemistry of Technetium" 1950; presented at Spring Meeting of Amer.Chem. SOC. 68 Fried and Hall Phys. Rev. 1951 81 741. 6 9 Magneli and Anderson Acta Chern. Scand. 1955 9 1378. 7 0 Colton Griffiths Dalziel and Wilkinson J. 1960 71. Busey and Bevan Unclassified Document 1960 ORNL 2983. 7 2 Busey and Larson Unclassified Document 1958 ORNL 2584. 308 QUARTERLY REVIEWS well known for rhenium in the solid state. The possible stability of TcO,~- in aqueous solution has been The pertechnetate ion absorbs strongly in the near ultraviolet region.The edge of the 2875 A peak is very close to the visible region of the spectrum and evidently only a small disturbance from the tetrahedral symmetry of the Tc04- ion is necessary to cause a strong colour such as is shown by the free acid. It is interesting to note that a persistent reddish- violet colour resembling that of the free acid is sometimes found in solid pertechnetates which have been dried under the infrared lamp; indeed a definite absorption band at about 5000 A was recorded by some of the earlier workers.33 The chemistry of potassium pertechnetate has been studied in hydro- chloric acid solutions and will be discussed later. Sulphides.-Technetium forms two sulphides Tc,S and TcS ,.These are prepared in analogous ways to those of rhenium which they closely re- semble. Technetium heptasulphide is formed by the action of hydrogen sulphide on a pertechnetate dissolved in 2-4~-hydrochloric The crude material is freed from sulphur by carefully washing it with carbon di- sulphide. The heptasulphide loses sulphur when heated at moderate temperatures and yields the amorphous disulphide. Crystalline disulphide is made by heating the heptasulphide with sulphur at 1000" for 24 hours in a bomb the excess of sulphur being sublimed away in a vacuum. Techne- tium disulphide is isostructural with rhenium disulphide7* and is thought to have a disordered molybdenum disulphide structure. When heated with either hydrogen or hydrogen sulphide at 1000" the disulphide is reduced to the metal.Halides and Complex Halides.-The known halides and oxide halides of technetium are listed with those of rhenium in Table 4. Technetium hexaJuoride. Technetium hexafluoride (m.p. 33.4"; b.p. 55.3") prepared by the action of fluorine on the metal at 400° is a yellow solid melting to a yellow liquid. The vapour is colourless and monomeric.75 As in other transition-metal hexafluorides there is a phase transition in the solid marking the change from the cubic high-temperature form to the orthorhombic low-temperature form ; for the hexafluoride this change occurs at -5.3 O (calculated from vapour-pressure measurements -4.54"). Vapour-pressure and spectroscopic measurements indicate that under the experimental conditions heptafluoride is not formed. The magnetic moment (pealc = 0.45 B.M.at 300°K) is very low and like rhenium hexafluoride technetium hexafluoride exhibits peculiar magnetic be- Rulfs and Meinke J. Amer. Chem. Suc. 1952 74 235. 74 Zachariasen quoted as private communication in ref. 5. 75 Selig Chernik and Malm J. Inurg NucZear Chem. 1961 19 377. COLTON AND PEACOCK TECHNETIUM CHEMISTRY 309 haviour at temperatures below 14 The infrared spectrum over the range 600-2000 cm.-l is similar to the spectra of other transition-metal hexafluorides and there is a strong fundamental absorption at 745 cm.-l. Upon hydrolysis with sodium hydroxide technetium hexafluoride yields TABLE 4. Halides and oxyhalides of technetium and rhenium. Tc(vrr) Re(vrr) Tc(vr) Re(vr) Tdv) Rdv) TC(IV) Re(1v) TC(III) Re(@ a black Fluorides Chlorides Bromides TcO~F ? Tc0,Cl ? - ReF7 ReOF, ReO,F ReO,F Re03C1 Re0,Br ReF ReOF ReF TcOBr ReCI ReBr TcC14 ReBr precipitate which dissolves upon addition of dilute hydrogen peroxide ; this behaviour is reminiscent of rhenium hexafluoride which disproportionates on hydrolysis to perrhenate and hydrated dioxide.Chlorides.. It was reported that chlorine had no action on heated technetium metal but the chlorination was carried out in a closed static system.65 It has recently been shown that a stream of chlorine readily reacts with technetium to produce both the hexachloride and the tetra- chloride. 77 Technetium hexachloride is a volatile green compound. It is readily decomposed thermally to the red tetrachloride and on hydrolysis gives technetium dioxide and the pertechnetate ion according to the usual disproportionation reactions of rhenium(v1) and technetium(v1) com- pounds :- 3Tc(v1) -+ 2Tc(v11) + TC(IV) Technetium tetrachloride is stable.It can be made by the action of chlorine on the metal and it can be sublimed unchanged in the chlorine stream. It was first prepared however by the action of carbon tetrachloride on technetium heptaoxide at 400" in a bomb.'* Technetium tetrachloride reacts with oxygen to give oxide chlorides of technetium(v~r)~~ and it 76 Selig and Malm Personal communication. 77 Colton Nature 1962 193 872. 78 Knox Tyree Srivastava Norman Bassett and Holloway J. Anzer. Chem. Soc. 7 9 Colton 1962 to be published. 1957 79 3358. 3 10 QUARTERLY REVIEWS dissolves readily in concentrated hydrochloric acid to give the hexa- chlorotechnate(1v) ion TcCI 62-.79 It dissolves in water to give a yellow solution which hydrolyses only slowly79 (cf.platinum tetrachloride). The magnetic susceptibility of technetium tetrachloride has been measured over a temperature range.8o It was found to obey the Curie-Weiss law fairly closely with ,ueff = 3-14 B.M. (25"c) and 8 = -57". No binary bromides or iodides of technetium have so far been charac- terised. Oxide halides. The existence of pertechnyl fluoride Tc03F was suspected from a mass-spectroscopic examination of the products formed from technetium heptaox ide and uranium tetrafluoride.81 This compound which has properties intermediate between those of Mn03F and Re03F has very recently been obtained by the direct fluorination of technetium dioxide.82 Pertechnyl chloride TcO,Cl is said to have been prepared by treating potassium pertechnetate dissolved in 1 8~-sulphuric acid with 12~-hydro- chloric acid.The compound was extracted into chloroform carbon tetrachloride or h e ~ a n e . ~ ~ Although its vibrational spectrum was recorded there is no record of it having been isolated and properly characterised. Two products which were thought to be oxide chlorides were observed during the reaction between chlorine and technetium The first was blue subliming at 80-90" and the second was a brown product subliming at 900". Neither was characterised but it has since been sug- gested that the brown product was technetium oxide trichloride TCOC~,.~ This has recently been confirmed.6o The only other oxide halide of technetium which has been definitely characterised is the brown oxide tribromide TcOBr, prepared by the action of bromine vapour on technetium dioxide at about 350".83 Under similar conditions rhenium gives an oxide tetrabromide R~OBI-~.~* The technetium compound can be sublimed in a bromine stream at 400".It hydrolyses in the usual way for compounds of rhenium(v) and techne- tium(v) 3Tc(v) -f 2Tc(rv) + TC(VII) CumpZex halides. The pink complex fluoride potassium hexafluoro- technetate(rv) K2TcF6 has been obtained from reaction between the hexachlorotechnetate(1v) and potassium hydrogen difluoride It i s isostructural with the hexafluororhenate &Gel? structure) but is more stable to water and like the hexafluororhenate can be recrystallised from aqueous solution. The free acid as well as other fluorotechnetates can be obtained from the potassium salt by ion exchange.The sodium and Knox and Coffey J. Amer. Chem. SOC. 1959 81 7. 81 Sites Baldock and Gilpatrick Unclassified Document 1952 ORNL 1327. 82 Selig 1962 Personal communication. s3 Colton 1961 to be published. 85 Boyd 1962 Personal communication. Colton J. 1962 2078. COLTON AND PEACOCK TECHNETIUM CHEMISTRY 31 1 amiiionium salts are very soluble; the barium salt like barium hexa- fluororhenate,s6 becomes less soluble on ageing. A solid silver salt could not be isolated from solution even by evaporation in the dark; this behaviour resembles that of the corresponding rhenium salt. Although potassium pertechnetate treated with concentrated hydro- chloric acid yields the hexachlorotechnetate K2TcC16 ;s7 this salt is more conveniently prepared by the action of concentrated hydrochloric acid and potassium iodide on potassium or ammonium perte~hnetate,~~’~~ In the latter reaction the oxygenated species Kp[TczOCllo] is obtained as an intermediates3 and is converted into the hexachlorotechnetate by further treatment with hydrochloric acid.Potassium hexachlorotechnetate(1v) forms golden yellow octahedra and is isostructural with the corresponding rhenate and platinate. The magnetic moment of the solid is 4.8 B.M.;65 two values have been obtained in solution 4-05 B.M. by the Gouy methodss and 3.83 B.M.89 by Evans’s nuclear magnetic resonance method.g0 The hexachlorotechnetate(1v) ion is not so stable in solution as its rhenium analogue. Thus the silver salt cannot be prepared by the reaction of silver nitrate with a neutral solution of the potassium salt because of rapid hydrolysis although this is the method used for making silver hexachloro- rhenate(~v).~~ Even qualitative observation shows that potassium hexa- chlorotechnetate(1v) is hydrolysed in dilute hydrochloric acid; more detailed study8’ has indicated that the potassium salt is unstable in 1 M-hydrochloric acid decomposing to an oxygenated species.Potassium hexabromotechnetate(rv) K2TcBr6 can be prepared quanti- tatively from the corresponding chloride by evaporation with hydro- bromic acid.88 The compound forms dark red or black crystals which have a face-centred cubic structure. The magnetic moment has been measured in solution only 3.94 B.M. was the value obtained by the Gouy method88 and 3.5 B.M. by the nuclear magnetic resonance rneth0d.8~ Potassium hexaiodotechnetate(rv) K JC16 is made by evaporating the corresponding chloride or bromide with hydriodic acid.The shiny black crystals are monoclini~,~~ but the structure is probably a modification of the potassium hexachloroplatinate(1v) type. The magnetic moment in hydriodic acid solution was found to be 4-14 B.M. by the Gouy method.88 The first crystals separating during the reduction of potassium pertechne- tate by potassium iodide in hydrochloric acid are red.s3 When digested with large quantites of hydrochloric acid these dissolve to give a reddish solution which slowly becomes light yellow and deposits crystals of the hexachlorotechnetate on cooling. The red crystals are a binuclear complex Kq[Tc20Cllol analogous to the well-known rhenium and ruthenium Peacock J. 1956 1291.87 Busey Unclassified Document 1959 ORNL 2782. 88 Dalziel Gill Nyholm and Peacock J. 1958 4012. 89 Colton 1960 Unpublished observations. go Evans J . 1959 2003. 312 QUARTERLY REVIEWS c ~ m p o u n d s . ~ ~ ~ ~ ~ The crystals are stable in the atmosphere but hydrolyse rapidly in water the technetium being deposited as Chloro-complexes of technetium(v) have been observed in solution.87 In 12~-hydrochloric acid potassium pertechnetate is reduced to techne- tium(v) directly by chloride ion without the formation of intermediates. It has been suggested that the oxyion TcOC14- is formed. The absorption spectrum of the complex shows bands at 2925 and 2300 A (E = 4700 and 10,400 respectively). Further reduction to TcCl,,- occurs only slowly. The technetium(v) complex while stable in 3hl-hydrochloric acid dis- proportionates to pertechnetate and the hydrolysis products of the hexa- chlorotechnetate(1v) ion in 1 M-hydrochloric acid.A different technetium(v) species is produced when a solution of potassium hexachlorotechnetate in 12~-hydrochloric acid is exposed to It is evident that the reduction of pertechnetate ion by and in hydro- chloric acid solutions is a complicated process. The exact nature of the technetium@) species remains to be resolved and the relation between them and the binuclear ion Tc,OC~,,~- which is not formed in pure hydro- chloric acid solutions requires investigation. Complex Compounds.-Apart from the halogenotechnetates and certain reactions of analytical interest the chemistry of technetium complexes has received little attention.Hydrated technetium dioxide Tc0,,2H20 dissolves in alkali cyanides to give an anion believed to be [TcIV(OH),(CN),] 3- which was isolated as the dark brown thallium Under similar conditions rhenium gives [ReV(OH)4(CN)4]3-. 94 Polarographic studies70 suggest that technetium should form quadrivalent rather than quinquevalent cyanide complexes so that the [Tc(OH),(CN),I3- ion merits further study on account of both oxidation state and co-ordination number. The complex oxycyanide was reduced by potassium amalgam to the technetium(1) complex cyanide K,TC(CN),,~~ similar to and isostructural with the corresponding man- ganese and rhenium compounds. By using o-phenylenebisdimethylarsine (L) as the ligand complexes of the types [TcWl,L,] [Tc~*~C~,L,]C~ and [TcvCl,L,]CIO have been p ~ e p a r e d .~ ~ ~ ~ ~ The first two are so far the only isolated and characterised compounds of technetium in the valency states 2 and 3. The technetium(v) compound affords the only example so far of eight co-ordination for technetium. The compounds are isostructural with the corresponding rhenium compounds 97 which they closely resemble. This complex shows a band at 3260 A ( E = 7800). 91 Croft Austral. J. Chem. 1956 9 184. 92 Mathieson Mellor and Stephens Acta Cryst. 1952 5 185. 93 How and Schwochau Angew. Chem. 1961 73 492. 94 Trzebiatowska and Danowska Z . phys. Chem. 1959 212 29. 95 Fergusson and Nyholm Nature 1959 183 1039. 96 Fergusson and Nyholm Chem. and Znd. 1960 347. g7 Curtis Fergusson and Nyholm Chem. and I d . 1958 625. COLTON AND PEACOCK TECHNETIUM CHEMISTRY 313 Organometallic compounds of technetium have only recently been prepared.Technetium tetrachloride reacts with sodium cyclopentadienide in tetrahydrofuran to give a reddish purple solution which on treatment at 50" with sodium borohydride yields air-sensitive yellow crystals of composi- tion TC(C,H,),.~* The infrared spectrum of this compound resembles that of (C,H,),ReH99~100 but the characteristic stretching frequency of a metal- hydrogen bond is absent. Molecular weight determinations indicate a dimer and nuclear magnetic resonance spectroscopy shows the compound to be diamagnetic and confirms the absence of a metal-hydrogen bond.98 Chemical evidence suggests that the bonding between the rings and the metal ion resembles that in (C,H,),ReH. There does not appear to be any similarity between the bonding in the technetium compound and that in the paramagnetic manganese cyclopentadienidelOl and it is probable that the molecule has a metal-metal bond.The dibenzenetechnetium cation [(C,H,),Tc]+ is said to be formed in trace amounts by the neutron irradiation of dibenzenemolybdenum.lo2 Technetium carbonyl has been prepared by the action of carbon monoxide on technetium heptaoxide at 220-275 O and 400 atmospheres p r e ~ s u r e . ~ ~ ~ * ~ ~ * The molecular weight is twice the formula weight that is Tc,(CO)lo and the presence of a metal-metal bond similar to those in manganese and rhenium carbonyls has been established from the infrared absorption spectrum. The colourless diamagnetic compound sublimes at moderate temperatures in a vacuum and slowly decomposes in the atmos- phere.Technetium carbonyl slowly reacts with iodine to form the dimeric carbonyl halide [Tc(CO),I] 2 but when treated with iodine under a pressure of 50 atmospheres of carbon monoxide the monomeric pentacarbonyl iodide Tc(CO),I is formed. When ammonium perrhenate solution labelled with technetium-99 is reduced by potassium in ethylenediamine the technetium like the rhenium is converted into a complex hydride.lo5 By analogy with the rhenium compound the formula of the hydride was thought to be KTcH,,2H20. The available evidence suggests that this hydride ion is less stable than the rhenium analogue. The more important compounds of technetium are shown in Table 5. Polarographic Studies.-Polarographic studies of technetium compounds in various supporting electrolytes have been described and some differences between technetium and rhenium have been found.98 Huggins and Kaesz J. Amer. Chem. SOC. 1961 83 4474. 99 Wilkinson and Birmingham J. Amer. Chem. SOC. 1955,77 3421. 100 Green Pratt and Wilkinson J. 1958 3916. 101 Wilkinson Cotton and Birmingham J. Inorg. NucZear Chem. 1956,2,95. 102 Baumgartner Fischer and Zahn Naturwiss. 1961,48 478. 103 Hileman Huggins and Kaesz J. Amer. Chem. Soc. 1961 83 2953. 104 Hieber and Herget Angew. Chem. 1961 73 579. lo6 Floss and Grosse J. Znorg. Nuclear Chem. 1961 16 44. 314 QUARTERLY REVIEWS Potassium pertechnetate in 2~-potassium chloride shows the same apparently eight-electron reduction as rhenium to give the “technide” Little is known about the constitution of this ion although a technide has been is01ated.l~~ The present position regarding the “rhenides” is chaotic6 and until the rhenides have been properly characterised it is not worth speculating on the structure of the technide.Surprisingly potassium pertechnetate shows no reduction wave in 4 ~ - perchloric acid although the perrhenate gives a good wave corresponding to a three-electron reduction to rhenium(~v).l~~* lo7 Similarly in 4 ~ - hydrochloric acid perrhenate ion is reduced to rhenium(1v) in a single wave106*107 but technetium shows evidence of an intermediate tech- netium(v1) state. O TC(VII) TABLE 5. Compounds of technetium. Tc207 HTcO, MTcO (M = K+ NH4+ Cs+ Ph,As+ etc.) Tc2S7 Tc0,F TcOBr, [TcL2C14]C104 (L = o-phenylenebisdimethylarsine) K2TCX6 (X = F c1 Br I) K4~c20Cllo] [TcL2Cl2]C1 Tc(CO),X [Tc(CO),X] (X = C1 Br I) TCF6 TCC16 TcO~ TcS, TcCl [TCL,C1,1 Tc,(CO) 10 In O*lM-pOtaSSiUm cyanide potassium pertechnetate gives only one wave corresponding to a three-electron reduction to technetium(~v).~o Perrhenate gives two waves; the first is to rhenium(v) and the second corresponds to reduction to rhenium(~).~O These results agree with the chemical behaviour of the elements; thus no technetium(v) cyanide has yet been prepared.Green technetium(II1) solutions have been prepared by the controlled potential electrolysis of pertechnetate solution at pH 7.0 in a phosphate buffer. The solutions are unstable and readily oxidise to technetium(Iv).lO* Analysis.-Radioactivity. The earliest method of determining techne- tium was by means of its radi0activity.l The method is sensitive as the specific activity of technetium-99 is 37,800 disintegrations/min./,ug.but difficulties associated with measurements of low-energy 18 particles arise due to self absorption in the specimens. Hence the method is not the best for weighable amounts of technetium. Gravirnetric methods. These usually depend on the formation of sparingly soluble pertechnetates. The two most common precipitants are the tetraphenylarsonium cation and nitron.,O Unfortunately neither is Io6 Lingane J. Amer. Chem. SOC. 1942 64 1001. lo7 Rulfs and Elving J. Arner. Chem. SOC. 1951 73 3281. lo* Thomason Unclassified Document 1958 ORNL 2453 7. COLTON AND PEACOCK TECHNETIUM CHEMISTRY 315 specific and many anions including perrhenate nitrate iodide and bromide interfere. Technetium can be determined by precipitating the sulphide under carefully controlled conditions.73 Spectrophotometric methods.The simplest is the measurement of the intensity of the characteristic peaks of the pertechnetate ion which occur at 2460 and 2890 This method has been used for the simultaneous estimation of technetium .and rhenium.log Other methods involve the formation of technetium complexes with thiocyanates,l1° furil -ol-dioxime,l1l tol~ene-3,4-dithiol,~~~ and thioglycollic acid.l13 The spectrophotometric methods are very sensitive-about 10 pg. of the element can be easily estimated-and are therefore to be recommended on the grounds of safety. Several other reagents such as potassium xanthate dimethylglyoxime and thiourea give colour reactions with technetium ions under special condi- tions and it is possible that from these other more specific quantitative methods may be deve10ped.l~~ Other methods.Traces o f technetium may be determined polaro- g r a p h i ~ a l l y ~ ~ J ~ ~ but the most sensitive method for very small amounts is neutron activation analysis. Technetium-99 has a moderate cross section for neutron capture (20 barns) to give the 15.8 sec. technetium-loo.115 However the method is only convenient if a reactor or neutron source is available. The Future.-Technetium is a relatively stable fission product and the working up of the quantities present in spent fuel elements from nuclear reactors is a matter of economics rather than of the scarcity of the element itself. No recent figure of the amount of technetium present in the waste from such reactors is available but there is little doubt that kilogramme or larger quantities could be isolated if the demand existed.The uses of technetium will be limited by its radioactivity. The pertechne- tate ion has been shown to be an efficient anticorrosion agent in solution in contrast to perrhenate,l16 and the concentration of pertechnetate required is sufficiently low molar) for the radioactivity hazard to be small. Alloys of technetium except those containing other fission products and uranium have been little studied but already they have been considered as superconductor^.^^^ A start has been made on the organometallic chemistry of the element and this field could well produce potential catalysts. We thank Dr. A. A. Woolf for a thorough reading of the manuscript. log Wolkowitz Unclassified Document 1955 ORNL 1880 4. ll1 Colton and Morley U.K.A.E.A. Report 1961 AERE R 3746 112 Miller and Thomason Analyt. Chem. 1961 33 404. 113 Miller and Thomason Analyt. Chem. 1960 32 1429. 114 Jasmi Magee and Wilson Talanta 1959 2 93. Boyd and Larson J. Phys. Chem. 1956,60 707. 116 Cartledge J. Phys. Chem. 1957 61 973. 11’ Buckel Metallurgy 1959 13 814. Crouthamel Analyt. Chem. 1957 29 1756.

ISSN:0009-2681    

DOI:10.1039/QR9621600299

出版商:RSC    

年代:1962

数据来源: RSC

摘要:

