6 Kinetics and Mechanism of Addition Po I ymer iza t io n By P. HYDE and A. LEDWITH Donnan laboratories University of Liverpool Liverpool 169 3BX 1 Free-radical Polymerization Of particular interest in free-radical polymerization is the direct evaluation of the rate constant for chain propagation (k,). Bresler and co-workers' have developed an e.s.r. technique which permits quantitative measurement of the stationary concentrations of growing chains. For styrene polymerization at 25 "C they give k = 66.5 1 mol-' s- ' and E = 9.0 kcal mol- ' values which are within the (wide) range determined earlier by various workers. The pre-exponen-tial factor (A,) may be determined from the equation ekT h A = - exp (AS*/R) AS* values have been estimated using a method of group contributions2 and give values reliable to 1-1.5 kcal mol-' for vinyl acetate (VA) styrene (STY), methyl acrylate (MA) and methyl methacrylate (MMA) polymerization.Sub-sequent calculations of A are generally within a factor three of values determined experimentally. A comprehensive study of the polymerization of vinyl acetate both in bulk and in glacial acetic acid has been made and the effect of temperature on k,/(k,)f and transfer constants eval~ated.~ Although reasonable rates of polymerization can be achieved in glacial acetic acid under the conditions used polymer molecular weights were lower than in the bulk system. Studies of branching in vinyl acetate polymers have established that transfer to polymer backbone at 60°C is 2.4 times more frequent than it is to the pendant acetoxy-group and 4.8 times more frequent at 0 "C.Data on transfer to monomer show that this occurs exclusively uia the acetoxy-gro~p.~-' ' S. E. Breslcr F. N . ' K i ~ b e k ~ ~ . V. N . Fomichev. andV. N . Shadrin Makrornol. Chem., 1972 157 167. ' M. Guaita Makromol. Chem. 1972 154 191. S. P. Potnis and A. M. Deshpande Makromol. Chem. 1972 153 139. ' S. Nozakura Y. Morishirna and S. Murahashi. J . Polymer Sci. Part A - I Polymer Chem. 1972.10 2767. ' S. Nozakura Y. Morishima and S. Murahashi J . Polymer Sci. Part A-I Polymer Chem. 1972 10 2781. S. Nozakura Y . Morishima and S. Murahashi J . Polymer Sci. Part A-I Polymer Chern. 1972 10,2853. S . Nozakura Y . Morishima and S. Murahashi J . Polymer. Sci. Part A - I Polymer Chem. 1972,10,2867.13 134 P. Hyde and A . Ledwith It is a long-held assumption in free-radical kinetics that reaction rates are in general little affected by the medium. An investigation of the AIBN-initiated polymerization of MMA at 60 "C in a variety of solvents shows that (i) Ri varies little over the range of solvents employed (q = 2.54-27.88 mP) (ii) k decreases markedly with increased viscosity and (iii) k reaches a maximum in diethyl malonate solution (q = 6.61 mP). (i) and (ii) are not surprising and the change in k seems to be a solvent effect rather than a viscosity effect but no explanation is offered.8 Solvent and temperature effects have been investigated for MMA polymerization.' Both solvent and temperature as well as initial monomer concentration affect the stereochemistry of the resulting polymers.The effect of bromotrichloromethane on the polymerization of MMA has been studied by radiotracer and g.p.c. methods." Transfer constants for the first seven radical intermediates increase up to the fourth by which time the value is equal to that for a normal macroradical. The polymerization of MMA in the presence of isotactic poly(MMA)" and finely divided silica12 has been studied. Of interest is the polymerization of n-butyl methacrylate on the surface of the synthetic zeolite Faujasite,' initiated by adsorbed benzoyl peroxide. Polymerization of several acrylates and methacrylates in dimethylformamide has been de~cribed.'~ Differences in reactivity are discussed in terms of the nature of the substituent 01 to the double bond.The preparation and polymer-ization of highly chlorinated ethyl acrylates and methacrylates has been reported.' These may find use as cross-linking agents for polymers containing t-amine groups. Telechelic polymers (polymers with reactive end-groups for use as pre-poly-mers) are increasing in importance. Free-radical telechelic polymers may be prepared by using appropriately substituted initiators. The polymerizations of butadiene with 4,4-azobis-(4-cyano-n-pentanol) and 4,4'-azobis-(4-~yanovaleric acid)' and of isoprene with di-(4-hydroxybutyl)-2,2'-azobis-isobutyrate'' have been described and give pre-polymers with highiy reproducible properties. Because of their possible use in the drug industry polymers which contain heterocyclic nuclei are receiving more attention.Emphasis at the moment is on the synthesis of large numbers of new monomers and evaluation of their polymer-ization and copolymerization characteristics. Studies on vinylated oxazoles (l), thiazoles (2) isoxazoles (3) and oxadiazoles (4)18 have been performed. The M. M. Zafar Makromol. Chem. 1972 157 219. P. Goldi and H.