KINETICS AND MECHANISM OF REPLACEMENT REACTIONS OF CO-ORDINATION COMPOUNDS By R. G. WILKINS (THE UNIVERSITY SHEFFIELD) 1. Introduction and Value of Subject THIS subject has been well documented in recent yearsl-10 and in greater detail than is the intention of this Reviewer. The salient features of the subject will be covered however and references to key papers and ac- counts will be included. Inevitably many of the ideas previously pro- pounded will be recounted. The Review will be mainly concerned necessarily with co-ordination compounds of the transition elements since it is this group of complexes with its variety of stereochemical behaviour which has primarily interested the investigator further stimulated in recent years by the application of the powerful ligand-field It is about fifty years since the earliest kinetic study of a complex- ion reaction.ll It is however only in the past decade that anything approaching a concerted attack on the problem of the mechanism of these reactions has been made.One of the challenges of investigating co- ordination compounds is understanding the variety of chemical inter- mediates and transition states possible for the reactions of four- (planar and tetrahedral) five- six- and eight-co-ordinate complexes. Many fundamental problems remain unanswered but increasing attention from a number of quarters is being paid to the subject. Apart from their intrinsic interest the reactions of co-ordination com- pounds have been investigated to test some of the theories of chemical reactivity. Thus the base hydrolysis of bromopenta-amminecobalt(II1) ion [CO(NH,),B~]~+ + OH-+ [CO(NH,),OH]~+ + Br- .. . . . . . (1 1 has been used to study the effects of hydrostatic pressure,12 ionic strength,13 Taube Chem. Rev. 1952,50,69. Basolo Chem. Rev. 1953 52,459. Basolo Rec. Chem. Progr. 1957 18 1. Basolo and Pearson “Mechanisms of Inorganic Reactions,” Wiley New York 1958. Pearson J. Phys. Chem. 1959 63 321. Ingold “Substitution of Elements other than Carbon,” Weizmann Science Press Jerusalem 1959 Chapter 1. Tobe Sci. Progr. 1960 48 483. Stranks in “Modern Coordination Chemistry,” ed. Lewis and Wilkins Interscience Publ. Inc. New York 1960 p. 78. Pearson J . Chem. E ~ u c . 1961 38 164. lo Basolo and Pearson in “Advances in Inorganic Chemistry and Radiochemistry,” ed. EmelCus and Sharpe Academic Press Inc. New York Vol. 111 1961 Chapter 1.l1 Lamb and Marden J . Amer. Chem. SOC. 1911 33 1873. l2 Burris and Laidler Trans. Farada-v SOC. 1955 51 1497. l3 Bronsted and Livingston. J. Amer. Chem. SOC. 1927 49 435. 316 WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 317 and ion-pairing14 on reaction rate. The rate of the second-order reaction (2) can be measured conveniently by using pM-concentrations of reagents.15 Salt effects can thus be studied in the region of low ionic strength in which the Bronsted theory is applicable and can thus be tested. Many of these oxidation-reductions whose rates have been measured involve complex ions and a knowledge of the lability* of these ions is often important for understanding the detailed mechanism of electron-transfer processes.16 Metal ions can act as important catalysts in organic and biological systems.Reactivity may form the basis for this behaviour; the profound difference between calcium and magnesium ions as enzyme-activators may arise1’ from the much faster reaction of calcium discovered recently with ligands such as adenosine 5’-diphosphate adenosine 5’-triphosphate,l8 and a metal indicator metal ~htha1ein.l~ The modus operandi of the catalytic effect often involves a metal ion’s attacking the substrate to form a transient complex in which the drain of electrons to the co-ordination centre results in facilitated reaction at another point of the substrate.20s21 A knowledge of reaction mechanism can aid in the synthesis of new metal complexes and improve older methods. Basolo,22 in a readable account outlines how for example the hitherto unknown nitrito-complex ions of rhodium(m) iridium(III) and platinum(Iv) [M(NH3),ONOln+ were prepared as a direct sequel to kinetic studies.The successful use of some of the procedures for complexometric estimation of metals depends on the difference in rates of complex reactions,23 although insufficient attention appears to have been paid to the rate aspect in analytical pro- cedures. One of the least ambiguous methods for assigning the solvation number of a metal ion involves exchange studies e.g. Fez+ + [Co(C2O4)Js- -f Fe3+ + Co2+ + 3C2042- . . . . . . [M(H20),lm+ + nH,*O + [M(H,*O),]“+ + nH,O . . . (3) Provided the rate of exchange is measurable and flow m e t h o d ~ ~ ~ y ~ ~ can *A kinetic term.l Complex ions reacting within 1 minute at room temperature involv- ing - 0.h-solutions are termed “labile”; otherwise they are “inert”.l4 Davies and Williams Trans. Faraduy SOC. 1958 54 1547; Davies “Progress in Chemical Kinetics,” ed. Porter Pergamon Press London 1961 p. 175. l5 Barrett and Baxendale Trans. Farachy SOC. 1956 52 210. l6 Halpern Quart. Rev. 1961 15 207. These processes will not be discussed further in this Review. l7 Diebler Eigen and Hammes 2. Naturforsch. 1960 15b 554. l8 Eigen and Hammes J. Amer. Chem. SOC. 1960 82 5951. l9 Czerlinski Diebler and Eigen 2. phys. Chem. (Frankfurt) 1959 19 246. 2o Ref. 4 Chapter 8. 21 Martell and Calvin “Chemistry of the Metal Chelate Compounds,” Prentice-Hall 22 Basolo Chem. Eng. News 1961,39 86. 23 Schwarzenbach “Complexometric Titrations,” transl. by Irving Methuen London 24 Baldwin and Taube J. Chem.Phys. 1960 33 206. 26 Sutter and Hunt J. Amer. Chem. Soc. 1960 82 6420. Inc. New York 1952 Chapter 8. 1957. 318 QUARTERLY REVIEWS help attain this the value of n can be determined. In this way the hexa-co- ordinated structures for [A1(H20),13+ and [Cr(H,O),I3+ in ~ a t e r ~ ~ ~ as well as for [C0(NH3)6l3+ and [cr(NH3),I3+ in liquid have been deduced. 2. Experimental Background (a) The Measurement of Rates.-A wide range of rates and a variety of reactions of metal complexes are known. For example cupric ions react "instantaneously" at - 100" with ammonia or pyridine in whereas the hydrolysis of [Ir(NH3),C112+ has to be studied at 95" to obtain measurable Some idea of the extent of rate constants encountered is shown in Table 1 where examples are also given of the variety of methods which must be used to encompass the large time scale for reactions.8 The impor- tance of the recently developed techniques for measuring very fast processes TABLE 1.Reactions rate constants and techniques used in kinetic studies of co-o r dina t ion compounds illustrative examples. Reactiona Cd2+ + Br- + C a r + Cd(CN) + CN- $ Cd(CN),- 'Mg2+ + ADP3- $ MgADP- co,' + soq- + COSO,~~ Ni en;+ + 2H+ -+ dcis-/-[Co en,CI,]+ + X- -t Ni en? + enH;+ [Co en2C1X]+ + C1- Rate constantb k2 = 1.4 X lo9 k = 1.OX lo7 k2 = 5 X 10' k2 = 0.3 X lo7 k = 2.5 x 103 k = 2 x 105 k == 86.6 k = 1.4 X lo-* Technique applied Ref. NMR (line broaden- 30 ing of Br nuclei) Polarographic 31 Temperature jump 18 Ultrasonic relaxation 32 spectroscopy Stopped- flo w 33 Polarimetric (X = 34 SCN or Cl-) spec- tral (X = SCN) and radiochemical (X = 35C1) a All reactions in water except last (MeOH).k first-order rate constant in set.-'; kz 1. mole-1 sec.-I; at -25" unless otherwise ADP = adenosine 5'-diphosphate; temperature of reaction 13". indicated. a Temp. 35.8". Hunt and Taube J. Chem. Phys. 1951 19 602. Wiesendanger Jones and Garner J. Chem. Phys. 1957 27 668. 88 Bjerrum and Poulsen Nature 1952 169 463. as Lamb and Fairhall J. Amer. Chern. SOC. 1923 45 378. so Hertz 2. Elektrochem. 1961 65 36. s1 Gierst and Hurwitz 2. Elektrochem. 1960 64 36. sz Eigen 2. Elektrochem. 1960 64 115; Eigen and Tamm ibid. 1962,66 93,107. ss Ahmed and Wilkins J. 1960 2901. s4 Brown and Ingold J. 1953 2680. WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 3 19 with half-lives of reaction in the micro- or millimicro-second range cannot be over empha~ised.~~ Estimates of the lability of certain metal complexes (e.g.aquo ammines halides) have been assessed by electrochemical and from nuclear magnetic resonance data.30,36,3i In the latter case the resonance line broadening of nuclei such as lH 1 7 0 35Cl and 12’1 which is produced by paramagnetic ions can be interpreted in terms of the rate at which these nuclei (as water or halide ions in the examples above) are entering the co-ordination sphere of the paramagnetic ion (i.e. the exchange rate). In addition the elegant relaxation methods pioneered by Eigen and his colleagues38 can be used in which a chemical equilibrium is rapidly perturbed by some physical process. Here the temperature-jump method appears to be particularly applicable to the study of co-ordination corn pound^^^^^^ (for reaction rates in the range 1 0-5-1 second) in which light-absorption changes arising from rapid temperature elevations can be related to a kinetic rate (for example dis- sociation).The interpretation of the results from these rapid methods can however be difficult for the mere chemist (see e.g. ref. 30) especially if more than one stage is involved. This is not so true of the well-known flow techniques39 but these cover only the millisecond-second time range although their extension to low-temperature work has considerable poten- tiality for study of a wide range of complex ions. Undoubtedly the most important property of a complex ion which has been used for kinetic measurements is its absorption spectrum and the changes often large which occur in the visible ultraviolet or even infra- red region can be extremely usefcll not only in measuring rate but also for clarifying detailed mechanism importance of intermediates etc.Thus an understanding of the steric course of reactions (4)40 and (941 was cis-[Co en,Br(SCN)]+ + H,O -+ 100% cis-[Co en,(H,O)(SCN)I2+ + Br- . . . (4) trans-[Co en,Br(SCN)]+ + OH- -+ 81% cis- 19% trans-[Co en,OH(SCN)]+ + Br- . (5) aided considerably by the observation during the reaction of a constant optical density at a certain wavelength. This isosbestic point indicated that the original substance was being replaced by either (a) one other substance or (b) two or more substances which.were in constant ratio and once formed underwent no further change. “ ( a ) Eigen and Johnson Ann.Rev. Phys. Chem. 1960 11 307; (b) Bewick and Fleischmann Ann. Reports 1960 57 90. 30 Pearson Palmer Anderson and Allred 2. Elektrochem. 1960 64 110. Connick in “Advances in the Chemistry of the Coordination Compounds,” ed. Kirschner Macmillan New York 1961 p. 15; Connick and Stover J. Phys. Chem. 1961 65 2075. 3* For a general account see Eigen in “Advances in the Chemistry of the Coordination Compounds,” ed Kirschner Macmillan New York 1961 p. 371. 39 Roughton and Chance in “Investigation of Rates and Mechanisms of Reactions,” ed. Friess and Weissberger Interscience Publ. Inc. New York 1st edn. 1953 Chapter 10. 40 Baldwin and Tobe J. 1960 4275. *l Ingold Nyholm and Tobe J . 1956 1691. 320 QUARTERLY REVIEWS Conductivity polarography and pH measurements have also featured in kinetic studies of co-ordination compounds and like the spectral methods allow continuous examination of the reaction system.This is not usually possible with the other methods which have been used involving chemical analysis and the important isotopic-exchange reactions (Table TABLE 2. Some isotopic exchange reactions of co-ordination complexes. Reaction Reactants Rate law (25") (see text) 2).42 (8) [Cr(NH3)s]3+ + *H20 R = 2.6 X 106[Cr(NH3),3+][OH-]a (8) [Cr(NH&I3+ f H2O R = 2.1 X 106[Cr(NH3)63+][OH-]b (23) lNi(CO),I + *CO R = 7.5 X 10-4[Ni(C0)4]C (31) [*Pt en2Cl2I2+ + [Pt en2I2+ R = 12-1 [Pt(rv)][Pt(~r)][Cl-]~ a L. mole-1 sec.-l from H exchange in DC2H3O2/C2H9O2- buffers. L. mole-' sec.-l from NMR line broadening in 0*02~-NaOH. c Sec.-l in toluene at 0". (31) [Pt en2Cl2I2+ + *Cl- R = 150[Pt(IV)] [Pt(II)] [Cl-ld (presence of [Pt en2I2+) (presence of C1-) L.2mo1e-2 sec.-l. (b) Kinetic Behaviour.-The rate data encountered so far in studies of co-ordination compounds are relatively straightforward. In the successive- replacement or decomposition reactions of complexes containing more than one ligand there is often sufficient difference in the reactivity of succeeding stages to prevent difficulties in interpretation. Thus the hydrolysis in acid solution of both [Ni en3I2+ and [Cr en3I3+ slows down as successive ethylenediamine molecules are r e r n ~ v e d . ~ ~ ~ ~ Even so if the properties of intermediates (e.g. spectral) are unknown then several interpretations of the data are possible and resort must often be made to pla~sibility.~~ In addition it is sobering to realise that the decomposition of hexa-ammine- chromium(II1) ion in acid solution [Cr(NH3),I3+ -l- 6H30+ -+ [Cr(H20),I3+ -/- 6NH4+ (6) consists of twenty consecutive and simultaneous exchanges of ammonia by water and cis-trans-isomerisations.The complete solution is complicated but can be partially resolved by resort to matrix a1geb1-a.~~ There has been little investigation of reactions involving successive formation of complex ions which being commonly of the second-order may well prove even less simple. An important mechanism noted in complex-ion studies involves slow rate(s) preceded by equilibrium e.g. in the formation of trisbipyridyliron- (11) ion :* 42 Stranks and Wilkins Chem. Rev. 1957 57 743. 43 Schlafer and Kollrack Z. phys. Chem. (Frankfurt) 1958 18 348.44 Jorgensen and Bjernun Acta Chem. Scand. 1959 13 2075. 45 Jargensen and Bjerrum Acta Chem. Scand. 1958 12 1047. 46 Baxendale and George Trans. Faraday Soc. 1950,46 736. WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 32 1 Fez+ + bipy + [Fe bipy12+ fast K . . . . . . . ( 7 4 [Fe bipy12+ + bipy + [Fe(bipy),12+ fast K . . . . . . . . . (7b) [Fe(bipy),12+ + bipy + [Fe(bipy),12+ slow k . . . . . . . . . (74 The rate of formation of [Fe(bi~y)~]~+ was shown to equal kt[Fe2+] [bipy13 with the value of kt independent of temperature from 0” to 35”. This unusual circumstance arises because kt( = KIK2k3) incorporates the heats or” (7a) and (7b) and the energy of activation of (7c) and these cancel one another.47 3. The Mechanisms of Replacement Reactions All reactions of co-ordination compounds with the exception of those involving redox processes may be regarded as replacements (substitutions).Reactions in which only part of the ligand is replaced include the important hydrogen exchange processes occurring between aquated and ammonated metal ions and solvent e.g. [Cr(NH,),I3+ + *H20 + [Cr(N*H3),I3+ + H20 - * (8) This reaction is catalysed by hydroxide ion with a rate equal to k2[Com- plex][OH-] over a wide range of hydroxide concentration from about 1 O - ~ O M (where it has been studied by conventional deuterium-exchange experiment^^^) to 1 0 - 2 ~ (where it has been studied by proton magnetic resonance line- broadening3,) (see Table 2). Hydrogen-exchange experi- ments have value in assessing lability of solvated ions,36y37v49 detecting metal-ligand bond rupture in certain chelate and disclosing processes of the type51 [Cu en2I2+ + en + [Cu en3I2+ .. . . . . . . (9) even where the thermodynamic equilibrium lies very much to the left-hand side. Reactions may involve the replacement of one metal by anofher and these are termed electrophilic reactions and designated SE e.g. [Ni EDTAI2- + Cu2+ + [Cu EDTAI2- + Ni2+ . . . (10) where EDTA = (02C-CH2)2NCH2CH2~N(CH2C02)24 -. By far the most important substitution reactions involve the ligand and (1)-(9) except (2) constitute examples of such nucleophilic S, reactions. 47 Bjerrum Poulsen and Poulsen “Proceedings of the Symposium on Coordination Chemistry,” Danish Chemical Society 1954 p. 51. 48 Palmer and Basolo J. Inorg. Nuclear Chem. 1960 15 279; they measured the exchange rate of a number of metal ammines studying the effect of electron configura- tion of metal charge and co-ordination number.E.g. Bernheim Brown Gutowsky and Woessner J. Chem. Phys. 1959 30 950. bo Ahmed and Wilkins J. 1959 3700. 61 Morgan Murphy and Cox J . Amer. Chem. Soc. 1959,81,5043; Cox and Morgan J. Arner. Chem. SOC. 1959 81 6409. 322 QUARTERLY REVIEWS It will emerge in subsequent discussion that there are several variants or sub-classifications. Most kinetic studies refer to aqueous solutions since water is by far the most common solvent for co-ordination compounds. Two types of reaction can be studied involving (a) replacement of ligand by water and (b) replacement of water in an aquo-complex which in- cludes formation reactions. (a) The Replacement of Ligand by Water (Hydrolysis* Reactions).- This is an important process since replacement of one ligand by another almost invariably proceeds initially by hydrolysis.Thus the substitution of chlorine by glycine aniline or pyridine in trans- [Pt(NH3)2C12] is controlled by the hydrolysis of the latter followed by rapid replacement of water by the substituting group. Chloride ion however appears to be able to effect direct entry as well as via hydrolysis judging from some 36Cl- exchange experiments (see Table 6).52 Similarly hydrolysis may play an important role in the reactions of certain cobalt(rrr) complexes containing quin- quedentate EDTA which revert in solution to complexes containing sexadentate EDTA,53s54 e.g. oc-0 0s-0 - t It is difficult experimentally to distinguish between two possible paths (1 la) or (llb) because equilibration is rapid compared with the slow This means therefore that the observation that compound (I) and its carboxylate form lose chloride at the same rate is not necessarily signifi- cant.Ingold Nyholm Tobe and their c o l l e a g ~ e s ~ ~ ~ ~ have investigated the orienting rate and mechanistic effect of a non-participating group A on the hydrolysis of octahedral complexes of cobalt cis- or trans-[Co en,ABIn+ + H20 3 cis- or trans-[Co en,A(H,O)]n+ + B . . (12) Some results are shown in Table 3 but a discussion of their significance will be deferred. In the reactions of type (12) which have been assigned the *The terms hydrolysis aquation and dissociation are synonymous in this field. The reaction is often referred to as solvolysis in non-aqueous solvents. 6z Martin and Adams in "Advances in the Chemistry of the Coordination Com- pounds," ed.Kirschner Macmillan New York 1961 p. 579. 63 (a) Shimi and Higginson J. 1958 260; (b) Dyke and Higginson J. 1960 1998. 54 Morris and Busch J. Phys. Chem. 1959 63 340. 55 Ingold Nyholm and Tobe Nature 1960 187 477 and references cited therein; Baldwin Chan and Tobe J. 1961 4637 and previous Parts of that series. WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 323 TABLE 3. reaction [Co en2AC1In+ + H,O + [ Co en2A(H20)](n+1)+ + Cl-.” First-order constants (kl sec.-l at 25”) and steric course of cis-A OH N3 c1 NCS NH3 H2O NO2 trans-A OH N3 c1 NCS NH3 NO2 1 07k1 130,000 2400 1250 114 -2 -1 50 1120 14,000 cis Suggested ( %) mechanism 1007 100 100 ? 100 75 7 SN2 SNl SN2 2500 160 -1 -2 go00 a Data taken from refs.7 40 and 5 5 which should be consulted for fuller details. mechanism S 1 (Substitution Nucleophilic Unimolecular) the Co-B bond is commonly pictured as lengthening to a critical distance whence water slips in to occupy the position held by B which is then expelled. Thus because the 5-co-ordination complex which would result from a “pure” SNl reaction is unstable water must play a significant part in the hydrolysis and indeed the term “solvent-assisted SN 1” has been suggested for this mechani~m;~~ it has however always been appreciated that solvation is important in an S,l reaction. In certain cases (B = halide) the hydrolysis can be assisted by metal ions and with certain of these e.g. Hg2+ the reaction may indeed be of SN1 type the bond to water not being made “until all memory of the effects of the halide ion removed has been wiped Similarly under the influence of mercuric ion reaction (1 la) is In an SN2 (Bimolecular) mechanism the binding of the water is considered more important than the liberation of B.The difference is thus often a question of degree dependent on whether bond-breaking or bond-making takes the lead. Since the reactant is always in large excess kinetics are not helpful in deciding the molecularity of hydrolytic reactions. This has been assigned by the usual procedures well tried in physical- organic chemistry of examining steric electron-displacement and ligand effects as well as from considerations of changes in entropy and volume of activation accompanying these reaction^.^^^^^^ However some doubt as to their correctness remains.Eflect of acid. These hydrolyses will not be acid-catalysed unless a protonated species can be formed as a reactive intermediate. Thus the 56 Jones Harris and Wallace Canad. J . Chem. 1961 39 2371. 57 Posey and Taube J . Amer. Chem. SOC. 1957 79 255. 58 Hunt and Taube J. Amer. Chem. SOC. 1958 80 2642. 324 QUARTERLY REVIEWS rates of hydrolysis of [Co(CN),N3I3- {but not [CO(CN),B~]~-}~~ and of [Co(Cr)en2F2]+ (but not the corresponding ~ h l o r ~ - ~ ~ m p l e ~ e ~ ) ~ are acid- dependent. A mechanism such as:6o [Co en2Fz]+ + H+ + [Co en,F(FH)I2+ fast K . . . . (1 3 4 [Co en,F(FH)I2+ + H,O -+ [Co en,F(H,0)I2+ + H F fast k . . . . (1 3 4 can account for the observed rate equation k observed = k2[Co en2F2+] + k3K [Co en2F2+] [H+]. Hydrolyses involving H20 as the entering ligand are often referred to as acid hydrolyses to distinguish them from base hydrolyses.Our discussion thus far has been concerned with the removal of uni- dentate ligands since this is the easier to understand and the associated complexes are used for molecularity assignment. In general the effect of acid on the hydrolysis of complexes containing multidentate even simple bidentate ligands is decidedly more complicated. There is a dependence on [H+] for the rates of hydrolysis of [ F e ( b i ~ y ) ~ ] ~ + ~ ~ [Ni en(H,0),]2+,50 and [Cr(C,0,)3]3-63 which can be explained on the basis of small amounts of reactive intermediates which have one ligand linked by only one donor atom nitrogen or oxygen e.g. [Co en,F,]+ + H,O 3 [Co en,F(H,0)]2+ + F- slow k . . . . (13b) The kinetic behaviour can also be explained by assuming larger amounts of a “protonated” species without invoking bond rupture but this idea appears chemically feasible only for the bipyridyl complexes.Although it is a less likely explanation it cannot be dismissed especially in view of recent 69 Grassi Haim and Wilmarth in “Advances in the Chemistry of the Coordination Compounds ed. Kirschner Macmillan New York 1961 p. 276. 6 o Basolo Matoush and Pearson J. Amer. Chem. SOC. 1956 78 4883. 61 Fehrmann and Garner J . Amer. Chem. SOC. 1961 83 1276; MacDonald and Garner J. Inorg. Nuclear Chem. 1961 18 219. 62 Baxendale and George Trans. Faraday SOC. 1950 46 736; Basolo Hayes and Neumann J. Amer. Chem. SOC. 1953,75,5102; K.rumholz,J. Phys. Chem. 1956,60,87. 63 Graziano and Harris J . Phys. Chem. 1959 63 330; Krishnamurty and Harris J.Phys.Chem. 1960,64,346; Krishnamurty and Harris Chem. Rev. 1961,61,213. WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 325 experimental support for such “protonated” species.s4 With the hydrolysis of [Cr en3I3+ in O-lM-perchloric acid [Cr en,I3+ + 2H,O+ 3 cis-[Cr en,(H,0),]3+ + enH22+ . . . . . . (1 5) the intermediate species (in which one ligand is attached to the chromium by only one -NH2 group) probably has little independent stability although the kinetic data are a m b i g ~ o u s . ~ ~ ~ ~ ~ The dissociative behaviour of metal complexes of polyamines66 and polyamino-carboxylates represented by ethylenediaminetetra-acetate6’ is examined in the neutral pH region,. where there is no net chemical reaction by isotopic exchange studies. One obtains a rate composed from hydrolysis and formation rate constants and its unravelling in terms of simple pro- cesses is extremely difficult.If alternatively net decomposition is examined e.g. by addition of proton^,^^^^*^^^ cyanide,70 or a different metal ion* [e.g. reaction ( l o ) ] complex rate data are obtained which must be in- terpreted in terms of intermediates containing the decomposing substi- tuent. It is thus a problem to get at the unassisted (or solvent-aided) dissociative rates. When it is difficult to produce stepwise rupture (which may arise because there is little flexibility in the chelate as in the metal phthalocyanines or porphyrins) kinetic inertness often results. Eflect of base. The hydrolysis of many acidoamminecobalt(II1) com- plexes is markedly accelerated by the addition of hydroxide ion (base hydrolysis).These reactions are often easily measurable at low alkalinity (e.g. pH -9) but resort must be made to continuous-flow methods to investigate the reaction rates in stronger alkaline solution.73 Concomitant with the base hydrolysis and with similar rate characteristics (i.e. Rate = k,[Complex][OH-1) and dependence of rate on substituent effects,74 is a very much faster hydrogen exchange analogous to reaction (8). Indeed the complex mechanism shown below (SN 1 CB ; substitution nucleophilic unimolecular conjugate base) which includes the hydrogen exchange as a rapid pre-equilibrium (16a) may be the actual route for base hydrolysis [Co(NH,),X]*+ + OH- + [Co(NH,),(NH,)X]+ + H,O . . . . (1 6 4 [Co(NHJ,(NHJXI+ + [CO(NH,),(NH,)]~+ + X- .. . . . (16b) [Co( N H3)4( N H2)l2+ + H,O -+ [Co(N H3),0HI2+ . . . . . . . . (1 6 4 *There have been several studies of this type; see refs. 67 71 72. 64 Murmann and Healy J. Amer. Chem. SOC. 1961 83 2092. 66 Schlafer and Kling 2. phys. Chem. (Frankfurt) 1958 16 14. 66 Hogg Melson and Wilkins in “Advances in the Chemistry of the Coordination Compounds,” ed. Kirschner Macmillan New York 1961 p. 391. 67 Margerum J. Phys. Chem. 1959 63 336. 68 Tanaka Tamamushi and Kodama 2. phys. Chem. (Frankfurt) 1958 14 141. 69 Papoff J. Amer. Chem. SOC. 1959 81 3254. 70 Margerum Bydalek and Bishop J. Amer. Chem. SOC. 1961 83 1791. 71 Tanaka and Kato Bull. Chem. SOC. Japan 1960,33 1236 and previous work cited 72 Bydalek and Margerum J. Amer. Chem. Sac. 1961,83,4326. 73 Pearson Meeker and Basolo J. Amer.Chem. Soc. 1956 78 709. 74 Basolo Palmer and Pearson J. Amer. Chem. SOC. 1960 82 1073. therein. 326 QUARTERLY REVIEWS This mechanism can give the observed second-order kinetics with certain condition^.^,^^ Attempts have been made to distinguish this from a simple SN2 mechanism34 by examining the effect of adjusting the molecular structure of the reacting molecule (displaced group changes steric and isotope effects etc.’) but the results can be usually explained on either mechanism. The arguments in support of these mechanisms- are well d o c ~ m e n t e d . ~ ~ ~ ~ ~ ~ It does appear that a simple SN2 mechanism Can be ruled out in certain solvents,7s e.g. dimethyl sulphoxide but for water the problem remains unsolved. Even with this question settled the behaviour of other metal complexes may be different.There is for example a striking difference in the onset of base hydrolysis of [Co(NH3),XI2+ and [Cr(NH,),XI2+ which is at pH 5 and 10 re~pectively.~~ An interpretation of this result in terms of the SNICB mechanism has been suggested.78 The base hydrolysis of metal chelates has been little studied. Since [Fe(~hen)~]~+ has no acidic hydrogen the marked base-catalysed hydro- lysis cannot proceed via an SNICB mechanism. Since in addition the hydrolysis of the very similar ion [Ni phen312+ is little affected by hydroxide other mechanisms can also probably be dismissed including SNl IP (see p. 339) and those involving stepwise rupture.79 Consequently attention is focused on the d6 configuration of iron(I1) and a 7-co-ordinate reaction intermediate stabilised by this particular electronic configuration is postulated for the reaction with OH- (and CN- and N3-) ions.79 A 7-co- ordinate species is also postulated for the base-catalysed hydrolysis of [Co EDTA]-80 and some credence is given to these ideas by the establish- ment (by means of X-ray structural analysis) of 7-co-ordinated metal in certain ethylenediaminetetra-acetate complexes.81 An illustration of the interpretative difficulties facing the investigator is the observation that the rate of hydrolysis of the corresponding co balt(II1) propylenediaminetetra- acetate complex is not affected by addition of hydroxide ion.8o Thus such an important difference is produced by simply placing a methyl group on the periphery of the complex ion.(b) The Replacement of Water in an Aquo-complex.-Insight into the mechanism of the replacement of co-ordinated water in aquo-complexes has been obtained from water and ligand exchange.Water exchange. The exchange of water between bulk solvent and the aquated metal ion is the simplest experiment in principle but because of its 75 Pearson and Basolo J. Amer. Clzem. SOL 1956 78,4878. 76 Pearson Schmidtke and Basolo J . Amer. Chem. Sue. 1960,82,4434. 77 Levine Jones Harris and Wallace J. Amer. Chem. Suc. 1961 83 2453. 78 Ref. 4 p. 136. 7 9 Margerum and Morgenthaler in “Advances in the Chemistry of the Coordination Compounds,” ed. Kirschner Macmillan New York 1961 p. 481. Busch Cooke Swaminathan and Young Ae tm in “Advances in the Chemistry of the Coordination Compounds,” ed. Kirschner Macmillan New York 1961 p. 139. 81 Hoard Smith and Lind in “Advances in the Chemistry of the Coordination Com- pounds,” ed.Kirschner Macmillan New York 1961 p. 296; Hoard Lind and Silverton J . Amer. Chem. Sue. 1961 83 2770. WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 327 rapidity generally it is most difficult to measure in practice. Resort is made to nuclear magnetic resonance experiments these however introducing difficulties of interpretation. The broadening of the nuclear magnetic resonance signal of 170 in water by the addition of paramagnetic metal ions sheds information on the mobility of oxygen and hence on the water- exchange process.37 Lower limits for the first-order rate constant (k1-1~0) for the process [M(H2*0)ln+ + H,O + [M(H,O)]n+ + H2*0 . . . . . . . (1 7) have been recently obtained (see Table 4).37 l / k ~ o represents the lifetime of a particular water molecule on a metal ion.By studying the temperature effect on the relaxation process it is possible in principle to determine the TABLE 4. Kinetic data for water exchange (reaction 17)24937 and for sulphate reaction (20)38 with metal aquo-complexes. AP+ Be2+ Mg2+ Mn2+ k,,,"(sec. -l). < 35 - - 2 x 107 Mt& kS,,2-(sec.-l) -1 1 x lo2 1 x 105 3 x lo6 Fe2+ co2+ Ni2+ CU2+ 3 x 105 3 x 104 3 x 106 - M2i k,,,"(sec.-l). kSo,2-(sec.-l) 1 x 106 2 x 105 1 x 104 > 107 a limit for first-order rate constants. actual rate constant and the agreement between this value for Mn2+ ( 5 x lo7 sec.-l) with the proton exchange rate from nuclear magnetic resonance data (4 x lo7 sec.-1)49 shows that with this metal ion at least protons do not move independently of the water molecules.The results of these experiments are important in the interpretation of the mechanism of formation reactions (see next section). By studying the methyl- and hydroxyl-proton magnetic resonance of methanol solutions of metal ions the actual exchange rates of copper(I1) and nickel(I1) solvated ions have been determined. Comparison with water-exchange data shows that methyl alcohol molecules undergo exchange more slowly.36 The slow exchange of water between certain chromium(rI1) or cobalt(Ir1) aquo-complexes and solvent has been measured by conventional oxygen- 18 exchange experiments. By studying the rate of the reaction (18) [CO(NH,),H,O]~+ + H2*0 + [CO(NH,),H,*O]~+ + H,O . . . . (18) at various temperatures and pressures the volume and entropy of activa- tion were determined as + 1.2 0.2 ml.(independent of pressure) and + 6.7 IJr 1 e.u. respectively. These values appear to eliminate a compact SN2 mechanism. The reasonable conclusion was drawn that an SNl designation more nearly applied in which the cobalt-water bond stretched to a definite critical distance in the activated complex and that this was rapidly followed by bonding with the incoming water.58 The higher positive 328 QUARTERLY REVIEWS entropies of activation for water exchange with cis- and trans- [Co en2(H2O),I3+ and [Co en,(OH),]+ is in agreement with this concept.82 The results of such water-exchange experiments can also aid in understand- ing isomerisationS2 and racemisations3 processes (see section 5). Formation” reactions. The important question posed by these reactions concerns the part played by the entering ligand Y [MX,(H,O),I + y -+ [MX,Y(H,O),-ll + H20 * * (19) where (m + n) commonly equals six and charges are omitted for clarity.The answer can be anticipated-there is little evidence in the studies so far for the reactant “forcing” an entry in a direct SN2 reaction and the rate appears to be controlled by the water-exchange process. For such an SN 1 mechanism [MX,(H20),1 + [MX,(H20),-,1 + H20 kl k- . . (19a) [MX,(H@),-i] + Y -+ [MXmY(H20),-1] k2 . . (19b) it is easily shown by assuming steady-state conditions that d klk2[MXm(H2o)nl [YI -([MXmY(H,O),-,I) = k-1 + k,[YI dt Various situations can arise (a) k- M k2. In this case the kinetics observed may well be dependent on the concentration of [Y]. At low [Y] k- > k2[Y] and a second-order kinetic expression rate = (k,k,/k-,) [MX,(H,O),] [Y] is obtained.At high [Y] the reaction becomes of the first-order with a rate = kl [MX,(H,O),]. Thus with this mechanism prevailing a rate should be obtained which is dependent on small concentrations of Y but independent of the concen- tration and nature of Y at higher concentrations and with a limiting formation rate constant kl equal to the corresponding value for water exchange since (19a) is the path for such exchange. This behaviour has been observed only recently in the reaction of [CO(CN)~H~O]~- with N3- and SCN- ions.59 Earlier attempts to show the effect in the reactions of cationic cobalt(II1) ammines with N3- and SCN- ionss4 or with ions57 and of Cr:; with SCN- ionss5 were thwarted by ion-pairing between reactants at the high anion concentrations apparently necessary to reach the limiting region.t Summarising then second-order kinetics do not necessarily indicate an SN2 direct replacement unless the rate can be shown to be :Often termed anation in the common circumstance that the entering substituent is anionic. ?The very little studied reactions with neutral ligands may as with the anionic re- actants circumvent this difficulty. 82 Kruse and Taube J. Amer. Chem. SOC. 1961,83 1280. a3 Bailar J . Inorg. Nuclear Chem. 1958 8 165. 84 Basolo Stone Bergmann and Pearson J. Amer. Chem. Soc. 1954,76 3079. 8s Postmus and King J . Phys. Chem. 1,955,59 1216. WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 329 very much faster than corresponds to kl and this so far has not been observed.