-G. Elias Makromol. Chem. 1972,153 81. R. Buter Y. Y. Tan and G. Challa J . Polymer Sci. Part A - I Polymer Chem. 1972, 10 1031. J. Rychly and M. LazAr European Polymer J. 1972 8. 7 1 1. H. Hrabhk and H. PivcovA ( a ) Coll. Czech. Chem. Comm. 1972,37,3279; (b) J . Polymer Sci. Part A-1 Polymer Chem. 1972 10 3 125. l o C. A. Barson A. R. Luxton and J. C. Robb J.C.S. Faraday f 1972,68 1666. " l 2 T. R. Manley and B. Murray European Polymer J.1972 8 1145. l 4 J. Safko and E. Turksa Makromol. Chem. 1972 156 297. l 6 S. F. Reed jun. J . Polymer Sci. Part A - I Polymer Chem. 1972 10 649. l 7 S. F. Reed jun. J . Polymer Sci. Part A- I Polymer Chem. 1972 10 2493. Y. Iwakura F. Toda H. Suzuki N. Kusakawa and K. Yagi J . Polymer Sci. Part A - I , Polymer Chem. 1972 10 1133 Kinetics and Mechanism of Addition Polymerization 135 C=CH, C=CH2 synthesis of various monomers containing the N-phenothiazinyl group (5) is of interest because of the electron-donating properties and redox susceptibility of this group. Of the various monomers prepared (substituted acrylates meth-acrylates and acrylamides) all were susceptible to free-radical polymerization. P-(N-Phenothiaziny1)thyl vinyl ether (5 ; R = -CH,-CH,-0-CH=CH2) was also prepared and found to be polymerizable by cationic initiators.I R Methods have been described for the preparation of a range ofpoly(acry1amides) and poly(methacrylamides)20 and of polymers containing cholesteryl groups either adjacent to the main chain21 or separated from it by a chain of eight or more atoms2, Thermal initiation of vinyl polymerization is generally accomplished using either peroxides or azo-compounds. The use of a series of acra'a'-tetra-substituted bibenzyls (6) has been de~cribed.,~ As with similar systems these employ higher temperatures (60-1 10 "C) than the common initiators owing to the greater strength of the central carbon-carbon bond. Ester derivatives of act'-dicyano-bibenzyls (6; R' = CN R2 = C0,Alk) were found to be good initiators (f x 0.3-0.6) whereas corresponding phenyl derivatives (6 ; R' = CN R2 = Ph) were inefficient.l 9 H. Kamogawa J . Polymer Sci. Part A-1 Polymer Chem. 1972 10 95. 'O P. Ferruti A. Bettelli and A. Ferk Polymer 1972 13 462. Y. Tanaka S. Kabaya Y . Shimura A. Okada Y. Kurihara and Y . Sakakibara, J . Polymer Sci. Part B Polymer Letters 1972 10 261. 2 2 H. Kamogawa J . Polymer Sci. Part B Polymer Letters 1972 10 7. 23 H. A. P. de Jongh C. R. H. I. de Jongh W. G . B. Huysmans H. J. M. Sinnige W. J. De Klein W. J. Mijs and H. Jaspers Makromol. Chem. 1972 157 279 136 P . Hyde and A . Ledwith In contrast to earlier work claiming that difuroyl peroxide was inefficient in initiation of vinyl polymerization M01nar~~ has estimated that it is comparable to dibenzoyl peroxide in efficiency ofinitiation of styrene polymerization (f x 0.9), and to dilauroyl peroxide in the polymerization of vinyl acetate ( f x 0.7).Kinetic analysis indicates that chain transfer to difuroyl peroxide is important. Interaction of dibenzoyl peroxide (BPO) with amines to produce initiating species is well known. Applications of this reaction to the curing of epoxy-resins25 and the preparation of a polymeric emulsifier capable of initiating polymerization,2 have been made. A charge-transfer interaction of BPO with N-vinylcarbazole (NVC) is suggested as being the reason for anomalous kinetics in the BPO-initiated polymerization of NVC.” Whereas AIBN gives conven-tional kinetics of the form R = K[M] BPO-initiated systems have R = K[MI2[I].Species (7) is assumed to be the active initiator. Because of their lack of susceptibility to induced decomposition reactions, azo-compounds are the most convenient to use at the high initiator concentrations employed for studies of primary radical termination. 2,2’-Azobis-(2,4-dimethyl-valeronitrile) has been used in STY28u and VAZ8’ polymerizations. In the latter case determinations of the initiator efficiency (f = 0.5) and the primary radical termination rate constant (1.4 x lo9 1 mol-’s-’) were made. 4,4-Azobis-(4cyanopentanoic acid) is a water-soluble initiator used in the polymerization of a~rylamide.~’ Data indicate an initiator efficiency of 0.65 at 25°C. At low monomer concentrations R is proportional to explained in terms of 2 4 S. Molnar J .Polymer Sci. Part A - I Polymer Chem. 1972 10 2245. 2 s S. S. Labana Y. F. Chang J . Polymer Sci. Part A - I Polymer Chem. 1972 10 1861. 2 6 S. N. Trubitsyna Kh. K. Ruzmetova and M. A. Askarov Polymer Sci. (U.S.S.R.), 1971 13 1950. 2 7 R. G. Jones E. Catterall R. T. Bilson and R. G. Booth J.C.S. Chem. Comm. 1972,22. 2 8 K. Ito ( a ) J . Polymer Sci. Part A - I Polymer Chem. 1972 10 931; ( b ) ibid. p. 1491. 2 9 M. Baer J. A. Caskey and A. L. Fricke Makromol. Chem. 1972 158 27 Kinetics and Mechanism of Addition Polymerization 137 cage effects. The polymerizations of MMA initiated by 9-diazafl~orene~~ and 4,4'-diazidoa~obenzene~ have also been investigated. AIBN decomposition has been shown to be accelerated in the presence of bis-( - )-ephedrine-copper(I1) chelate ;3 the mechanism of this unusual behaviour is by no means clear but is suggested as involving reductive decyanation of the AIBN when co-ordinated to the chelate.Substituted tetrazenes (8) have been shown to be efficient initiators of vinyl polymerization either alone,33 in the presence of CCl and water,34 or in the presence of nucleic acid bases.35 R2 R3 \ / N-NyN-N / \ R' R4 Initiation of vinyl polymerization has been shown (Scheme 1) to be feasible with the systems dimethylaniline N-oxide-tosyl chloride [via radicals (9) and ( lO)I3 or organic acids-tetraphenylborate salt (uia phenyl radi~al).~ Me I I Me Ph-N-0 + Ts'C1- -+ Me I Me 1 Me I 'CH, (10) (9) monomer I -H+ I monomer Polymer - Ph-N +- Ph-Nf + -0Ts + Polymer Ts =Me Scheme 1 3 0 T.Nakaya M. Tanaka and M. Imoto Makromol. Chem. 1972 155 169. 3 1 T. Doiuchi T. Nakaya and M. Imoto Makromol. Chem. 1972 161 231. 3 2 V. Horanska J. Bartofi and Z. Mafiasek J . Polymer Sci. Part A-I Polymer Chem., 1972 10 2701. 3 3 K. Sugiyama T. Nakaya and M. Imoto Makromol Chem. 1972 161 219. 3 4 K. Sugiyama H. Watanabe T. Nakaya and M. Imoto Makromol. Chem. 1972 162, 3 5 K. Sugiyama T. Nakaya and M. Imoto Makromol. Chem. 1972 161 207. 36 T. Sato M. Yoshioka and T. Otsu Makromol. Chem. 1972 153 47. 3 7 T. Sato E. Kashino N. Fukumura and T. Otsu Makromol Chem. 1972,162 9. 291 138 P. Hyde and A . Ledwith Photo-initiation is of great importance since polymerizations can then be carried out at much lower temperatures than generally necessary for thermal initiation.The use of aromatic carbonyl compounds is well known and has recently been reviewed.38 Normal sensitization uses wavelengths up to approx-imately 40Onm but it has been shown that combinations of various dyes and ascorbic acid are effective up to 54611m.~~ Amines are reported to have an accelerating effect on the photopolymerization of MMA.,' This is thought to be due to the formation of a complex between excited monomer (M) and amine (A) which then generates a radical capable of initiating polymerization : M* + A (MA)* --+ Radical Polymer The reaction is similar in principle to that for the photoreduction of aromatic ketones with aliphatic amine~.~' Tetramethyltetrazene (8; R' = R2 = R3 = R4 = Me) is an efficient photo-sensitizer for polymerization of MMA and STY.42 The initiating dimethylamino-radical is electrophilic and reactivities of this radical towards various monomers correlate well with the e (polar) parameters of the vinyl monomers tested.Photopolymerizations using SS'-diphenyl dithi~carbonate,~~ which involve both initiation and termination by the phenylthiyl radical (PhS-) and the cyclic azo-compound 3-acetoxy-3,5,5,-trimethylpyrazoline (1 l), have been described. Me Mixtures of ferrocene and carbon tetrachloride are effective photoinitiators of MMA polymerization by the mechanism shown in Scheme 2.,' (C,H,),Fe + CCl tea+ ***Cl-Ca-CI I I monomer Polymer C-- CCl + (C,HS)2Fe+ + C1-(12) Scheme 2 38 H. G. Heine H. J. Rosenkranz and H. Rudolph Angew. Chem. Internat. Edn.1972, 3 9 T. Nagabhushanam and M. Santappa J . Polymer Sci. Part A - I Polymer Chem., 11 974. 1972 10 151 1 Kinetics and Mechanism of Addition Polymerization 139 Styrene is not polymerized by this system probably owing to a termination reaction with the ferricinium cation (12). Styrene is however polymerized radically in a titanocene dichloride [(Cp),TiCl,] sensitized system.46 This is unusual in that this sensitizer normally polymerizes via a cationic mechanism (e.g. vinyl ethers) but also produces copolymers of 2-chloroethyl vinyl ether and styrene by an apparent radical mechanism. Metal carbonyls and acetylaceto-nates can also act as photo initiator^,^^^^ the mechanism usually involving an abstraction reaction by the photoexcited metal derivative. Initiation of styrene polymerization by high electric fields5' has been shown to involve a cation-radical species.G.p.c. and inhibition data indicate that cationic and free-radical polymerization occur independently and it is proposed that the cation-radical transfers its cationic activity to styrene monomer enabling the resulting growing radical and styrene cation to propagate separately. Tetramethylthiuram disulphide (1 3) is commonly used both as an initiator and Me. Me \ / N-C-S-S-C-N I1 \ S Me / II Me S as an inhibitor of free-radical p~lymerization.