(b) k2 9 ke1. The limiting condition appears to have been reached with certain reactions. Thus Hamm and his co-workerssg found that the rates of reaction of Crii with certain unidentate and bidentate ligands e.g. acetate lactate oxalate were similar and their energies of activation close to that for water exchange. An even more penetrating example of reactions where water release dominates the entry of ligand is well shown by some forma- tion-rate studies of complexes of certain bivalent metal ions particularly of the first transition series in which sound-absorption relaxation methods were ~ ~ e d . ~ ~ ~ ~ The rate constant for the formation of the sulphate complex (from the ion pair; sulphate considered a unidentate ligand) [M(H20),I2+. . . . + [M(H,O),-lSO,] + H20 kl,k- .. (20) is similar to that for water exchange of the corresponding aquo-ion3' (see Table 4). The value is also independent of the entering anion in the light of some limited results for other ligands containing oxygen donor atoms namely thiosulphate chromate and EDTA. This emphasises the small specificity and role of the entering ligand. The corollary of these observa- tions is that it is the reverse reactions (hydrolysis or dissociation repre- sented by k-l) which are responsible for the differences in stability constants usually observed for complexes of a particular metal with various l i g a n d ~ ~ ~ ~ ~ ~ ~ ~ ~ or of a particular metal with different numbers of l i g a n d ~ . ~ ~ The values for the second-order rate constants for the formation of [Ni(SCN)]+ 87 and [Ni(NHy)12+ s8 suggest that here also metal-aquo bond-breakage is an almost controlling factor.The same may be true (although the situation is at present unclear) of some formation reactions involving iron(II1) which although rapid have been studied by using rapid- flow devices [Fe(H20)6]3+ -+ X(2 Or ')- + [Fe(H,O),X](l Or 2)+ + H20 . . (21) with X = SO, C1 Br or SCN.38,89-91* The common rate-law for these reactions rate (forward) = k [Fe3+] [Xn-] + k2[Fe3+] [Xn -1 [H+]-l can be interpreted in terms of two reacting iron(rI1) species the aquo- and the (significantly more reactive) monohydroxy-species. The rough parallelism between kinetic and thermodynamic parameters observed for the reaction of the aquo-complex (for example between rate and stability constant of p r ~ d u c t ; ~ ~ ~ ~ ~ see Table 5 ) suggests that the transition and the final state * These reactions have recently been studied by the pressure-jump relaxation method (Wendt and Strohlow 2.Electrochem. 1962 66 228). 86 Hamm Johnson Perkins and Davis J. Amer. Chem. SOC. 1958 80 4469 and previous work cited therein. 87 Davies and MacF. Smith Proc. Chern. SOC. 1961 380. 88 Melson and Wilkins unpublished observations. 8 9 Connick and Coppel J. Amer. Chem. SOC. 1959 81 6389. 91 Below jun. Connick and Coppel J. Amer. Chem. SOC. 1958 80 2961. Matthies and Wendt 2. phys. Chem. (Frankfurt) 1961,30 137. 330 QUARTERLY REVIEWS TABLE 5. Kinetic data for formation of [FeXIn+ at 25” (reaction 21). X kl k2 Ka Ref. (1. mole-l sec.-l) (set.?) (1. mole-l) c1 9.4 18 4 89 Br 20 31 0-6 90 NCS 127 20 150 91 -3000 -500 100 38 F 5400 - 105 92 so4 a Approximate value for association constant appropriate for the conditions of the kinetic experiments as used in cited reference.have similar characteristics Le. that a true S N 2 reaction is involved. Similar arguments had been used with the Feg-F- reaction which however perhaps not surprisingly with this ligand has different rate character- i s t i c ~ . ~ ~ However whether an SN1 mechanism can be ruled out at least with the faster reactions of the series awaits further study of the Fei:-water exchange rate from nuclear magnetic resonance meas~rements.~~ There is a relation between rate (k,) and the basicity of the ligand for the aquo- complex. In addition the hydroxy-species is the more reactive (the second- order rate constants for the reaction k2/Kh,g1 are all approx.104-105 1. mole-1 sec.-l). Eigen38 has therefore suggested a mechanism for the reaction of the aquo-species in which the hydroxy-species is generated by transfer of a proton from the aquo-complex to the substituting ligand. This idea has affinities to the SNlCB mechanism which has been proposed for base hydrolysis. This phenomenon of co-ordinated ligands being labilised more by a hydroxy- than by an aquo-group has also been ob- served in the Cr(m)-SCN- reaction 85 and in the hydrolysis of [Co(NH,) (H20)(N03)]2+ ion. 93 4. The Geometry and Characteristics of Intermediates and Transition States The discussion of this aspect of the mechanism of reaction of co- ordination compounds has become more profitable as more results become available. In addition resort is being increasingly made to con- sideration of available orbitals and the crystal-field theoretical approach.*$ 94 The configuration of the possible transition states and chemical inter- mediates for reactions of octahedral and planar complexes has been con- sidered by various investigators.(a) Seven-co-ordinate.-In the transition state for a bimolecular reaction the incoming and the outgoing group must have a similar geometrical relation to the rest of the molecule so as not to violate the principle of 92 Pouli and MacF. Smith Canad. J. Chem. 1960 38 567. 93 Bronsted 2. phys. Chem. (Leipzig) 1926,122 383. 94 Orgel “An Introduction to Transition-Metal Chemistry Ligand-Field Theory,” Methuen London 1960 Chapter 7. WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 33 1 rmcroscopic reversibility .Two structures which satisfy this requirement for an SN2 reaction for an octahedral complex are as shown in (11) and (III).95 Any kind of observed stereochange (e.g. cis-trans; D-L) can be ascribed to an edge-displacement or edge-shfP or trans-attack.* For such a reaction the transition state is pictured as (11). It is not possible to accom- modate a trans -+ trans-conversion within this concept so that the transi- tion state for this (and possibly the other non-stereochemical changes) must be represented differently for these processes involving non-edge- displacement or cis-attack (111). Since different orienting effects can be observed in similar molecules the energies associated with (11) and (111) must be similar. A third possible transition state the pentagonal bi- pyramid g~ in which entering and the displaced ligands are symmetrically located represents a somewhat uneven change in the original octahedral structure and is therefore less favoured.In the special caseof solvolysis assumed specific hydrogen-bond interaction between the entering hydroxy- lic substituent departing ligand and amine group may modify the simple concept of a bimolecular mechanism so that a six-co-ordinate metal complex more closely represents the transition ~ t a t e . ~ * ~ ~ ~ ~ (b) Five-co-ordinate.-This concerns the intermediate and the transition state for both an SNI mechanism with an octahedral complex and an SN2 mechanism with a planar complex. Indeed the four-co-ordinate complex can be considered as an octahedral complex with easily replace- able solvent molecules occupying the two extra positions at longer bond distances.The mechanism of the reaction is then “dissociative,” the solvent molecules moving in to help push off the departing ligand.g8 X and L represent the departing and the remaining ligands and S a solvent molecule. The five-co-ordinate species then rapidly reacts with various potential ligands. O5 ASperger and Ingold J. 1956,2862. O6 Basolo Stone and Pearson J. Amer. Chern. Soc. 1953 75 819. O7 Adamson J. Amer. Chem. SOC. 1958 80 3183. O 8 Banerjea Basolo and Pearson J. Amer. Chem. Suc. 1957 79 4055; cf. ref 52. 2 332 QUARTERLY REVIEWS There are two plausible structures associated with this co-ordination number. These are the tetragonal pyramid and the trigonal bipyramid both probably modified by a ligand in a sixth distant position.Both structures have support and which pertains may well depend on the particular reaction. The tetragonal pyramid is the more easily attained and is favoured on crystal-field and orbital c ~ n ~ i d e r a t i o n ~ . ~ ~ ~ ~ In the trigonal bipyramid arrangement ,53 956 O0s1 O1 however there is less repulsion between ligands and in the special case that one of the five groups can form n-bonding it is favoured in many respects.75 (c) Three-co-ordinate.-There is little evidence for planar complexes [and this refers currently and specifically to platinum(11)1 reacting by an SNl mechanism. Isomeric change is usually very slow with platinum(I1) in the absence of catalysts. Examination of Table 6 shows that the charge on TABLE 6. Kinetic data at 25" for the reactions of certain platinurn(I1) complexes showing aquation charge and ligand e f l e c t ~ .~ ~ J O ~ Reactants Products Rate law (R =) trans- [Pt(NH,),CI,] [Pt(NH,),CI(H,O) I+ + C1- 9.8 x [Pt(NHd,CI,] trans- [Pt(NHd,CI,] [Pt(NHd,(aniline)Cl]+ 9-5 x 10-5[Pt(NH,)2C12] trans- [Pt(NH,),CI,] [Pt(NH,),Cl*Cl] + C1- 9.8 x [Pt(NH,),CI,] + (sec.-l or 1. mole-l sec.-l) [PtCl4I2- + H20 [PtCl,(H,O)]- + C1- 3.9 x 10-5[PtC1,2-] + H2O + aniline + c1- + *c1- 7.8 x lO"[Pt(NH3),Cl,] [CI-] [Pt(PE5),ACIl + PY + [Pt(PEQ,A(pY)l+ + Cl- A = c~s-CGH~ A = c~s-C,H~(CH& A = cis-CH A = cis-Cl A = trans-H A = trans-CH A = trans-C1 kl + k2bYl k,a = 4.2 x 10-7 kla = 3-8 X lo- kla = 6.0 x kla = 1.7 x low2 kla = 1.8 x loh2 kl = 1.7 x lo4 k = 1.0 x a At 0'. the platinum(I1) complex has little effect on the rate4s5 while steric hindrance above and below the metal atom has a profound influence.lo2J03 The latter is shownlo2 by examining the reaction (22) The pseudo-first-order rate constant kobs = kl + k2[py] where kl refers [M(PEt&ACII -I- Py + [M(PEtJJ(pY)]+ + CI- .. . . . 9 9 Dyatkina and Syrkin Russ. J. Iriorg. Chem. 1959 4 579. loo Hamm Kollrack Welch and Perkins J. Amer Chem. SOC. 1961 83 340. lol Orgel J. Inorg. Nuclear Chem. 1956 2 137. lo2 Basolo Chatt Gray Pearson and Shaw J. 1961 2207. lo3 Pearson Gray and Basolo J. Amer. Chem. Soc. 1960 82 787. WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 333 to the solvent(ethano1)-controlled reaction and k is a second-order rate constant for reaction with pyridine. For M = Pt A = phenyl k is some lo5 higher than for M = Pt A = mesityl two methyl groups occupying positions in the latter above and below the plane of the complex.All the above facts militate against a dissociative mechanism for reactions of platinum(I1). On the other hand the little study that has been made of tetrahedral complexes suggests that these reactions involve an SNl mechanism. Thus the rate of exchange of carbon-14 between tetra- carbonylnickel and carbon monoxide in toluene is independent of the concentration of carbon monoxide (Table 2) :*04 [Ni(CO),] + *CO + [Ni(CO),(*CO)] + CO . . . . . That panacea of inorganic ills (n-bonding) can be invoked to account for this difference in behaviour. 5. Stereochemical Changes accompanying Replacement Reactions One of the interesting facets of co-ordination chemistry is the study of geometrical isomerisation and racemisation,lo5 e.g.M utarotation d-cis-[Co en,F,]+ z d-cis-[Co enzF(H2O)Iz+ Racemisation + trans-[Co en,F(H,0)]2+ . . . . . . . (24) Only the relevance of the results to the understanding of the mechanism of substitution will be mentioned here. Although all possible stereochemical changes have been observed in cobalt(rI1) chemistry,lo6 it is important from an interpretative point of view that the initial product is observed and not one produced later in the reaction or one chemically isolated. This care is necessary since cist-ttrans and D++L changes can often occur as readily as the substitution process itself. Even armed with the correct stereo- chemical result it is difficult to assign mechanism. The calculation of the likely proportions of isomers produced with octahedral complexes from an S,l (with trigonal-bipyramidal or tetragonal-pyramidal intermediates) or an SN2 mechanism has been ~ndertaken.~ A number of other factors are however involved and there are at present many limitations to the stereochemical approach to elucidation of mechanism.Although an SN2 reaction always produces quantitative stereochemical changes with tetrahedral carbon the same is not true of octahedral complexes [e.g. base hydrolysis of cobalt(rI1); Table 81 and with plati- num(~~) reaction with retention of configuration is usual this itself aiding a mechanistic discussion (see previous section). In general however Io4 Basolo and Wojcicki J. Amer. Chem. SOC. 1961 83 520. Io5 Wilkins and Williams in “Modern Coordination Chemistry,” ed. Lewis and lo6 Werner Annalen 191 2 386 1.Wilkins Interscience Publ. Inc. New York 1960 p. 174. 334 QUARTERLY REVIEWS stereospecific reactions will result more from SN2 than SNl mechanisms. Both cis- and trans-cobalt(m) complexes aquate with full retention of configuration for an SN2 but only cis for an SNl reaction (Table 3). Circumstantial evidence for mechanism can sometimes be obtained from racemisation and isomerisation studies. The fact that for example the rate of reaction of optically active cis-[Co en2C12]+ with certain anions in methanol (see Table 1) equals the rate of concurrent racemisation supports the postulate of an intermediate [Co en2C1I2+ or [Co en2C1(CH,-OH)]2+ ion which is symmetrical or can rearrange before reaction. Contrariwise the observation that [Co en2(NH,)(H2O)I2+ exchanges water without racemis- ing suggests that the intermediate must be asymmetric in which case it is more likely to have the tetragonal-pyramidal configuration.83 Experiments in which H21s0 was used show that in the isomerisation of trans- [Co en2(H20)J3+ one molecule of water is brought into exchange with sol- vent water whereas in the isomerisation of [Co en,(OH),]+ only a fraction of one oxygen per ion undergoes exchange for each act of isomerisation. This and the fact that the energy of activation varies with temperature indicate that more than one mechanism operates in the isomerisation of K O en2(OH>21+. 6. Important Factors influencing Rate and Mechanism5 (a) Nature of Central Metal Atom.-It would be expected from electro- static considerations that there would be a slowing down of rate as the charge of the central metal increases and its size d e c r e a s e ~ .~ ~ ~ It is thus understandable that e.g. Be2+ and A13+ react more slowly than Mg2+ Cu2+ and Zn2+ ions with S042- (see Table 4) and thenoyltrifluoroa~etone.~~~ Some attempt has been made to correlate the rates of these reactions with the ionic charge and radius of the reacting metal aquo-ion.* Such relations will however be disturbed by considerations of the electronic configura- tion of the metal involved. Although attention had been drawn28y108 to the importance of the electronic configuration of the metal ion on the rates of reaction of its complexes it was Taubel who first tabulated the scattered and fragmentary data on the rates of reaction of co-ordination complexes. Attention was focused on the relation between lability and electronic configuration on the basis of the valence-bond theory.Although these ideas have been some- what superseded by the crystal-field approach their importance in drawing attention to this area of research cannot be exaggerated. The systems which are highly stabilised crystal-fieldwise are likely to *The “effective ionic charge” allows for ions with a large number of d-electrons to have effective charge some 10-20 % greater than the nominal value and in part explains why metals in second and third transition series react more slowly than those in the first.* Io7 Taft jun. and Cook J. Amer. Chem. Soc. 1959 81 46. lo* Adamson Welker and Volpe J. Amer. Chem. SOC. 1950 72 4030; Adamson Welker and Wright J. Amer. Chem. SOC. 1951 73 4786.WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 335 react slowly on either an S,l or an SN2 basis since in formation of the transition state there will be a loss of crystal-field stabilisation energy which will be an extra (but important) contribution to the energy of activation compared with that for similar ions not so stabilised. This is the basis for understanding the slow reactions of d3 [ ~ ~ ( I I I ) ] low-spin d6 [CO(III) and certain F~(II)] complexes and d8 [N~(II)] systems. The advantage of the crystal field over the valence-bond treatment (in the simple form visualised by Taube) is its ability to predict at least semi- quantitatively differences in energies of activation for the reactions of a series of similar complexes. The difference dE in the crystal-field stabilisa- tion energy (C.F.S.E.) has been calculated for the octahedral (reactant state) and square-pyramidal (transition state) configurations for various dn systems (Table 7).4 For the reactions of the bis(terpyridine) complexes10g TABLE 7.Kinetic data and C.F.S.E. f o r the dissociation of terpyridine complexes at pH - 7 at 25" k = A exp (-E/RT).loS Electronic system d5 dc6 dyo dc4 dy2 d7 da d9 d10 Complex ion [Mn terpy12+ [Fe W P Y 2 l 2 + [Fe terpy12+ [Co terpy2I2+ [Ni terpy2I2+ [Cu terpy12+ [Zn terpy12+ 105k (Sec . -1) Fast 0.017 670 67 0-167 Fast Fast log A (seC.-l) - 1 4 3 11.0 7.7 9.5 - E AE(D4) (kcal. mole-l) 0 28.7 4 18.0 0 14.8 0 20.8 2 0 0 - - - there are increasing energies of activation in the sequence cobalt nickel and iron. The increments are about 6-8 kcal.mole-1 and this is in good agreement with the estimated increasing C.F.S.E. losses (dE M 2Dq M 6 kcal. mole-l). The estimation of the energies of activation for the d5 d9 and d10 systems (which should resemble that for d7) will have to be made by using rapid-reaction techniques. A really striking difference is between low-spin [Fe terpy2I2+ and high-spin [Fe terpyI2+. On either a displacement or a dissociation mechanism a much larger C.F.S.E. loss would be expected for the diamagnetic than for the paramagnetic iron complex. The difference 11 kcal. mole-1 in the energies of activation agrees nicely with prediction. From considerations then of both charge and crystal field cobalt(m) complexes would be expected to react slowly. This is often observed and any digression from such behaviour can be usually ascribed to the fact that the cobalt-ligand link is not actually being broken during the reac- ion,ll0 e.g.in the rapid decomposition of the amminecobalt(I1x) hydro- gen carbonate complex by acid:111 log Hogg and Wilkins J. 1962 341. 11* Murmann and Taube J. Amer. Chem. SOC. 1956,78 4886. '11 Hunt Rutenberg and Taube J. Amer. Chem. Soc. 1952 74 268; Bunton and Llewellyn J. 1953 1692. 336 QUARTERLY REVIEWS f //o [(NH3),Co-0-j-Cy 13+ + [(NH,),CO-OH,]~+ + CO . . . (25) i \O-i-H Most comparisons have been made so far between elements of the first transition series. Only fragmentary results exist for the second and the third row but when similar compounds can be compared it is usually found that the heavier elements react the more slowly. This is certainly true of the Ni Pd Pt triad.lo2J12 For example the ratio of the rates of the reaction (22) for A = o-tolyl and M = Ni Pd and Pt is 5 x 10g:105:I.102 Some very early kinetic studies showed that rhodium(II1) and iridium(@ complexes react much more slowly than the corresponding ones of cobalt(m).8 This increasing inertness may have crystal-field and “effective ionic charge” behaviour as its Lest one should become too con- fident of this truth however it is worth recalling some recent results of Hertz,30 where the order of formation rates for bromide and iodide complexes is HE > Cd > Zn.Finally the stereochemistry of the metal complex will have some influence. Apart from the considerations outlined above metals which tend to form planar complexes (or tetragonal struc- tures) e.g.copper(II) may be expected to react rapidly for reasons already considered (see Table 4).38 (b) Ligand Specificity.-The effect of ligand change is rarely as drastic as that of the metal. However in the reaction [MAnB] + X -+ [MA,X) + B we find that A B and X can influence the rate and mechanism of the reaction. Predictions as to the effect on the rate of charge and size of these groups can be made.4gD In all cases we must rely on experiments with cobalt(@ and platinum(I1). Systematic work on this aspect has been carried out on the hydrolysis of [Co en,ABIn+ (reaction 12). The rates are very dependent on the nature and to a smaller extent the position (Le. cis or trans) of the non-participating group A (Table 3).* As the electron supply of A diminishes-and this may operate through conjugative (electron-transfer) or inductive (electron-deformation) pro- cesses-the ability to promote an SNl mechanism e.g.Substituent present (A) but not replaced. by assisting the departure of B diminishes. The rate thus falls to a minimum OH > N3 > Cl > SCN - NH,. A group which can withdraw electrons from cobalt can promote a bimolecular mechanism e.g. *A full discussion has been given of the polar effects which influence rate and orienta- tion.6s 11 (a) Basolo Gray and Pearson J. Amer. Chem. Soc. 1960 82 4200; (b) Basolo Bergmann and Pearson J. Phys. Chem. 1952,56,22. WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 337 and since a conjugative effect such as this is more important than an inductive one the increased rate can be understood. Base hydrolysis is much less dependent on the nature of A (see Table 8) and since there is now a reduced rate with the nitro-group the S,2 process assigned is considered one of synchronous bond-fission and -formation rather than reagent in the lead as in the hydrolysis reaction.Reasons for this behaviour have been suggested.s The steric course of base hydrolysis TABLE 8. Second-order rate constants (k2 1. mole-1 sec.-l at 0’) and steric course of reaction [Co en,ACl]* + OH- + [Co en,A(OH)]”+ + C1- (ref. 7,113). A OH c1 Br NCS NH3 N3 NO2 k2 0.37 15 23 1.4 0.50 0.17 0.03 cis-A cis (%I 97 37 40 80 85 51 66 trans-A 0.017 94 85 5 110 0 0-36 76 1 *25 76 0.41 13 0.08 6 k2 cis (%I is however profoundly influenced by the non-participating ligand. It has been known for many years that with platinum-(11) and -(Iv) complexes a trans-effect operates which reflects the tendency of a ligand to direct an incoming group into the trans-position.An order of trans-effect has been constructed on the results of preparative studies. The kinetic basis of the effect is currently being explored. By examining the ease of replacement of chlorine by pyridine with different A substituents (reaction 22 M=Pt) it was shown in avery interesting studylo2 that there are probably two electronic mechanisms of the trans-effect. These produce polarisation of metal ion by (a) .T.r-bonding and/or (b) electrostatic effects. Both of these enhance nucleophilic attack at the trans- and have little effect on the cis- position. Examples of (a) and (b) are phosphine and hydride ion respec- tively both of which are strongly labilising.Table 6 illustrates these points. Substituent replaced (B). In the reaction [Pt dien XI+ + py -+ [Pt dien pyI2+ + X- . . . . . . . . (26) the observed rate constant equals k + kz(py),112u much as with (22). By using either k or k2 the following order of decreasing ease of removal is observed NO > Cl > Br > I > N3 > SCN > NO2 > CN. This indicates that strong trans-activating groups are themselves difficult to 113 Chan and Tobe personal communication. 338 QUARTERLY REVIEWS dislodge. A similar order obtains with cobalt(rr1) except that the halides are reversed.112a However changes of B (a series of substituted acetates112b) in [Co(NH3),BI2+ influence markedly the rate of base but not of acid hydrolysis so that the nature of entering nucleophile will also have an influence.From their studies on exchange reactions of platinum(I1) com- plexes Martin and his colleagues conclude that H20 > C1 > NH3 in decreasing ease of rep1a~ement.l~~ Groups already present may however modify these sequences.l15 The steric course of acid or base hydrolysis of cobalt(II1) appears little affected by the nature of B. Chelated ligands are usually replaced with more difficulty than uni- dentate ligands unless strain factors are Substitution within a ligand has predictable effects on the rates of bond cleavage involving it or other ligands. 02s116 Entering substituent (X). It is difficult to set up a scale of nucleophilic power for cobalt(n1) because of the overriding complexing power of water except via the rate of substitution of an aquo-complex.By studying the rates of reaction of [Co(CN),H20I2- with a series of ligands a nucleophilic scale was built up.59 The value for k2/k- (see equation p. 328) was used and thus the nucleophilicity refers to the 5-co-ordinate intermediate OH > N3 > SCN > SC(NH2)2 > I > NH3 > Br > S,03 > CNO > H20. This agrees with the more limited series in methanol for the reaction34 [Co en,CI,]+ + X-t [Co en,CIX]+ + CI- . . . . . . (27) where although the usurping power of water is avoided complications due to ion-pairing may arise. The situation is simpler with platinum(i1) since the complexes are often soluble in organic solvents and SN2 reactions are more common. It is thus easier to construct an order of nucleophilic power roughly:98 R3P SC(NH2)2 SCN I > N3 > NO2 > py > aniline > olefin NH3 Br > C1> glycine OH H20 which shows a general similarity to the CO(III) series and for that matter the sequence found with organic halides,l17 except in the position of hydroxide which is a very much weaker nucleophile for platinum(I1).The removal of bromine from cobalt(m) complexes [CoEDTA BrI2- -f [CoEDTAI- + Br- . . . . . . . (28) in which the ethylenediaminetetra-acetate changes from a 5- to a 6-co-ordinate ligand see (1 l) is accelerated by metal ions. The order of increasing electrophilic effect Fe3+ > Cd2+ > TI+ is expected on charge considerations.l18 114 Elleman Reishus and Martin jun. J. Amer. Chem. SOC. 1959 81 10. 115 Grinberg and Kukushkin Russ. J. Inorg. Chem. 1959,4 139. 116 Ellis Hogg and Wilkins J. 1959 3308; Aprile Caglioti and Illuminati J.Znorg. 117 Edwards and Pearson J. Amer. Chem. SOC. 1962 84 16; they compare nucleo- 118 Dyke and Higginson personal communication. Nuclear Chem. 1961 21 325. philic reactivities at saturated carbon and platinum@) atoms. WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 339 (c) Change of Solvent.-It is apparent that a study of reactions in non- aqueous solvents would obviate the problems of ubiquitous water so apparent from previous sections. At the same time ion-pairing might then become a more important consideration and the necessity to remove even traces of water be important.l19 The very few systematic studies which have been undertaken of reactions in a variety of ~ o 1 ~ e n t ~ ~ ~ ~ ~ emphasise the key role that solvent plays in the reactions of co-ordination compounds.Adamsons’ showed that the rate constant for removal of the first thio- cyanate group from [Cr(NH,),(SCN),] - by solvolysis was approximately constant for hydroxyl solvents but negligibly small for solvents such as nitromethane where hydrogen bonding was impossible. The kinetic importance of the solvent and the inability of strong nucleophiles to accelerate the rate suggested to Adamson that “pure” SN 1 or S N 2 reactions could be ruled out and an alternative mechanism was proposed in which the role of the hydroxyl group was paramount. The strong interaction of solvent and metal ion in the rate-determining step (as required by the “dissociation” mechanism for a planar complexs8) is shown by the importance of the solvent on the reaction order for the simple exchange process :lo3 (29) Solvents with potentially vacant orbitals e.g.nitromethane were partic- ularly effective in aiding the rate probably because of their ability to n-bond with the platinum(u) and thus approach nearer and help displace the chlorine. The second-order substitution of Ns- into cis- [Co en2C1,]+ (but not the trans-isomer) in methanol can be interpreted as either an SN2 or an sN21P (or SNIIP) reaction with ion-pairing (30a) an important pre- equilibrium in the latter cases [Co en,CI,]+ + X- + [Co en,CI,]+ X- (304 . . . . trans-[Pt pyeCI,] + *CI- + trans-[Pt py2*Clz] + CI- ... . . . . . . . . Slow [Co en,CI,]+ ... X- - [Co en,CIX]+ + CI- . . . . . . . (30b) In the special case of X = OMe- an &lCB mechanism is also possible. For these reactions in methanol first-order kinetics observed with certain nucleophilic reagents (see Table 1) are more likely to arise from an S,1 than from an SN2 reaction involving the weakly nucleophilic methano1.120 The advantages of thus studying substitution racemisation and isomerisa- tion phenomena in non-aqueous solvents without accompanying hydro- lysis are still offset by interpretative difficulties.(d) Catalysis.-Homogeneous heterogeneous and photo-catalysis have all been observed in kinetic studies involving co-ordination compounds. Homogeneous. Acceleration of rate by added ligand or metal ion can 119 Slaten and Garner J. Phys. Chern. 1959,63,1214. lZo Pearson Henry and Basolo J. Amer. Chem. SOC. 1957,79,5379,5382. 340 QUARTERLY REVIEWS occur in certain circumstances.121 10-%-Hg(II) ion accelerates the hydro- lysis of Fe(CN)2-.122 Probably the most important catalysis however involves redox processes.Thus Cr2; (labile) catalyses Cr3,i-H20 exchange via the rapid Cr:i-Cr:; electron transfer,123a and cobalt@) catalyses reactions of c o b a l t ( ~ ~ ~ ) . l ~ ~ ~ A study of the reaction kinetics of platinum(1v) is fraught with dangers from traces of Pt(111)~~J~~ or Pt(11).l~~ Thus exchange of chloride with [Pt en2Cl2I2+ occurs only in the presence of Pt(II) and the rate law R = k3 [Pt(~v)] Ipt(11)1 [Cl-] can be explained by :126 [Pt en2]2+ + *CI- + [Pt en,*CI]+ . . . . . . . . . . . (31 4 en en en en [Pt en,*CI]+ + [Pt en,Cl,12+ + [*CI-Pt-CI-Pt-C1I3+ + [Pt en,*CI2l2+ + [Pt en2CI]+ (31 b) Heterogeneous. Certain substances (for example active charcoal and mercury) appear capable of labilising metal-ligand bonds.This has been observed particularly with cobalt(II1) complexes although even here the studies are fragmentary. Isomeri~ationl~~ and racemisationl2* of cobalt(n1)- ethylenediamine complexes can be markedly accelerated by addition of active charcoal. This solid appears to have a dual role:129 (a) it converts small amounts of the low-spin [Co en,],+ into a high-spin state which it can then reduce to high-spin [Co en,I2+ and (b) it catalyses the electron- transfer process d-[Co en3I3+ (low-spin) + I-[Co en3I2+ (high-spin) -f I-[Co enJ3+ + d-[Co en3I2+ + I-[Co en3I2+ . . . . (32) Small amounts of cobalt(I1) are found. That the charcoal is involved in a redox process is indicated by the fact that I-[Pt en,I4+ is less readily racemised while I-[Rh en3],+ is stable to charcoal treatment even on boiling for some tirne.l3O Certain polarographic observations on the rate of reduction of [Co(NH3) 5X]2+ at the dropping-mercury electrode can be understood on the basis that the substitution [Co(NH3),X]2+ + Cl- -+ [Co(NH3),C1I2+ 3.X- occurs at the mercury surface much faster than in bulk Photocatalysis. Once again little systematic investigation of this often- 121 Beck J. Inorg. Nuclear Chem. 1960 15 250; he classifies and discusses homo- geneous and heterogeneous catalysis of complex formation. 128 Asperger and Pavlovic J. 1955 144-9. 123 ( a ) Plane and Taube J. Phys. Chem. 1952,56,33; (6) Ellis Wilkins and Williams J. 1957 4456. 124 Rxh and Taube J. Amer. Chem. SOC. 1954,76 2608; Poe and Vaidya J. 1961 2981. 126 Ellison Basolo and Pearson J.Amer. Chem. SOC. 1961 83 3943. Basolo Wilks Pearson and Wilkins J. Inorg. Nuclear Chem. 1958 6 161 ; see also Cox Collins and Martin jun. J. Inorg. Nuclear Chem. 1961,17 383. 12' Bjerrum and Rassmussen Acta Chem. Scand. 1952,6 1265. 12* Douglas J. Amer. Chem. Soc. 1954,76 1020. 129 Dwyer and Sargeson Nature 1960 187 1022. l 3 0 Sen and Fernelius J. Inorg. Nuclear Chem. 1959 10 269. 131 Vlcek and Kuta Nature 1960,185,95; Watt and Vaughn Nature 1960 186 309. WILKINS KINETICS AND CO-ORDINATION COMPOUNDS 34 1 hv [MIIIA,X] + [MIIA,*.*X] - observed phenomenon can be reported. The photochemical reactions of metal complexes can be understood on the basis of a chemical in which absorption of a light quantum results in homolysis of the ligand- metal bond. If the products are formed with sufficient energy to escape completely the net result is a redox reaction (33a).If re-formation occurs the total quantum yield is reduced and if “back-electron transfer” occurs (33b) aquation results e.g. This simple idea can account for the facts that photochemical reactions of cobalt(II1) and iron(II1) are often of redox type while those of chromium(II1) and iron@) usually result in aquation. In certain cases the energy of the light quantum and the ease of oxidation of the acido-group will be im- ~ 0 r t a n t . l ~ ~ However some role surely must be ascribed to the photo- chemically excited state and with chromium(II1) at least photocatalysis may involve reaction when the complex is in the lowest spin-forbidden electronic state.13* (e) Thermodynamic Stability and Kinetic Reactivity.-It is often stressed that there is not necessarily any connection between kinetic and thermo- dynamic behaviour and there are certainly examples with co-ordination compounds where this is true.Thus the inertness of complex cyanides does not parallel their ~tability.l*~J~~ Nevertheless since in simple cases the stability constant is equal to the ratio of formation and dissociation rate constants (K = k,/k-,) K A + B + AB kl k-l . . . . (34) should kl (or k-,) remain sensibly constant with change of structure in A or B then obviously the other rate constant will parallel concomitant changes in K. This behaviour is observed with the formation of metal sulphate and nickel diamine complexes and has been referred to in Section 3(b). Some other examples necessarily incomplete of observed relations between kinetic and thermodynamic parameters are shown by rates of reaction of diamine complexes of nickel,13s ethylenediaminetetra-acetate complexes of metals,g7 halide complexes of iron,89,90 cobalt lse Adamson and Sporer J.Amer. Chem. SOC. 1958 80 3865; Adamson J. Znorg. ls3 Adamson Discuss. Faraday SOC. 1960,29 163. 13* Plane and Hunt J. Amer. Chem. SOC. 1957 79 3343; Edelson and Plane J. ls6 MacDiarmid and Hall J. Amer. Chem. SOC. 1954 76 4222. ls6 Ahmed and Wilkins J. 1960 2895. Nuclear Chem. 1960,13,275. Phys. Chem. 1959,63,327. 342 QUARTERLY REVIEWS and p l a t i n ~ m ~ ~ ~ p ~ ~ ~ ~ and zinc cadmium and mercury.30 In the last case for example attention is drawn to a rough relation between increasing formation rate and stability with bromide and iodide complexes.7. General Conclusions and Future Developments It will be apparent from the foregoing that a surplus of conflicting mechanisms exists even for the simple hydrolytic processes in acid and base solution. Obviously it will be necessary to devise key experiments to resolve these problems. Here there will be scope for experiments with both conventional apparatus and the techniques for fast reactions. Indeed in the latter respect the subject is on the threshold of exciting developments since so many of the reactions of complexes especially of the first transition series are rapid and hitherto unmeasured. The impact of the results on the understanding of inorganic analytical and biological problems can be imagined. More intensive examination of complexes of elements other than those of the transition series is also required (a start having been made with for example boron,137 silicon,13* and arsenic139) and of complexes containing ligands (such as carbon monoxide and pho~phinesl~~) where interesting questions of bonding arise and which may be answered in part by kinetic studies.The examination of polynuclear hydroxy-complexes by rapid techniquesl4I has begun but here as elsewhere the kinetic aspects have lagged behind the thermodynamic work. There are signs that this has been recognised and an attempt is being made to reduce the leeway. The rewards will surely be great. The author thanks Dr. M. L. Tobe for helpful criticism of this Review. 13' Ryschkewitsch J. Amer. Chem. SOC. 1960,82 3290. 138 Dhar Doron and Kirschner J. Amer. Chem. SOC. 1959 81 6372. 139 Craddock and Jones J. Amer. Chem. SOC. 1961 83 2839. 140 Ref. 10 p. 68; Strohmeier and Mittnacht,Z. phys. Chem. (Frankfurt) 1961 29 141 Schwarzenbach and Meier J. Inorg. Nuclear Chem. 1958 8 302; Wendt Z. 339. Electrochem. 1962 66 235.

ISSN:0009-2681    

DOI:10.1039/QR9621600316

出版商:RSC    

年代:1962

数据来源: RSC

摘要:

TOPOTACTIC REACTIONS IN INORGANIC OXY-COMPOUNDS By L. S. DENT GLASSER F. P. GLASSER and H. F. W. TAYLOR (DEPARTMENT OF CHEMISTRY UNIVERSITY OF ABERDEEN) IN some reactions of crystalline solids the crystal structures are partly preserved and in this way the structure and crystallographic orientation of a starting material may control the orientation and even the nature of the products. Such reactions can therefore be adequately explained only if the relevant crystal structures are taken into account. Two differing degrees of structural control can be distinguished epitaxy which is a two- dimensional effect and topotaxy which is three-dimensional. In epitaxy crystals grow on the surface of another substance (the host) in certain definite orientations. The host need not be a reactant and may even be chemically unrelated to the overgrowth.Thus certain organic compounds crystallise on the surface of mica in definite orientations con- trolled by a fortuitous correspondence of the lattice-repeat distances. This kind of epitaxy perhaps better called oriented overgrowth is not relevant to this discussion. When the host is also a reactant the product forms partly or wholly at its expense and grows on the surface in an oriented way. Some oxides form on metals in this manner and these will be briefly discussed as they have some relevance to the question of topotaxy. In topotaxy a single crystal of a starting material is converted into a pseudomorph" containing one or more products in a definite crystallo- graphic orientation ; the conversion takes place throughout the entire volume of the crystal.For true topotaxy there must be some three- dimensional correspondence between the structures of the product and its host (in contrast to epitaxy in which the correspondence need only be two- dimensional). We shall confine ourselves to reactions involving major first-order phase transitions. This eliminates from consideration such minor changes as the a-p quartz inversion ferroelectric transitions or the onset of ionic rotation in ammonium nitrate. Pseudomorphs formed from single crystals vary greatly. When more than one phase is formed the regions occupied by each may be macro- scopically visible or they may range in size down to intimate mixtures in which the individual phases are detectable only by X-rays. There may be no change in the bulk composition as in polymorphic transformations or exsolution phenomena,? or there may be a net gain or loss of material as in oxidations or dehydrations.When a loss occurs the crystal does not usually shrink in external dimensions but becomes porous. The pseudo- morph sometimes has crystallinity comparable with that of the starting material but usually there is a marked deterioration. It may consist of * A pseudomorph is a crystal which has become converted into another substance or mixture of substances without change in its external form. t Exsolution describes the process in which a single homogeneous solid phase breaks up into two solid phases thus becoming heterogeneous. 343 344 QUARTERLY REVIEWS crystallites which are small or poorly crystalline or imperfectly oriented (or any combination of these).If there are two or more phases these may differ in crystallinity; amorphous material may be formed. Differing degrees of crystallinity cause characteristic X-ray diffraction effects the reflections being broadened or streaked along powder lines (Fig. 1). In general examples of pseudomorph formation may be found which provide a continuous gradation from very well-ordered topotactic changes down to changes in which the structure is completely disrupted and crystallites in random orientation are formed. Partly because of this continuous gradation opinions differ as to exactly which reactions should be regarded as topotactic. Lotgeringl proposed the term topotaxy “for all chemical solid state reactions that lead to a material with crystal orientations which are correlated with crystal orientations in the initial product” (sic).Mackay2 defines topotaxy more narrowly to cover transformations in which “the majority of the atomic positions in the original and in the transformed material are substantially the same and there is accord in three dimensions between the initial and final lattices”. He regards as epitaxy those transformations in which “crystals . . . grow on certain internal . . . crystal planes of the initial material so that there is two dimensional accord between the packings in these planes but otherwise the structures are dissimilar the second phase appearing as an inter- growth in the first phase”. Mackay’s definitions lead to practical difficulties as many transformations do not fall clearly into either category; a detailed knowledge of their mechanisms would be necessary to classify them.Lotgering’s definition on the other hand ignores what may be a valid distinction. We have adopted an intermediate position. Experimental Methods.-Although some studies have been made with the polarising microscope the most powerful tools are X-ray and electron diffraction and only these methods will be described here. Some additional and more specialised techniques will be mentioned later. A simple X-ray method using only standard single-crystal apparatus is to determine the orientation of a crystal of the starting material which has easily recognisable outlines. The crystal is then treated in the desired way and the orientation of the product relative to the crystal outline afterwards determined. In this way orientation of product relative to starting material is found.This method can be applied to topotactic reactions of all kinds,1,3,*,5 but its accuracy depends on the ease with which the outlines of the crystal can be recognised and the accuracy with which it can be set visually. If the reaction can be halted before it is complete the pseudomorph will give reflections from both starting material and product (Fig. l) and the accuracy is improved. In thermal transformations it is sometimes 1 F. K. Lotgering J. Inorg. Nuclear Chem. 1959 9 11 3. a A. L. Mackay Proc. 4th Internat. Symp. Reactivity of Solids Amsterdam 1960,571. * H. Saalfeld Roc. 4th Internat. Symp. Reactivity of Solids Amsterdam 1960 310. 4 I,. S. Dent Glasser and F. P. Glasser Acta Cryst. 1961 14 818. 6 G. M. Faulrhg W. K. Zwicker and W.D. Forgeng Amer. Min. 1960 45 946. DENT GLASSER GLASSER AND TAYLOR TOPOTACTIC REACTIONS 345 possible to do this by heating one end only of a prismatic crystal. It is often preferable to heat the crystal without removing it from its mounting; a device for doing this using a Weissenberg goniometer has been described.s Various forms of high-temperature single-crystal cameras have been described7 which enable reactions reversible on cooling to be studied too. A high-pressure single-crystal camera has also been described.8 Selected-area electron diffraction has only limited application in these studies but is occasionally very useful when single crystals large enough for X-ray work cannot be obtained. Some materials decompose in the electron beam and topotactic reactions occurring thus can be studied dire~tly.~ Others have crystals or cleavage fragments with an easily recognisable shape which persists in the product and can therefore be reasonably assumed to be pseudomorphic; the orientation relationship can be deduced from a comparison of the electron micrographs and diffraction patterns of product and starting material.l0 Some different classes of topotactic reactions will now be considered.Oxides and Mixed Oxides In some simple topotactic reactions of oxides or mixed oxides cations migrate while the oxygen packing stays approximately unchanged. Thus FeO Fe,O (magnetite) and y-Fe20 can be interconverted by heating in suitable atmospheres. All have structures based on cubic close packing of oxide ions and differing in the number and kind of interstices occupied by the cations.It has long been accepted that reaction proceeds topotaxially by addition or removal of oxygen layers with appropriate migrations of cations,ll though direct crystallographic evidence for this has only recently been c ~ b t a i n e d . ~ ~ ~ ~ ~ ~ ~ Another example is provided by the Occurrence in Nature of pseudomorphs in which rutile (TiO,) has been formed from ilmenite (FeTiO,). Both of these structures are based on hexagonally close packed oxygen and the change evidently occurs by migrations of iron and titanium.l4 Slightly more complex cases occur where there is also a change in the type of oxygen packing. Thus Fe304 can be oxidised to a-Fe,O (haema- L. S. Dent J. Sci. Instr. 1957 34 159. J. D. Donaldson J. Sci. Instr. 1961 38 286; J. V. Smith and W.L. Brown 2. * L. F. Vereshchagin S. S. Kabalkina and V. V. Evdokimova Pribory i tekhnika @ J . F. Goodman Proc. Roy. SOC. 1958 A 247 346. lo J. A. Gard and H. F. W. Taylor Amer. Min. 1958 43 1. 11 A. F. Wells “Structural Inorganic Chemistry,” 3rd ed. Oxford University Press London 1962,490. 12 J. D. Bernal D. R. Dasgupta and A. L. Mackay Nature 1957 180 645; Cluy Minerals Bull. 1959 4 15. 13 W. Feitknecht and H. W. Lehmann Helv. Chim. Acta. 1959 42 2035. l4 S. W. Bailey E. N. Cameron H. R. Spedden and R. J. Weege &on. Geol. 1956 51 263; K. P. Yanulov and I. V. Chulkova Dokludy Akad. Nauk S.S.S.R. 1961 140 215. Krist. 1961 115 93. eksperimenta 1958 No. 3,90. 346 QUARTERLY REVIEWS tite) in which the oxygens are hexagonally close packed. Optical studies on partly oxidised magnetite crystals show that the haematite forms as exsolution lamellae oriented so as to allow the best possible fit between the close packed oxygen planes of the two structures [haematite (OOO1) // magnetite (1 1 l>].l5 A single-crystal study of the oxidation of a manganese- zinc-iron spinel similarly showed formation of haematite as exsolution I amellae.l6 An X-ray study of the y-a Fe203 transition gave a result similar to that found when a-Fe,O is formed from magnetite; the orientation relation- ship was such as to give the best possible fit between close packed oxygen planes.12 Polymorphic transitions in alumina occur in the same way and are discussed later. An anomalous result was obtained in the Fe203 transition when selected-area electron diffraction was used a different orientation relationship being observed.This was attributed to re- crystallisation in the thin flakes used (probably 100-500 A) in such a way as to cause the close packed oxygen planes to re-form parallel to the plane of the flake. This seems to be the only known case of an orientation relation- ship being controlled by external morphology and not by internal structure. In the above reactions the cations which migrate (e.g. Fey Ti) are much smaller than the oxide ions. With oxides of large electronegative cations topotactic processes may occur by a different mechanism. It has been suggested that in redox processes in actinide and lead oxides,17 oxygen migrates through a relatively unchanged cation framework though no single-crystal studies have been made to support this. Redox changes in manganese oxides have been studied using single-crystal X-ray methods starting from cryptomelane which is approximately MnO,.The sequence 1 2 3 MnO -+ Mn,O - Mn,04 - -+ Mn,O cryptomelane hausmannite spinel occurs topotactically ;5 the orientation relationships were determined but no attempt was made to suggest a mechanism. In stage 3 the oxygen packing is clearly preserved but this may not be true of stages 1 and 2; the behaviour of manganese is probably intermediate between that of “small” and “large” cations. Topotactic changes can also occur in which both oxygen and metal ions remain approximately fixed and only hydrogen ions and electrons move. y-Manganese dioxide can be reduced topotactically with hydrazine ;I8 hydrogen ions and electrons are added and the reaction can proceed as l5 J.W. Gruner Econ. Geol. 1926 21 375; J. W. Greig E. Posnjak H. E. Menvin l6 R. E. Carter W. L. Roth and C. A. Julian J. Amer. Ceram. Soc. 1959,42,533. L. E. J. Roberts Quart. Rev. 1961 15 442; J. S. Anderson and M. Sterns J. 18 W. Feitknecht H. R. Oswald and U. Feitknecht-Steinmann Helv. Chim. Acta and R. B. Sosman Amer. J. Sci. 1935,30 239. Inorg. Nuclear Chem. 1959,11 272. 1960,43 1947. FIG. 1. Typical experimental data obtained in the study of a topotactic reaction an X-ray Weissenberg photograph from a crystal which has Lndergone such a reaction. The sharp spots are from traces of unchanged starting material the reflections streaked horizontallv (i.e. along powder lines) are from the major product and a few additional broad reflections of which the strongest is indicated by an arrow are from a minor product.From such photographs the relative orientations of products and starting material can be found. (a) (b) Fig. 2. Crystal structure of (a) magnesium hydroxide and (b) magnesium oxide showing the relative orientations observed when magnesium oxide is formed by heating magnesium hydroxide. Large open circles represent nearly close packed hydroxyl or oxide ions; small shaded circles represent magnesiums. (001) planes are horizontal in (a) and (1 11) planes are horizontal in (b). Corundum 1100' in air crystals Gibbsite <o.2p crystals or hydrothermally 300' in air X - A I ~ O ~ 970' in a i r K-AI 2 3 0 AI(OH)~ larger < 300° in air 9 ~i~~~~~~ 500' in air (?I,/ --- - - -- ** 300' in a i r Al O.OH i Bayerite AI(OH)j ( ? hydrothermol action within crystals) '1 3 0 0 ' i n a i r -1 400' in a i r 200' inN2 200' ' m s Feg04 Aqueous alkaline oxidation Goethite 300'in air > &FeO.OH CC -FoO.OH Fe(OH)* c Haematito a-Fe2O3 FIG.3. Some topotactic reactions of aluminium and iron compounds. Phases enclosed in rectangles have structures based at least approximatel.v on cubic close packed oxygen layers. All other phases shown have structures based at least approximately on hexagonally close packed oxygen layers. I -Mq- [-+- H20 -Mg - -0- -Mq - -Mq- - 0 - H2° - 0 - -Mq - 7 A. Homogeneous mechanism -0- -Mg- -Mq- -0- - Mg - -0- -Mq - r - a -OH- 4 H 2 0 .- .s P -0H- B. lnhomoqeneous mechanism FIG. 4. Homogeneous and inhomogeneous mechanisms for the dehydration of magnesium hydroxide. Horizontal lines represent layers of atoms or ions.3 0 3 0 3 OH 3 0 2 O+OH 1 2 <Si,Al) 2 O+OH 2 0 +OH 1 2 Al 3 0 3 OH Kaol i ni t e 1 6 0 6 0 6 0 6 0 6 0 6 0 3 S i 4 Al 3 Si 4 Al 3 Si 3 Mq 2O+OH 3 0 3 0 2 Si Talc 3 0 3 0 3 0 3 0 3 0 3 0 2 M9 2 Si 2 M9 2 Si 2 M9 3 M9 2 <Si,Al) 2 O+OH 3 0 3 OH 3 OH 3 0 3 CM~,AI) Chlo r i tor I 1 4 0 4 0 4 0 4 0 2 Mq +Si Si + 2Mq 2 Mq+Si Al-Si spinel Enstatitc Forsteritc FIG. 5. Structures of some clay minerals and of their dehydration products. Horizontal lines represent layers of oxygen or hydroxyl. Square brackets show the thickness of the elementary layer. A FIG. 6. Parts of the structures of A pyroxene; B rhodonite; C wollastonite showing in each case how the chains of SiO tetrahedra are linked to the octahedral parts of the structure (after N. V. Belov).B and C are drawn in the relative orientations found experimentally when rhodonite is converted topotactically into a wollastonite solid solution. DENT GLASSER GLASSER AND TAYLOR TOPOTACTIC REACTIONS 347 far as MnO,. 4s(OH),.64. The substitution of Mn3+ for Mn4+ causes a small lattice expansion and in the later stages of the reduction there is a change in symmetry. Exsolution Phenomena.-MgAl,O (spinel) has a structure based on cubic close packed oxygen with cations in tetrahedral and octahedral interstices. At high temperatures it forms extensive solid solutions with alumina. These have the same arrangement of oxygen ions but some of the Mg2+ ions are replaced by A13+ and others omitted to preserve electrical neutrality. If these solid solutions are maintained at lower temperatures (850-1 300") excess of alumina exsolves.Initial distortion of the structure is followed by the appearance of a metastable phase which exsolves topotaxially in lame11ae.19 This phase has a distorted spinel structure and contains Mg2+. y-Alumina which has an oxygen framework similar to that of spinel is not formed because of its instability at these temperatures. Finally the metastable phase disappears forming a-alumina (corundum) and nearly stoicheiometric MgA1,04 but the lamellar texture remains. In corundum the oxygen is hexagonally close packed and the lamellae are oriented so that the close packed planes are parallel to those of the spinel. Reactions Between Oxides.-Wagner's hypothesis put forward early in the study of solid-state reactions states that when two solid oxides react they do so by counterdiffusion of the cations through the layer of product.Many attempts have been made to verify this but few have used single crystals and orientation relationships have rarely been explicitly considered. The largest groups of studies have been diffusion or self- diffusion experiments with radioactive tracers to determine diffusion coefficients and activation energies and marker experiments in which an inert marker such as a molybdenum wire is placed between blocks of the reacting materials. The results have often been conflicting; for example Wagner's hypothesis is confirmed for the formation of ZnFe,O, but not for ZnA1,0,.20 Recent work on the formation of MgA1204 and MgFe,O, using pores within the structure as markers supports Wagner's hypothesis for both reactions;,' in the same paper it was suggested that earlier con- flicting results from marker experiments could be attributed to splitting of the marker.Recent work on the formation of ferrimagnetic oxides provides more direct evidence of topotaxy together with a possible technical application of topotactic reactions.1,22 A series of compounds exists having the general formula xBaO,yMeO,zFe,O (where Me is a bivalent metal such as Mn Fe Co or Zn). These have related hexagonal crystal structures which may be regarded as blocks of a spinel (with its cubic [I 111 direction parallel to the c-axis) alternating with layers of a characteristic structure containing l9 H. Saalfeld and H. Jagodzinski Z. Krist. 1957 109 87; H. Jagodzinski ibid. 109,388; A. M. Lejus and M. R. Collongues Bull.SOC. chim. France 1961,65. 2o R. Lindner 2. Elektrochem. 1955 59 967. 2l R. E. Carter J. Amer. Ceram. Soc. 1961 44 11 6. 22 F. K. Lotgering J. Znorg. Nuclear Chem. 1960 16 100. 348 QUARTERLY REVIEWS barium ions. These compounds can be formed by solid-state reactions such as BaOdFe,O + ZFe,O + Ba0,2Fe0,8Fe20 BaO,GFe,O is hexagonal and has strong magnetic anisotropy. When pellets of the starting materials were pressed in a strong magnetic field its crystallites became oriented with their c-axes parallel while the Fe,O remained unoriented. After firing the product was a polycrystalline aggregate in which the c-axes of the individual crystallites were more or less parallel to those of the Ba0,6Fe20 originally present. It was concluded that a topotactic reaction had occurred but the mechanism was not elucidated.Redox Processes involving the Metal.-A few examples of the topotactic reduction of oxides to metals are known; single crystals of copper(1) oxide may be reduced to copper topotaxiallyZ3 and iron-containing specimens of brucite [(Mg,Fe)(OH),] can be first dehydrated and then partly reduced to give metallic iron.24 Ag,Si,O can be reduced topotaxially to silver.25 In all these cases the orientation relationship is such that the positions of the metal atoms roughly correspond in the two structures. This contrasts with many of the mechanisms previously described in that oxygen atoms or ions are required to move. A more complex example is provided by the thermal decomposition of silver oxalate at 100-140°.26 Silver is formed with good preferred orientation but although both structures are known the mechanism of the reaction is not obvious.The reverse process namely oxidation of metals barely falls within the scope of this Review. Oxide films or crystals are often formed epitaxially. Many diffusion and other studies have been made especially with iron and copper; the results are complex and no unique interpretation seems possible. More specialised reviews must be Hydroxides Oxide Hydroxides and Silicates of Small Cations (Mg9 4 Fe) Dehydration and Redox Processes in Hydroxides and Oxide Hydroxides.- Many of these compounds such as magnesium hydroxide (Fig. 2a) have layer structures based on sheets of hydroxyl and sometimes also oxide ions. The arrangement of these ions approximates in varying degrees to either cubic or hexagonal close packing; the cations occupy octahedral interstices.23 A. Goswami and Y. N. Trehan Trans. Furuduy Soc. 1956,52 358; M. R. Piggott 24 M. C. Ball and H. F. W. Taylor Min. Mag. 1961 32 754. 25 F. Liebau F. Wodtcke and H. Bunge Actu Cryst. 1960 13 1016. 26 R. L. Griffith J. Chern. Phys. 1946 14,408. 27 J. €%nard Bull. Soc. frunc. Min. 1954,77 1061 ; J. A. Hedvall Plansee Proc. 1955 1 (Publ. 1956); R. Lindner Inst. intern. chim. Solvay 10e Conseil chim. Brussels 1956 459; K. Hauffe Kinetic High-temperature Processes Conf. Dedham Mass. 1958,282 (Publ. 1959); M. Wyn Roberts Quart. Rev. 1962 16 71. Actu Cryst. 1957,10,364. DENT GLASSER GLASSER AND TAYLOR TOPOTACTIC REACTIONS 349 Dehydration to the oxides usually occurs topotaxially and below 500" on heating in air. The oxides also have structures based on approximately close packed oxide ions (e.g.magnesium oxide; Fig. 2b). Because there are no hydrogen ions more interstices are occupied by metal ions than in the hydroxides; tetrahedral as well as octahedral interstices may be occupied. The orientation relationships observed in the dehydration reactions seem always to allow the best possible fit between close packed oxygen planes of starting material and product. Fig. 2 illustrates this for the dehydration of magnesium hydroxide. The dehydration of magnesium hydroxide was studied by X-rays28 in 1919 and this was probably the first time that a topotactic reaction was investigated in this way. Many subsequent X-ray studies of this reaction have been made.g~24~29 In it the type of oxygen packing changes from hexagonal to cubic.Such changes seem usually to occur quite easily when Mg2+ is the predominant cation. The hydroxides and oxide hydroxides of aluminium and iron show many interesting topotactic reaction^,^,^,^^^^^ some of which are summarised in Fig. 3. In the case of iron these include redox processes as well as dehydra- tions. The behaviour of Fe2+ is probably closely similar to that of Mg2+ but Fe3+ and A13f show an important difference in that changes in the type of oxygen packing occur only with difficulty at any rate in reactions effected by heating in air. Thus diaspore gives corundum at 50O0 but with boehmite y-alumina is formed at this temperature and corundum (the stable phase) only appears above 1000°.32 In diaspore and corundum the oxygen atoms are in hexagonal close packing while in boehmite and y-alumina they are in cubic close packing or nearly so.The tendency for the oxygen framework to persist unchanged thus determines the sequence of phase changes. Intermediate metastable phases such as y-alumina frequently occur in these reactions. Sometimes as in the above example their occurrence can at least partly be attributed to the persistence of the oxygen packing but in the dehydration of magnesium hydroxide an intermediate phase has been detected which has the same type of packing (cubic) as that of the stable form of magnesium oxide which is afterwards f ~ r m e d . ~ ~ ~ ~ Metastable arrangements of cations must therefore also be possible and probably come into existence during a process of cation migration. The intermediate phases in general have structures with simple oxygen frameworks but with m G.Aminoff Geol. Form. Forh. (Stockholm) 1919,41,407. 29 J. Garrido Ion. Rev. Espan". Quim. Applic. 1951 11 206 220,453. 30 G. W. Brindley and J. 0. Choe Amer. Min. 1961 46 771. *l M. Deflandre Bull. SOC. franc. Min. 1932 55 140; M. R. Tertian Compt. rend. 1950,230,1677; M. H. Francombe and H. P. Rooksby; Cluy Minerals Bull. 1959,4,1; H. P. Rooksby and C. J. M. Rooymans ibid. 1961 4 234; G. W. Van Oosterhout Acta Cryst. 1960 13 932. 33 G. Ervin Acta Cryst. 1952,5 103; K. Sasvhri and A. Zalai Actu Geol. (Hungary) 1957 4 415. 350 QUARTERLY REVIEWS complicated and sometimes disordered arrangements of ~ a t i o n s . ~ ~ ~ ~ 0 Considerable local disturbances in cation arrangement due to stacking faults have also been po~tulated.~~ These intermediate phases are not always truly anhydrous ; X-alumina in particular contains much hydroxyl ion which is gradually lost as the temperature rises.30 Hydrothermal reactions* of these hydroxides and oxide hydroxides may also occur topotaxially and some examples of these are included in Fig.3. With aluminium compounds hydrothermal conditions facilitate transformations where the type of oxygen packing must change as for instance the conversion of gibbsite into boehmite which occurs below 300”. Hydrothermal reactions may occur inside crystals even on heating in air if evolved water cannot escape easily; thus gibbsite often yields boehmite as well as ~-alumina.~~ It has generally been taken for granted that in the dehydrations the water is lost more or less uniformly from all regions of the crystal.Fig. 4a illustrates this for the dehydration of magnesium hydroxide. In a recent study of this reaction this type of “homogeneous” mechanism was rejected in favour of an “inhomogeneous” mechanism which was con- sidered to account better for the occurrence of the spinel-like intermediate According to the new hypothesis (Fig. 4b) Mg2+ and Hf ions migrate in opposite directions; oxygen atoms are not lost from those regions of the crystal where the magnesium oxide forms but only from those regions which change into pores. Such pores might be formed initially by loss of oxygen near a dislocation or other fault in the structure. Similar mechanisms can be postulated for the other dehydration and redox reactions given in Fig. 3 but further work is needed to see whether they are more nearly correct than the previously accepted homogeneous mechanisms.The dehydration product of calcium hydroxide unlike that of mag- nesium hydroxide is poorly oriented.36 This may be a consequence of the larger size of the Ca2+ ion which could make a process dependent on easy migration of calcium impossible. Ca2A1(OH),,3H,0 gives oriented calcium hydroxide when heated.37 Clay Minerals Serpentine and Micas.-These compounds are struc- turally derived from hydroxides by replacing most of the hydroxyl ions on one or both sides of each elementary layer by highly condensed lamellar * Hydrothermal reactions are ones occurring above 100” under water pressures 1 G. W. Brindley ( J . Japan. Ceramic Assoc. 1961 69 189) has expressed a similar 33 J. D. C.McConnell and J. Lima-de-Faria Min. Mug. 1961 32 898. 34 J. M. Cowley Acfu Crysf. 1953 6 53. 35 J. H. de Boer J. M. H. Fortuin and J. J. Steggerda Proc. Acad. Sci. Amsterdam 36 b. D. West Amer. Min. 1934 19 281. 37 F. G. Buttler L. S. Dent Glasser and H. F. W. Taylor J. Amer. Cerum. SOC. greater than 1 atmosphere. opinion. 1954 B 57 170,434. 1959 42 121. Topotactic reactions caused by heating some clay minerals in air (some minor products omitted) Kaolin-like structures Talc-like structures Chlorite Kaolinite 600" Metakaolin 900" Al-Si spinel* 1100" Mullite* A12Si205(OH) - A12Si207 - A14Si,012 -4 A16S1'2013 Serpentine 600" Forsterite* 1 100" Forsterite + Enstatite Mg3Si2O5(OH) - Mg,Si04 - Mg2Si04 MgSiO Reference a 3 P % B b v1 275 " Oxy-cronstedtite 700" Fe-Sispinel 800" Haematite* c P Cronstedtite (Fe11,Fer11)(SiFe1~r05)(OH)4 --f Fe11~,(SiFer1~05)02(OH) --f Fe1I1,SiO -+ a-Fe,03 I Ferr-chamosite 350" FelIr-chamosite (Fexr~.,Mgo.2Alo.,)(Sil.,Alo.,~,~~O~~4 -+ ~ ~ ~ r r ' l .~ M ~ 0 . 2 A 1 0 . 8 ~ ~ S i l . 3 ~ 0 . 7 0 5 ~ ~ 2 . 3 ~ ~ H ~ Pyr op hylli te 800" Meta-pyrophyllite 1200" Mullite* A12Si401 o(OH)2 4 A12Si401 --t A16Si2013 900" Enstatite* r J Talc 1 Mg3Si4010(OH)2 -f MgSiO Saponite (a smectite or montmorillonoid) 500" Al-talc 950" Enstatite* g -+ MgSiO -4 Mg3(Si,A1010)(OH)2( Mg2Al)03 -+ Mg2Si04 2 Penninite 600" A meta-phase 800" Forsterite* h ij E Mg6(Si,A1020)(OH)4,Mgo. 5," H2° - Mgfj(Si7A1020)(0H)4 MgO' 5 - I Mg,(Si,AI010)(0H)2(Mg2A1)(0H) * With separation of silica or silica-rich material. h 3 2 VJ a G. W. Brindley and M . Nakahira J. Amer.Ceram. SOC. 1959,42,311,314,319; J. Grofcsik and F. Tamils "Mullite its Structure Forrna- tion and Significance," Akademiai Kiado Budapest 1962. R. Steadman and R. F. Youell Nature 1957,180 1066. G. W. Brindley and R. F. Youell Min. Mag. 1953,30,57. E. Thilo and H. Schunemann Z. anorg. Chem. 1937 230 321 ; W. F. Bradley and R. E. Grim Arner. Min. 1951 36 182; L. Heller ibid. 1962 47 156. f E. Thilo and G. Rogge Ber. 1939 72 341. g H. G. Midgley and K. A. Gross Clay Minerals Bull. 1956,3,79. G. W. Brindley and S. 2. Ali Acta Cryst. 1950,3,25. G. W. Brindley and J. Zussman Arner. Min. 1957 42 461. 352 QUARTERLY REVIEWS silicate or aluminosilicate ions (Fig. 5). They undergo dehydration reactions similar to but more complex than those of the lamellar hydroxides. These reactions are probably all topotactic.In some clay minerals such as halloysite [A12Si20,(0H)4,2H20] the layers are separated by sheets of water molecules. The first stage of dehydra- tion is topotactic and usually complete below 500"; the water molecules are lost with one-dimensional lattice shrinkage.38 Above 500" more drastic but still topotactic changes accompany the dehydroxylation of all clay minerals; typical examples are listed in the Table. Some such as kaolinite pyrophyllite and chlorites first lose all or part of their hydroxyl without changing into basically new structures; products known as meta- phases are formed (e.g. kaolinite -+ metakaolin). These meta-phases are similar in many respects to the intermediate phases formed during the dehydration of aluminium hydroxide and other hydroxides.Other com- pounds such as talc and chrysotile rapidly change to new structures on dehydroxylation. The compounds in the first group mostly have a high Al3+ content while those in the second have a high Mg2+ content. The difference in behaviour parallels that observed with the hydroxides and can perhaps be attributed to the greater ease with which the oxygen packing changes in the magnesium compounds. The principal high- temperature phases formed are mullite forsterite enstatite and defect spinels. In all of these the cations other than Si" (i.e. AP+ Mg2+ Fez+) are largely or wholly in octahedral sites. The orientation relationships observed when the various clay minerals are heated in air can be summarised by the rule that the octahedra have the same orientations in the product as in the starting material.In some cases as with talc or saponite the arrangement of SiO tetrahedra in the product also resembles that in the starting material but sometimes (e.g. with serpentine) the orientations of the tetrahedra change during the reactions. In general the regions where topotactic change occurs become progressively poorer in silica. This silica sometimes crystallises as cristo- balite (usually unoriented) above 1 OOO" though sometimes the existence of amorphous silica or silica-rich material must be presumed. On heating the Fe2+-containing minerals in air oxidation of Fe2+ to Few occurs first. Electrical neutrality is maintained by loss of protons and the lattice contracts slightly because Few is smaller than Fe2+. Detailed mechanisms have been suggested for most of these reactions.It has generally been assumed that the mechanisms are homogeneous and also that the SiO tetrahedra are more likely to be stable than are the cation- oxygen octahedra. There is some positive evidence for the first assumption in the case of chlorites where the observed intensities of the 001 X-ray reflections for the meta-phase are compatible with the postulated struc- ture.39 There seems to be no evidence for the second; on the contrary both 38 "X-Ray Identification and Crystal Structures of Clay Minerals," ed. G. Brown 2nd edn. Mineralogical Society London 1961. 39 G. W. Brindley and S. 2. Ali Acta Cryst. 1950 3 25. DENT GLASSER GLASSER AND TAYLOR TOPOTACTIC REACTIONS 353 the greater tendency for the octahedra to be preserved and also the tendency for Si02 to be expelled suggest that the SiO tetrahedra are less stable at high temperatures than the octahedra.Studies on hydrated calcium silicates discussed later also indicate that silicon atoms or ions are mobile at high temperatures. Inhomogeneous mechanisms might provide a better explanation of some of these reactions. The chrysotile dehydration has recently been approached in this way,4o as was that of kaolinite in some early though in the latter case a homogeneous mechanism was later assumed.42 Ame~ite,~~ ~hloritoid,~~ micas,45 ~epiolite,~~ and chrysotile under hydro- thermal conditions40 are also dehydrated topotaxially. Interconversion and acid leaching. There is geological evidence for various reactions in which one clay mineral is changed into another and such processes are possibly topotactic.Smectites have been changed into chlorite-like materials in the laboratory by reaction with magnesium hydroxide or other bases.47 Attempts to reverse the process by acid leach- ing are successful only in certain cases.48 With typical chlorites acid leach- ing causes disintegration of the entire structure but octahedral aluminium is attacked more readily than tetrahedral aluminium.49 Reactions of Pyroxenes Amphiboles and Olivine.-These important rock-forming minerals are silicates in which the predominant cations are Mg2+ Fe2+ or others of similar size (about 0.8 A). They have structures based on fragments of octahedral layers similar to those in the metal hydroxides and clay minerals which are condensed with separate SOp tetrahedra in olivines (e.g.forsterite Mg,Si04) with infinite single chains of tetrahedra in the pyroxenes (e.g. enstatite MgSiO,) and with infinite double chains in the amphiboles [e.g. anthophyllite Mg,(Si,022)(0H)2]. The chains (Fig. 6a) in each case adopt a form which is repeated at intervals of two tetrahedra (5.2 A) and allows them to fit on to the cation-oxygen parts of the structures; such chains have been called Z~eierketten*~O to distinguish them from the different types of chain that arise when other * No satisfactory translation exists for this. 40 M. C. Ball and H. F. W. Taylor Min. Mug. 1963 in the press. 41 M. Nakahira Cluy Minerals Bull. 1954 2 206. 42 G. W. Brindley and M. Nakahira J. Amer. Cerum. SOC. 1959 42 31 1 314 319. 43 G. W. Brindley B. M. Oughton and R. F. Youell Actu Crysf. 1951 4 552; H.Steinfink and G. D. Brunton ibid. 1956,9,487. 44 H. G. Bachmann 2. Krist. 1956 108 145. 45 N. Sundius and A. M. Bystrom Trans. Brit. Cerum. Soc. 1953,52,632; H. Wilman and T. de S. Mutucumarana Actu Cryst. 1954,7,666; J . B. Holt I. B. Cutler and M. F. Wadsworth J. Amer. Cerum. SOC. 1958,41,242. 46 G. Kulbicki Amer. Min. 1959 44 752. 47 S. Caillere S. HCnin and R. Guennelon Cumpf. rend. 1949 228 1741; R. F. Youell Cluy Mineruls Bull. 1950 1 174; M. Slaughter and I. H. Milne Proc. 7th Nat. Clay Conf. (U.S.A.) 1958 114. 48 S. Caillkre S. HCnin and J. Esquevin Cluy Minerals Bull. 1954,2 166. G. W. Brindley Proc. Int. Symp. Reactivity of Solids Gothenburg 1952 349; G. W. Brindley and R. F. Youell Actu Cryst. 1951 4,495. 6o F. Liebau 2. phys. Chem. 1956,206 73. 354 QUARTERLY REVIEWS cations predominate.The sheets in clay minerals represent a higher degree of condensation of Zweierketten. Pyroxenes and amphiboles can ac- commodate limited proportions of larger cations as in diopside (CaMgSi 206) and tremolite [Ca ,Mg,Si,O ,(OH) 2]. Examples of topotactic processes involving these minerals have been studied in the laboratory and also found in Nature. The history of the natural specimens must be inferred from indirect evidence and it is not always possible to distinguish between epitaxy and topotaxy. (1) Naturally occurring pyroxene crystals have been found51 in which inclusions occur either of a different pyroxene mineral or of other minerals such as magnetite (Fe3O4),ilmenite (FeTiO,) haematite (a-Fe203) and amphiboles. In each case the included mineral bears a definite crystall- ographic orientation relative to the pyroxene.(2) Iddingsites are naturally occurring alteration products of olivine in which the latter has been wholly or partly converted topotaxially into other products especially haematite goethite and chlorite or other lamellar silicates ; unoriented quartz may also occur.52,53 (3) Naturally occurring composite crystals of talc and tremolite often occur with the two minerals in a definite relative orientation; the talc seems to have been formed topotaxially from the trem~lite.~* (4) Dehydration of amphiboles occurs topotaxially. As the main crystalline products anthophyllite55 gives a polymorph of the pyroxene MgSiO ; tremolite gives a calcium-magnesium pyroxene and riebeckite [Na,(Fe~I,Mg),Fe~~~,Si,O,,H,I gives pyroxene iron oxides and cristobal- ite the product depending to some extent on whether an oxidising or neutral atmosphere is (5) Topotactic transformations in enstatite have been studied in the laborat (6) Oriented exsolution lamellae of willemite (a-Zn2Si04) have been found in natural olivine All the phases mentioned above have structures based on nearly close- packed oxygen frameworks and the observed orientation relationships are broadly speaking such as to give the best possible fit between the oxygen frameworks of starting material and product.It has been suggested53 that the formation of iddingsites from olivine occurs by diffusion and replace- 61 M. G. Bown and P. Gay Amer. Min. 1959,44 592. sz G. Brown and I. Stephen Amer. Mia 1959 44 251; W. W. Smith Min. Mag. 63 P. Gay and R.W. Le Maitre Amer. Min. 1961 46 92. 64 I. S . Stemple and G. W. Brindley J. Amer. Ceram. SOC. 1960,43 34. 66 E. Thilo and G. Rogge Ber. 1939 72 341. 66 A. G. Freeman and H. F. W. Taylor Silikattech. 1960 11 390; and unpublished 67 L. Atlas J. Geol. 1952 60 125; W. L. Brown N. Morimoto and J. V. Smith 68 C. S . Hurlbut Amer. Min. 1961 46 549. 1959,32,324; ibid. 1961,32,823. work. J. Geol. 1961 69 609. DENT GLASSER GLASSER AND TAYLOR TOPOTACTIC REACTIONS 355 ment of cations within a relatively unchanged oxygen framework and that it is probably catalysed by protons. In natural iddingsite the cations are not rearranged perfectly enough to give an intergrowth of distinct phases even in regions of submicroscopic extent; instead they are arranged in a disordered irregular way so that in small domains the structure ap- proximates to that of one or other crystalline product.53 The mechanism of this process is basically similar to the inhomogeneous one proposed for the dehydrations of hydroxides and clay minerals.A similar explanation has been advanced for the tremolite dehydrati~n,~~ and many of the other processes described in the previous paragraph probably occur in the same way. Silicates of Larger Cations Calcium Manganese and Alkali Silicates.-The structures of these are usually analogous to those of the small cations only to the extent that many are based on fragments of octahedral cation-oxygen layers condensed with the silicate anion. The types of anion derived from Zweierketten found in pyroxenes amphiboles and clay minerals will not fit the larger octahedra of the calcium and manganese compounds; different anions are found based on chains repeating at intervals of three tetrahedra (Dreier- ketten) or five (Funferketten) (Fig.6b and c).50959 Xonotlite [Ca6(Si6O&OH),] contains double Dreierketten analogous to the double Zweierketten of amphiboles; it is dehydrated topotaxially at about 800" in air to wollastonite (/3-CaSi03) which contains single Dreierketten.60 It was concluded that the fragments of octahedral layers were preserved and that migration of silicon and loss of some of the oxygen not attached to calcium resulted in the reconstruction of the silicate anions. Thus the Si-0 skeleton generally assumed to be the most important part of a silicate structure was apparently less stable in a high-temperature reaction than the Ca-0 part.Rhodonite [(Mn,Ca)SiO,] is transformed into a wollastonite solid solu- tion of the same composition on heating. Rhodonite contains Funferketten which do not run parallel to the rows of octahedra; the crystals are prisms elongated parallel to the chains (Fig. 6b). In wollastonite the Dreierketten are parallel to the rows of octahedra (Fig. 6c). X-Ray study of pseudo- morphs formed by heating rhodonite at 1100" showed that the chains were no longer parallel to the prism axis but that the orientation of the octahedra had been preser~ed.~ This proves conclusively that the trans- formation proceeds by the destruction and rebuilding of the chains through migration of silicon. This probably entails a series of movements of silicon from initially filled tetrahedra to initially empty ones with which they share a face ("filled-empty tetrahedron migration").5D N. V. Belov Zhur. Strukt. Khim. 1960 1 39. L. Heller Proc. 3rd. Int. Symp. Chem. Cement London 1952 237; L. S. Dent and H. F. W. Taylor Acta Cryst. 1956,9 1002. 356 QUARTERLY REVIEWS Foshagite [Ca4(Si03),(OH),] is dehydrated topotaxially at 750" to wollastonite and /3-Ca2Si0,.10 The structure of foshagite was unknown when this study was made but on the assumption that the mechanism of its dehydration was similar to that of xonotite positions could be postulated for the calcium and most of the oxygen atoms and a probable arrangement suggested for the (OH)- groups and silicate anions (which were suspected to be single Dreierketten). This provisional structure was confirmed by Fourier methodsG1 Other hydrated calcium silicates which undergo topotactic changes at 650-800" include okenite nekoite tobermorite gyrolite reyerite zeophyllite and possibly others.These reactions have not been fully interpreted but it seems likely that they occur by mechanisms essentially similar to those of the xonotlite and foshagite dehydrations.62 Many of the above dehydrations will proceed at a lower temperature (about 400 ") under hydrothermal conditions. "Filled-empty tetrahedron migration" is thus catalysed by water at high pressures (or perhaps more accurately by protons) and this hastens the attainment of equilibrium. As already stated workers on iddingsite reached a similar conclusion. Possibly Si-OH-Si groups exist temporarily and this facilitates the breaking of Si-0 bonds. Sometimes as with tobermorite hydrothermal dehydration does not give the same product as heating in air.63 Si-OH groups decompose in air below 500".The dehydrations of afwillite [Ca3(HSi04)2,2H20]64 and of bultfonteinitess are topotactic. The hypothesis of silicon migration within a relatively stable Ca-0 framework does not apply to these reac- tions. With afwillite the process seems to involve loss of molecular water and migration of calcium. With bultfonteinite both of these processes and also silicon migration seem to occur. a-Na,Si,O has a layer structure; the Si,O sheets are composed of fused rings of six tetrahedra as in the clay minerals but the rings are kinked in a way that permits condensation with the large Na-0 poly- hedra.66 Treatment with fused silver nitrate at 280" causes cation exchange; Ag,Si,05 is formed topotaxially the Si205 sheets being ~naltered.~' This provides a further example of the movement of large metal cations rather than of silicon and aluminium at low temperatures.Similar processes of cation-exchange occur readily in clay minerals3s and zeolites,ss but will not be discussed here as they probably do not fall within the present definition of topotaxy. Hydrated calcium silicates containing J. A. Gard and H. F. W. Taylor Acta Cryst. 1960 13 785. 82 H. F. W. Taylor J Appl. Chem. 1960 10 317. 63 H. F. W. Taylor Min. Mag. 1959 32 110. 64 H. F. W. Taylor Acta Cryst. 1955 8 440. 8s E. J. McIver Mineralogical Society Notice of Meeting 117 1962. 88 F. Liebau Acta Cryst. 1961 14 395. 13' F. Liebau Acta Cryst. 1961 14 537. 88 R. M. Barer Proc.Chem. Soc. 1958 99. DENT GLASSER GLASSER AND TAYLOR TOPOTACTIC REACTIONS 357 FeZdspars.-These have three-dimensional aluminosilicate frameworks of empirical formula (Si,A1)02 with relatively large cations such as sodium potassium or calcium. Naturally occurring feldspar crystals are often pseudomorphous after a former high-temperature solid-solution phase which has undergone a complicated series of changes on cooling. Sometimes the details of these processes are obscure. One type of change which results in exsolution in of the homogeneous high-temperature phase is due to ordering of the large cations e.g. Na-K.69 This process involv- ing Na-K migration is topotactic. The reverse process namely homo- genisation of oriented intergrowths of KAlSi308 and NaAlSi,O, occurs at 350-750".70 Treatment of a feldspar Na,.,CaO. 6(Si,Al),08 with aqueous potassium chloride at 500" and 600" bars resulted in replacement of sodium by potassium and exsolution of the homogeneous phase to give KAlSi,O and CaAl,Si,O,. Experiments with single crystals showed that this did not result from dissolution and recrystallisation but from a topotactic change involving migration of sodium potassium aluminium and silicon. Treatment with fused potassium chloride in contrast caused replacement of sodium by potassium but no change in the degree of Al-Si ordering. It was concluded that the large cations could migrate under anhydrous conditions but that water was essential for migration of silicon and aluminium.71 The action of water was explained by a mechanism involving diffusion into the structure of both H+ and OH- ions; experiments with oxygen- 18 showed that considerable exchange of oxygen between crystal and solution had Catalysis of silicon migration by water was also postulated in the reactions of olivine and of calcium silicates pre- viously discussed but a different mechanism involving only protons was assumed.From the study of rhodonite, also previously discussed it was concluded that migration of silicon can probably also occur under an- hydrous conditions. Acid Leaching.-Apophyllite [KCa4(Si802,)F] and gillespite [BaFe (Si,O,,)] have related structures containing rings of four and eight tetrahedra arranged in layers of Si205 composition. Leaching with dilute acid leaves the Si205 sheets unchanged and still crystalline; the residue from gillespite has the composition H4Si40,,.73 Most silicates which are attacked by acids yield either gelatinised or amorphous silica residues depending mainly on the degree of condensation of the silicate anion,' and the behaviour of apophyllite and gillespite is thus unusual.69 0. F. Tuttle and N. L. Bowen "Origin of Granite in the Light of Experimental Studies in the System NaAlSi,O,-KAlSi,O,-SiO,-H,O," Geological Society of America Memoir 74 New York 1958. 70 E. Spencer Min. Mag. 1937 24 453. 71 J. Wyart and G. Sabatier Bull SOC. franc. Min. 1958 81 223. 72 G. Donnay J. Wyart and G. Sabatier 2. Krist. 1959 112 161. 73 E. H. Bailey Amer. Min. 1941 26 565; A. Pabst ibid. 1958 43 970. '* K. J. Murata U.S. Geol. Survey Bull. No. 950 1946 25. 358 QUARTERLY REVIEWS Other Oxy-salts There have been very few systematic studies of topotactic reactions in other oxy-salts even though judging from the examples described below such reactions must be common.Gypsum is dehydrated topotaxially in three steps CaS0,,2H20 e CaSO,,iH,O + y-CaSO -+ CaSO The first two reactions are reversible. Orientation is well preserved in the second and third steps but only partly preserved in the first. The reaction has been interpreted in terms of the preservation of the chains of Ca2+ and S042- ions which occur in all four structures. The structures of hemi- hydrate y-calcium sulphate and anhydrite can alternatively be described in terms of analogous sheets of Ca2+ and SO,2- ions. One structure can change into another by small relative displacements of these sheets together with loss of The occurrence of y-calcium sulphate metastable under all conditions is an example of structural control over the phase formed.Potassium chlorate crystals grown on an electron-microscope grid were decomposed in the electron beam.76 Potassium chloride was formed as a well-oriented pseudomorph with its (100) planes parallel to (001) of the potassium chlorate but no interpretation was given. The chlorate ion is only slightly larger than the chloride ion and potassium chlorate can be regarded as having a distorted potassium chloride- or sodium chloride- like structure. A homogeneous mechanism involving loss of oxygen and repacking of the large residual K+ and Cl- ions might be possible. Several calcite-type carbonates decompose topotaxially when heated. The calcite structure is a rhombohedra1 variant of the sodium chloride structure with C1- replaced by coplanar COS2- groups and Na+ by the appropriate cation.Ferrous carbonate decomposes topotaxially to FeO and Fe,0,.12 The orientation of the FeO suggests that carbon dioxideis expelled from each C032- group (cf. potassium chlorate). The decomposi- tion of calcium carbonate appears to be unoriented although the existence of an intermediate metastable phase designated CaO* with structure and orientation derived from the calcium carbonate has been postulated from kinetic evidence.77 Dolomite CaMg(CO,), decomposes in two stages the first giving oriented calcite and unoriented magnesium oxide; in the second unoriented calcium oxide is formed from the calcite.78 Little is known about the decomposition mechanisms of carbonates having the aragonite structure.Strontium carbonate-barium carbonate solid solu- gypsum hemihydrate anhydrite 75 0. W. Florke Neues Jahrb. Mineral. 1952 84 189. 76 M. Ross and C. L. Christ Amer. Min. 1958 43 1157. 77 E. P. Hyatt I. B. Cutler and M. E. Wadsworth J. Amer. Ceram. SOC. 1958 41 78 R. A. W. Haul and H. Wilsdorf Acta Cryst. 1952 5 250. 70. DENT GLASSER GLASSER AND TAYLOR TOPOTACTIC REACTIONS 359 tions on being heated are said to exsolve into the pure phases before decompo~ing;~~ this is unusual behaviour since solid solubility normally increases with rising temperature. Zinc carbonate also decomposes topotaxially to zinc oxide; the orientation relationships have not been given explicitly.80 No single mechanism can be suggested to explain both the topotactic decomposition of ferrous carbonate and CaMg(CO,) and also the apparently non-topotactic decomposition of calcium carbonate.Tempera- ture may be an important factor;12 calcium carbonate has the highest decomposition temperature of any member of the group. The topotactic decompositions may proceed not by the homogeneous mechanism suggested for ferrous carbonate but by an inhomogeneous one similar to that suggested for hydroxides.,* The calcite structure can be regarded as approximately close packed oxygen atoms with cations in octahedral and carbon in triangular interstices ; decomposition might then occur by migration of cations and carbon. It is tempting to correlate the non- topotactic decomposition of calcium carbonate with the relative difficulty with which calcium diffuses (as with the hydroxides) but calcium must diffuse during the first stage of the dolomite decomposition; more studies are needed before generalisations can be made.Summary Although the study of topotactic reactions in inorganic oxy-compounds is still in its infancy a few tentative generalisations may be suggested. (1) Topotactic processes are probably quite commonplace. For a trans- formation to proceed topotaxially there must be a three-dimensional similarity between the structures of starting material and product. This similarity need not be very great and even the compositions may differ radically. The mere similarity between structures is however not in itself sufficient to guarantee that a transformation will occur topotactically . Experimental evidence is always needed. Some reactions are topotactic under all conditions some only under certain conditions and others not at all.Factors which may influence the degree of orientation in any particular reaction include temperature pressure physical state (e.g. grain size) and specific experimental conditions such as the time and rate of heating used to induce thermal changes. (2) The sizes and electronegativities of the cations partly decide which ions move and which do not. With small cations (e.g. Mg2+ AP+ Few Si4+) the oxygen framework tends to stay relatively unchanged while cations migrate. The number of oxygen atoms in a given volume tends to remain constant in the regions where topotactic change actually occurs. However the type of oxygen packing may change; this happens readily with R. Faivre and G. Chaudron Compt.rend. 1948,226,249. A. Rose Compt. rend. 1939 208 1914. 360 QUARTERLY REVIEWS Mg2+ but less readily with Alw or Fe3+. At the other extreme large electro- negative cations (e.g. UQ- Pb4+) probably remain fixed while oxygen atoms move. Large electropositive cations (e.g. Ca2+) represent an inter- mediate case. In low-temperature reactions of hydrated calcium silicates calcium migration is important but reactions occurring above about 500" take place mainly by migration of silicon the Ca-0 framework staying nearly unaltered. (3) Thus in high-temperature reactions Si04 tetrahedra rarely behave as units; Si-0 bonds are often less stable than the other cation-oxygen bonds. The concept of the silicate anion as the most important element of the structure is not a good starting point for an understanding of these reactions.The packing of oxygen atoms and of any large cations present is more important than that of silicon. (4) Caution should be used in applying the above generalisations to redox processes involving the metal (e.g. FeO + Fe) because it may be incorrect to use ionic radii. ( 5 ) The effects of catalysts have been little studied. Water seems to catalyse silicon migration. This work was sponsored by the U.S. Office of Aerospace Research through its European Office whose financial assistance is gratefully acknowledged. The photograph for Fig. 1 was kindly supplied by Dr. A. W. Nicol.

ISSN:0009-2681    

DOI:10.1039/QR9621600343

出版商:RSC    

年代:1962

数据来源: RSC

摘要:

STEREOREGULAR ADDITION POLYMERISATION By C. E. H. BAWN and A. LEDWITH (DEPARTMENT OF INORGANIC AND PHYSICAL CHEMISTRY UNIVERSITY OF LIVERPOOL) Stereoisomerism in high-polymer molecules . . Optical activity in stereoregular polymers . . Nature of the stereoregulating forces in controlled propagation reactions. . . . . . . . Free-radical propagation . . .. .. Ionic polymerisations general . . .. cationic polymerisation . . . . .. anionic polymerisation . . .. .. diene polymerisation . . . . . . Ziegler-type polymerisation . . .. .. aldehydes . . . . . . . . . . Molecular structure and properties . . . . Melting points . . . . * . .. . . Characterisation of stereoregular polymers . . Newer developments . . . . . . . . Stereoregular polymerisation of olefin oxides and Molecular configuration of stereospecific poly- mers .. . . . . . . .. .. Page 362 366 367 367 374 375 380 386 391 408 414 415 420 423 433 THE most spectacular developments in polymer chemistry during the past five years have been concerned with the synthesis of previously known polymer molecules in which the backbone chain of carbon atoms shows considerable structural and stereochemical regularity. These developments which have resulted from a concentrated research programme in industrial and university laboratories were stimulated by Professor K. Ziegler’s brilliant discovery1 of a catalyst for the polymerisation of ethylene at room temperatures and normal pressures. In Italy Professor G. Natta and his collaborators2 developed the use and scope of so-called “Ziegler catalysts” and showed that they could be modified to produce three kinds of polymeric cc-olefins (alk- 1 -enes) which had different steric arrangements of the carbon backbone chain.Before discussing the nature and scope3 of the various stereospecific catalyst systems it is necessary to consider in detail the various types of stereo-order in a polymeric molecule. Ziegler Holzkamp Breil and Martin Angew. Chem. 1955,67 541. Natta Angew. Chem. 1956 68 393; Natta Chem. and Ind. 1957 1520; Natta J. Polymer Sci. 1955 16 143. Gaylord and Mark “Linear and Stereoregular Addition Polymers,” Interscience Publ. Inc. New York 1959; Bawn J . Inst. Petroleum 1960 46 374; Furukawa and Tsuruta J . PoZymer Sci. 1959 36 275; Ledwith Ind. Chemist 1961 37 71; Cooper “Progress in High Polymers,” Heywood and Co. London 1961 p. 279; Korshak Russ.Chem. Rev. 1960 269. 361 362 QUARTERLY REVIEWS Stereoisomerism in High-polymer Molecules Mono-olefin Polymers.-The addition polymerisation of a mono- (CHz=CHR) or 1,l-di-substituted ethylene (CH2=CRR’) can be re- presented as follows n(C H,= C R R ) -+ -C H,C R R’. [ CH,C R R’] -,.CH,.C RR’- where R’ may be H; and if R + R’ every alternate carbon atom in the polymer chain is an asymmetric centre since in theory the two polymeric substituent groups are of different chain length. Consequently these asymmetric carbon atoms can exhibit two different configurations con- veniently referred to as d- and I-configurations.* Diagrammatic representa- tion of these different configurations in a polymer molecule is sometimes difficult. The simplest representation is that which has the plane projection of the planar zig-zag carbon backbone as a straight sequence of alternate CH and CHR groups as shown in Fig.1A. If the first substituent R is below the chain (e.g. d-configuration) then we have the possibilities (a) that the arrangement of d- and I-configurations is random (b) that the substituents are all d- or all I- and (c) that the substituents alternate regularly above and below the planar zig-zag back-bone i.e. are alter- nately d- and I-. These structures were called by Natta et aL4 (a) atactic (b) isotactic and (c) syndiotactic. An alternative representation which the present authors have found very convenient when considering the mech- anism of stereospecific synthesis is the Newman projection illustrated in Fig. 1B.t t i l t j H ~ l i i ~ t i t H Y l $ (I) H A H H H H H H H - y-c-t-c-c-c-c- I I I I I i-73- FIG.1A. (1) Atactic. (2) Isotactic. (3) Syndiotactic. More recently Natta5 and Breslow6 and their collaborators have suc- ceeded in polymerising 1,2-disubstituted ethylenes of the type RHC= CHR’. This kind of monomer gives rise to polymer molecules with two * Throughout this article the word configuration denotes a particular order of substituent groups around an asymmetric carbon atom as viewed from along the back- bone chain. t In the Newman perspective the bond joining adjacent carbon atoms in the main chain is not shown. Natta Farina and Peraldo J. Polymer Sci. 1960 43 289. Natta Makromol. Chem. 7960 45 93. Vandenberg Heck and Breslow J. Polymer Sci. 1959 41 519. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 363 -RHC -RHC -RHC iH ct FIG.1B. (1) Atactic. (2) Tsotactic. (3) Syndiotactic. different asymmetric carbon atoms in the backbone chain and con- sequently the original definitions of the various tactic fonns have been extended to include these modifications. When the two different asymmetric carbon atoms in the polymer chain occur with the same configuration the polymer is called erythro-di-isotactic (Fig. 2). If the two asymmetric carbon atoms have alternating configurations the polymer is said to be threo-di- isotactic (Fig. 2). The erythro-polymer will be obtained for example by FIG. 2. (a) erythro-Di-isotactic. (b) threo-Di-isotactic. (c) Disyndiotactic. stereospecific polymerisation of a trans- 1,2 disubstituted ethylene and the threo-derivatives will arise from a similar polymerisation of the cor- responding cis-olefin.The nomenclature corresponds to that conventionally used7 to describe the products of stereospecific addition to 1 ,Zdisubstituted ethylenes. It will be observed that only one form of disyndiotactic polymer is possible and this occurs when adjacent pairs of asymmetric carbon atoms have alternating configuration. Whilst the above nomenclature is unambiguous for polymers obtained by polymerisation of ethylene derivatives confusion may arise when naming stereoregular polymers obtained by polymerisa- Newman “Steric Effects in Organic Chemistry,” Wiley New York 1956 p. 10. 3 364 QUARTERLY REVIEWS tion of diazoalkanes.* Thus polyethylidene [-CH(CH,)-] 12 is formally the same as poly- 1,2-dimethylethylene and polyben~ylidene~ could be regarded as poly-l,2-diphenylethylene.If we consider the structure of the crystalline form of polyethylidene which appears to be syndiotactic i.e. to contain adjacent asymmetric carbon atoms having alternating configurations then this is best represented by Fig. 2b (R = R2’ = CH,) and should therefore be named threo-di-isotactic poly- 1 ,Zdimethylethylene. Conjugated Dioleh Polymers.-The monomers principally encountered in this class are buta-l,3-diene 2-methylbuta-l,3-diene (isoprene) and 2-chlorobuta- 1,3-diene (chloroprene) viz. CH3 CI 1 2 3 4 1 1 3 4 1 1 3 4 CH,=CH-CH=CH, CH2=2C-CH=CH, CH2=T-CH=CH2 Butadiene Isoprene C hloroprene Butadiene can give rise to five types of polymer three of which are the isotactic syndiotactic and atactic modifications of poly- 1 ,Zbutadiene (Fig.1; R = -CH=CH2. The other forms of polybutadiene arise from 174-addition to the monomer molecule and this produces an unsaturated carbon backbone in which the unsaturated linkages can have either a cis- or a trans-conformation viz. -H,C C H2- c=c \ / H ’ ‘H 1 &cis- -H2C H \ / c=c CHZ- / \ H 1,4trans- With isoprene and chloroprene the situation is further complicated by the additional possibility of both 1,2- and 3,4- modes of polymerisation. Theoretically there are three forms each of poly-1 ,Zisoprene and poly-3,4- isoprene corresponding to isotactic syndiotactic and atactic placements [Fig. 1; R = CH, with -CH=CH2 in place of H; and R = -C(CH,)= CHJ and a similar number for chloroprene. More important however are the polymers obtained by 1,4-addition viz. \ /CHz- -H2C c=c H,C / ‘H 1,4-cis- -H,C H \ / c=c CH2- / \ H3C 1 ,rl-trans- Polyisoprene occurs in both forms in Nature the cis-derivative being Hevea rubber and the trans-derivative gutta-percha (balata).Very recently Professor Natta and his collaboratorslO have succeeded in * Nasini Trossarelli and Saini Mukromol. Chem. 1961 44 550. lo Natta Farina Peraldo and Bressan Mukrornol. Chem. 1961 43 68. Bawn Ledwith and Matthies J. Polymer Sci. 1958 28 21. BAWN AND LEDWITII STEREOREGULAR ADDITION POLYMERISATION 365 obtaining crystalline polymers from methyl ethyl and butyl trans-trans- sorbate and methyl 5-phenylpenta-2,4-dienoate. All these monomers have the general structure RCH= CH-CH= CHCO 2R’ and on polymerisa- tion by 1,4-addition yield polymers [-*CHRCH= CH.*CH(CO,R’)-1 in which each repeating unit has three centres for variation; there are two asymmetric carbon atoms (marked*) which could exist as d- or I-forms and the ethylenic bond which can be either cis- or trans-substituted.The original workers have suggested the name “tritactic” for this type of polymer although to date only one modification has been isolated. This is thought to have the structure (I) and Natta has named it erythro-di-iso- trans-tactic. This type of reaction is therefore a composite of mono-olefin and diene polymerisation and polymers can be obtained showing optical activity.1° Polyethers.-The principal monomers in this class are of the type R-CH-CH2 and more recently acetaldehyde CH,CHO. In the former case polymerisation occurs by 1,3- addition (p. 408) to the cyclic system giving a polymer with a repeating unit [-CH,CHR.O-] while the latter monomer undergoes polymerisation by 1 ,Zaddition (p.408) to the car- bony1 group leading to a repeating unit [-CH(CHJ.0-]. In each case it is theoretically possible to have three polymer modifications corresponding with the isotactic syndiotactic and atactic forms of poly-ct-olefins and these would be named in a similar manner. \O/ Copolymers.-Two or more different monomer units may be copoly- Typical structure are represented as follows merised to produce either a random copolymer or a block copolymer. -AABABBABAA- random copolymer -ABABABAB- 1 1 copolymer -AAAAAABBBBBAAAA- block copolymer Since both monomer units A and B are capable of yielding asymmetric carbon atoms in the backbone chain the nomenclature for this kind of system can have no obvious general rules but must be adapted to fit each particular situation.In addition it is theoretically possible to polymerise one monomer in 366 QUARTERLY REVIEWS such a manner that the resulting polymer is a block copolymer of d- and 1-configurations which may be represented This type of block copolymer is likely to occur in most stereospecific polymerisations and the relative lengths of the various blocks will have a pronounced effect on the physical properties of the resulting polymer (see below). A syndiotactic polymer could of course be regarded as a perfect 1 :1 copolymer of d- and I-configurations from the same monomer unit and this point has been elaborated by Schuerch.ll Optical Activity in Stereoregular Polymers The amount of optical activity a polymer exhibits will depend not only on the specificity of polymerisation but also on the final structure of the polymer.Optical activity is determined mainly by groups which are in the immediate vicinity of an asymmetric carbon atom and as the point of structural dissimilarity is moved further from the asymmetric centre the optical activity decreases rapidly to a negligible value. In a homopolymer the differences in structure between the two chains of the polymer attached to an asymmetric centre are limited to end-groups the lengths of the chains and the configurations of the substituted carbon atoms on adjacent monomer units. These would be expected to cause negligible optical activity since the environment immediately round the asymmetric centre is so similar. Further if the polymer chains were comprised wholly of all d- or all 2-centres then each asymmetric carbon atom would have a mirror image on the opposite side of the central carbon atom of the main chain.It is however unlikely that the chain will be built up of d- or I-centres ex- clusively and will normally contain chain sequences of d- and I-centres of varying length. If on the other hand the near environment of the asymmetric carbon atom is grossly dissimilar then measurable optical activity would result. Thus polyalkylene oxides can show considerable optical activity when prepared in the isotactic form (11) from optically active monomer since every asymmetric carbon atom is surrounded by four groups which do not H H I I I R (11) I depend on chain length for their dissymmetry. The tritactic polymers recently prepared by Natta and his collaborators are also capable of showing optical activity since the repeating unit in the polymer backbone -*CHRCH= CH.*CH(C02R')- contains two asymmetric carbon atoms.Finally optical activity may result if two different monomers are l1 Schuerch J. Polymer Sci. 1959 40 533. -CHz-C-O-CHz-C-O- R BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 367 copolymerised in a stereoregular manner. This can best be described by considering the random copolymer formed from maleic anhydride and a methacrylate monomer e.g. The carbon atoms metric and indeed 1 2 and 4 in the copolymer backbone are all asym- there is evidence that the copolymer obtained from maleic anhydride and ( -)Z-cc-methylbenzyl methacrylate shows stereo- regularity in the backbone chain.12 Stereoregularity of the backbone chain produced in this manner is of course an example of asymmetric induction and this phenomenon is likely to find increasing use in the synthesis of stereoregular macromolecules.Nature of the Stereoregulating Forces in Controlled Propagation Reactions Until about 1953 the mechanisms for addition polymerisation could be classified in three main groups conventionally referred to as cationic anionic and free-radical polyrneri~ation.~~ Since then we have what is commonly referred to as a Ziegler-type polymerisation although perhaps this would better be termed co-ordination polymerisation this type of polymerisation allows most control of the propagation reaction and cannot readily be defined as anionic or cationic in the conventional sense.Stereoregular Polymers from Free-radical Propagation.-Of all the modes of propagation the so-called free-radical polymerisation is best understood and it is free from such complicating factors as solvation which must necessarily occur in ionic polymerisation. For this reason a propagating free radical is the nearest approach to a free propagating species and recently several authors have used such a model to calculate specific non- bonded interactions likely to occur in the growth of a free propagating species. As long ago as 1944 Huggins14 predicted on theoretical grounds that a decrease in reaction temperature should increase the stereospecificity of a free-radical propagation. Recently it has been shown15 that simple radical addition of hydrogen bromide and of deuterium bromide to the isomeric but-2-enes proceeds with complete stereospecificity at -80”.Consequently it is not surprising that free-radical polymerisations carried out at such low temperatures readily give polymers with a much higher degree of crystallinity and stereoregularity than pertain to the polymers l2 Beredjick and Schuerch J. Amer. Chem. SOC. 1956,78,2646; Arcus J. 1957,1189; Schmitt and Schuerch J. Polymer Sci. 1960,45 313; Arcus and West J. 1959 2699. l3 Flory “Principles of Polymer Chemistry,” Cornell Univ. Press. 1953. l4 Huggins J. Arner. Chem. SOC. 1944 66 1991. l5 Skell and Allen J . Amer. Chem. SOC. 1959 81 5383; Goering and Larsen ibid. p. 5937. 368 QUARTERLY REVIEWS obtained by the same reaction at room temperature or above.I6 In addition it is now becoming well established that low-temperature propagation by a free radical or a solvent-separated ion-pair yields predominantly syndio- tactic placernent~.~~-~~ These indications have received theoretical support from independent calculations by Coleman20 and Hughes.21 Fig.3 represents a typical step in a free-radical propagation. The active radical-end of the propagating species will be essentially planar. The monomer can attack either from the front* or from the back the former leads to form (111) the latter to form (IV). It will be seen that in this act of addition the configuration of the former free-radical end becomes fixed. Thus in (111) the configuration is opposite to that of the previous unit in the chain and we can describe the step as a “syndiotactic placement;” repetition of a sequence of such steps would give a syndiotactic polymer -t CH,=CHX h a c k in which the stereochemical configuration alternates regularly.Con- versely (IV) represent an “isotactic placement” and by repetition will give an isotactic polymer. Following Coleman20 we can define quantities a /3 as representing the respective probabilities of isotactic and syndiotactic placements a + p = 1. In these terms the problem of control of polymer structure reduces to that of determining the factors on which cc and fl depend. Ideal isotactic and syndiotactic polymers will be represented by cc = 1 and /3 = 1 respect- ively. Between these two limits lies a complete spectrum of possibilities * Since the radical end of the growing chain can rotate fre&Iy attack from the front is meant to imply attack from the same side as the substituent in the penultimate monomer unit as shown in the figure.I6 Fox Goode Gratch Huggett Kincaid Spell and Stroupe J. Amer. Chem. Soc. 1958 80 1768. Miller and Rauhut J. Amer. Chem. Soc. 1958,80,4115; J . Polymer Sci. 1959 38 63. Mill&- Mills Small Turner-Jones and Wood Chem. and Znd. 1958 1323. Is Garrett Goode Gratch Kincaid Leversque Spell Stroupe and Watanabe 2o Coleman J. Polymer Sci. 1958 31 155; see also Miller and Nielsen ibid. 1960 21 Hughes Abs. Papers 133rd Meeting Amer. Chem. SOC. San Francisco April J. Amer. Chem. Soc. 1959 81 1007. 46 303. 1958 p. 10R. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 369 most of which have hitherto been embraced within the general title “atactic.” It is evident that the two modes of addition may well differ in probability for steric reasons if for no other.The completely random growth re- presented by a = p = Q is therefore unlikely to hold exactly by accidental cancellation of opposing factors. Study of models leads to the conclusion that steric factors will in general favour syndiotactic placements especi- ally ifX is bulky. If no other factors are important we might thus expect to find /3 > & as a general rule. If /3 approaches 1 we shall obtain long syndiotactic sequences and the product will be stereoregular. In the above discussion it has been assumed that the probability of a syndiotactic placement was determined only by the propagation step in- volved. A more complex situation arises when the mode of addition is influenced by two or more earlier units.In such a case once a sequence of identical units has been established it will be self-perpetuating i.e. /3 or a will be nearly 1 but will fall to a considerably lower value as soon as an addition happens to occur in the “wrong” sense. Highly regular blocks will then be separated by regions of a lower order. If there is a finite chance of establishing sequences of either configuration a stereo-block polymer will result with less ordered regions between the blocks. It seems reason- able therefore to assume that the stereochemical structure of a free-radical polymer should be describable by means of a single parameter /I. If this is controlled entirely by steric factors it will be independent of solvent concentration and conversion but in general will be temperature-dependent.The higher the temperature the less significant will be the small differences between the alternative reactions so that a and /3 will approach 8. At lower polymerisation temperatures one or other process will be favoured and if steric factors are assumed to favour syndiotactic placement p will increase as the polymerisation temperature is lowered and may ultimately become large enough to permit the forma- tion of a crystallisable polymer. If the difference in enthalpy of activation for the two alternative reac- tions is AH the temperature-dependence of /3 should be of the form In [p/(1-/3)] = A + dHS/RT. Since the probability of n successive placements for a given value of /3 is /In we can calculate the sequence length n whose chance of formation is 50% i.e. = 0.5.Table 1 records n as a function of a linear scale of ln[/3/(1-/3)] together with relative values of Tcalculated on the quite arbitrary basis of ANSIR = 1000°(~). TABLE 1. Efect of temperature (T) on stereoregularity (illustrative).20 II T ( K ) 2.2 333 O In [B/(1-p)l B 1.0 0-730 1.5 0.818 3.5 285 2.0 0.881 5.5 250 2.5 0.924 8.8 222 3.0 0.950 14-2 200 370 QUARTERLY REVIEWS While these figures are purely illustrative they bring out clearly how sharply the probability of forming long regular sequences can be increased by lowering the temperatures. The actual coefficient in a particular case will of course be determined by the magnitude of AH:. Fordham22 has extended these general considerations to more specific cases and also has made a valuable attempt to correlate in a qualitative manner stereoisomerism in vinyl polymers and the phenomenon of rota- tional isomerism in simple compounds since both are concerned with the relative positions and interactions of substituent groups.In rotational isomerism the terms that are analogous to isotactic and syndiotactic configurations are gauche- and trans-conformations. Fordham pointed out that in a representative free-radical polymerisation the activation energies of the syndiotactic and the isotactic propagation are very pro- bably related to the relative potential energies of the respective configura- tions since the activation energy of radical addition is lowered with increased heat of the exothermic reaction. The specific case of polyvinyl chloride was then treated in detail and approximate calculations made for the steric and the electrostatic components of the relative potential energy levels for the two possible configurations.In this manner it was shown that in polyvinyl chloride only three structures were likely these are shown in Fig. 4. There are two syndiotactic structures (a) and (b) and *CI :'"\ "'.. c.." .C 6' c cc c' 'c' ' c ci' 'c' 'a c' \c' \a c c.'" FIG. 4. Estimated range for energy contents of forms ofpolyvinyl chloride .from a steric consideration. (a) trans-trans 0 kcal./mole. (b) gauche-gauche 0 - 1 .O kcal./mole. (c) trans-gauche 0-0.5 kcal./mole. one isotactic structure (c). Structure (a) has a lower energy and remains the most probable arrangement for the syndiotactic configuration while (c) represents the most probable isotactic structure. Obviously then the preferred conformational arrangement for the syndiotactic configuration of vinyl polymers is trans-trans with respect to chain carbon atoms yielding the planar zig-zag backbone observed for the syndiotactic vinyl polymers 1 ,Zpolybuta-1 ,3-diene23 and polyvinyl chloride.24 Similar calculations have been made by Ferstandig and G o o d r i ~ h ~ ~ for the special case of propene polymerisation They conclude that syndiotactic polymer should result from a radical or carbonium-ion polymerisation 22 Fordham J.Polymer Sci. 1959 39 321. 23 Natta Porri Zanini and Fiore Chimica e Industria 1959 41 526. 24 Fordham Burleigh and Sturm J. Polymer Sci. 1959,41,73; George Grisenthwaite and Hunter Chem. and Ind. 1958 1114. Ferstandig and Goodrich J. Polymer Sci. 1960,43 373. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 37 1 and isotactic polymer should result from anionic propagation.For propene however it is not possible to obtain high polymer by other than Ziegler-type catalysts and as mentioned previously this type of polymer- isation does not involve a free propagating species in the conventional sense and hence it is not possible to test the predictions of Ferstandig and Goodrich." BoveyZ7 has made a detailed study of the number and length of syndio- tactic sequences in poly(methy1 methacrylate) using nuclear magnetic resonance techniques. By measurements of the peak areas in the a-methyl region of the spectra of 15 % chloroform solutions of methyl methacrylate polymers prepared by free-radical initiators over a 178" range of poly- merisation temperatures it was shown that syndiotactic propagation becomes increasingly dominant as the polymerisation temperature is decreased and that propagation tends towards randomness as the poly- merisation temperature is increased.Bovey found that there is no difference in the activation entropies of syndiotactic and isotactic monomer place- ment but that isotactic placement requires an additional activation en- thalpy of 775 5 7 5 cal./mole. In a later paper Fordham and his collaborator^^^ confirmed the finding that syndiotactic propagation in free-radical polymerisation is enhanced over isotactic propagation by decreasing polymerisation temperature. However the temperature effect is small with the activation-energy difference estimated to be of the order of 0.5 kcal./mole agreeing with the theoretical estimate.And finally Fordham McCain and Alexander,28 from a study of the free-radical polymerisation of a series of chlorinated and fluorinated vinyl acetates suggest that the electrostatic factor is more important than the steric factor in the stereo-regulated polymerisation of vinyl esters. In each case however only structures of the syndiotactic type were obtained and the authors suggest that syndiotactic propagation may be preferred for all free-radical polymerisations. The last conclusion is borne out by the recent synthesis of syndiotactic polymers of methyl methacrylate isopropyl acrylate and cyclohexyl a~ry1ate.l~ The conclusion that low-temperature free-radical polymerisation would lead to a predominance of syndiotactic placements was arrived at in- dependently by Cram29*30 from a consideration of related steric effects in the reactions of small molecules.Thus in the polymerisation of a-methyl methacrylate for example Cram assumes that the effective bulk of the substituent groups near the reactive centre falls in the order P > C02Me * Addition of triphenylphosphine to a typical Ziegler catalyst26 yields a polypropene which has a crystal structure quite different from that of isotactic polypropene and so is presumably syndiotactic. However it is not yet possible to define the nature of the propagating species. 26 Eastman Kodak Co. Belgian P. 577,214/1959. 27 Bovey J. PoZymer Sci. 1960 46 59. 28 Fordham McCain and Alexander J. Polymer Sci. 1959,39,335. 2 9 Cram and Kopecky J. Amer. Chem. SOC. 1959 81 2748. 30 Cram J. Chem. Educ. 1960 37 317.372 QUARTERLY REVIEWS > Me where P represents the polymer chain. Accordingly it is proposed that least internal steric compression within the polymer chain will obtain when the growing radical end and the penultimate monomer unit adopt the conformation shown in Fig. 5. In this manner the syndiotactic structure will result and will be independent of the direction of approach of the monomer. This of course provides a qualitative picture of the conclusion H,C= CMe Me FIG. 5. Cram's explanation of free-radical polymerisation. (A) Transition state. (B) Syndiotactic polymer. P = growing polymer chain. * denotes a negative charge or a lone electron. reached by Fordham on theoretical grounds. Recently a very specific type of stereoregulating influence by the solvent on free-radical propagation has been reported by B ~ r l e i g h ~ ~ who found that when vinyl chloride is polymerised at 50" with conventional free- radical initiators in an aliphatic aldehyde as solvent syndiotactic polyvinyl chloride results.Details of the mechanism for this type of polymerisation are as yet very scarce but it appears that butyraldehyde and 2-ethyl- hexanaldehyde give the best results. In the foregoing discussion the main stereoregulating force was assumed to arise as a direct consequence of lowering the temperature of polymerisation. There is however one other general stereoregulating influence to be considered and this is the effect of solvent on the growing polymer. The steric consequences of polymer- solvent interaction have been discussed by S ~ w a r c ~ ~ and by Ham.33 Both emphasise the well-known tendency by macromolecules to form a regular helix in the crystalline state or in solvents of very low solvating power.Thus it is suggested that if the polymerisation solvent is a very "poor" solvent. i.e. if the polymer-solvent interactions are extremely weak then a growing polymer molecule could take up a helical form such as might be obtained in the crystalline polymer. If a regular helix is formed during 31 Burleigh J. Amer. Chem. Soc. 1960 82 749; Rosen Burleigh and Gillespie J. Polymer Sci. 1961 54 31. 32 Szwarc Chem. and Ind, 1958 1589. 33 Ham J. Polymer Sci. 1959 40 569; 1960 46 475. Helical stereospec$c polymerisation. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 373 chain growth then a particular steric configuration of asymmetric carbon atoms could result because of the desirability of preserving the symmetry of the helix.The stereoregular placement of monomer in helical polymerisa- tion might be influenced not only by the lowered potential-energy barrier resulting from the symmetry of the adduct but also by the favourable environment of other regularly placed helices and potentially favourable interactions with groups comprising these helices. Now it is well established that use of poor solvents e.g. alkanes invariably gives a greater yield of crystalline polymers in almost any kind of polymerisation but this cannot as yet be ascribed to helix formation because very often use of a poor solvent for the growing polymer necessarily ensures that the system containing the catalyst or growing end of the chain becomes heterogeneous.Heterogeneous catalyst systems (see below) are undoubtedly the most effective means of stereoregulating a propagation step. To illustrate this point the work of Williams Laakso and Dulmage3* may be cited. These workers studied the polymerisation of styrene catalysed by triphenyl- methylpotassium. When the polymerisation was carried out in benzene which is a relatively good solvent for polystyrene amorphous atactic polymer resulted. If however the reaction was performed in an aliphatic hydrocarbon in which polystyrene is virtually insoluble then crystalline and probably isotactic polymer was obtained. The authors attribute the difference to the fact that triphenylmethylpotassium is insoluble in alkanes but soluble in benzene and not to the formation of a helix in the alkane solvent.Perhaps the stereospecific reaction is a result of both these influences. The best evidence for a helical structure in a dilute polymer solution and in a molten polymer was reported recently by Pino and Lorenzi.35 These workers studied the polymerisation of cc-olefins containing optically active substituents e.g. (S)-3-methylpent-l-ene (S)-4-methylhex-l-ene and (S)-5-methylhept- 1 -ene in conditions known to yield isotactic polymer. The crystalline and presumably isotactic polymer obtained from these monomers showed molar optical rotation referred to the monomeric unit 5-28 times greater than the molar optical activity of the paraffins of low molecular weight whose structures closely resemble that of the mono- meric units of the polymers under consideration.* In addition it was found that polymer fractions having increasing crystallinity showed increasing molar optical rotations and the value of the temperature-coefficient of their optical rotation in solution was much higher than the coefficient found for the paraffins of low molecular weight.The temperature-coefficient was of the same order of magnitude as those for some spiral proteins and * Bailey and Y a t e ~ ~ ~ have independently reported similar studies on the polymerisa- 34 Williams Laakso and Dulmage J. Org. Chem. 1958 25 638. 35 Pino and Lorenzi J. Amer. Chem. Suc. 1960 82 4745; see also Kulkarni and 36 Bailey and Yates J . Org. Chem. 1960,25 1800. tion of optically active 3-methylpent-l-ene. Morawetz J. Polymer Sci. 1961 54,491. 374 QUARTERLY REVIEWS poly-L-glutamic acid.37 These observations led Pino and L ~ r e n z i ~ ~ to conclude that the optical rotation in the poly-a-olefins does not arise solely from the existence of asymmetric carbon atoms in the lateral chains.In order to explain the high optical rotation and its remarkable depend- ence on stereoregularity and temperature it was suggested that in dilute solution and in the molten state the isotactic and block polymers of optically active a-olefins are at least in part in spiral form and that helices of a single screw sense largely prevail. The effect of helix formation on optical activity is however still not well under~tood.~~ Polymerisation in canal Complexes. Recently it has been that certain monomers especially diene monomers can be polymerised in a highly stereospecific manner by first preorientating the monomer molecule as a urea or thiourea complex.Thus 2,3-dimethylbuta-l,3-diene when irradiated as a thiourea canal complex gave highly crystalline trans-1,4- polymer. The polymerisation is shown schematically in Fig. 6. V n FIG. 6. Canal polymerisation. Similarly by using the urea inclusion complex it was possible to obtain highly crystalline all-trans- 1,4-polymer from buta- 1,3-diene. These elegant results are important examples of template polymerisations. Initiation of polymerisation in these canal complexes could not be achieved by conventional means for many obvious reasons and was finally accomplished by use of high-energy radiation such as X-rays. Ionic poly- merisations are normally classified as cationic and anionic polymerisation according to the nature of the ion occurring in the propagation reaction.13t39 The former involves the growth of a carbonium carboxonium or oxonium ion and initiation is usually achieved by means of Lewis acid-cocatalyst complexes e.g.H+ [BF3X]- and R+ [AlCl,]-. The polymerisation can be represented thus Stereoregularity in Ionic Po1ymerisations.-(a) General. H+[BF,X]-+ CH2=CHR + CH,*CHR+[BF,X]- CH,*CHR+[BF,X]- + nCH,=CHR 4 CH3-CHR.[CH2*CHR],-,*CH2*CHR+[BF3X]- [BF,X]- is the gegen- or counter-ion. Anionic polymerisation involves the growth of a propagating carbanion 37 Doty Wada Yang and Blout J. Polymer Sci. 1957 23 851. and White J. Amer. Chem. Soc. 1960,82 5671 ; White ibid. p. 5678. 38 Pepper Quart. Rev. 1954 8 88. Klasen 2. Electrochem. 1956,60,982; Yen J. Polymer Sci. 1959,38 272; Brown BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 375 or alkoxide anion and the usual initiators are metal alkyls metal amides and metal hydroxides e.g.M+R- + CH,=CHR' -+ R*CH,.CHR' - M+ R.CH,*CH R'-M+ + nCH :CH R' -+ R* [ CH,.C H R'] .*CH ,*CH R-M+ In this case the metal cation M+ is the gegen-ion. Polymerisations involving ionic intermediates are subject to several stereoregulating forces which do not occur in typical free-radical polymer- isation the most important of which is the influence of the attendant gegen-ion. Unfortunately the extent of this influence on reaction rates and stereochemistry is all too little understood. The best approach to such problems is undoubtedly to consider each ion-pair as able to exist in several distinct forms depending upon the degree of separation.Win- stein and his collaborator^^^ have formulated such a scheme for ionisation leading to carbonium ions namely RX + R+X- + R+//X- + R+ + X- Covalent Intimate Solvent-separated Free ions ion-pair ion-pair (V) (VI) (VW (VIII) It is envisaged that there will be a definite energy barrier between an intimate ion-pair (VI) and a solvent-separated ion-pair (VII) although the exact nature of these species cannot yet be adequately defined. Such a general scheme for ionisation does not take into account any specific solvation effects or salt effects on either component of the ion-pair but it suffices to point out that in solvents of low dissociating power ionisation will probably lead to intimate ion-pairs. The precise limit of dissociation in an intimate ion-pair will of course depend entirely upon the nature of its components and the solvent.On account of the high reactivity of carbonium ions and carbanions ionic polymerisations are necessarily performed in non-solvolysing inert solvents such as hydrocarbons ethers nitro-compounds and halogenated hydrocarbons. The use of such solvents will in many cases preclude the formation of dissociated ions and consequently it is desirable throughout to formulate mechanisms which involve the participation of ion-pair inter- mediates. (b) Cationic polymerisation. Although it was not then fully realised cationic polymerisation provided the first synthesis of an isotactic polymer. In 1948 Schildknecht and his associatesg1 were investigating the cationic polymerisation of alkyl vinyl ethers and discovered that from isobutyl vinyl ether two polymeric modifications could be obtained.If this monomer was polymerised in a hydrocarbon solvent (liquid propane) at -70° with 4 0 Winstein and Robinson J. Amer. Chem. SOC. 1958 80 169. 41 Schildknecht Zoss and McKinley Ind. Eng. Chem. 1947 39 180; Schildknecht Gross Davidson Lambert and ZOSS ibid. 1948 40 2104; Schildknecht Gross and Zoss ibid. 1949 41 1998; Schildknecht Zoss and Grosser ibid. 2891. 376 QUARTERLY REVIEWS gaseous boron trifluoride as catalyst a flash-type polymerisation resulted and the product was a tacky resin. If however the boron trifluoride- ether complex was used as catalyst the polymerisation was slow and pro- liferous and the product was a non-tacky solid which showed a typical X-ray crystallinity diagram.Natta Bassi and Corradini4 in 1956 showed that the latter polymer was indeed isotactic poly (isobutyl vinyl ether) although by this time many other isotactic polymers were already charac- terised. S~hildknecht~~ also showed that crystalline polymers could be obtained from other branched-alkyl vinyl ethers by the same process although it is significant that ethyl and isopropyl vinyl ethyl ether did not readily yield crystalline polymers in the same conditions. Methyl vinyl ether yielded a crystalline polymer only when choroform was added to the solvent and this observation remains enigmatic. More recently Japanese have made a thorough study of the polymerisation of alkyl vinyl ethers using the boron trifluoride-ether complex as catalyst and have extended the original findings of Schildknecht et aL41 In the original the solvent used was always an aliphatic hydrocarbon in which the boron trifluoride-ether complex is insoluble and consequently it was thought that the stereospecificity of this reaction was largely attributable to the heterogeneous nature of the solvent-catalyst system.Schildknecht and D ~ n n ~ ~ cast doubt on this theory and recently it has been unambiguously demonstrated4* that the stereospecific polymerisation can proceed under entirely homogeneous conditions. It appears that insolubility of the catalyst is only one of the problems and that insolubility of the polymer is also very important. By suitable choice of mixed solvents the whole reaction can be carried out in a homogeneous system and stereospecific polymers of isobutyl vinyl ether and methyl vinyl ether are still readily obtained.In all cases where crystalline polymers are produced low reac- tion temperatures (ca. -70") were used and it was observed that crystallin- ity decreased as the temperature increased. Very recently Natta and his collaborator^^^ have shown that crystalline isotactic alkyl vinyl ethers can be obtained by using other kinds of catalyst both those giving homogeneous and those giving heterogeneous systems. For example homogeneous stereospecific polymerisation of isobutyl vinyl ether in toluene as solvent at ca. -78" resulted when the following catalysts were used AlEt,CL AlEt, TiCl,(OPr),. Similar results were ob- tained when using the soluble crystalline complexes (C5H5),TiCl,~AlCl and (C,H,),TiCl,.AlEtCl. The latter complex is aso active as a Ziegler- type catalyst for the polymerisation of ethylene and Natta and Mazzanti47 42 Natta Bassi and Corradini Makromol.Chem. 1955,18-19,455. 43 Schildknecht Ind. Eng. Chem. 1958 50 107. 44 Okamura Higashimura and Sakurada J. Polymer Sci. 1959 39 507; Okamura Higashimura and Yamamoto ibid. 1958 33 510. 45 Schildknecht and Dunn J. Polymer Sci. 1956 20 597. 46 Natta Dall'Asta Mazzanti Giannini and Cesca Angew. Chem. 1959 71 205. 47 Natta and Mazzanti Tetrahedron 1960,8,66. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 377 have made a comparison of the catalytic activities of this type of organo- metallic complex in olefinic and vinyl ether polymerisation. Natta has also announced the synthesis of crystalline polymers from isobutyl propenyl ether BuiOCH=CHMe e.g.a threo-di-isotactic polymer but without giving details of the catalyst used. Independently however Heck and Breslowg8 have also reported the synthesis of crystalline polymers from monomers of the general formula RO.CH= CHMe. When R was ethyl or n-propyl it was possible to investigate the polymerisation of both cis- and trans-isomers and in both series the same polymer was obtained from each isomer. This highly significant result indicates that the cis- and trans-isomers yield a common intermediate as they enter the polymer chain. Once again however details of the catalyst function remain shrouded in Patent secrecyg9 although it is clear that the essential reactants are precisely those employed for Ziegler type catalysts (see below) but that temperatures around -78" are employed.The authors believe that the polymerisation involves cationic intermediates and it is clear that if one accepts the probability that Ziegler-type polymerisation does not involve a propagating cation then this dual function of certain complex organometallic catalysts is likely to provide many new advances in polymer chemistry during the next few years. It is pertinent to point out here that the Ziegler-type catalysts used by Heck and Breslowg8 to polymerise alkyl propenyl ethers were first de- veloped by Vandenberg6* 49 for the stereospecific polymerisation of simple alkyl vinyl ethers. The catalysts function heterogeneously since they are based on reduced halides of titanium and vanadium (see below) and at low temperatures yield much more crystalline and presumably stereoregular polymers than do the typical Schildknecht or Natta catalysts.Thus the melting point of poly(isobuty1 vinyl ether) obtained by using the boron tri- fluoride-ether complex as catalyst is reported to be 110" whereas the crystal- ine polymer obtained by Vandenberg Heck and Breslow6 melted at 165 '. This implies a higher degree of stereoregularity in the latter since the two polymers show similar X-ray fibre diagrams. Heck and. Breslow50 also showed that with these insoluble catalysts 1-methoxybutadiene gave a highly crystalline trans-polymer in which the methoxy-substituent was regularly oriented along the polymer backbone. This polymer (IX) appears to be the first isolated form of what NattalO now calls a tritactic 48 Heck and Breslow J. Polymer Sci. 1959 41 520. 4 9 Vandenberg Italian P.571,741/1958; B.P. 820,469/1959. 5 0 Heck and Breslow J. Polymer Sci. 1959 41 521. 378 QUARTERLY REVIEWS polymer. Crystalline poly(isobuty1 vinyl ether) has also been obtained by use of other alkylmetal-transition-metal halide catalyst^.^^ Attempts to provide a mechanism for the homogeneous stereospecific polymerisation of alkyl vinyl ethers have been made by Cram and K o p e ~ k y ~ ~ ~ ~ ~ Higashimura Yonezawa Okamura and F u k ~ i ~ ~ and more recently by Bawn and L e d ~ i t h . ~ ~ All these authors recognise that because of the presence of a gegen-ion near the charged carbon atom of the last monomer unit the electron configuration of this carbon may not be adequately described by sp2 (trigonal) hybrids but rather by sp3 (tetra- hedral) hybrids especially in solvents where dissociation of the ion-pair does not take place to an appreciable extent.Alkyl vinyl ethers have long been considered as exhibiting mesomerism and consequently there is also the possibility that this tendency would result in the occurrence of two rotational isomers (Fig. 7). Infrared and other techniques have not provided conclusive evidence for rotational isomerism.54 Recent studies of the nuclear magnetic resonance spectra of the vinyl have clearly shown the occurrence of mesomerism in alkyl vinyl ethers but no evidence for rotational isomerism could be found over a temperature range from -100” to +loo”. This could mean either that only one form of mesomeric structure occurs [presumably the trans- form (Xa)] or that the interconversion of forms (Xa) and (Xb) is too fast to be detected by nuclear magnetic resonance differences even at -100”.