~ Incorporation of the sulphur-containing groups in polymers is shown by elemental analysis or even by U.V. spectro~copy.~~ The associated thermal instability of these groups leads to a possible use of these polymers as macromolecular initiators.Sulphur-containing compounds are generally good inhibitors of free-radical polymerization ; thiols have been used as molecular-weight regulator^,^ and in suitable cases dye partition techniques can be used to establish transfer constant^.'^ 'O K. Yokota H. Tomioka T. Ono and F. Kuno J. Polymer Sci. Part A - I Polymer 4 1 4 2 K. Sugiyama T. Nakaya and M. Imoto J. Polymer Sci. Part A - I Polymer Chem., 4 3 K. Tsuda and K. Kosegaki Makromol. Chem. 1972 161 267. 44 T. Nakaya H. Ikeda and M. Imoto Makromol. Chem. 1972 161 241. 4 5 K. Tsubakiyama and S. Fujisaki J. Polymer Sci. Part B Polymer Letters 1972 10, 46 K. Kaeriyama and Y. Shimura J. Polymer Sci. Part A-1 Polymer Chem. 1972 10, 4 7 C. H. Bamford and M. U. Mahmud J.C.S.Chem. Comm. 1972 762. 4 8 C. H. Bamford and A. N . Ferrar J.C.S.Furaday I 1972 68 1243. 49 C. H. Bamford and J. Paprotny Polymer 1972,13 208. '" M. Lambla R. Koenig and A. Banderet European Polymer J. 1972 8 1. " J. Beniska Polymer Sci. (U.S.S.R.) 1971 13 2009. '' J. Beniska E. Staudner and E. Spirk Makromol. Chem. 1972 161 113. 5 3 A. V. Ryabov L. A. Smirnova U. M. Soldatov and L. M. Orlova Polymer Sci. " K. K. Roy D. Pramanick and S. R. Palit Makromol. Chem. 1972 153 71. Chem. 1972,10 1335. S . G. Cohen and H. M. Chao J. Amer. Chem. SOC. 1968,90 165. 1972 10 205. 341. 2833. (U.S.S.R.) 1971 13 165 140 P. Hyde and A . Ledwith A method has been described for calculating transfer constants of very reactive transfer agents (thiols aldehydes) by the use of an integrated Mayo equation, which allows for depletion of transfer agent during rea~tion.’~ Values of the transfer constants for a number of reactive transfer agents in the polymerization of ethylene are given.Nitrobenzenes have been shown to be good inhibitors of the polymerization of vinyl a ~ e t a t e . ~ ~ ’ ~ Reactivities correlate with inductive parameters for -CH,R and -SO,R substituents p-substituents being slightly more reactive than rn-substituents. For groups which show a mesomeric effect (halides keto-groups amines) the p-effect is approximately four times greater than the rn-effect. Nitroso-compounds (ArNO) are also efficient inhibitors of free-radical polymer-i ~ a t i o n ~ * but detailed analysis of the systems may be complicated by side reactions.2,2,6,6-Tetramethyl-4-oxopiperidine 1-oxyl (14) is a stable free radical reported to be a good inhibitor for STY and MMA polymerization. Scavenging of both initiator radicals (Me,CCN) and short polymer chains seems to be very efficient.59 :$7 N I Estimates of AIBN efficiency in these systems were in good agreement with those of other workers (STY f = 0.6; MMA f % 0.5). Ferric nitrate normally an inhibitor has been shown to be an initiator of MMA polymerization when used at low concentration ;60a at high concentrations the ferric salt terminates polymeriza-tion as does ferric chloride in the polymerization of vinyl chloride.60b Model compounds are often used to assess the degree of transfer to polymer in systems which may be taken to high conversion.Vinyl acetate copolymers with fumarate esters are important commercial oil additives ; studies on model compounds (succinate esters and ethyl acetate) indicate that transfer to polymer via the fumarate units is the probable cause of branching in high-conversion 5 5 G. A. Mortimer J . Polymer Sci. Part A-I Polymer Chem. 1972 10 163. 5 6 T. Foldes-Berezhnykh S . Szakacs and F. Tudos European Polymer J . 1972 8 1237. 5 7 T. Foldes-Berezhnykh F. Tudos and S. Szakacs European Polymer J. 1972,8 1247. 5 8 I. Kende L. Sumegi and F. Tudos European Polymer J . 1972,8,1281. 5 9 Y. Miura S. Masuda and M. Kinoshita Makromol. Chem. 1972 160 243. ‘O ( a ) H. Narita Y. Sakamoto and S. Machida Makromol. Chem. 1972 152 143; ( 6 ) W. I . Bengough and N. M. Chawdry J.C.S. Faraday I 1972,68 1807.J . C. Bevington M. Johnson and J . P. Sheen European Polymer J . 1972 8 209 Kinetics and Mechanism of Addition Polymerization 141 Copolymerization.-The analysis of normal low-conversion binary copolymers is well understood and needs no further elaboration. The copolymerization equation has however been extended to account for systems which include acetylenic monomers where differences between radicals (15) and (16) need to be taken into account.62a A modified equation62b enables reactivity ratios in binary -CH=C-CH=C I R I R and ternary copolymerizations to be estimated from residual monomer concen-trations. Solvent effects in copolymerizations may be quite pronounced. The addition of water or glacial acetic acid to the copolymerization of MMA and acrylamide in dimethyl sulphoxide or chloroform changes the reactivity ratios.63 The copolymerization behaviour of MMA and STY is also affected by changes in solvent type.64 Such solvent dependence generally infers some form of complex-ing behaviour in the system either monomer-monomer or monomer-solvent.The effect of the contribution of the charge-transfer complex in the styrene-maleic anhydride system has been e ~ a l u a t e d . ~ ~ In strong donor solvents (acetone THF) there is a lot of solvent-maleic anhydride interaction and the monomer-monomer complex plays little part in the copolymerization. In benzene or CCl, styrene-maleic anhydride interactions are strong and it is suggested that the copolymer-ization is controlled by this complex. The effect of temperature on copolymer composition and microstructure is not easily studied since activation energy differences between the various reactions tend to be small and temperature changes have little effect.Such effects are best studied at low temperatures and only recently have suitable initiators become available. The copolymerization of STY and MMA between + 60 and - 60 "C has been studied using AIBN at +6O"C and the system CoH(N,)(PPh,),-dichloroethane at - 30 and - 60 0C.66 The reactivity ratios are found to fall slightly with decreasing temperature (r = 0.51-4.44; r = 0.46-4.29) leading to a greater degree of alternation and the low-temperature polymers also have a larger degree of syndiotacticity. Maleic anhydride is commonly used to form alternating copolymers with vinyl monomers.It has also been shown to produce high-molecular-weight copolymers with phenyla~etylene~~ and heptafluoroisopropyl methylallyl ether,6 both of 6 2 ( a ) B. A. Zaitsev A. G. Zak R. G. Luchko and G. A. Shtraikhman European Polymer J. 1972 8 1121 ; (6) V. Jaacks Makromol. Chem. 1972 161 161. 6 3 M. Jacob G. Smets and F. de Schryver J . Polymer Sci. Part A - I Polymer Chem., 1972 10 669. 64 G. G. Cameron and G. F. Esslemont Polymer 1972,13,435. 6 5 E. Tsuchida T. Tomono and H. Sano Makromol. Chem. 1972 151 245. 6 6 A. D. Jenkins and M. G. Rayner European Polymer J . 1972,8 221. 6' 6 8 W. L. Wasley and A. G. Pittman J . Polymer Sci. Part B Polymer Letters 1972 10, H. Block M. A. Cowd and S. M. Walker Polymer 1972 13 549. 279 1 42 P.Hyde and A . Ledwith which are difficult to homopolymerize to high molecular weight. Terpolymer-izations involving maleic anhydride STY and MMA or MA do not conform to the normal terpolymerization expression^,^^ and it is assumed that complex formation between STY and maleic anhydride is responsible for the anomalous behaviour. Terpolymerization of maleic anhydride with ethyl vinyl sulphide and ethyl vinyl ether has also been described;70" both monomers complex with maleic anhydride to a small extent and it was found that the reactivity of free vinyl sulphide to PMAn- was 4-6 times greater than that of free ethyl vinyl ether. Vinyl sulphides are also reported as compolymerizing thermally with weakly electron-accepting monomers such as MMA diethyl maleate and ethyl acryl-ate.70b Alternating copolymers may also be prepared by the copolymerization of an electron-rich olefin with an electron-deficient olefin in the presence of a Lewis acid.By this method alternating copolymers of STY and 1-substituted bicyclo-butanes [e.g. (17)] have been ~repared.~' The mechanism is thought to involve the interaction of a complexed growing radical with both monomer types which are then incorporated into the chain (Scheme 3). OMe C=O,ZnCl + CH,=CH 0 j)-3\~=o,ZnC12 U ' OMe (17) OMe OMe I Scheme 3 Addition of zi'nc bromide to the STY-diethyl fumarate system gives rise to 1 1 alternating copolymers for both ele~troinitiated~ and ph~toinitiated~~ systems. Similarly zinc chloride induces a 1 1 copolymerization of acrylonitrile and b~tadiene,~* although here the reaction is complicated by the formation of significant amounts of the cyclo-adduct of the two monomers 4-cyanocyclo-hexene.6 9 J. C. Bevington and C. Nicora Polymer 1972 13 249. 70 H. Inoue and T. Otsu ( a ) Makromol. Chem. 1972,153,37; (6) ibid. p. 21. H. K. Hall jun. and J. W. Rhoades J . Polymer Sci. Part A - I Polymer Chem. 1972, 10 1953. 72 D. C. Phillips D. H. Davies and J. D. Smith Makromol. Chem. 1972 154 317. 73 D. C. Phillips D . H. Davies and J. D. Smith Makromol. Chem. 1972 154 321. 