(Xa) trans FIG. 7. If as believed the trans-forms of vinyl ethers predominate then it is obvious that the alkyl substituent containing at least three consecutive carbon atoms will cause substantial steric blocking of one side of the ole- finic bond. Fig. 8 illustrates this for isobutyl vinyl ether and also shows analogous steric effects for t-butyl acrylate and certain NN-dialkylacryl- amides. In the special case of vinyl ethers this steric blocking which is a form of the “rule of six” proposed by N e ~ m a n ~ ~ will be greatest for neopentyl vinyl ether and will decrease through isobutyl and n-propyl vinyl ether. For isopropyl and ethyl vinyl ether it is clear that such steric blocking is not possible. These predictions are substantiated by examina- 51 Lal J.Polymer Sci. 1958 31 179; JSray ibid. 1960 44 264. s2 Higashimura Yonezawa Okamura and Fukui J. Polymer Sci. 1959,39,487. 63 Bawn and Ledwith Polymer in the press. 54 Brey and Tarrant J. Amer. Chem. SOC. 1957 79 6533. s5 Feeney Ledwith and Sutcliffe J. 1962 2021. 56 Newman “Steric Effects in Organic Chemistry,” Wiley New York 1956 pp. 201 cet seq. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 379 (A) FIG. 8. (A) t-Butyl acrylate. H \ . I’ CH2- CH,-CH H,C -C-CH H ... -”‘ H \2 N- R g=c H 5 .H ,CH2 \ ,c=”\ G / 0 (6) (0 (B) NN-Dialkylacrylamides. (C) Isobutyl vinyl ether. tion of molecular models and also coincide with the experimental observa- tion that isobutyl vinyl ether readily yields crystalline polymers whereas isopropyl vinyl ether does not.Bawn and L e d ~ i t h ~ ~ postulate that the growing cation can be stabilised by a form of neighbouring-group interaction or intramolecular solvation through the oxygen atoms in the penultimate monomer unit (formation of a four-membered ring) or more probably by formation of a six-membered ring with the oxygen atom of the last-but-three monomer unit as shown in Fig. 9. Such “backside” solvation or stabilisation of the growing cation leads to reactions at the cation which involve attack from the opposite side i.e. that side on which is found the gegen-ion. The polymerisation of alkyl vinyl ethers by this mechanism is shown in Fig. 9 and can be regarded as a form of S N 2 attack with retention of con- figuration. The latter is assured by the probable interaction of the gegen- ion A- with both existing and newly forming carbonium ions in the transi- tion state.This allows for the immediate intramolecular solvation of the FIG. 9. Bawn and Ledwith’s mechanism. newly formed carbonium ion on the same side as was the original reacting carboiiium ion. Indeed it is possible that solvation of the new cation occurs before it is completely formed and in this manner the configuration of the growing carbonium ion is maintained throughout provided that the monomer molecules enter in the manner indicated in Fig. 9. The above mechanism takes no account of the precise nature of the gegen-ion A- although obviously this can exert its own influence on both the rate and the stereospecificity of the polymerisation. 380 QUARTERLY REVIEWS The mechanism proposed by Cram and kopeck^^^ is essentially similar to that described above except that these authors assume the complete formation of a six-membered-ring oxonium ion in the transition state instead of the much less rigid intramolecular solvation proposed by Bawn and L e d ~ i t h .~ ~ The polymerisation is represented as in Fig. 10. Cram and H ‘T1 c;1 7 + H P -C-CH-C-CH2-C=OR -C n I 2 1 H OR OR / Transition state H ...OR H ,.... OR H \,...‘OR C C C p ‘C< ‘Cc ‘CH,-CH=i)-R Isotactic polymer FIG. 10. Cram and Kopecky’s mechanism. K o p e ~ k y ~ ~ pay strict regard to the steric aspects of conformation in a six- membered ring; since the growing polymer chain P, is larger than OR they suggest that P occupies an equatorial position as does OR attached to the growing end of the chain.In the reaction of monomer with the cyclic oxonium ion which would logically proceed in the direction in- dicated the relative configurations of the first two asymmetric centres formed in each polymer chain determine the overall configuration of that chain. If the configurations are similar the polymer chain becomes isotactic; if different syndiotactic. Molecular models suggest that iso- tactic placements are more likely. Although this mechanism has con- siderable merit it does not indicate why different groups R have their own influence on stereospecificity . Examples of cationic polymerisation to stereospecific structures are largely confined to the vinyl ethers. However Natta5’ has recently reported that the ether complexes of boron trifluoride and aluminium tribromide polymerise aldehydes at -40” to - 100” to very stereoregular molecules and also that 2-methoxystyrene unlike styrene itself is polymerised homogeneously and cationically by dichloro(ethyl)aluminium AlCl ,Et to an ordered non-crystallisable isotactic polymer.(c) Anionic polymerisation of vinyl monomers. In the last few years anionic polymerisation has received extensive study particularly in the use of metal alkyls and sodium-naphthalene as initiator~.~~-~l This work 5’ Natta J. Polymer Sci. 1960 48 219. 58 Szwarc Makromol. Chem. 1960,35,132; Szwarc “Advances in Chemical Physics,” 59 Wenger Chem. and Ind. 1959 1094; Wenger Shiao-Ping and Yen Makromol. 6o Worsfold and Bywater J. Polymer Sci. 1957 26 299; Canad. J. Chem. 1958 36 61 McCormick J. Polymer Sci. 1959,36 341 ; 1959 41 329.Vol. 11 Interscience Publ. Inc. New York 1959 pp. 147 etseq. Chem. 1961,43 1 ; Wenger ibid. 1960,37 143. 1141 ; J. 1960 5234. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 38 1 has been mainly concerned however with the non-stereospecific nature of the catalyst systems and consequently will not be reviewed here in detail. The recent and spectacular developments of stereospecific anionic polymerisation are related almost entirely to the polymerisation of vinyl monomers and dienes and these will be summarised in this and the next section. Apart from those initiated by catalyst mixtures involving transition-metal components almost all polymerisations initiated with organo-metallic com- pounds are commonly referred to as anionic polymerisation. It is now possible to prepare stereoregular polymers from styrene and its deriva- tives,s2 various acrylate ester monomers,63 and NN-dialkyla~rylamides~~ by use of organometallic catalysts the most effective being the lithium alkyls although Grignard reagents dialkylmagnesium compounds and sodium and potassium alkyls have all been used successfully.65~ For a-methyl methacrylate a detailed study of the relative efficiency of many organometallic compounds in promoting stereospecific polymerisa- tion has shown that lithium alkyls are most efficient although in this case low temperatures and a hydrocarbon solvent are req~ired.~' In these con- ditions essentially isotactic polymer is produced but addition of ethers and other Lewis bases to the reaction mixture reduces the degree of isotacticity and depending on the temperature of polymerisation produces atactic or syndiotactic poly(a-methyl methacrylate).On the other hand the use of butyl-lithium as initiator for the polymerisation of NN-dibutylacryl- amide in heptane gives a good yield of isotactic polymer at room tempera- t ~ r e . ~ ~ Ambient temperatures can also be employed with organomagnesium compounds as initiators,18*6s but in such cases there is an added stereo- regulating force induced by the formation of complexes between the alkyl magnesium and the magnesium halide which occurs in solutions of Grignard reagents.68 The full understanding of the mechanism of these polymerisations is made difficult by the complex nature of the species used to bring about initiation. In many cases the initiating organometallic complex is insoluble in the reaction medium but dissolves as soon as the polymer begins to be formed.Lithium alkyls have been widely used as initiators and before discussing 62 Kern Nature 1960 187 410; Braun Betz and Kern Makrornol. Chem. 1960 39 42; Sinn Lundborg and Kirchner Angew. Chem. 1958 70 744. 63 Miller and Rauhut J. Polymer Sci. 1959 38 63; J. Amer. Chem. Soc. 1958 80 41 16; Garrett Goode Gratch Kincaid Levesque Spell Stroupe and Watanabe ibid. 1959 81 1007. 64 Attfield Butler Radcliffe Thomas Thompson and Tyler Chem. and Ind. 1960 263; Butler Thomas and Tyler J. Polymer Sci. 1960,48 357. 65 Kawasaki Furukawa Tsuruta Inoue and Ito Makromol. Chem. 1960 36 260; Inoue Tsuruta and Furukawa ibid. 1959,32,97; Morton and Taylor J. Polymer Sci. 1959 48 7. 66 Goode Owens Fellmann Snyder and Moore J.Polymer Sci. 1960 46 317; Goode Owens and Myers ibid. 1961 47 75. 67 Glusker Stiles and Yoncoskie J. Polymer' Sci. 1961 49 297; Glusker Lysloff and Stiles ibid. p. 315. 68 Dessy and Handler J. Amer. Chem. SOC. 1959 80 5824. 382 QUARTERLY REVIEWS the various mechanisms proposed for the stereoregulated propagation reaction it is necessary to consider their structure. Neglecting for the moment the effect of association we prefer to describe the ionisation of a lithium-carbon bond in a manner analogous to that used by Winstein and Robinson40 for carbonium-ion formation RLi R-Li+ R-/ Li+ R- + Li+ Covalent s Intimate + Soknt-separated + Free ions ion-pair ion-pair (XI) (XII) (XIII) (XW In hydrocarbon solvents lithium alkyls can be expected to behave as forms (XI) and (XII) initiating polymerisation at a slower rate and with more stringent steric requirements than do the ion-pairs (XIII) and free ions (XIV) which are likely to prevail in more solvating solvents such as ethers and amines.Temperature will also affect the equilibrium between (XII) and (XIII). Reaction conditions favouring species (XIII) and (XIV) i.e. polar or solvating solvents lead to truly anionic polymerisations similar to those discovered by S z w a r ~ ~ ~ when using sodium-naphthalene as initiator. In these conditions the growing polymeric anions can be imagined almost as a free propagating species. The requirements for stereospecific poly- merisation will be similar to those necessary to produce stereospecific polymerisation by a free-radical mechanism and will normally obtain only at very low temperatures.Furthermore syndiotactic placements are to be expected. This view is supported by the ob~ervation~~ that the alkyl- lithium-initiated polymerisation of a-methyl methacrylate in 1,2-di- methoxyethane yields predominantly syndiotactic polymer when carried out at temperatures around -60". When the reaction conditions favour species (XI) and (XII) i.e. in essentially non-solvating media such as aliphatic hydrocarbons stereo- specific polymerisation leading to isotactic polymers is commonly en- countered with acrylic monomers. The temperature at which isotactic polymers can be prepared depends however on the structure of the monomer. Thus t-butyl acrylate and methacrylate yield isotactic polymers in heptane at room temperature or above.63 Similar behaviour is shown by NN-dibutylacrylamide.64 Isopropyl acrylate cyclohexyl acrylate and methyl methacrylate can most easily be polyrneri~ed~~ to isotactic polymer at temperatures around or below -50". The driving force for propagation is an initial complex-formation or co-ordination between the lithium atom in species (XI) and (XII) and the 7-electron system of the monomeric olefin. This will undoubtedly involve overlap of the olefinic n-electrons with vacant s- or p-orbitals in the lithium and is analogous to the reaction of an olefin with a carbonium ion. The most probable approach conformation is shown in Fig. 1 1. Co-ordination of the olefin will be followed by an intramolecular rearrangement involving migration of the carbanion R- to the most electrophilic carbon atom of the BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 383 unsaturated molecule.The product of this reaction will then be another alkyl- lithium possessing forms (XI) and (XII) and the process will be repeated until either the monomer is used up or the metal-alkyl bond is deactivated in some manner. This type of polymerisation is analogous to the stepwise growth of aluminium alkyls by reaction with ethylene,69 and of boron ... - L i t FIG. 11. P - c . T 0 R- M? ,OMe FIG. 12. OMe FIG. 13. OMe alkyls by reaction with ethylene'O or with dia~omethane.'~ In the case of acrylic and other conjugated monomers this process is complicated but it is no doubt facilitated by the possibility of incipient 1,4-addition as shown in Fig. 12 for methyl methacrylate. Once a simple alkyl-lithium has reacted with say methyl methacrylate in this manner the growing polymeric alkyl-lithium can then be expected to have some enolic character as shown in Fig.13 and furthermore the lithium atom can co-ordinate with the carbonyl oxygen of the penultimate monomer unit as shown. The cyclic intermediate can be regarded as involving intramolecular solvation of the lithium and because of the intramolecular shielding of one side of the lithium atom nucleophilic attack by the monomer molecule will occur from the opposite side giving a transition state formally analog- ous to that in an SN2 reaction (Fig. 13). As the bond-forming reaction between lithium and the incoming olefinic 6s Ziegler Angew. Chem. 1952 64 323 330. 70 Koster Annulen 1958 618 31 ; Koster and Gunter ibid. 1960 629 89. 71 Bawn Ledwith and Matthies J.Polymer Sci. 1959 34 93. 384 QUARTERLY REVIEWS molecule develops the oxygen-lithium co-ordination link will break and simultaneously the polymer anion will migrate from the lithium atom to the methylene group of the monomer molecule. The newly formed lithium enolate system is then immediately stabilised by intramolecular solvation as before and in this manner retention of configuration will be assured provided that the incoming monomer molecule always presents the same conformation towards the lithium atom. Examination of molecular models shows clearly that for methyl methacrylate the conformation involving least steric interaction is that in which the a-methyl group is trans to the a-methyl group of the carbanion (Fig. 14). FIG. 14. Preferred monomer conformation in an S~2-type reaction with a lithium atom.(The bond between lithium and the growing polymeric enolate is not shown in this projection.) With monomers such as t-butyl acrylate the important steric repulsion is between the bulky olefinic ester group and the growing polymer chain C ( I ) and this leads to the same mode of attack as in the diagram with the ester group and the polymer chain always in trans-relation. Without the cc-methyl group the last condition would not be fulfilled and methyl acrylate for example should prove difficult to polymerise stereospecifically in this manner i.e. it should not readily give an isotactic polymer. In general for stereospecific polymerisation involving a free propagating species the various approach configurations of the monomer molecule will differ only slightly in the amount of steric repulsion or non- bonded interactions between substituent groups and consequently the small differences in activation energy of the various modes of addition will normally be significant only at low temperatures.With other acrylate monomers in which there is branching at the a-carbon of the ester group e.g. t-butyl acrylate or isopropyl acrylate the same general requirements for stereospecific polymerisation will hold in such cases however the conformation of the monomer is determined by the bulky nature of the ester group which shields one side of the double bond and causes the opposite face to be presented to the lithium atom. This effect which is a form of steric hindrance is also found with vinyl alkyl ethers having /3-substituents on the alkyl group.With t-butyl acrylates3 the effect is so pronounced that it leads to an isotactic polymer even at temperatures above 0". Similar behaviour is shown also by NN-dib~tylacrylamide,~~ which can be polymerised to isotactic polymer with butyl-lithium in hexane at room temperature. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 385 Cram and Kope~ky"*~~ proposed an alternative mechanism which has much to commend it. They assume that the growing lithium enolate has complete alkoxide character as in (XV). This can be stabilised by the y e yeti+ ?Me Ye P,-V-C%- CqTpO == q=y-CH*-$-Pn L?O- Me C0,Me C02Me he (XV) formation of a six-membered ring as a result of alkoxide attack on the carbonyl group of the penultimate monomer unit as shown in (XVI). Stereospecific propagation then proceeds by a Diels-Alder type of reac- tioni2 in which the six-membered ring is destroyed and a new ring made all in one transformation.The transition state for this reaction is shown in (XVII) which can be regarded as formed by 1,4-dipolar addition to a Me0 'C=O MeO' Me02C Me Me CO$hz \: **..-I MeO / c 9 Li+& Me C=C-CH2 CH2 Pn I ow Isotactic polymer polarised double bond. The stereoregulation is determined essentially by the desirability of having the lithium atom co-ordinated simultaneously with the two oxygen atoms as shown in (XVII). This of course requires that each monomer molecule shall approach from below the plane of the ring and in this manner isotactic placements will be favoured. Once again however there is doubt whether it is correct to assume a rigid six-membered cyclic alkoxide structure for a propagating lithium enolate.If a rigid structure is not allowed then this mechanism becomes very similar to the previous one outlined above. In an elegant series of experiments involving the use of radioactive tracers D. L. Glusker and his have studied the kinetic and molecular-weight behaviour of methyl methacrylate polymerising in an essentially hydrocarbon solvent using 9-fluorenyl-lithium as initiator. It was concluded that (1) chains of different steric configuration may propagate at different rates even in a homogeneous medium; (2) a high 72 Woodward and Katz Tetrahedron 1959 5 70. 386 QUARTERLY REVIEWS percentage of the total number of chains started undergoes “pseudo- termination” whose probability is a function of the configuration of the last three monomer units in the chain; (3) when a chain attains a minimum length of 8-10 monomer units the probability that further addition will take place in isotactic sequence is greatly increased; and (4) the source of this increased probability lies it was thought in the inherent asymmetry of a preferred helical conformation which may exist for chains of this minimum length in solution at -60”.It was found that initiation is rapid and complete and is followed by a steadily decreasing rate of polymerisa- tion until about eight monomer units have been added whereupon a constant and highly stereospecific polymerisation ensues until polymer of very high molecular weight is obtained. Throughout the foregoing discussion it has been assumed that the most effective propagating species is a monomeric alkyl-lithium or lithium enolate.Alkyl-lithiums in solution are highly as~ociated’~ and it has been shown that in hydrocarbon solutions there is an equilibrium between associated butyl-lithium and the monomeric species [LiBu] + LiBu + [LiBu],, and that initiation of styrene polymerisation is brought about only by monomeric The presence of Lewis bases such as ethers and amines will grossly affect this kind of equilibrium by specific solvation effects on both monomeric and associated metal alkyls and it is interesting that addition of Lewis acids such as alkylaluminium compounds decreases the rate of polymerisation by complex formation with the monomeric alkyl- lithium LiR + AIR + LiAIR Polymerisation by the mechanism described above involves an essentially covalent stepwise growth of an alkyl-lithium chain.This and related polymerisations might then be termed “covalent insertion polymerisation” or “co-ordinate insertion polymerisation,” whereas growth of an essentially ionic form of alkyl-lithium is more likely to involve attack by an anion on the monomer molecule and is truly representative of an “anionic poly- merisation” in the conventional sense. The latter and also conventional “cationic polymerisation,” really imply the repeated addition of a separ- ated ion-pair to a monomeric double bond and as such should correctly be described as “anionic addition polymerisation,” or “cationic addition polymerisation,” respectively. (d) Anionic diene po2yrnerisation.-The previous considerations concern- ing association and complex formation of lithium alkyls used as catalysts apply equally when diene polymerisation is being studied and should be borne in mind throughout the following discussion.73 Coates “Organometallic Compounds,” Methuen London 1960 p. 20. 74 O’Driscoll and Tobolsky J. Polymer Sci. 1959 35 259; Welch J. Amer. Chem. Soc. 1959,81 1345. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 387 Isoprene and butadiene are readily polymerised with metallic lithium and lithium alkyls as initiator^,^^ and the overall reactions can be re- presented as illustrated. k (metal) k2 2Li + CH,=CR.CH=CH 3 LiCH,CR=CH.CH,Li LiR' + CH,= CR.CH= CH -f Li.CH,-CR= CH.CH,R' k3 LiCH,CR= CH.CH,R' + n(CH,=CRCH=CH,) -+ Li-CH,-CR= CHCH,.[CH,CR= CHCH,],-,.CH,.CH R= CHCH,R' The rate constants kl and k2 represent rates of initiation when metallic lithium and lithium alkyl respectively are used as initiators.It is to be expected that kl < kz and this is borne out experimentally. With these monomers it is possible to obtain cis- and trans-l,4-addition polymers as well as 1,2- and 3,4-polymers. (cf. introductory section). As described above the precise distribution of these various modes of addition in the final polymer has a pronounced effect on the physical and mechanical properties of the synthetic rubber and it is now quite clear that solvent and the cation from the metal alkyl are the chief causes of variation in the polymer microstructure. Polymerisation of isoprene has been studied extensively by Tobolsky and his collaborator~,~~ and also by Stearns and F ~ r m a n ~ ~ and typical effects of solvent and metal cation on polymer composition are shown in Table 2.The drastic modifications of polymer structure caused by addition of small amounts of active solvents such as tetrahydrofuran are illustrated in Table 3. In the case of butadiene Kuntz and G e ~ b e r ~ ~ found that polymerisation in heptane initiated by butyl-lithium yields a product resulting from approximately equal amounts of cis-1,4- and trans-1,4- and about 10% of 1 ,Zaddition. The actual composition lies within the range trans-1,4- 48-58 % cis-1,4- 33-45 % and 1,2-addition 7-10 %. This composition was independent of catalyst and monomer concentrations over the range studied and showed a remarkable independence of temperature of poly- merisation from 4" to 80". Small amounts of ethers changed the polymer composition markedly increasing the amount of 1,2-addition to such an extent that polymerisation in tetrahydrofuran yielded a polymer with more than 80% of 1,Zaddition and small but equal amounts of cis- and trans- 1,4-addition.The effect of changing the organometallic initiator also has been investigated by Russian but the results are consistent 75 Bawn Rubber and Plastics Age 1961 42 267. '* Tobolsky and Rogers J. Polymer Sci. 1959 40 73; Hsieh Kelley and Tobolsky " Steams and Forman J. Polymer Sci. 1959,41,381. Kuntz and Gerber J. Polymer Sci. 1960 42 299. '' Korotkov Internat. Symposium Makromol. Chem. Prague 1957 paper 66; Angew. Chem. 1958 70 85; Kropachev Kolgoplosk and Nikolaev Doklady Akad. Nauk S.S.S.R. 1957 115 516; Chem. Abs. 1958 52 3386. ibid.1957 26 240; Morita and Tobolsky J. Amer. Chem. SOC. 1957 79 5853. 388 QUARTERLY REVIEWS TABLE 2. Microstructure of polyisoprene catalysed by alkali metals and their organo-derivatives in pentane and ether.77 Modes of addition (%) disclosed by infrared analysis Catalyst In pentane. Lithium E t h yl-li thium Butyl-lithium Sodium Ethylsodium Butylsodium Potassium E t hylpo t assium Butylpotassium Rubidium Casium “Alfin” (sodium) Emulsion Cationic E thyl-li thium Ethylsodium Lithium In diethyl ether. cis- 194 94.4 94.2 92-6 0.0 6.3 4.1 0.0 23.7 19.6 5.0 4-0 27.0 22.0 36.7 6.0 0.0 4.0 trans- 194 0.0 0.0 0.0 43.0 42.0 34.9 52.0 39.2 40.8 47.0 51.0 52.0 65-0 50.6 29-1 14.3 26.7 0.0 0.0 0.0 6.0 7.1 6.7 8.0 5.7 6.2 8.0 8.0 5.0 6.0 3.8 5.3 10.2 5.7 394 5-6 5.8 7.4 51.0 44.6 54.4 40.0 31.4 33.5 39.0 3 7.0 16.0 7.0 8-9 59.6 75.6 63.5 Total 91.7 92.0 88.9 99.4 100.7 96-7 94.2 - - - 78.9 95-2 98.9 100.4 TABLE 3.Eflect of tetrahydrofuran (THF) on the polymer microstructure76 (catalyst n- bu tyl-lithium ; solveif t hep tane) . Vol. fraction of heptane 1*OOO 0.999 0-998 0.996 0.984 0-968 0.o00 Ratio THF/BuLi Modes of addition (%) 192 394 174 0 0 7 93 0.25 1 13 86 0.50 1 20 79 1 0 40 67 32 4 30 8 - 16 54 30 - - - - with those found for isoprene polymerisation as expected on general considerations (see below). First as regards isoprene polymerisation initiated by lithium and its alkyl derivatives there is common agreement that the very high proportion of cis- 1,4-polymer which results from polymerisation in hydrocarbon solvents is largely the result of stepwise growth of an essentially covalent or intimate ion-pair lithium-carbon bond.We can envisage interaction of an isoprene and an essentially covalent alkyl-lithium molecule as BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 389 analogous to that proposed for the interaction of an acrylate monomer with alkyl-lithiums (see Figs. 12 and 12A). In order to account for the high FIG. 12A. cis- 1,4-content of polyisoprenes prepared under these conditions it is commonly assumed that a pseudo-six-membered ring is formed. This Me Me H\ /Me H\ / -CH H’i c..-*-*..-. Li - -CH a2w ‘cH;L~+ etc. H\ ,C=C / ,H ,c=c /c=c type of complex (as XVIII) is especially favourable for lithium alkyls (as compared with other organometallic compounds) because of the relatively small ionic radius of lithium and the high proportion of p- character in its outermost orbitals.When the lithium-initiated polymerisation is performed in solvating or basic solvents such as an ether or amin.e the resulting polyisoprene con- tains mainly 3,4-linkages with about 25 % of trans-1,4-linkages. Similar structures result from polymerisations initiated by other alkali metals even in hydrocarbon solvents. These observations are easily rationalised in terms of the relative ionic radii and ionic character in alkali-metal organo-derivatives. The organo-metallic compounds of sodium potassium rubidium and caesium will be much more polar and more highly dissociated than the corresponding lithium derivatives. We can imagine therefore that the basic structures of these metal alkyls approximate even in hydrocarbon solvents to the solvent-separated ion-pair structure proposed for lithium alkyls.The catalyst reacts essentially as a solvated anion and in diene polymerisation it will always be an allylic anion. The behaviour of allyl-metal derivatives of this type will be similar to that of ally1 Grignard reagentsSo and it is significant that of the three extreme structures (XIX- XXI) for such derivatives the reaction with carbonyl CompoundP yields M+ M+ om) (XI0 (xx 1) Roberts Nordlander and Young J. Amer. Chem. SOC. 1961 83 494; Roberts Wilson Roberts and Young J. Amer. Chern. SOC. 1950 72 218. and Nordlander ibid. 1959 81 1769. 390 QUARTERLY REVIEWS products derived from species (XX) whereas polymerisation of isoprene with the corresponding organometallic derivatives likewise yields mainly 3,4-addition which indicates that the growing allyl-metal derivative also reacts as if it had a structure analogous to (XX).Szwarca2 has rationalised the polymerisation studies by assuming that in a resonating allylic anion the form containing a secondary carbanion (XX) is the more reactive. He also outlines steric considerations which show that when 1,4-addition takes place in such systems it must do so in a trans-manner. This follows since the actual structure of a dissociated or solvent-separated allyl- metal derivative is likely to be as shown in (XXII) and only when the reacting diene approaches in a trans-conformation is it possible for the resulting allylic anion to have the required structure. Much of this type of speculation however depends upon the structure of allyl-metal derivatives and while the recent nuclear magnetic resonance evidence obtained by Roberts and his collaborators80 does not conflict with any of the ideas outlined so far there is still some doubt as to the precise nature of allyl-metal systems.For butadiene Raman spectroscopy has indicated 83 that there is a very great difference in the relative stabilities of the cis- and the trans-conforma- tion. In this case the equilibrium mixture contains less than 4% of the cis-conformation at room temperature and the planar trans-form has been reported to be more stable than the cis-form by about 2000 cal./mole. Despite these differences in the stabilities of the cis- and trans-rotational isomers of butadiene the observations of Kuntz and Ge~ber'~ show con- clusively that there is no stereoselective polymerisation to a trans-l,4- polymer in heptane solution when lithium alkyls are used as catalyst.Instead the polymer contains comparable amounts of cis- and trans-1,4- polymer with about 10% of 1,Zpolymer; the composition was constant over a wide range of temperatures and it was independent of polymer conversion and molecular weight so that structural homogeneity was maintained during the entire polymerisation. It is clear therefore that trans-butadiene molecules in solution are easily converted into cis-confor- mations in the activated complex. The latter possibility is substantiated by the results of Diels-Alder reactions with butadiene where the molecule invariably reacts in the cis-c~nforrnation.~~ Kuntz and G e ~ b e r ~ ~ conclude that in heptane solution the carbon- lithium bond of the growing polymer chain is involved in a reversible 82 Szwarc J.Polymer Sci. 1959,40 583. 83 Richards and Nielson J. Opt. SOC. Amer. 1950 40,438. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 391 equilibrium in some aggregate structure involving butadiene molecules and is fairly insensitive to association with organolithium compounds. This kind of polymer-monomer aggregate was also suggested by Korot- k ~ v ' ~ and it is possible that it involves several polymer chains at the same time. If one assumes that the 10% or so of 1,2-addition occurs always from a transition state involving a trans-conformation then this is compatible with the prevalence in solution of the trans-l,4-conformation of butadiene. When ethereal or other solvating solvents are used with lithium alkyl catalysts or when other alkali-metal derivatives are used the product contains a higher proportion of l,Zlinkages and this is to be expected since under these conditions the propagating species will be a substituted ally1 anion similar to that encountered with polymerisation of isoprene.Szwarc82 has pointed out that 1,Zplacements are to be expected under such conditions. Although the polymeric microstructures referred to above are thought to exist within individual polymeric molecules this point has not yet been unambiguously established. Consequently it could happen that at least some of the above polymerisations are much more stereospecific than appears at first sight and that the variations in microstructure are brought about by mere traces of impurity in the catalyst systems.This point has been fully discussed by Natta5 in connexion with diene polymerisation by Ziegler-t ype catalysts. Ziegler-type Polymerisation After first outlining the scope of Ziegler-type catalyst systems an attempt will be made to compare the more plausible mechanisms proposed for the polymerisation of oc-olefins and dienes. The original discovery by Ziegler et aZ.l related to a catalyst for low- pressure polymerisation of ethylene and was prepared by adding a solution of triethylaluminium to a solution of titanium tetrachloride in hydro- carbon solvents such as heptane or hexane. The product of this reaction is a brown-black insoluble compound which is essentially a reduced halide of titanium e.g. TiC1,,TiCl2 and this precipitate together with its super- natant liquor is essential for catalytic activity.Nowadays the patent liferat~re~9~~ covers a vast number of variations on the original Ziegler catalyst and it appears that catalytic activity can be obtained by mixing metal alkyls or aryls from Groups I to 111 in the Periodic Table e.g. (RMX, where R = alkyl or aryl M = metal and X = halogen alkoxy etc.) with compounds (usually halides or esters) of the transition metals in Groups IV-VI. Thus titanium tetrachloride and tetra-alkoxide and vanadium tetrachloride are typical examples and in all of these cases the solvent is preferably an aliphatic hydrocarbon. 84 Stille Chem. Rev. 1958 58 541. 392 QUARTERLY REVIEWS When the two selected catalyst components are mixed there is immediate reduction of the transition metal to a compound with the metal in a lower valency state and these reduced halides etc.are often insoluble in the reaction mixture. However it is quite clear that the effective catalyst is made up of some complex between this precipitate and the metal alkyl used to effect reduction. Indeed Natta has concludeda5 that the active Ziegler-type polymerisation catalyst is a complex containing organo- metallic bonds and more than one metal atom. For cc-olefins NattaS5* has shown that the most active and stereospecific catalysts are those formed by reaction between an alkyl of a highly electropositive metal having a small diameter e.g. beryllium aluminium or lithium and a crystalline halide of a transition metal from Groups IV-VI in which the metal is in a valency state less than the maximum e.g.titanium tri- or di-chloride or vanadium trichloride. When the Ziegler catalyst is prepared from the unreduced transition-metal halide,86 then the reduction is thought to proceed through a series of states exemplified for the reaction between triethylaluminiurn and titanium tetrachloride as follows AIEt + TiCl -+ AIEt,CI + EtTiC1 EtTiCI -+ TiCI + Et. EtTiC1 + AlEt -+ Et,TiCI + Et,AICl Et,TiCI -+ EtTiC1 + Eta TiCI + AlEt 3 EtTiCI + Et,AICl EtTiCl -+ TiC1 + Eta Thus previously unknown (and presumed unstable) organo-derivatives of titanium are thought to occur during the reaction; and indeed trichloro- (ethyl)titanium EtTiCl, and similar compounds have now been isolated from reactions similar to that described above.87 The effective Ziegler catalyst might then be formed by complex formation between unused trialkylaluminium and either titanium trichloride or titanium dichloride.For example it is plausible to propose the formation of a bridged complex (XXIII) such as (XXI11) which (XXIV) is fo&aliy analogous 85 Natta. Internat. Conference on Chemistry of Co-ordination Compounds September; 1957; J . Inorg. Nuclear Chem. 1958 8 589. 86 Ziegler Martin and Stedefeder Tetrahedron Letters 1959 No. 20 Rodriguez Gabaut and Hargitay ibid. 1959 No. 17 p. 7; Malatesta Canad. J. 1959 37 1176. 87 Beerman and Bestian Angew. Chem. 1959 71 618; Bawn and Gladstone Proc. Chem. SOC. 1959,227; Karapinka Smith and Carrick J. Polymer Sci. 1961.50,143. to the Rome p. 12; Chem., BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 393 well-established bridged dimersss of aluminium trialkyls (XXIV) and aluminium halides (XXV).89 That a complex is formed during Ziegler-type polymerisation seems extremely likely in view of the electrophilic nature of the catalyst com- ponents.However it is still largely a matter for speculation whether the complex is involved in the actual propagation steps. Ziegler’s original patentg0 referred specifically to polymerisation of ethylene and it was Natta and his associatesg1 who extended the scope of Ziegler’s work and showed that Ziegler-type catalysts could be used to prepare linear and in many cases stereoregular polymers of high molecular weight from almost any olefinic hydrocarbon with a basic structure CH2=CHR. Dienes also could be polymerised in a stereospecific manner and the physical properties of these new products are discussed later p.419. It was quickly realised however that in general Ziegler-Natta- type catalysts could not be used for the polymerisation of monomers containing polar and reactive groups such as vinyl chloride acrylic esters acrylonitrile etc. Obviously the reactive nature of the monomer substituent would destroy or deactivate by co-ordination the highly electrophilic catalyst components. Mechanisms for Ziegler-type Polymerisation.-It should be emphasised at the outset that most mechanisms considered or proposed under this general classification refer mainly to the chemical nature of the propagation reaction without attempting to explain its highly stereospecific qualities. The latter are usually assumed to be a direct consequence of the hetero- geneous nature of the catalyst system.That the surface of the catalytic system plays a large part in controlling the stereochemistry of the propaga- tion cannot be disputed since no homogeneous Ziegler-type catalyst system which will produce stereoregular polymers of or-olefins has so far been reported. However surface effects cannot be the sole stereoregulating force as will be seen below. Natta and his collaborator^^^^^ have shown convincingly that the crystal size and type of the reduced transition-metal halide and the reducing agent play very important roles in controlling the rate molecular weight and stereochemistry of the polymerisation (Table 4; ref. 92); they also showed that the heterogeneous catalyst can actually change its particle size during the polymeri~ation.~~ All of this is entirely consistent with a surface-controlled heterogeneous reaction and consequently kinetic studies are not readily carried out with any degree of reproducibility.However several kinetic studies have been made and it 88 Lewis and Rundle J. Chein. Phys. 1953 21 986. 89 Palmer and Elliot J. Amer. Chem. SOC. 1938 60 1852. 91 Natta J. Polymer Sci. 1955 16 143; Natta Pino Corradini Danusso Mantica Mazzanti and iMoraglio J. Amer. Chem. SOC. 1955 77 1708. 92 Natta Pino Mazzanti and Langi Guzzetta 1957 87 570. 93 Natta Pasquon and Giachetti Angew. Chem. 1957 69 213; Mukramol. Chem. 1957 24,258. Ziegler Belgian P. 533,362/1954. 394 QUARTERLY REVIEWS will be sufficient here to report the findings of Natta Pasquon and Giachettig3 in a study of the polymerisation of propene in the presence of triethylaluminium and the crystalline a-form of titanium trichloride.93 The trichloride crystals were reduced by grinding until the average particle size was less than 2p. Heptane was used as solvent and the polymerisation temperature was varied from 31 O to 70". The rate of polymerisation was then proportional to the amount of solid catalyst (TiC13) indepen- dent of the concentration of triethylaluminium independent of the atomic ratio Al:Ti and proportional to the partial pressure of the olefin. The activation energy measured by means of the reaction rate was 11,500 & 500 cal./mole and the most important chain-termination process at temperatures below 80 O and pressures slightly above atmospheric was a process that of the first order with respect to monomer and was most likely to be a chain transfer with the monomer.These observations are in general agreement with those of other workers and it is pertinent to point out that polymerisation of propene in heptane with a triethylaluminium- titanium tetrachloride catalyst was reportedg4 to have an activation energy of 5.7 kcal./mole while the polymerisation of styrene in the same solventg5 with a triethyl aluminium-titanium trichloride catalyst had an overall activation energy of 9.4 kcal./mole. Natta Pasquon and Giachettig3 concluded that the kinetics of the polymerisation of propene leading to atactic and isotactic polymers were essentially the same. Molecular weights TABLE 4. Influence of the transition-metal compoundg2 and metal alkyl on the yield of crystalline polypropene. Compound polymer TiCI TiBr TiCl a y or 6 ZnC1 vc1 c~cI VCl VOCl Alkyl polymer AlPr m u n 3 mu' m t 3 AlEt3 AlMe,Cl BeEt MgEt2 ZnEt NaR AlEt Y Crystalline Polymer (%I 48 51 30 60 42 80-92 94-96 78-85 30-40 0 40-50 55 73 36 48 32 96-98 94 Kodama Kagiya Machi Shimidzue Yuasu and Fukui J.Appl. Polymer Sci. gs Burnett and Tait Polymer 1960 1 151. 1960 3 20; see also Feldman and Perry J. Polymer Sci. 1960 46 217. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 395 in this system were found to be independent of the polymerisation time and so it was assumed that in the condition specified the life of a growing polymer chain was a fraction of a minute at 70”. Monoalkylaluminium dihalides and titanium trichloride do not provide stereospecific catalysts for a-olefins but if these catalysts are linked with 0.5 mol.of certain electron-donors or ’onium salts highly specific catalysts are produced (see Table 5).96 TABLE 5. InzJEuence of additive to TiCl (a y or 6)-A1EtX2 catalyst on yield of polypropene at 70”. Pyridine + 2AlEtBr NEt + 2AlEtC1 NHEt + 2AlEtC1 NBu41 + 2A1EtBr2 NBu4Br + 2AlEtC1 Stereo-index (% not extractable by ether or boiling heptane) > 98.5 95 93 > 99 96 Although the major investigations on the influence of catalyst structure have been carried out with propene most of the other a-olefins (straight- and branched-chain) and styrene and substituted styrenes have been polymerised by analogous catalyst systems. The properties of many of these olefins some which are of technical importance have been sum- marised in the literat~re.~’ Though the polymers from the a-olefins usually have the isotactic structure the synthesis of syndiotactic polypropene has recently been announced although no preparative details are yet a~ailable.~ Other than with the a-olefins the most successful application of the co- ordination catalyst has been in the polymerisation of butadiene isoprene and other conjugated dienes.The results in Table 6 show the highly specific nature of the catalyst and the large variation in 1,2- or 1,4-enchainment with change in ratio of components and in catalyst species. Co-ordination catalysts have also been used to afford polymers of cis- and trans-2,3-dimethylbutadiene piperylene and trans-polypenta- 1,4-diene. Of the several stereoisomers of the polypenta-l,4-dienes only the isotactic trans- 1,4-polymer has been synthesised.This has an amorphous structure owing presumably to disorder in the configuration of the tertiary carbon In addition to affording polymers of high molecular weight 1,3-diolefins and certain Ziegler-type catalysts can also react to give 01igomers.~~ Thus O6 Natta Pasquon Zambelli and Gatti J. Polymer Sci. 1961,51 387. O7 Cooper “Progress in Polymer Chemistry,” Heywood and Co. London 1961 O8 Natta Porri Corradini Zanini and Ciampelli J. Polymer Sci. 1961 51 463. p. 302. Wilke J. Polymer Sci. 1959,38,45. 4 396 QUARTERLY REVIEWS TABLE 6 . Catalyst systems for conjugated dienes. Catalyst TiCI,-AlR, A1 :Ti < 1 Al:Ti> 1 TiCl,( a)-AlEt TiCl,(P)-AlEt TiCl,(/?)-AlR2Cl TiBr4-A1Bui Ti14-AIR3 Co stearate-AlR,Cl Butadiene CoC12-AlR&l VCI3-AIR3 VOCl,-AlR VCl4-AlR3 V(AcAc)*,-AlR, Al :V-4 Ti(OBu),-AlEt (unaged) (aged) Al:V-10 MOO~(OR),-AIR~ A1:Mo < 6 Isoprene TiC1,-AIR, Al:Ti > 1 AI:Ti < 1 TiCl,( a)-AlEt VC13-AlEt3 V(AcAc),-*AlEt cis- 1,4 6 21-57 57 37 55-60 88 93-94 96-97 98 - - - - - - - 95 - - - - Modes of addition (%) tram- 1,4 91 3 1-69 87 60 36-41 3 2.5 1 99 97-98 97-98 0-10 1 -5-2 - I - - 95 91 99-100 - 172 3 2-1 1 8 3 4 9 1-1.5 1 1 2-3 2-3 99-100 1-3 60 75 45-5 - - - - butadiene reacts exclusively by 1,4-addition to form cyclododecatrienes in over 90 % yield.2,3-Dimethylbutadiene on the other hand becomes linked together by hydrogen transfer to form open-chain trimers in 70-80”/ yield. Isoprene reacts by both mechanisms. The actual number of mechanisms proposed or considered8*> for Ziegler-type polymerisation is surpassed only by the number of catalyst variations employed and before going on to consider some of these mechanisms it is convenient here to describe the well-established “Aufbau” reaction (i.e.chain extension) of alkylaluminium compounds. Ziegler and his collaborators have made extensive studies of the reaction between alkylaluminium compounds and a-olefins especially ethyIene.69~100 These reactions require relatively high temperatures and pressures but are easily typified as follows * AcAc = Acetylacetone. loo Ziegler Anpew. Chern. fiternat. Edn. Sample Tssue May 1961 Number 0 p. 1 “Organo-Metallic Chemistry ed. Zeiss Reinhold Publ. Corp. New York 1960 p. 220. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 397 R,AI + CH2=CH2 + R2AI.CHZ*CH2R R2AICHaCH2R + nCH2- CH -+ R2AI*[CH2*CH2],+,*R R2AI.[CH2.CH2],+,.R + mCH,= CH2 -f R.[CH2CH2] n+1-AIR.[CH2CH2] m-R etc.In this manner it is possible to grow three polyethylene chains simul- taneously from the same aluminium atom. A similar reaction was described by Bawn Ledwith and Matthies71 who studied the polymerisation of diazoalkanes especially diazomethane using a variety of boron compounds as catalyst and found that when trialkyl borons were used the overall reaction could be represented by R,B + CH2N2 + R2B.CH2R + N R2B*CH2R + nCH2N2 -+ R2B*[CH2],+,*R + nN2 R,B.[CH,],+,.R + rnCH2N -+ R-[CH,].+,.BR-[CH,] m.R This reaction and the Ziegler “Aufbau” reaction can thus be considered as prototypes for the stepwise polymerisation of or-olefins in Ziegler-type polymerisations. Although aluminium trialkyls 2re known to be dimericSS in hydrocarbon solution [see e.g.(XXVI)] Ziegler has repeatedly pointed out that the (XXVI) “Aufbau” reaction does not occur with the dimerised aluminium alkyls since these are no longer sufficiently electr~philic.