7 4 W. Kuran S. Pasynkiewicz and 2. Floriahczyk Makromol. Chem. 1972 162 53 Kinetics and Mechanism of Addition Polymerization 143 Copolymerizations of vinyl monomers with sulphur dioxide give 1 1 alternat-ing copolymers (polysulphones) and can be initiated by conventional free-radical techniques.Polymerization of SO and but-1-ene in the gas phase can be initiated by electron radiation to give poly(but-1-ene sulphone) which has the repeat unit (18).” In the gas phase the ceiling 0 II I II Et 0 -CH2 -CH-S-(18) temperature (temperature at which propagation and depropagation reactions have equal rates and no polymer-ization occurs) is about 60 “C lower than it is in the liquid phase. The copolymer-ization of norbornene (19) and SO has been thought to be ~pontaneous,~~ initiation supposedly occurring via a biradical (20) which similar species to propagate a biradical chain (Scheme 4). then combines with Recent experiments combination n l 1 18 Scheme 4 using highly purified norbornene suggest that although the polymerization is extremely rapid even in the presence of very small amounts of added initiator, initiation in the ‘spontaneous’ reaction occurs via peroxidic i m p u r i t i e ~ .~ ~ The exact nature of the regulating mechanism for alternation in these systems is not known ; studies on styrene7 and chloroprene” copolymerization and various 7 5 J . R. Brown and J. H. O’Donnel J . Polymer Sci. Part A-1 Polymer Chem. 1972, 10 1997. 7 6 N. L. Zutty C. W. Wilson G. H. Potter D. C. Priest and C. J. Whitworth J . Polymer Sci. Part A General Papers 1965 3 278 1. 7 7 G. Sartori and R. D. Lundberg J . Polymer Sci. Part B Polymer Letters 1972,10 583. 7 8 M. Matsuda M. Iino T. Hirayama and T. Miyashita Macromolecules 1972 5 240.7 9 M. Matsuda and Y . Hara J . Polymer Sci. Part A-I Polymer Chem. 1972 10 837 144 P . Hyde and A . Ledwith terpolymerizationss0~8 have shown that although certain monomers do form complexes with SO, such complexes are unlikely to be involved in the propaga-tion steps. The copolymerizations of styrene and various a-substituted ~ t y r e n e ~ ~ ~ . ~ have been shown to be normal although retardation of rate and chain transfer occur in some systems. Of particular interest is the styrene-a-methylstyrene system where the reaction scheme needs to be modified to account for the rever-sible addition of P(a-MeSTY). to its own monomer at the temperatures used (Scheme 5). At higher temperatures (1 50 "C) reaction (2) becomes reversible, Me Me Me Me I I I I I Ph I Ph H Me H Me I I I I I Ph Ph Ph -CH,-C* + CH -C -CH,-C-CH,-C* (1) I Ph ,- I Ph -CH,-C.+ CH -C -CH,-C-CH,-C* (2) I Ph I ,- I Scheme 5 the equilibrium constant being approximately 1/30th that for a-methylstyrene homopolymerization i.e. the equilibrium lies well to the left. a-Methylstyrene-MMA copolymerization between ceiling temperatures has also been investi-gated.84b Styrene-divinylbenzene copolymers form the basis for many commercial ion-exchange resins. It has been shown that at very low conversion with only small quantities of divinylbenzene large numbers of rings are formed by intramolecular capture of pendant vinyl groups (Scheme 6).85 Intermolecular cross-linking will Scheme 6 M. Iino K. Seki and M. Matsuda J . Polymer Sci.Part A - I Polymer Chem. 1972, 10 2993. M. Matsuda M. Iino and S.-I. Numata J . Polymer Sci. Part A-I Polymer Chem., 1972 10 829. 8 2 J. P. Fischer Makromol. Chem. 1972 155 211. 8 3 J. P. Fischer Makromol. Chem. 1972 155 227. 8 4 (a) J. P. Fischer and W. Luders Makromol. Chem. 1972 155 239; (6) M. Izu K. F. O'Driscoll R. J. Hill M. J. Quinn and H. J. Harwood Macromolecules 1972 5 90. 8 5 B. Soper R. N . Haward and E. F. T. White J . Polymer Sci. Part A-I Polymer Chem., 1972 10 2545 Kinetics and Mechanism of Addition Polymerization 145 occur when the polymer concentration becomes sufficiently large. Di-isopro-penyl benzene is similar to divinylbenzene and forms cross-linked copolymers with STY in the same fashion.86 Acrylonitrile (AN) is an important monomer for the production of artificial fibres and copolymerization of AN with monomers containing functional groups is useful in improving d ~ e a b i l i t y .~ ~ Reactivity ratios of several acrylates towards AN have been determined88 and lie in the order a-Br-methyl acrylate > a-C1-methyl acrylate > MMA > MA. Copolymerization of acrylamide with a number of cis and trans 3-substituted acrylic acids8’ has shown that the relative reactivity of the cis-isomer is larger than the trans- when the substituent on the acid is electron-donating but that the order is reversed when the substituent is electron-withdrawing. In this system steric effects on reactivity are unimportant, and it is suggested that in the general case of 1,2-distributed ethylenes the order of reactivity of cis- and trans-isomers can be predicted from the difference in inductive parameters for the substituents.In copolymerizations of acrylic acid salts with acrylamide the nature of the metal cation has been shown to have a significant effect upon the monomer reactivity ratios and overall copolymerization rates.” This is ascribed to inter-actions between the growing radical and the fully dissociated acrylate anion, which are modified by the bonding of the metal cations (Ba2+ Sr2+ Ca2+) to acrylate segments in the polymer. Diallyl compounds are known to be unreactive in copolymerizations with styrene ;91 determinations of chain-transfer constants92 show that hydrogen abstraction by polystyryl radical to produce the stable ally1 radical (21) is an important process in these systems.PSTY- + H2C=CH-CH2R -+ PSTY-H + H2C=CH-eHR (3) (21) N-Vinyl compounds are very reactive in free-radical polymerization. Molecu-lar orbital calculations have shown that the various N-vinyl monomers should have similar Q and e parameters i.e. the H,C=CH-N < structure is the major contributor to the reactivity of these corn pound^.^^ As a test of this hypothesis, it is shown that copolymerization curves of STY-NVC and STY-N-vinyl-pyrrolidone are superimposable. Formation of a polyester by free-radical copolymerization of cyclohexene and formic acid in the presence of iodine and hydroperoxides is the first reaction of this type reported to yield high-molecular-weight The repeat unit 8 6 H. S. Kolesnikov A.-S. Tevlina L.Ye. Frumin and A. I. Kirilin Polymer Sci. ( U . S . S . R . ) 1971 13 625. *’ U. Bahr H. Wieden H.-A. Rinkler and G. Nischk Makromol. Chem. 1972 161 1 . * * J . Szaflco and E. Turska Makromol. Chem. 1972 156 311. 8 9 K. Matsuo and S. Machida J . Polymer Sci. Part A-I Polymer Chem. 1972 10 187. 90 K. Plochocka and T. J. Wojnarowski European Polymer J. 1972 8 921. 9 1 For example see A. Matsumoto and M. Oiwa Kogyo Kagaku Zusshi 1970,73 228. 9 2 A. Matsumoto and M. Oiwa J . Polymer Sci. Part A-I Polymer Chem. 1972,10 103. 9 3 I . Negulescu D. Feldman and Cr. Simionescu Polymer 1972 13 149. 94 C. G. Gebelein J . Polymer Sci. Part A-I Polymer Chem. 1972 10 1763 146 P. Hyde and A . tedwith in the polymer seems to be (22) retaining the original unsaturation of the cyclo-hexene.The exact mechanism of the polymerization is not yet known but is suggested as involving interaction of a cyclohexene-iodine complex with C 0 2 H radicals. Ph~toinitiation~’ of chloroprene polymerization by the system manganese carbonyl-poly(viny1 trichloroacetate) can be used to prepare cross-linked net-works of known constitution. Telechelic copolymers of chloroprene isoprene, and butadiene with p-chlorostyrene may be prepared by use of appropriately substituted azo-initiators ; chain extension via the functional end-groups is then possible and it is suggested that these materials may be of use as encapsulation resins.96 The copolymerization of styrene or maleic anhydride with a monomer (23) bearing an inhibitor function is surprising but may lead to polymers with improved oxidation re~istance.~’ Olefin (23) is only of low reactivity in copoly-CMe, H2C=CH-O-CH2-CH2-O-C -QOH (23) CMe, merization with STY (monomer 2) r l = 0.0; r 2 = 30.In the copolymerization of (23) with MMA,98 it was found that initiation by AIBN gave a normal copoly-merization but initiation by cumene hydroperoxide gave very little polymeriza-tion owing to the preference for the ketyl radical (24) to abstract the phenolic hydrogen of the monomer (Scheme 7). Me Me I Me (24) Ph-C-0. I + R 4 ; H + Ph-C-OH+ I R I Me CMe.3 CMe, R = H2C=CH-O-CHzCH2-02C-Scheme 7 9 5 J. Ashworth C. H. Bamford and E. G. Smith Polymer 1972 13 57. 9 6 S. F. Reed jun. J . Polymer Sci. Part A-1 Polymer Chem. 1972 10 2025. 9’ 9 8 M. Kato and Y.Nankano J . Polymer Sci. Part B Polymer Letters 1972,10 157. M. Kato and Y. Takemoto J . Polymer Sci. Part B Po[ymer Letters 1972 10 489 Kinetics and Mechanism of Addition Polymerization 147 Ethylenes are best polymerized with co-ordination catalysts but the free-radical copolymerization of ethylene and tetrafluoroethylene has been shown to give a 1 1 alternating copolymer of high chemical and thermal stability.99 The alternating tendency in this system is extremely high ; (r1r2 is 0.O001 at - 30 "C and 0.006 at + 65 "C) and polymers tend to be fairly crystalline (50-60 %). 2 Anionic Polymerization Initiation of anionic polymerization is normally via simple unifunctional low-molecular-weight organometallics (e.g. n-butyl-lithium) or bifunctional species such as a-methylstyrene tetramer (dianion).Significant differences in rates of initiation and propagation cause deviations from the ideal Poisson distribution for the molecular weight of the resulting polymer. Hacker''' has shown that, when a bifunctional initiator is used the dispersity (M,,,/M,) of the polymer should not exceed 1.4 even when k,/ki is as large as lo6. The sodium adduct of pyridine N-oxide has been shown to be capable of initiating the anionic polymerization of STY MMA and acrylonitrile."' From a study of the reaction products of sodium and pyridine N-oxide it was concluded that the active species in initiation are pyridyl 2,2'-bipyridyl and 4,4'-bipyridyl anion-radicals. Lithium di-n-butyl cuprate' O 2 polymerizes MMA to a polymer of tacticity similar to that produced via conventional organolithium initiators.Attempted MMA copolymerization with STY gave polymers containing no STY a result again consistent with conventional anionic initiation.'03 The cuprate salt is exceptional in that it will polymerize NVC copolymerizations with MMA and vinyl acetate show that a radical mechanism is not operative and it is suggested that the NVC anion is co-ordinated by the copper enabling a nucleophilic addition to a co-ordinated monomer molecule to take place ; see (25). - -9 9 100 1 0 1 1 0 2 103 R I I - - * ' - * - . - I p!., -cH,-CH ,,=-' .'