~~~ This point is of considerable importance since many workers (see below) have assumed that the active Ziegler-type catalyst is a stable complex formed between alkyl- aluminium derivatives and reduced transition-metal halides. As mentioned earlier these complexes would be formally analogous to the dimeric aluminium alkyls and therefore any growth reaction in or from such a complex could not involve the aluminium atom since this already has a complete electron shell. However the titanium atom with its vacant d-orbitals could still be the centre of chain growth even in a stable complex.Since the majority of publications concerning stereospecific Ziegler- type polymerisation have come from Natta’s laboratories it is appropriate to consider first the mechanism proposed by these workers. Natta believess5 that the initially incomplete co-ordination of titanium in the reduced di- and tri-chloride facilitates chemisorption of organometallic com- pounds of strongly electropositive metals having a small ionic radius (aluminium beryllium magnesium). This chemisorption leads to the formation of electron-deficient complexes between titanium and the other metal which contain alkyl bridges similar to those present in dimeric aluminium and beryllium alkyls.** Although it is now quite clear that lol Ziegler Angew. Chem. 1959 71 623 ; Tnternat Conference on Co-ordination Chemistry London 1959 Chern.Soc. SpecialPubl. No. 13,1959 p. 1. 398 QUARTERLY REVIEWS similar systems containing only titanium atomsS7* lo2 can be active catalysts for ethylene polymerisation (see below) Natta firmly believes that in the complexes containing titanium and aluminium the polymeric chain grows from the aluminium-carbon bond and not from the titanium-carbon bond.lo3J04 Many workers however disagree with this idea believing that chain growth occurs at the titanium-carbon bond and consequently it is relevant at this point to summarise the evidence which Natta uses in support of his mechanism. The isolation of soluble crystalline complexes of the general formula (C5H5)2TiC12AlRR’ which are known to contain titanium-carbon-aluminium bridges obviously lends weight to Natta’s theory of a bimetallic electron-deficient complex.103*104 However the activity of these complexes is controlled to some extent by the oxidation state of the titanium,lo5 and this point is considered in detail later.It has been established that in polymerisation of ethylene with catalysts derived from dichlorobiscyclopentadienyltitanium (C,H,),TiCI, and triphenyl- aluminium the polymeric chains contain phenyl end groups.103~104 Terminal phenyl groups cannot be detected however when catalysts derived from biscyclopentadienyldiphenyltitanium (C 5H5) 2Ti( C6H 5) and triethylaluminium are used in comparable conditions.103 Natta and his collaborators have further shown that when triethylaluminium containing 14C-labelled ethyl groups is adsorbed into the surface of a-titanium trichloride and when this whole system is used as a polymerisa- tion catalyst in the presence of non-labelled triethylaluminium then the resulting polymer contains as terminal groups practically all of the labelled alkyl groups.1o6 Obviously all these ethyl groups could not have been in- volved in alkylation of titanium and in addition chemical analysis of the resulting polymers showed the presence of aluminium.107 According to Natta and Mazzantilo4 the latter result is probably due to a “dissociation” H,C= I CH X H+HX lo2 Natta Pino Mazzanti and Lanzo Chimica e Industria 1957 39 1032.lo3 Natta Pino Mazzanti Gianini Mantica and Peraldo Chimica e Industria lo4 Natta and Mazzanti Tetrahedron 1960 8 86. lo5 Breslow and Newburg J. Amer. Chem. SOC. 1959 81 81. lo6 Natta Pajaro Pasquon and Stellacci Rend.Accad. Naz. Lincei 1958 24 479. lo’ Natta Giuffre and Pasquon Rend. Accad. Nuz. Lincei 1958,25,417. 1957 39 19; J. Polymer Sci. 1957 26 120. BAWN AND LED WITH STEREOREGULAR ADDITION POLYMERISATION 399 of each catalytic complex into two parts one containing the titanium atom the other containing alkyl groups including polymeric chains bound to aluminium. This dissociation is followed by reassociation of the titanium “fragment” with other aluminium alkyls present in the solution. Natta’s mechanism can then be formulated as in the annexed scheme. This mechanism is in full accord with the kinetic data obtained by Natta Pasquon and Giachettig3* loa and provides a very satisfactory explanation for the experimental observation that the rate of certain transfer processes and the rate of chain propagation are both of the same order with respect to monomer and behave as if they were two parallel reactions proceeding through the same transition state.The driving force for propagation is of course the primary co-ordination of olefinic n-electrons with vacant hybrid orbitals in the metal (see below). Recently however Carrick and Karapinkalog have re-investigated the use of biscyclopentadienyldiphenyltitanium as a catalyst for the polymerisation of ethylene and although they confirm the findings of Natta ef al. ie. that no phenyl groups are incorporated into the polymer they concluded that this does not constitute any real evidence for growth on aluminium rather than on titanium. It appears that in the reaction of biscyclopenta- dienyldiphenyltitanium with alkylaluminium derivatives there is an immediate exchange of alkyl and aryl groups between aluminium and titanium.Chain growth then occurs more readily at alkyl-substituted metal atoms than at phenyl-substituted sites. Furthermore it was also shown that the n-bonding of the cyclopentadienyl compounds is dis- rupted wholly or in part during the polymerisation and that at least part of the catalyst reactivity is due to the decomposition products. Patat and Sinn1lo~ll1 have proposed a mechanism which is essentially the same as that envisaged by Natta and has been severely criticised by ZieglerlOl on the grounds that by analogy with dimeric aluminium alkyls electron-deficient complexes involving titanium and aluminium are not likely to be involved in the actual “growth” reaction even though such complexes may occur in polymerising systems.Ziegler’s objection is both forceful and plausible in view of his experimental findings which are discussed in the next paragraph. At this point however it is pertinent to point out that any such objections to Patat and Sinn’sllO mechanism apply equally to Natta’s and to other similar suggestions even though Ziegler confined his criticism exclusively to the mechanism proposed by the former authors. ZieglerlOl has frequently referred to a Belgian patent of the American Goodrich Gulf Company which describes112 the catalysis of a normal- pressure growth reaction due to diethylaluminium chloride by a trace of titanium tetrachloride in xylene solution. This system is obviously closely lo* Natta Pasquon and Giachetti Makromol.Chem. 1957,24,258. lo@ Carrick and Karapinka J. Polymer Sci. 1961 55 145. n1 Patat and Sinn Naturwiss. 1958 45 312. 112 B.P. 553,721/1955. Patat and Sinn Angew. Chem. 1958 70 496. 400 QUARTERLY REVIEWS related to typical Ziegler-type catalysts for polyethylene formation yet in this case the amount of titanium necessary is much too small to yield an appreciable quantity of organotitanium compound. Ziegler goes on to point out that the function of highly reactive organotitanium compounds will be considerably assisted (although sometimes obscured) by alkyl exchange with alkyl aluminium molecules present in the solution. Thus we can envisage equilibria such as Al-alkyl + Ti-alkyl + AITi-alkyl complex Alkyl exchange occurs readily in the complex but the actual growth reaction could take place only at the monomeric titanium alkyl.Although Ziegler has not yet formulated a specific mechanism or structure for growth and alkyl-exchange it is the opinion of the Reviewers that most of the proposed mechanisms discussed below can be reconciled on the basis of the general picture proposed by Ziegler. In 1958 Ludlum Anderson and Ashby113 suggested that ethylene poly- merisation initiated by titanium tetrachloride-trialkylaluminium catalysts proceeded by co-ordination of ethylene with a bivalent titanium alkyl compound i.e. CITi.[C,H,];R + C,H ClTi [C,H,],.R -+ CITi-[C,H,],+,-R CH,LCH The bivalent titanium compound was formed by reduction of the added tetrachloride by the alkylaluminium. That a bivalent species was necessary for polymerisation seemed evident from a consideration of the variation in catalytic activity with the measured average valency state of titanium in the catalyst complex.More recently Carrick et a1.l1* and Badin115 have confirmed the high activity of bivalent titanium species in typical Ziegler- type catalyst preparations. The former workers also showed conclusively that when vanadium compounds are used instead of titanium once again the lower-valency states are necessary with V2+ the most active. In all the above cases there is much evidence that changing the nature of the reducing agent e.g. trialkylaluminium alkyl-lithium or lithium aluminium hydride does not significantly alter the nature of the propagating species but changing the transition metal has a most pronounced effect. This is best illustrated by the work of Carrick et aZ.lle who showed that the relative reactivity ratio of two monomers ethylene and propene in copolymerisa- tion changed when different transition-metal compounds were used but was not affected by use of different reducing agents with a common transi- tion-metal compound.Such evidence clearly indicates that chain growth occurs at the transition metal although there are still no definite indications 113 Ludlum Anderson and Ashby J . Amer. Chem. SOC. 1958 80 1380. 114 Carrick Chasar and Smith J . Arner. Chem. Soc. 1960 82 5319. lle Carrick Karol Karapinka and Smith J . Amer. Chern. SOC. 1960,82 1502. Badin J. Phys. Chem. 1959 63 1791. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 40 1 whether a transition-metal reducing-agent complex is catalytically active or not.Stable organotitanium compounds have been well characterised during the last few years although most of the known compounds are derivatives of quadrivalent titanihm and decompose at appreciable rates even at room t e m p e r a t ~ r e . ~ ~ - ~ ~ ~ The products of this thermal decomposition often include free-radical fragments although the exact mechanism of decomposition is not yet very clear. Tri-isopropoxyphenyltitanium on decomposition initiates free-radical polymerisation of styrene but does not initiate polymerisation of ethy1ene.lls On the other hand alkyltitanium trichlorides (alkyl = Me or Et) can be used to initiate polymerisation of ethylene but only after considerable thermal decomposition.87 The latter decomposition will of course produce solid titanium trichloride and perhaps lower- valent titanium compounds by the annexed reactions.RTiCI -f R. + TiCI TiCI + RTiCI 3 RTiCla + TiCI RTiCI -+ R- + TICI RTiCI + TiCI -+ RTiCl + TiCI, etc. In addition it is plausible to envisage complexes such as (XXVII) between RTiC13 and reduced titanium halides which also might be catalytically active. The obvious thermal instability of higher-valent organo-transition- metal compounds appears to rule them out as active catalysts by themselves but it is equally obvious that lowering the valency state of the metal will greatly increase the stability of the metal-carbon bond. Carrick et al.ll* have shown that a decrease in the metal valency causes an increase in the polarity of the metal-carbon bond. This follows since decreasing the transitior inetal valency decreases the electronegativity of the metal e.g.the electronegativity of titanium changes from 1.6 in the quadrivalent to 1.1 in the bivalent state whilst that of vanadium changes from 1.8 in the quinquevalent to 1.2 in the bivalent state. Clearly then it is possible to have a stable organo-transition-metal compound which could be the principal active species in Ziegler-type catalyst systems. The idea that an alkylated reduced halide of titanium (or other transi- tion metal) is the active catalyst has received much support of late and is further borne out by the recent findings that alkyltitanium halides can 11' Herman and Nelson J . Amer. Chem. SOC. 1953 75 3877 3882. 11* North Proc. Roy. Soc. 1960 A 254,408. 402 QUARTERLY REVIEWS themselves act as catalysts for polymerisation of ethylene ;*' also active catalysts can be formed by ultraviolet irradiation or electron bombardment of reduced titanium halides in the presence of ethylene.119J20 Most of these ideas are incorporated in the comprehensive mechanism proposed by Cossee.121 This mechanism will be described rather fully here since it appears to have special merit in its explanation of stereoregular polymerisation at a crystalline surface.Cossee121 assumed that polymerisa- tion occurs at one titanium ion in the surface layer of a titanium trichloride (or presumably dichloride) lattice of which one surface chlorine atom is replaced by an alkyl group R while an adjacent chlorine site is vacant in order to accommodate the incoming monomer molecule. Polymerisation then proceeds in a manner similar to that suggested by Ludlum Anderson and Ashby,l13 as shown in Fig.15. FIG. 15. After this step the active centre retains a titanium-alkyl bond and a vacant site which however have changed places. During the polymerisa- tion the aluminium alkyl is thought to act as a chain-transfer agent in re-establishing lost centres and as a scavenger for adventitious impurities but it is not essential for the propagation. Such a mechanism has con- siderable merit and provides an adequate explanation for several important experimental observations. First however it permits a plausible picture of the driving force of the reaction. This is represented by Cossee as a 7-type olefin complex similar to the platinous-olefin complexes ;122 a diagrammatic representation of the orbital overlap is shown in Fig.16. The .rr-electrons of the olefin overlap with the vacant dxe-ya orbital of titanium forming a transitory n-bond. Simultaneously the d,,-orbital of the metal can over- lap with vacant antibonding orbitals in thde olefin and consequently decrease the distance between the highest filled bonding orbital and the empty (or nearly empty) d-orbitals of r,,-type. Chatt and have shown that such a situation in transition-metal alkyls leads to a weakening of the 119 Schustze Suttle and Watson Belgian P. 551,330/1956. 120 Oita and Nevitt J. Polymer Sci. 1960 43 585. lZ1 Cossee Tetrahedron Letters 1960 No. 17 12 17; Trans. Faraday SOC. 1962. 58. 122 Chatt and Duncanson J. 1953 2939. 123 Chatt and Shaw J. 1959 705. 1226 see also Fontana and Osborne J. Polymer Sci. 1960,47 523.BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 403 I Y -fl Open W The rr-bond between titanium and ethylene. FIG. 16. circle titanium. Full circles carbon. Broken circles hydrogen. carbon-metal bond and consequently facilitates migration of the alkyl group to one end of the in-going olefin molecule. In order to show the desired catalytic effect the size of the empty d-orbitals of the metal must enable them to overlap sufficiently with anti- bonding orbitals of the olefin. Therefore only ions with a comparatively low effective nuclear charge are expected to be good catalysts. It is well understood that the crystal structure of the solid phase is of direct im- portance for stereospecific polymerisations in this type of catalyst and Cossee goes on to show how his mechanism takes account of this fact.Fig. 17 shows a three-dimensional schematic picture of a titanium tri- chloride lattice with the active titanium ion carrying the alkyl group R and FIG. 17. Part of the a-titanium trichloride lattice showing an “active centre” with a monomer propene molecule in the chlorine vacancy. Small circles titanium. Large open circles chlorine. Large full circles alkyl. 4c4 QUARTERLY REVIEWS FIG. 18. Z Y-Cross-section through reacting monomer. FIG. 19. XZ-Cross section through reacting monomer. a molecule of propene inserted in the chlorine vacancy in the right orienta- tion to form a n-bond with the active titanium ion. Figs. 18 and 19 are cross-sections through the mmomer in the chlorine vacancy parallel to the YZ- and the XZ-plane respectively of Fig.16. In these diagrams the van der Waals radii are drawn to scale in order to show how the monomer molecule exactly fits into the chlorine vacancy. Fig. 19 shows clearly that there is only one possibility for insertion of the propene molecule i.e. with the = CH2 group pointing into the lattice. This would explain why the polymerisation is exclusi\ ely head-to-tail and also allows for only two different orientations of the monomer one as shown in Fig. 19 the other BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 405 with the C=C bond horizontal. Presumably the former configuration will be preferred since this involves minimum steric interactions with other titanium ions in the lattice. Repetition of this process clearly leads to isotactic polymers as shown in Fig.20. It is clear from the diagrams that FIG. 20. The situation after migration of the alkyl group according to the reaction path indicated by the arrow in Fig. 17. The next propene monomer is placed in the new chlorine vacancy. Reaction path of second alkyl group is indicated by the arrow. Small circles titanium. Large open circles chlorine. Large full circles alkyl. only a-olefins may be polymerised also that the other carbon atom of propene will obtain a well-defined configuration when cis- or trans-l- deuteropropene is polymerised. Such a result is already indicated by Natta Farina and Peraldo’s findings in the field of di-isotactic poly- mers.124~5 Optical microscopical e~arninationl~~ of active crystalline titanium trichloride catalysts shows that chain growth occurs only at the edges of the crystal structure and such a result is in accord with the mechanism proposed by Cossee.The stereospecific polymerisation of a-olefins at a surface has been explained in general terms by Cram and K ~ p e c k y ~ ~ on the basis of steric repulsion between the monomer substituent and the substituent in the penultimate monomer unit (see schematic illustration in Fig. 21). The basic requirement of this mechanism is that the growing polymer chain protrudes from the edge of the catalyst surface and whilst it is insufficient by itself this mechanism can easily be superimposed on any other so far proposed and it should be considered as complementary to any other mechanism. In addition the idea of chain growth at the edge of a catalyst surface is in complete agreement with recent electron microscopical examination of an 124 Natta Farina and Peraldo Rend.Accad. Naz. Lincei. 1958,25,424. 125 Hargitay Rodriguez and Miotta J. Polymer Sci. 1959 35 559. 406 QUARTERLY REVIEWS R ..v R ..H \ * c‘* p,’ ‘c< ‘CH,-MX,MY~ FIG. 21. Cram and Kopecky’s scheme. The catalytic edge of the metai acts as a buttress; it is tilted down towards the hj7droTen attached to the asymmetric carbon atom thus the incoming monomer is best able to react $R is on the same side as H of the asymmetric carbon atom. (A) lsotactic polymer. P > R > H in bulk. activated titanis m trichloride crystal made during polymerisation of eth~1ene.l~~ That stereospecificity might be induced entirely by steric repulsions as indicated was also proposed independently by Furukawa and Tsuruta.126 Cf the other mechanisms proposed for Ziegler-type polymerisation by far the most plausible are those involving ion-pair structures for the catalyst complex such as those which were first proposed independently by U e l ~ m a n n l ~ ~ and Bier.128 It is well established that al~.mini~ m alkyls and titanium tetrachloride form a deep red ionic complex at -78”.This complex has been f~rm’cllated,~~~ and assumed to decompose as follows AIRS + TiCI -f [TiCI,]+[AIR,CI]- -+ RTiCI + AIR,CI - 780 - 30” Titanium trichloride (or dichloride) might be expected to react similarly to produce ion pairs such as [TiCl,]+[AlR,CI]-. Essentially the various ion-pair mechanisms can then be typified by the mechanism proposed by U e l ~ m a n n ~ ~ ~ i.e. TiCI + AIR + ~iCl,]+[AIR,CI]- -+ TiCI,CH,CH,+ [AIR,CI]- -+ TiCI,.CH,CH2R + AIR,CI CH,= CH TiCI,.CH,CH,R + AIR2CI + [TiCI,]+[AIR,CICH,-CH,R]- etc.The Reviewers believe that such a mechanism might well serve as a basis for the rationalisation of most other mechanisms for the following reasons (a) The actual chain growth takes place by co-ordina- tion of the monomer with the transition metal and thus accommodates the findings of Carrick116 and of Ludlum et. al.l13 (b) The aluminium 126 Furukawa and Tsuruta J . Polvmer Sci. 1959 36 275. 1,’ Uelzmann J . Org. Chem.. 1960 25 671 ; J . Polymer Sci. 1958 32 457. 12* Bier Kmststofe. 1958 48 354. 129 Uelzmann J. Polymer Sci. 1959 37 561. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 407 alkyl is essentially a chain carrier and this could account for the conclusion of Natta et aZ.lo3 with regard to end groups and in addition dispose of the objection that titanium-alkyl bonds are not sufficiently stable to exist for the lifetime of a growing polymer chain.(c) Mechanisms based entirely on titanium alkyls in a crystal lattice e.g. that of Cossee,lZ1 would require only slight modification to incorporate an ion-pair pro- pagation since the fundamental reaction between monomer and reduced titanium halide cation would be essentially the same as that proposed for the neutral alkylated titanium halide molecule. (d) Ion-pair mechanisms of this type adequately explain the enormous catalytic influence of small amounts of titanium halides on the “Aufbau” reaction of aluminium alkyls. The latter point has been suggested by Ziegler.lol So far nothing has been said of the mechanism of diene polymerisation using Ziegler-type catalysts but obviously the various catalyst structures proposed could easily account for the observed diene microstructure along the lines discussed earlier for alkyl-lithium catalysts provided one assumes that under normal conditions the effective Ziegler catalyst behaves essentially as an intimate ion-pair similar to that encountered when using lithium alkyls in hydrocarbon media.Although most Ziegler-type catalysts are heterogeneous in nature there are several well-known homogeneous catalyst systems for polymerisation of ethylene which are regarded as being also Ziegler-type catalysts.130 Most of these are formed by reaction of aluminium alkyls with cyclo- pentadiene “sandwich” complexes of titanium.131 The overall reaction can be represented (C,H,),TiCI + AIR + (C,H,),TiCI,AIR (R = alkyl or chlorine) These complexes are stable hydrocarbon-soluble crystalline compounds and were first reported by Breslow and Natta and their respective collabora- tors.lo3-lo5 As written above the complexes contain titanium in the terval- ent state but Breslow and Long132 have shown that much higher catalytic activity.is obtained when the soluble complex is oxidised by admission of a little oxygen together with the ethylene.In these conditions the complex is thought to give rise to a product containing titanium in the quadri- valent state and this could be represented as e.g. (C,H5),TiEtC1,EtAlCl,. At any rate all these various complexes which could easily be written in the form of ion-pair structures are active only towards ethylene p l y - merisation although they undoubtedly relate to more usual Ziegler catalysts in the mechanism of ethylene polymerisation.Perhaps the very fact that such complexes can be isolated lends additional weight to the ion-pair mechanisms rather than to those depending entirely on titanium alkyls. 130 Bawn and Symcox J. Polymer Sci. 1959 34 139. 131 Drucker and Daniel J. PoZymer Sci. 1959 37 553. Breslow and Long J. Amer. Chem. SOC. 1960 82 1953. 408 QUARTERLY REVIEWS Stereoregular Polymerisation of Olefin Oxides and Aldehydes Olefin Oxides.-Three- four- and five-membered cyclic ethers can all be polymerised to material of high molecular weight with various catalyst systems and this subject has been reviewed recently by Ea~tham.1~~ The only such stereoregular polymerisation so far studied in any detail is that of propylene oxide.In 1955 Pruitt and Baggett134 reported that catalysts prepared from ferric chloride or ferric hydroxide are capable of converting propylene oxide into a new crystalline polymer of high molecular weight. Propylene oxide contains an asymmetric carbon atom in the monomer and also in the addition polymer as illustrated in formulz (XXVIII-XXX). (XXVIII) (I?) (XXIX) (s) (xxx) (s) Optically active forms of (XXVIII) (R)- and XXIX) (S)-propylene oxide and (XXX) the Price and OsgarP5 extended the work of Pruitt and Baggett and showed in the following manner that the crystalline poly(propy1ene oxide) was actually isotactic polypropylene. Optically active (-)-propylene oxide was synthesised and polymerised with solid potassium hydroxide as initiator.This was thought to be a conventional anionic polymerisation in which the CH,-0 bond is broken i.e. isotactic (&polymer * HO’ + H2C-?HMe - HO-CHCCHMe 0- \ / 0 CH,-HW I ‘O HO.CH~-~HM~-O*CH~~HM~O- etc. Consequently there is no possibility for loss of optical activity and Price and O ~ g a n l ~ ~ obtained an optically active but highly crystalline polymer of m.p. 55-5-56.5 ’. When racemic propylene oxide was polymerised under identical conditions the product was a syrupy liquid thus demonstrating the remarkable effect of stereoregularity on physical properties. When (-)-propylene oxide or its racemate was polymerised in ether at 80° with a ferric chloride catalyst essentially the same product was obtained from both monomers except that the crystalline portion of the polymer from (-)-oxide was optically active.The crystalline polymers obtained in this Eastham Fortschr. Hochpolym. Forsch. 1960 2 1. 184 Pruitt and Baggett U.S. P. 2,706,181/1955. 136 Price and Osgan J. Amer. Chem. SOC. 1956,78,4787; J. Polymer Sci. 1959 34,153. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 409 manner had a higher melting point (70") than the crystalline polymer obtained by using potassium hydroxide as catalyst but it was thought that this difference was due entirely to the great difference (a hundred-fold) in molecular weight between the two crystalline products. Support for the view that these polymers differ only in molecular weight comes from their X-ray diffraction patterns their infrared spectra and their optical rotation.Powder X-ray patterns for all the samples of crystalline polymer active and racemic were identical and indicated an extremely high degree of crystalline order. The infrared curves for all samples of crystalline polymer were indistinguishable and the high- and low-melting active polymers had the same molar optical rotation within the limits of experimental error. The fact that ferric chloride-catalysis of either the active or the inactive oxide gave mixtures of amorphous and crystalline polymer virtually indistinguishable except for optical activity has much influence on any proposed reaction mechanism. This type of polymerisation has also been studied extensively by Colclough Gee and their collaborators136 in Manchester. These workers showed that the product of the initial reaction between ferric chloride and propylene oxide was a monochlorodialkoxide in which 4-5 mol.of propylene oxide are combined It is now thought that this or a related product is the actual catalyst for the stereoregular polymerisation of propylene oxide and that it normally yields a mixture of crystalline and amorphous fractions. Further it was shown that the yield of crystalline polymer is controlled by the presence of water so that the proportion of crystalline to amorphous product in- creases smoothly from 0.13 with no added water to 0.86 with a molar ratio H,O:Fe = 1.8. With slightly higher water ratios the catalyst is precipitated. If water is added slowly to a preformed solution of formu- lated complex a heterogeneous catalyst is obtained which appears to be exclusively ClFe(OH) ,with only a small residue of alkoxide.That the stereoregular polymerisation of propylene oxide proceeded at a heterogeneous surface was first pointed out by Price and O ~ g a n l ~ ~ who proposed a mechanism which is almost identical with that proposed by Cram and K ~ p e c k y . ~ ~ It is assumed that the essential propagation reaction consists of the stepwise growth of a ferric alkoxide chain i.e. :Fe.OR + H,C-CH.CH -f :Fe-O-OCH,.CHMe.OR 'o/ :Fe.OCH,CHMe-OR + n(H,C-CHCH,) -+ :Fe.[OCH,.CHMe],+,.OR '0' The product can be crystalline (isotactic) or amorphous depending upon whether the ferric-alkoxide bond is part of a heterogeneous surface or Colclough Gee Higginson Jackson and Litt J. PoZyrner Sci. 1959 34 171, 410 QUARTERLY REVIEWS whether the system is truly homogeneous respectively.Usually a mixture of both would be obtained. Stereoregularity can then be assured by assum- ing that transition state shown below in which P,>Me>H in bulk (P,=growing polymer chain). The surface causes the transition state to be more compressed and requires that the steric repulsion between the in- coming and the final monomer unit in the chain be minimised by ensuring that the methyl substituent of the incoming propylene oxide molecule is trans to the methyl group of the previous monomer unit. Such a con- formational approach also ensures that steric repulsion between the in- coming monomer and the large bulky growing polymer P is at a minimum as illustrated in (XXXI). (XXXI) A previous mechanism proposed by Corey13’ requires that throughout the polymerisation the iron atom maintains an octahedral disposition of its valencies and that this is achieved by retaining all three chlorine atoms (in FeCl,) attached to the metal.Since the latter condition seems most improbable in view of recent experimental results the mechanism is not discussed further although the possibility of an octahedral iron complex formed during propagation cannot yet be ruled out. Colclough Gee and J a g g e ~ l ~ ~ have also proposed a mechanism similar to that outlined above but they make the additional and most important comment that ferric alkoxides are known to be highly associated in non- polar solvents and that the catalyst would therefore have a structure similar to that (XXXII) of simple ferric alkoxides. However in the case of 0 Fe/ >C R’o-CHMe R (XXXI I) (xxxr It) ClFe(.[OCH,CHMe];Cl) intramolecular chelation as in (XXXIII) can be expected to reduce the degree of association of the alkoxide catalyst.In addition the stereoregularity-promoting effect of water on the catalyst system may well be the direct result of increased association after partial 137 Corey Tetrahedron Letters 1959 No. 6 1. 13* Colclough Gee and Jagger J. Polymer Sci. 1960 48 273. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 41 1 hydrolysis of the ferric alkoxide. Although the heterogeneous catalysts [presumably based on ClFe(OH),] can undoubtedly produce isotactic polymers the latest work by Gee139 shows that the ferric alkoxide catalyst can be made highly stereospecific by partial hydrolysis and still remain dissolved in the polymerisation solvent (ether).This observation has led to the idea that perhaps the active catalyst contains Fe-0-Fe bonds formed by condensation of partially hydrolysed The polymerisation can then be represented as in alkoxide derivatives. the annexed scheme. 0 +O OR / \ 0 > Fe’ )Fe - 0 / \ ;f ‘T- --t \OR 4 H2C- CHMe H,C -’CH Me + H2C- CHMe cis-Opening of the epoxide ring would be assured by the forces of inter- action between the iron atoms and the various oxygen atoms as shown by the dotted lines. The actual stereochemistry is of course decided ultimately by the conformation of the in-going propylene oxide molecule and this will be influenced by the considerations outlined by Cram and K~pecky,~ and discussed above in connexion with their proposed mechanism. Further- more the present mechanism involves a type of SN2 reaction on the ferric alkoxide so that as with methyl methacrylate and isobutyl vinyl ether there will be increased restrictions on the conformation of the in-going monomer unit as it approaches the reaction centre.Catalysts based on iron alkoxides are by far the most effective for stereo- specific polymerisation of propylene oxide ; other catalysts have been employed but with very poor results. Of these the most important are aluminium alkoxides and alkyls and again the presence of water appears essential for high stereospecificity. Since the Lewis acid and associating properties of these substances are very similar to those of ferric alkoxide there seems little doubt that the mechanism of polymerisation will be similar to that eventually established for the ferric alkoxide polymerisa- tion.Vandenberg140 has recently shown that the aluminium alkyl-water catalyst is considerably improved if the aluminium alkyl is first treated with acetylacetone. The resulting chelate derivative (XXXIV) activated with water thus provides a very efficient catalyst for the stereoregular polymerisation of olefin oxides and in addition can give new amorphous 13@ Gee Trans. J. Plastics Inst. 1960 28 89. 140 Vandenberg J. Polymer Sci. 1960 47 486 489. 412 QUARTERLY REVIEWS polymers of high molecular weight from styrene oxide and 3,4-epoxybut-1- ene. Vandenberg has further demonstrated that this powerful catalytic system produced di-isotactic polymers from cis- and trans-2,3-epoxy- butane. Aldehydes.-Polymers of aldehydes other than formaldehyde have been extensively studiedlgl since the original independent discoveries of Letort142 and T r a ~ e r s l ~ ~ of the polymerisation of acetaldehyde.These workers discovered that when acetaldehyde was cooled below its melting point or when its vapour was condensed on a glass surface cooled at liquid-air temperature then warming the solid to room temperature produced a viscous liquid or gel. This rubber-like material was of high molecular weight and completely amorphous and depolymerised slowly at room temperature. Recently it has been found that acetaldehyde may be polymerised at temperatures well above its melting point by the aid of suitable catalysts to give both crystalline and amorphous polymers For example Furukawa et aPg4 found that activated alumina was an effective catalyst at -80".By analogy with formaldehyde it was expected and observed that boron trifluoride was a powerful catalyst. Thus the pure monomer and solutions in inert solvents such as toluene ethylene and propene were polymerised to products of high molecular weight by Lewis acids such as AICI, ZnCI, SbF, PSCI, AsF, AsCI, BF, and phosphoric and trifluoroacetic acid. These cationic catalysts gave a tacky rubber-like material similar to that obtained by polymerisation at low temperature. Novak and Whalleylg5 in 1959 succeeded in obtaining crystalline polymers of isobutyraldehyde and heptanal by operating at very high pressures and in the following year Natta et Furukawa et aZ.,14' and V0gl148 independently observed that crystalline polymers were formed by poly- merising them in the presence of small amounts of organometallic com- pounds and other types of anionic initiator.Thus initiation by a trialkyl- aluminium or triarylaluminium in toluene or heptane at -80" gave crystalline polymers of acetaldehyde n-butyraldehyde and propion- aldehyde. Natta Corradini and Bassi149 have described the structures of these polymers which are all isotactic and have a four-fold helical con- formation with a repetition period of 4.8 A. Furakawa et aZ.1449147 observed that stereo-block and crystalline polymers of acetaldehyde and other 141 Bevington Quart. Rev. 1952 6 141. 142 Letort Compt. rend. 1936 202 767; 1947,224 50; Letort and Mathis ibid. 1959 143 Travers Trans. Faradav SOC. 1936. 32 206. 144 Furukawa Saegusa Tsuruta Fujii Kawasaki and Tatano Makroml. Chenz. 145 Novak and Whalley Canad.J. Chem. 1959 37 1710 1718. 146 Natta Mazzanti Corradini. and Bassi Makronrol. Chern.. 1960 37 156. 14* Vogl J. Polvmer Sci.. 1960 46 261. lJD Natta Corradini and Bassi J. Polynier Sci. 1961 58 505. 249 274. 1959 33 32 1959. 36. 546. Furukawa Saeguqa and Fujii Makromd. Chem. 1961,4446 398. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 41 3 aldehydes were formed when aluminium oxide which itself gave polymers of high molecular weight was activated by metal alkyls such as dietbyl- zinc. On the other hand noted that lithium alkoxide gave a crystal- line polyacetaldehyde m.p. 165" which was insoluble in common solvents. In 1961 Furukawa et aZ.14' considered that the active species in the alkyl metal-initiated polymerisation was the metal alkoxide which is readily formed from the alkyl metal MR by reaction with the monomeric aldehyde RM + R'CHO -+ RR'CH-OM.This polymerisation is thought to be closely related to other well-known synthetic reactions (Meenvein- Pondorff etc.) of metal alkoxide and carbonyl compounds some of which are represented in the annexed formula. RM + Me.CHO - MOCHMeR / r' ,O-C.eR "0 = CH Me M ... \ ..H 0-CHMe R \O-CHMe \ Meewein- Ponndorff . - Opprnauu 0-CHMeR 4 P-CHMeR i ..6=CMeR '0-CHMe Me- /H (XXXVI) 0-CHMeR / .. O=CMe M - '0-CH,Me (xxx 1x1 0-~HMQ 1 M '0-CHMe 0-CH MeR I O*CHMeR 1 I :O-CHMe :x IO-CH M e:x I I ,0-C.Me 4 Chain O=CMe (XLI I) + M ... ..H transfer *' O= C'H Me M -O*CH$CH The Meenvein complex (XXXIV) is converted into (XXXV) by hydride ion transfer from the alkoxy-group to the carbon atom of the aldehyde.(Meenvein-Ponndorf reduction160). In the complex (I) the alkoxide anion lSo McGowan Chem. and I d . 1951,601. 414 QUARTERLY REVIEWS can also be transferred to the carbonyl group to produce a new metal alkoxide (XXXVII) with which another aldehyde forms a complex (XXXVIII). The hydride transfer in the complex corresponds to the Tischtschenko reaction151 which gives ester (XXXIX) and the metal alk- oxide. On the other hand transfer of the alkoxide anion in compound (XXXVIII) produces another metal alkoxide (XL). By repetition of two consecutive processes e.g. co-ordination of aldehyde and transfer of alkoxide anion the polymerisation continues to be propagated until hydride-ion transfer happens in the Meerwein complex of higher order (XLI) giving then a metal alkoxide and a polymer molecule with an ester end group.The mechanism of stereoregularity with metal alkoxide catalysts such as aluminium alkoxides is thought to be largely controlled by the known tendency of these alkoxides to form aggregates. The aggregates increase in size with decrease in temperature and co-ordination of monomer to aluminium atoms provides additional steric influence. Molecular Structures and Properties The difference in properties of the various stereospecific forms of polymers is considerable. In general the atactic forms are amorphous and non-crystalline and have low softening temperatures. Isotactic and syndio- tactic polymers show higher melting points lower solubilities higher crystallinities and markedly different mechanical properties. These re- markable differences are partly determined by the molecular arrangement in the crystals of these polymers and the understanding of this aspect has an important bearing on the much mider question of the relation between polymer properties and molecular structure.This problem has long been familiar to polymer chemists-the soft extensible natural hevea rubber and the stiff crystalline gutta percha the fibre-forming cellulose and the non-fibrous amylose are examples of isomeric polymers differing only in molecular architecture but having widely different properties. It is well established that polymer properties depend not only upon gross chemical structure but also upon the more detailed isomeric or stereoisomeric arrangement in the molecule. A major consequence of the regularity of the steric structure is that the polymer molecules may pack together into an ordered three-dimensional structure in which they display the properties of crystalline materials.This crystallisation phenomenon was of course well known previously for many polymers e.g. in polyethylene and polytetrafluoroethylene in which all the substituents were of one kind of atom or in poly(viny1idene chloride) and polyisobutene in which stereoisomerism is impossible. Polystyrene poly(viny1 chloride) poly(methy1 methacrylate) and most vinyl polymers as normally synthesised by free-radical or ionic reactions lSL Lin and Day J. Amer. Chem. SOC. 1952 74 5133. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 41 5 show largely an amorphous structure due to the random occrrrence in the chain of possible stereoisomeric structure.The stereoregular forms of these polymers which have now been synthesised (p. 419) show properties very markedly different from the randomly arranged structures e.g. isotactic polystyrene melts at 230" compared bith the atactic form at 85" and material of medium molecular &eight is insoluble in solvents such as ethyl methyl ketone which dissolve the atactic polymer. In order to understand the particular properties of the stereoregular polymer it is necessary to know the molecular arrangement in their crystals. Most of the results summarised in the following paragraphs were obtained by Natta Corradini and their co-worker~l~~ in a series of brilliant studies. Molecular Configuration of Stereospecific Polymers.-As the definition of a crystal implies a three-dimensional order a crystalline linear polymer must show at least a regularity in the succession of its monomer units.It has been observed that in all known structures of crystalline homo- polymers all the monomer units in a crystal occupy geometrically equivalent positions mith respect to each chain axis. From this equivalence postulate Natta and C ~ r r a d i n i l ~ ~ have shown that two different types of regular sequences may be expected for vinyl head-to-tail crystallisable polymers. Isotactic polymers are those capable of forming a helix-type crystalline structure ; syndiotactic polymers assume either a helical or a plane structure in the crystalline state. Provided the equivalence postulate is satisfied the conformation of the chain in the crystal may be assumed to approach the conformation of minimum potential energy for an isolated chain oriented along an axis (minimum potential energy postulate).Lateral packing effects betu een neighbouring chains are also important in determining the conform2tion of the chain but these generally play a less important role than the packing effects inside the chain i.e. van der Waals forces between the chains are such as to permit unstrained packing. These general rules are not always fully obeyed and in actual strictures the form is modified slightly by the necessity to fill the space uniformly. In the crystal the chains are arranged so that they are parallel to one another and spaced at such an inter- molecular distance that they fill as much as possible of any gap between them. Isotactic polymers. Tsotactic polymers are the cis-stereoisomers of vinyl head-to-tail polymers whereas syndiotactic polymers are in the trans-conformation.The minimum-energy configuration for polyethylene is the trans-planar zigzag since this ensures that the non-bonded atoms are not closer than the van der Waals contact distances.153 The first third fifth etc. carbon atoms are stacked 2.5 A apart and this is precisely the 152 Natta and Corradini J. Polymer Sci. 1959 39 29. 153 Bunn and Holmes Discuss. Faraday Soc. 1958 25 95. 416 QUARTERLY REVIEWS theoretical spacing for carbon-carbon bonds of normal length (1-54 A) and normal tetrahedral bond angle (109.5”). The hydrogen atoms on every second carbon are also 2.5 A apart; this is a “comfortable” spacing and the hydrogen-hydrogen contacts are near their energy minimum.With the a-olefins the bulky dimensions of the lateral group do not permit their polymers to assume the trans-planar zigzag configuration-such planar structures are impossible. For instance in polypropene a planar structure would put the side groups only 2.54 A apart which is much too close for a rnethylene or methyl group which has an effective diameter of 4.0 A. The overcrowding in the all-trans-configuration is relieved by twisting each FIG. 22. Possible types of isotactic chain according to the nature of the lateral group. Smallest circles CH. Medium circles CH2. Largest circles R. Left diagram R = CH3 C2H6 CH =CH2 CH2-CH2CH(CHS), OCHs Centre diagram R = CH2-CH(CH3)-C2Hs CHaCH(CH3)B Right cbagram R = CH(CH,),*C,H O*CH2.CH(CHJ2 CsH6 BAWN AND LEDWlTH STEREOREGULAR ADDITION POLYMERISATION 41 7 successive carbonxarbon bond away from the precise trans-position and this leads to a spiral or helix.The helical structure may be derived from the planar one by rotations of 120" around alternate bonds. The separation of the methyl groups is then preater than 4 8 and a minimum-energy configuration is attained. In practice the identity period is 6.5-6.65 A compared with the theoretical 6-2 A and the C-C-C bond angle is slightly increased aboke the tetrahedral angle since this allows better accommoda- tion of the hydrogen atoms. Inisotactic polymers of the olefins with branching side groups which are mmh more bulky than methyl the steric hindrance of the side groups is Freater and leads to a larger separation for the minimim energy of interaction. This resiilts in a looser helix of greater pitch and lower crystal- line density.With polypropene the repeating pattern along the chain axis requires three monomer units and occupies one turn of the helix (3-helix). FIG. 23. Left 1,2-Isotactic poli,b tad,et,e. Rig I Syndiotactic 1 ,?-polybutadieiie. Large circles carbon. Small circles hydrogel-. 418 QUARTERLY REVIEWS The crystalline forms of a number of these isotactic chains are shown in Fig. 22. Increase in the size shape and complexity of the side groups is reflected in variations of the characteristic parameter of the helix which tends to a more open or less spiralised form. Thus 7 1 1 3 and 4 sequen- cies of helices have been observed with isotactic FIG. 24. Left set 1,4-trans-PoIybutadiene. Right set 1,4-cis-Polybutadiene.Large circles carbon. Small circles hydrogen. Syndiotactic polymers. These are the all-trans-conformations of vinyl head-to-tail polymers and two polymers known to possess a syndiotactic chain structure viz. polyvinyl chloride and “ 1,2”-polybutadiene adopt this The bulky side groups do not hinder such an arrange- ment which however would not be expected for polymers with still bulkier side groups. Thus the recently announced syndiotactic polypro- pene5 is forced to adopt a non-planar helical configuration because the trans-form would put the neighbouring methyl groups 3.57 A apart whereas the methyl diameter is 4 A. Whilst much of the earlier study on stereospecific polymers and that which clearly emphasised the relation between chain Polar polymers. 15* Natta and Corradini .I.Polymer Sci. 1956 20 251. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 4 19 structure and physical properties was concerned with the a-olefins and other hydrocarbon polymers more recent work has shown that this dependence is general also among polar polymers most of which in the crystalline state adopt the helical-chain arrangement. Table 7 summarises some typical results. Crystalline “1,4”-polydienes. Natta and C ~ r r a d i n i l ~ ~ have studied the structure of all four crystalline polybutadienes with the results shown in Table 8. The structure of the crystalline forms are shown in Figs. 23A and B. Natural hevea rubber is the regular head-to-tail polymer built up of TABLE 7. Polymer of Structure Helix type Me methacrylate6’* Isotactic Syndiotactic Atactic But a~rylate~~ lsotactic 31 Bui vinyl ether42 Isotactic 31 N-Isopropylacrylamidet Syndiotactic Amorphous NN-Di-is~propylacrylamide~~ Isotactic Atactic Propylene oxide Isotactic Atactic A~etaldehydel~~ Isotactic 41 Pr opionaldehyde Isotactic 41 Period 10.55 (A) 6-45 6.50 7-16 4-8 4.8 Density M.p.1 *23 1.19 1-15 1.078 0.93 1.1 18 1 -070 0-97 1 50-1 60 O 190-200 82 165 170-200 100-125 -350 -120 72-75 - 165 * Stroupe and Hughes J. Amer. Chern. SOC. 1958 80 2341. f Shields and Coover J. Polymer Sci. 1959 39 532. # Natta Corradini and Dall’Asta Rend. Accad. Naz. Lincei 1956,8,408. TABLE 8. Isomeric polybutadienes. No. of monomers Form per unit cell M.p. trans-1 4 Planar 1 148” - cis-1 4 9 9 2 1 2 Syndiotactic 39 2 155 1 2 Isotactic Helical 3 125 cis-“ l,4”-isoprene units and natural balata (gutta percha) of trans- “1,4”-isoprene units.The structure of the natural polymers was shown by Bun1-9~~ to be completely planar. No data yet exist on isotactic and syndiotactic polyisoprene although Natta has announced their existence. cis-“ 1,4”-Polyisoprene shows a great similarity in structure to cis-“ l,4”- polybutadiene and this is reflected in corresponding similarities between their physical and mechanical properties. lS5 Natta and Corradini Angew. Chem. 1956 68 615. lS6 B u n Proc. Roy Sac. 1942 A 18 40 67 82. 420 QUARTERLY REVIEWS Melting Points.-The most striking property consequent upon the crystallinity lies in the melting points. The following Tables summarise the melting points of some of the isotactic crystalline p01yolefins.l~~ Many of the results are surprising especially among the branched stereospecific polymers which have remarkably high melting points.This is contrary to the usual behaviour since normally incomplete side-chain substitution in a linear polymer provides a hindrance to crystallisation and a lowering in the melting point. The reason for this apparent reversal lies in the geometrical shapes of the polymeric molecules in the crystallites. As we shall discuss in detail below the asymmetric synthesis gives highly ordered chains which pack very well and are stiffened by suitable arrange- ment of the alkyl groups. The data in Table 9 show the striking effect of change of the lateral group R in the polymer [CH,CHR*] on the melting point and on the TABLE 9. Melting points of isotactic poIyole_fis [CH,CHR.],. Monomer Ethylene Propylene But-1 -ene Pent- 1 -ene Hex- 1 -ene Dodec- 1 -ene Octadec- 1 -ene 3-Methylbut-1-ene 4-Methylpent- 1 -ene 4,4-Dimethylpent- 1 -ene 4,4-Dimethylhex- 1 -ene 4-Methylhex-1-ene 5-Methylhex-1 -ene 5,5-Dimethylhex- I -ene 4-Phenylbut-1 -ene 3-Phenylbut-1 -ene 5-Phenylpent- 1 -ene Styrene 4-Methylstyrene M.p.136" 165 125 75 ( - 50) 45 70 100 280-285 200-240 320 350 1 60 110 130 158 360 240 - - - Crystalline Crystalline Crystalline Crystalline Rubber non-cryst. Crystallinity due to crystn. of long lateral groups Hard highly cryst. Very highly cryst. Hard cryst. Hard cryst. Hard cryst. Rubber non-cryst. Slightly rubbery but cryst. Intractable but cryst. Rubber non-cryst. Brittle cryst. Amorphous transition from rubbery amorphous to hard crystalline polymer. If one considers the effect of substitution in the side chain in terms of the general monomer formula (XLIII) a number of broad genera lisations hecome apparent.(1) Linear side chains. In the series beginning with but excluding ethylene the melting points decrease through polypent- 1 -ene and higher 15' Campbell and Haven J. Appl. Polymer Sci. 1959 1 73. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 421 members are amorphous rubbers up to polyoctadecene which is again crystalline. Long flexible side chains bring down the melting point since they contribute to increasing molecular asymmetry. Olefins with linear sub- stituents longer than ten carbon atoms crystallise in the normal paraffinic structure as has been observed previously with the higher methacrylates. The materials behave like normal paraffins tied at one end.(2) Side-branching on the a-carbon atom. Single branching gives high melting points (250-360") and the highest crystallinity. The polymers are hard and readily form film and fibre. (3) Side-branching on the /!I-carbon atom. All polymers of the sym- metrical branched hydrocarbon melt in the range 220-240" and give hard crystalline polymers. Unsymmetrical branching as in poly-4-methyl- hex-1-ene lowers the m.p. Double branching on the /3-carbon gives very high-melting polymers. (4) Side-branching on the y-carbon atom. The two examples of this class are rubbery but crystalline polymers melting in the range 110-160". ( 5 ) Phenylalkenes. The polymers decrease in crystallinity melting point and stiffness as the number of carbon atoms between the double bond and the phenyl group is increased.Replacement of a terminal hydrogen by methyl increases the melting point of the polymer. It is interesting that the para-substituted styrenes although isotactic do not crystallise. This is an example of a wider principle that crystallisability is not necessarily a measure of tacticity. Crystallisation does not seem to be possible when the side chains of the polymer are mobile or where the packing of the ordered structures because of position and bulkiness is such as to lead to a crystalline polymer having a density lower than that of the same polymer in the amorphous phase. The influence of substituents on the properties of crystalline polyhydro- carbons has also been widely examined for the methylstyrenes,lS the p-tol),lpropene and the arylbutene Typical results are summarised in the Table 10.TABLE 10. Polymer M.p. Polymer Styrene 240" 2,4-Dimethylstyrene 2- Methyls tyrene 260 2,5- 9 3- Y9 21 5 3,4- 9 9 4- 9 9 Amorphous 3,s- 9 15* Natta SOC. Plastics Enr. J. 1959 15 373. 150 Price Lytton and Ranby J. Polymer Sci. 1961 51 541. M.p. 310" 330 240 290 422 QUARTERLY REVIEWS The influence of a methyl group in the benzene ring on polymer proper- ties when the alkylaryl group is separated from the polyethylene chain by one or two methylene groups is clearly shown in Table 11 where A indi- cates no order B some order C a lattice of low order D good order TABLE 11. Polymer of Ally1 benzene o-Allyltoluene m- 7 9 P- Y 2-Allyl-p-xylene 4-All yl-o-xylene 5-Allyl-m-xylene 4 Phenylbut-1 -ene 4-o-Tolylbut-1 -ene 4-p-Tolylbut- 1 -ene Crystallinity A-B D D B D-E D-E D D D D Glass M.p.( T,) (Tm) 60" 204-208 O 80 290 3540 180 60-65 240 334-338 268-275 24 7-2 52 10 162-168 234-239 189-196 and E excellent order. The effect of structure on the melting points of the poly(ary1propenes) and poly(ary1butenes) appear to agree with variation in melting points of poly(viny1 toluenes) and poly(viny1- xylenes). A m-methyl group in styrene or allylbenzene polymer brings the melting point close to that of the unmethylated polymers and at least 100" below those of the position isomers. A p-methyl group has a relatively mild effect on the transition temperature and an adverse effect on crystallinity. Introduction of two methyl groups into the ring has a substantial effect on the melting points of both the polystyrenes and poly(ally1benzene) derivatives.The melting point of a crystal is of fundamental interest in relation to its molecular structure and technically it is important for instance in the spinning of fibre and in the manufacture of plastics of good high-temper- ature properties. The process of melting of high polymers is not yet well enough under- stood for precise quantitative treatment but the knowledge that has been accumulated enables us to formulate at least in an empirical way the molecular characteristics which control melting point. It is well known that the melting point is related to the heat of melting AH and the entropy of melting AS by the thermodynamic equation Tm oc AH/AS and the problem is therefore to discern the correlations between molecular struc- ture and the heats and entropies of fusion.It is therefore clear that the factors which will be expected to influence melting point in a series of hydrocarbon polymers will be (a) interchain cohesion since other things being equal a decrease in intermolecular forces will decrease the melting point and (b) molecular flexibility and (c) symmetry. A flexible molecule which may be essentially linear in the crystal will coil on melting and thus AS will be higher for a rigid molecule than for a more flexible one i.e., BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 423 for a given AH AH/AS and thus 7'' will be lower. The degree of flexibility is determined by rotary movements around single bonds which occur in the face of an energy barrier. It is known that the height of this barrier varies with the nature and degree of substitution in the hydrocarbon chain and may vary from 1 to 5 kcal./mole.High barriers to rotation lead to high melting points. In these qualitative terms can now be understood the data of Table 9. Thus it is clear that the longer more flexible side chains as one goes from polypropene to polypentene would bring down the melting point since they increase the molecular asymmetry. Again if one compares the poly-a- olefins with branched side groups then the overcrowding will be less severe (greater rotational freedom) when the branch is farther away from the main chain and this is in line with the decrease in melting point along the series poly-3-methylbut-1-ene (m.p. 3 lo") poly-4-methylpent-1-ene (m.p. 240") and poly-4-inethylhex- 1 -ene (m.p. 125 ").For gem-disubstituted derivatives such as poly-4,4-dimethylpent- 1 -ene (m.p. 300°) the reverse is true the chain stiffness leading to a very high melting point. It is evident from these observations that we are now in a position to make reliable estimates of the melting points of polyolefins and thus to predict some of the properties of a given polyolefin even though it may not have been synthesised. Characterisation of Stereoregular Polymers Since the discovery of isotactic and syndiotactic polymers crystallinity as measured by X-ray diffraction has been the most common criterion of the degree of stereoregularity or tacticity. Much of this work comes as a result of the brilliant and intuitive X-ray studies of the crystal forms of poly-a-olefins carried out by Natta and C0t~adini.l~~ X-Ray methods are dependent on regularity of the main chain structure in large regions and it has become common practice to classify a polymer as stereoregular or otherwise primarily on the basis of whether it shows a characteristic sharp X-ray fibre diagram.Some polymers however are known to be highly stereoregular and yet do not show X-ray ~rystallinity.~ Furthermore the X-ray procedures commonly employed have limitations (p. 409) and consequently there is a need for independent methods for determining the stereochemical structure of large molecules which are directly governed by the probabilities of isotactic and syndiotactic addition of monomers viz. the "microtacticity of the chain." Tacticity of a polymer will affect physical properties other than gross crystallinity and indeed variation in melting point or glass transition temperature,laO mechanical properties,lal density,ls2 and Flory 8 tempera- 160 Natta J.Polymer Sci. 1955 16 143. 161 Sauer Wall Fuschillo and Woodward J. Appl. Phys. 1958 29 1385; Gall and lfi2 Reding J. Polymer Sci. 1956 21 547; Natta Danusso and Moraglio ibid. 1957 McCrum J. PoZymer Sci. 1961,50,489. 119 25. 424 QUARTERLY REVIEWS t ~ r e l ~ ~ have all been used to characterise the tacticity of polymeric modifica- tion (see below). However recently progress has been made in develop- ing a theoretical relationship between physical properties in solution and stereochemical structure from studies of infrared di~horisml~~ and optical anisotropy.le5 The most general techniques which can be used for tacticity measurement are infrared166 and nuclear magnetic resonance spectro- s c ~ p y ~ ~ ~ and these together with other more specialised methods are reviewed below.Definitions.-The description of a tactic polymer requires a knowledge of the distribution of isotactic and syndiotactic sequences in the chain i.e. the detailed structure of the polymer chain. Natta2 considered that two adjacent units in the chain were isotactically bonded when neighbour- ing asymmetric carbon atoms had the same configurations dd or 11 and syndiotactically bonded when the configurations were in the opposite sense dl (or Id). Any polymer may be regarded as consisting of isotactic sequences linked together by syndiotactic bonds or sequences and the description of the polymer consists of a knowledge of the distribution of isotactic and syndiotactic sequence (two syndiotactic bonds as in the configuration .. . llldlll . . . define a syndiotactic sequence of length unity). The mathematical description of such polymers has been developed by Coleman20 and by Miller and Nielsen168 in terms of generalised copolymer theory. Thus whereas a normal copolymer of monomers A and B consists of a mixture of A and B randomly linked together in an analogous manner a tactic homopolymer may be described as a mixture of d- and Z-sequences randomly linked together. It may be s ~ o w ~ ~ ~ ~ that the fraction of isotactic bonds in the chain is given by la = P d l l / ( P i l d -+ Pdll) where Pdll is the instantaneous probability of adding an 1-monomer to a growing chain ending in dl etc. Similarly the fraction of syndiotactic bonds is S = Plld/(Plld + Pdld).In this sense the degree of istotacticity I, can be defined as the fraction of monomer units isotactically bonded and similarly for So. According to these definitions an isotactic polymer would be one for which I > So rather than as more restrictively defined by Natta namely I = 1.0. Generalised theory168 also allows calculation of number- and weight- average sequences of material of type d or I and the weight fraction of material d or I in sequences of length y7? units. Bovey and Tiers170 showed for methyl methacrylate that this simple 163 Kinsinger and Wessling J. Amer. Chem. SOC. 1959 81 2908. 164 Volchek and Roberman Vysokomol. Soedenini-vva 1960 2 1 157. 166 Elliot “Advances in Spectroscopy,” Interscience Publ. Inc. New York 1959 16’ Powles Polymer 1960 1 219.168 Miller J. Polymer Sci. 1962 57 975. 170 Bovey and Tiers J. Polymer Sci. 1960 44 173; Bovey ibid. 1960 46 59; Tsvetkov Magarik Boitsova and Okuneva J . Polymer Sci. 1961 54 635. Vol. I p. 214; Krimm Forschr. Hochpolym. Forsch. 1960 2 18. Miller and Nielsen J. Polymer Sci. 1960 46 303. Odajima Woodward and Sauer ibid. 1961 55 181. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 425 concept of tacticity is inadequate and that it was necessary to consider the configuration of any asymmetric carbon atom relative to the configura- tions of the two adjacent carbon atoms. A given monomer unit would be considered to be in an isotactic placement if it were isotactically bonded to its neighbours viz. ddd or 111 and in a heterotactic placement if it were bonded isotactically to one of its neighbours and syndiotactically to the other ddl d l Zdd or IEd.As before the fractions of each of these types are determined by the probabilities of forming the respective configurations. If (T is the probability that a polymer chain will add a monomer unit to give the configuration of that of the last unit at its growing end and if (T is controlled only by the configuration of the end unit and if propagation can be defined by a single value of (T then Pi = a2; Ps = (1 - a)2; P h = (1 - Pi - Ps)= 2((T - a2); where Pi Ps and p h are the probabiIities of forming an isotactic syndio- tactic or heterotactic placement respectively. Nuclear magnetic resonance measurements of methyl methacrylate have been interpreted in terms of these three kinds of configuration (see p.428). It is evident that if the probability of forming an isotactic bond is lo (i.e. the fraction of bonds of type i)-the definition originally adopted by Natta-then the probabilities of forming two isotactic bonds in succession is Io2. Therefore the definitions of tacticity consistent with Bovey’s termin- ology are r = ro2 (1) s = so2 (2) H = 2z0s, (3) where I S and H are the isotactic syndiotactic and heterotacticity respectively and lo So are the Miller-Nielsen tacticities. The two tacticities are of course equivalent. In addition stereoregular polymers are often stereo-blocks. A stereo- block copolymer might be composed of alternating sequences of isotactic and syndiotactic units. Such a polymer in which 1 = So = Q will not be atactic and has proposed that it be called equitactic to distinguish it from a truly atactic polymer.In a stereo-block polymer the tactic sequences may be randomly linked and one must distinguish between an isotactic random polymer in which Pllz = I, and an isotactic block polymer in which Pllz > Z,. To be atactic a polymer must be not only equitactic but also random i.e. I = So = Q. Miller has shown that for stereo-block polymers the relations 1-3 for a random polymer have to be modified to 1 = IOP111 s = S O P d t d H = (IOplld + s $ d l l > = 21$11d. 426 QUARTERLY REVIEWS The problem is to consider the experimental methods suitable for the characterisation of stereoregular polymers and the determination of the quantities I S and H or I and So. We propose to summarise these methods in terms of their general applicability rather than consider individual polymers in detail.Crystallinity and Melting Point.-In general molecules with ordered conformations af the same type pack together to form a stable crystalline lattice and it should therefore be possible to relate crystallinity to the degree of tacticity of the polymer. However with such polymers crystallinity is never complete and generally a polymer with a very regular siructure may exhibit at most 80-90 % of crystallinity as measured by the J :ray method. The reasons for this are (a) that the completely regular chains of varying length will not be confined to any single crystallite since each individual chain orders itself in more than one crystallite thus creating amorphous discontinuities between crystallites and (b) that a crilical length of ordered sequences in the chain may be necessary for crystallisation and the minimum length may be different for different polymers.Measurements indicate that crystallites as short as 50 A are detectable by X-rays (5-10 monomer units). For tactic poly(methy1 methacrylate) estimates that a tactic length of 20 is necessary for crystallisation and this seems to be a reasonable value to use for vinyl polymers. Also if short stereo-blocks are formed the chance of their coming together to form a three-dimensional network is remote and consequently an estimate of tacticity would be highly inaccurate. Miller,169 assuming the copolymer model referred to in the next paragraph and a minimum crystallisable sequence of 10 units has derived a relation between maximum crystallisability and sequence probabilities.Melting-point curves calculated for polypropene agree satisfactorily with the experimentally measured ones. The method gives at least some quantitative information on the distribution of tactic sequences for semicrystalline polymers. The extent to which the perfect crystalline state is reached is reflected in the sharpness and reproducibility of the melting point. Polymers generally melt over a temperature range but with the proper precautions of measurement a well-defined melting point may be obtained which approximates closely to the equilibrium melting temperature. The intro- duction of a second component or an irregularity into the polymer chain depresses the melting point and the theoretical treatment of the melting-point depression has been given by F10ry.I~~ The crystallisation of partially tactic polymer is the same in principle as that of conventional copolymers.Crystallisation is confined to sequences of isotactic or syndio- tactic units and is interrupted by a placement of the wrong kind. Coleman,20 N e ~ m a n l ~ ~ and Miller1es have analysed crystallisation and melting data 171 Fox J. Polymer Sci. 1958 31 173. 172 Flory Trans. Faraday Suc 1955,51 848. Newman J. Polymer Sci. 1960 47 1 11. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 427 in terms of Flory's theory. According to this theory the melting tempera- ture of a copolymer Tm is given by 1 1 -R -R Tm Tmo AH ' - A H u log x - - logp - where AH is the heat of fusion per crystallising unit Tm is the melting point of the pure polymer and Xct is the mole fraction of crystallisable units.For a stereo-random copolymer Xa == p = a wherep is the sequence propagation probability and a refers to the fraction of the total monomer units that are in isotactic placements with their nearest neighbours. For a pure isotactic polymer p = 1 . Applying this relation Ne~manl'~ has calculated the minimum tacticities for a series of propene fractions. Infrared Absorption.-Measurements of the infrared absorption of stereoregular polymers are of two types (1) the detailed assignment of the infrared absorption bands; and (2) the difference in spectra of different samples variously attributed to crystallinity tacticity and chain configura- tion. Various workers have considered the fundamental vibrations of a three-fold helix as found in isotactic p01ypropene.~~~ This assumes that intermolecular interaction is absent and that we are interested only in nearest-neighbour intermolecular interaction along the chain.McDonald and Wa~dl7~ have pointed out that this leads to a definition of tacticity similar to that derived from nuclear magnetic resonance measurement. On this view the infrared spectrum is not related to or determined by crystallinity and is determined only by the chain configuration. Much success in resolving this problem has been obtained by using isotactic polypropene and many of the possible related deuterated polypropenes in which 1-3 hydrogen atoms are replaced by deuterium in the 1- 2- or 3-po~ition.l~~ Many studies have recently been reported on the use of infrared absorp- tion to measure crystallinity to distinguish isotactic and atactic parts of the sample and to measure the fraction of the chains existing in a helical configuration.McDonald and Ward,175 following Pera1d0,l~~ analyse the bands in terms of the three-fold helix and do not assume order in the solid or any other form of aggregation. In agreement with they con- clude that some bands are characteristic of isotactic chains existing in a helical configuration and can be used as a measure of tacticity. Miller,176 from measurements with crystalline and non-crystalline samples also concludes that molecular and not crystalline symmetry in the main chain governs the appearance of the spectra and that isotactic portions of 17p Peraldo Gazzetta 1959 89 798. 175 McDonald and Ward Polymer 1961 2 341. 176 Liang and Watt J.Polymer Sci. 1961 51 51 5; Liang Lytton and Borne ibid. Brader J. Appl. Polymer Sci. 1960 3 37C. 17* Miller Polymer 1960 I 135. 1961,54,523. 5 428 QUARTERLY REVIEWS the chains in the non-crystalline material also exist in the three-fold form found to exist in the crystal. Similarly measurements of the intensity or intensity ratio of character- istic bands have been utilised for the quantitative measurement of the isotactic content of polystyrene,179 and of the stereoregularity of poly- (vinyl chloride),lS0 methyl methacrylate,lsl p~ly-(NN-dialkylacrylamide),~~ poly(propy1ene lS2 and poly(isobuty1 vinyl ether).46 The general problem however is not so simple asit at first appears and much difference in the interpretation of spectra exists. Our understanding of the relation between the infrared absorption and local chain conformation is still far from complete and much more work is necessary before these methods may be regarded as suitable for routine analysis.For diene polymers infrared methods are the only satisfactory way in which an estimate of the relative amounts of cis- and trans-“1,4”-,-“1,2”- and -“3,4”- linkages can be made. The characterisation of unsaturated linkages by infrared methods is of course well established and its applica- tion to diene microstructure has been discussed fully by Richardson,ls3 Binder,ls4 Hampton,lss and Silas Yates and Thornton.lsa Since in isotactic and syndiotactic macromolecules of the type [CH,CHR.] the oriented units are respectively segments of a 3,-helix and a flat trans-chain the two forms should show differences in their infra- red dichroism and this has been demonstrated by Volchek and Rober- man164 who have used the difference to estimate microtacticity in polypro- pene.Nuclear Magnetic Resonance Spectroscopy.-Nuclear magnetic reson- ance spectra of crystalline polymers in the form of solvent-swollen gel or in dilute solution have recently been used with considerable success in the measurement of tacticity. The most striking and spectacular results have been obtained with syndiotactic and isotactic poly(methy1 rneth- acrylate) by Bovey and They observed that the high-resolution spectrum in chloroform at 90” showed three a-methyl proton peaks corresponding severally to isotactic (ddd or ZIZ) syndiotactic (Zdl or did) and heterotactic (Idd dl ddl or 116) placements of monomer units.The method provides a direct measure of these three sorts of triad in the polymer backbone viz. the values of I S H ; and CT (see p. 425) enables the tactic polymer to be completely characterised Nuclear magnetic resonance 179 Natta Chimica e Industria 1955 37 888. l.soKrimtfl Berens Folt and Shipman Chem. and I d . 1958 1512; 1959 433; lE3 Kawasaki Furukawa Tsuruta Saegusa Kakogawa and Sakata Polymer 1960 Ia4 Binder Analyt. Chem. 1954 26 1877. la6 Hampton Analyt. Chern. 1949,21,923. lE0 Silas Yates and Thornton An~l’t. Chem. 1959 31 529. Grisenthwaite and Hunter Chern. and Ind. 1958,719; 1959,433. 1 315. Baumann Schrieber and Tessman Makromol. Chern. 1959,36 81. Richardson J. PoZyrner Sci. 1954 13 229. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 429 measurements have also been reported for stereoregular po1ypropene,ls8 and poly(methacry1ic anhydride),lS9 but up to the present the method has been applied successfully only to the direct measurement of tacticity for poly(methy1 methacrylate); it is likely to develop into a most valuable technique for identification of stereoregularity.Solubility Behaviour of Crystalline Polymers.-Known processes for the production of stereoregular polymers always produce simultaneously appreciable quantities of atactic or amorphous polymer. The amorphous polymer is usually more soluble than the crystalline product and the normal practice in handling polymer thought to be stereoregular is to extract the more soluble and presumably less crystalline fractions under reflux by suitable solvents. Solvent-extraction is generally used as a preliminary to other independent measurements of tacticity.Extraction with a single solvent does not usually bring about a satisfactory separation and it is desirable to extract each fraction with a series of solvents of in- creasing solvent power and also at a series of increasing temperatures. For example with polypropene it is well-established that the amounts of isotactic stereo-block and amorphous linear polymer are respectively the amount not extracted by boiling n-heptane the amount extracted with boiling n-heptane and the amount extracted with ether. In fractionations of this type the determining factor is the change of solubility with solvent power and the changes in solubility with temperature. However any fraction however narrow obtained by solvent-extraction may be composed of a mixture of chemical components of widely differing molecular weight and crystallinity (structure).Further developments by Natta and his have utilised chromatographic processes for the fractionation of stereo-block polymers by using a highly crystalline polymer as column packing. In this way they claim that fractions are separated according to crystallinity and almost independent of molecular weight the more highly crystalline fractions of the stereo-block being more strongly adsorbed than the less crystalline. However as with other similar solubility pro- cedures fractionation is complicated by the fact tbat amorphous fractions of low molecular weight may have similar solubility characteristics. For poly(isobuty1 vinyl ether) Okamura et aZ.lgl showed that although discrimination of stereo-structure by direct differences in solubility was inadequate precipitation temperatures depended directly on steric structures but not on the degree of polymerisation.The turbidity point has been used by Elias et a1.lg2 for the determination of tacticity for poly- (methyl methacrylate) and polyvinylpyrrolidone. la7 Kern and Partinger Nature 1960 185 236. la* Miller Polymer 1960 1 135. lED Bovey and Tiers J. Polymer Sci. 1961 47 479; Miller Brey and Bulter ibid. lD0 Natta Pegoraro and Peraldo Ricerca sci. 1958 28 1473. lD1 Okamura Higashimura and Sakurada J. Polymer Sci. 1959,39 507. lD2 Elias Dobler and Wyss J. Polymer Sci. 1960,46 265. 1961 54 329- 430 QUARTERLY REVIEWS Tacticity and Properties in Solution.-It would be expected that the physical properties of macromolecules in solution would be related to the stereo-structure of the chain.However the magnitude of the differences may be so small that these methods may not be suitable for detailed analysis of structures. Studies of a series of fractions of isotactic polystyrene by Danusso and Moragliolg3 and by Trossarelli Campi and Sainilg4 showed that the intrinsic viscosity-molecular weight relation was the same as for the atactic polymer but that the second virial coefficient was smaller for the isotactic material. In agreement with this conclusion they noted that the isotactic polymer is less soluble than the atactic one of the same molecular weight. Similar conclusions have been reached from measure- ments with isotactic and atactic p01ypropene.l~~ Krigbaum Carpenter and Newman,lg6 whilst agreeing with these observations conclude that the configurational similarity of the two kinds of polystyrene as implied by viscosity measurements holds only in thermodynamically good solvents.They deduce that in a 0 solvent the atactic polymer has a larger mean- square radius. Kinsinger and Wesslinglg7 support these views by measure- ments in poor solvents and observe that 8 in phenyl ether is 8" higher for the isotactic polymer. Related properties such as precipitation temperature have been previously referred to as a means of discriminating between different steric structures. In a series of papers Birshstein Gotlib Ptitsyn and their co- ~ o r k e r s ~ ~ ~ J ~ ~ have shown theoretically that the electrical and optical properties of polymer molecules depend on the microstatic state of the chain.Their theories lead to the following expressions for the mean- square end-to-end distance hi-2 the mean square dipole moment Ui2 and the mean optical anisotropy dAi of isotactic molecules of the type [CH,CHR*], hi-2 = 2nZ2 [(l + cos C L ) ~ / ( ~ - cos a)]/pi; ui2 = nm2 - (1 - cos a) (1 - cos fl) 2 (1 + cos a) (7 + 3 cos a) . ( 5 sin2 a) pi 9 AAi a where n is the degree of polymerisation Z is the carbon-carbon bond length rn is the dipole moment of the monomer unit d y is the optical anisotropy of the monomeric unit in the axes of the helix (T- a) is the valency angle on the main chain (v-fl) is the valency angle in the side group and pi is the ratio of the probability of having different or identical conformations of lBS Danusso and Moraglio J.Polymer Sci. 1957 24 161. 19* Trossarelli Campi and Saini J. Polymer Sci. 1959 35 205. lB5 Danusso and Moraglio Mukromol. Chem. 1958 28 250. lB6 Krigbaum Carpenter and Newman J Phys. Chem. 1958 62 lg7 Kinsinger and Wessling J. Amer. Chem. SOC. 1959 81 2909. lo* Birshstein Gotlib and Ptitsyn J. Polymer Sci. 1961 52 77. 19* Birshstein Gotlib and Ptitsyn Vysokornol. Soedineniyu 1960 2 627. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 43 1 neighbouring monomeric units. It has been shown that the above expres- sion describes the experimental results for isotactic and syndiotactic polystyrene and its halides. Electric Polarisation.-Debye and Bueche200 and Ptitsyn and Sharnov201 have pointed out that isotactic and syndiotactic polymers would be expected to have different dipole moments in view of rotational-barrier differences.For poly(methy1 methacrylate) solution measurements of the molar polarisability by Bacskai and Poh1202 showed that the polarisability depended on the steric order along the backbone chain. It was found that the value for the syndiotactic polymer was 32 & 0.5 c.c./mole and that for the isotactic form was 41 & 0.5 c.c./mole. Atactic modifications had values which lie between these extremes. These conclusions are strongly supported by preliminary measurements for polystyrene by Krigbaum and Rorg20a who found the dipole moment of the poly- styrene unit to be 0.60 and 0.73 for atactic and isotactic units respectively. Once again this method is capable of rapid development but as with hydrolytic studies the method is essentially limited to polymers containing polar or functional groups.Chemical Methods for Determining Polymer Tacticity.-Recently con- siderable attention has been focused on the chemical reactivity of func- tional groups which are attached to polymeric backbone chains. As might be expected different orders of reactivity have been observed when the polymeric structures had different degrees of stereoregularity. This approach is a very useful and highly interesting one since it provides a method of investigating short chain segments of a polymer chain for inherent stereoregularity. Morawetz and G a e t j e n ~ ~ ~ ~ by studying the hydrolysis of copolymers of methyl methacrylate and 1-2% of p-nitro- phenyl methacrylate (or p-methoxyphenyl different rates of hydrolysis which could be chemical arrangements of the monomer solvolysing unit i.e.the isotactic (XLIV) ye M e Me methacrylate) observed two attributed to different stereo- units on either side of the and the syndiotactic (XLV). -CH-C-CH-C- CHr$- -CH,-$-CI-$-$-CH 2 1 -t- 2 1 2 1 co; C0.p co Me C02R Me (XLI v) ow Presumably the faster hydrolysis occurs by intramolecular solvation and this is more probable in the isotactic sequences. Similar results have been Debye and Bueche J. Chem. Phys. 1951 19 589. 201 Ptitsyn and Sharnov Zhur. Tekh. Fiz. 1957 27 2767. 202 Bacskai and Pohl J. Polymer Sci. 1960 42 151; see also Salovey ibid. 1961,50 203 Krigbaum and Rorg J. Chem. Phys. 1959,31 544. 204 Morawetz and Gaetjens J. Polymer Sci. 1958 32 527; J. Amer. Chem. Soc. 57. 1961,83 1738. Sf 432 QUARTERLY REVIEWS described by Smets and Loeder205 for the hydrolysis of methacrylic acid- methyl methacrylate and acrylic acid-acrylamide copolymers.In each case two distinct rates of hydrolysis were observed and it was shown that stereo- regularity in the carbon backbone had a pronounced effect on that rate attributable to intramolecular attack. Glavis20s studied the alkaline hydrolysis of methyl methacrylate polymers and found that the rate of hydrolysis of isotactic poly(methy1 methacrylate) was much higher and proceeded to a greater final conversion than did that of either the syndiotactic or the atactic modification. The acid hydrolysis of stereoregular polymers of NN-dialkylacrylamides has been investigated by Chapman207 and again it was found that the iso- tactic were hydrolysed much faster than atactic polymers.The hydrolyses were carried out in a N-solution of hydrogen chloride in 60 % v/v dioxan- water and for NN-dimethylacrylamide the reaction was followed by determining the liberated dimethylamine ; and tacticity values were esti- mated from the respective rates of hydrolysis. Chemical investigation of a purely hydrocarbon stereoregular polymer is obviously more difficult but some progress has been reported.208 Case and Atlas209 studied hydrogen and deuterium exchange in isotactic and atactic polypropenes using a nickel-kieselgular catalyst of large pore-size in cyclohexane. Although appreciable deuterium exchange occurred and the amount of exchange corresponded to a theoretical value of 18 % for the number of tertiary hydrogen atoms reacting no change in density or stiffness of pressed films could be detected in the exchanged polymers.Since the tertiary hydrogen in polypropene is the most likely site for deuterium exchange (an estimate borne out by infra- red measurements of the polymers) and since this hydrogen is attached to the asymmetric carbon atom of the polypropene backbone so by analogy with work on model reference compounds racemisation should occur on deuterium or hydrogen exchange i.e. Me Me D d- I- Racemisation in this manner would of course lead to a loss in isotacticity of the polymer and hence to a loss in density and stiffness. Case and Atlas209 have used these results in an attempt to cast doubt on the concept of stereoregularity in polymer molecules suggesting that the various tactic modifications may be random (i.e.atactic) polymers with different degrees 205 Smets and Loeder J. Polymer Sci. 1960 45 461. 206 Glavis J. Polymer Sci. 1959 36 547. 207 Chapman J. Polymer Sci. 1960 45 237; 1960 47 529. 208 Braun Makromol. Chem. 1961,44 269. 209 Case and Atlas J. Polymer Sci. 1960 45 435. BAWN AND LEDWITH STEREOREGULAR ADDITION POLYMERISATION 433 of branching. This explanation is however hardly tenable and in any case the lack of racemisation during exchange reactions of polypropene is readily understandable if one considers that in a poor solvent such as cyclohexane the stability of the polymer helix would surely cause any isolated and incipient tertiary carbon radical or ion to react with retention of configuration. Finally although the application of monolayer techniques showed no appreciable difference between atactic and isotactic polypropylene oxide,210 it has recently been shown211 that the pressure-area isotherms for isotactic sndiotactic and atactic poly(methy1 methacrylates) showed marked difference and suggest that this method may be used to measure tacticity in polymers containing polar groups.Newer Developments The polymerisation of carbonyl compounds is likely to be the focus of much effort during the next few years and further success has already been achieved by Natta and his collaborators in Italy212 and by several groups in Japan.213*214 These workers have shown that relatively high polymers can be obtained from keten keten dimer and dimethylketen. Keten itself apparently gives a regular polyketone formed by simple vinyl polymerisa- tion.Dimethylketen has been polymerised by Natta's group212 and apparently gives two distinct polymeric products depending upon the catalyst em- ployed. In each case essentially hydrocarbon solvents were employed and when aluminium tribromide was used as catalyst the resulting polymer had a structure analogous to that proposed for polyketen i.e. Me,C=C=O + .Me,CCOCMe,.CO- - - . However when triethylaluminium was em- ployed as catalyst the resulting polymer was a polyester having structure CMe CMe II II -C-O-(CO-CMe,-C-O).- (XLVI) (XLVI). Clearly much work remains before a full understanding of these reactions can be arrived at. Keten dimer yields a product analogous to the polyester (XLVI). The polymerisation of aliphatic diazo-compounds although of no com- mercial significance can still help to throw light on the mechanisms of stereospecific polymerisation.These monomers usually yield completely atactic materials on polymerisation with a variety of catalysts,'l but recently Nasini and his collaborators215 have shown that diazoethane can be polymerised to a high-melting crystalline polyethylidene (-CHMe-) 210 Hibberd and Alexander J. Polymer Sci. 1958,28,455. 211 Beredjick Ahibeck Kwei and Ries J. Polymer Sci. 1960 46 268. 212 Natta Mazzanti Pregaglia and Binaghi Makromol. Chem. 1961. 44 537. 213 Oda Munemiya and Okano Makromol. Chem. 1961,43 149. 814 Furukawa Saegusa Mise and Kawasaki Makromol. Chem. 1961 39 243. s15 Nasini Trossarelli and Saini Makromol. Chem. 1961. 44 550. 434 QUARTERLY REVIEWS when colloidal gold is the catalyst. So far however it has not beer possible to decide the precise nature of the stereoregularity.Here it is pertinent to point out that on any polymerisation mechanism. stereoregularity of a polymeric backbone could be caused by asymmetric induction. Previous attempts to investigate the scope of this effect by polymerising acrylate monomers containing optically active substituents have met with only moderate success. Lately however Natta and his collaboratorsl0 have produced an optically active polymer from a totally inactive monomer by using typical cationic initiators modified or pre- sented as a complex with asymmetric or optically active proton donors. The monomer used for this purpose was benzofuran (XLVIII) whose polymerisation gives (XLIX). The catalyst was made by treating dichloro- (ethyl)aluminium EtAlCl, with optically active amino-acids and the (XLV I I I ) (XLIX) polymerisation was carried out at -80".Clearly then introduction of an asymmetric influence at the site of propagation has a pronounced influence on the stereoregularity of chain growth. This effect is likely to provide many more interesting stereoregular polymerisations in the very near future. Furukawa and his c~-workers~~~* 217 have reported that some complex hydrides and organo-derivatives of Group 1-111 metals such as lithium aluminium hydride calcium tetraethylzincate and the complex Li+ [ZnEt ,Bu]- formed by reaction of butyl-lithium and diethylzinc exhibit excellent catalytic and stereoregulating activity towards acrylic2l6 and methacrylic esters.217 Furukawa218 has also announced the synthesis of a crystalline polymer of acetone.Stable crystalline polyacetone was prepared under an atmos- phere of propene with a modified Ziegler catalyst consisting of triethyl- aluminium titanium tri- or tetra-chloride and a metal salt such as calcium chloride sodium acetate or magnesium acetate. The last component was essential for polymerisation of acetone since in absence of these salts only the propene polymerised. The product which melted at 58-60" was stable at 200° and was shown by infrared and elemental analysis to be a block copolymer having 14% of propene blocks. Permission from authors and publishers to reproduce illustrations is gratefully acknowledged Figures 16-20 P. Cossee Tetrahedron Letters 1960 No. 17 12 17; Figure 21 D. J. Cram J. Amer. Chem. Soc. 1959 81 2752; Figures 23 and 24 G.Natta Chimie et Industre 1957 77 1009. 216 Makimoto Tsurata and Furukawa Makromol. Chem. 1960 36 116. 217 Kawasaki Furukawa Tsuruta Inoque and Ito Makromol. Chem. 1961,43 76; 218 Furukawa Second World Congress on Man-made Fibres London May 1962; 1961 49 112 113 163. Polymer 1962 3 487.

ISSN:0009-2681    

DOI:10.1039/QR9621600361

出版商:RSC    

年代:1962

数据来源: RSC