- HC=CH, M. Modena C. Garbuglio and M. Ragazzini J . Polymer Sci. Part B Polymer Letters, 1972 10 153. H. Hocker Makromol. Chem. 1972 157 187. K. Yamaguchi T.Yoshida and Y. Minoura J . Polymer Sci. Part A - I Polymer Chem., 1972 10 2501. M. E. Londrigan and J. E. Mulvaney J . Polymer Sci. Part A - I Polymer Chem., 1972 10 2487. W. Fowells C. Schuerch F. A. Bovey and F. P. Hood J . Amer. Chem. SOC. 1967, 89. 1396 148 P. Hyde and A . Ledwith The first photo-initiated anionic polymerization has been reportedlo4 and involves the polymerization of nitroethylene in THF. From a study of the wavelengths of light which activate the polymerization it is concluded that initiation occurs via a photoexcited THF-nitroethylene charge-transfer complex. Reports of initiation by phosphorous ylides,'05 metal xanthates,lo6 and potas-sium-graphite inclusion comp~unds'~ have also appeared. Because of the variety of propagating species (ion pairs contact ion pairs, solvent-separated ion pairs free ions) which may occur in a given system rates of polymerization for a given monomer-initiator system are found to be extremely solvent dependent.For the polymerization of STY in tetrahydropyran it has been confirmed that the large differences in rate observed between this system and other STY-ether systems are attributable to shifts in the equilibrium between contact ion pairs solvent-separated ion pairs and free ions rather than to any solvent effect on the individual rate constants for each ionic species."* The type of ionic species present in a system can be determined from studies of conductivity which may be supported by U.V. spectral data."'. '' using a variety of initiators with THF and 2-methyl-THF as solvents show that the free-ion rate constants for derivatives (0- rn- p-methyl- and p-isopropyl-STY) are generally smaller than for STY and also that these rate constants are reduced by changing from THF to 2-methyl-THF.Hammett plots for the propagation rate constants were dependent on the system for all but the free-ion rate constants. It was concluded that in the case of sodium salts of living polymers the value of k is a reflection of the dissociation state of the polymer ion whereas for the caesium salts the major influence on k is the reactivity of the free ion. Oligomers of or-methylstyrene may be used as initiators for anionic polymeriza-tion. Ageing of these compounds has been shown to result from either hydride transfer to the counter ion or hydrogen transfer from solvent (Scheme 8).'12 Olefins (26) and (27) may then react with further oligomers uia hydrogen transfer.The rate of polymerization of MMA initiated by the disodium salt of a-methyl-styrene tetramer is found to depend largely on the method of initiation.' When the monomer is added in one step the polymerization proceeds mainly via Kinetic studies on the polymerization of styrenes,' l o 4 M. hie S. Tomimoto and K. Hayashi J . Polymer Sci. Part B Polymer Letters 1972, 10 699. H. Klippert and H. Ringsdorf Makromol. Chem. 1972 153 289. 1972 10 63. 10 1267. L. L. Bohm and G. V. Schulz Makromol. Chem. 1972,153 5 . l o 6 K. Yamaguchi 0. Sonoda and Y . Minoura J . Polymer Sci. Part A- I Polymer Chem., l o ' I. M. Panayotov and I.B. Rashkov J . Polymer Sci. Part A-1 Polymer Chem. 1972, l o g H. Hirohara M. Nakayama R. Kawabata and N . Ise J.C.S.Faraday I 1972 68 51. D. K. Polyakov N . I. Baranova A. R. Gantmakher and S. S. Medvedev Makromol. Chem. 1972 152 1 . B. J. Schmitt Makromol. Chem. 1972 156 243. P. E. M. Allen R. P. Chaplin and D. 0. Jordan European Polymer J . 1972.8 271. l o H. Hirohara M. Nakayama and N . Ise J.C.S. Faraday I 1972,68 5 8 Kinetics and Mechanism of Addition Polymerization Me Me I / I I \ -c-c=c + /" Ph H Ph NaH (26) M e H Me I l l I l l I l l Ph H Ph \ Ph H Ph -C-C-C-Na+ + -C-C-C=CH + NaH \" M e H Me n I l l H Ph H Ph 149 Scheme 8 solvent-separated ion pairs (k > 25 1 mol-' s-' at 200 K) whereas initiation via the two-step technique (addition of a trace of monomer followed shortly after by the bulk) results in propagation via contact ion pairs ( k = 1.2 x 10- ' 1 mol- ' s-' at 283 K).The discrepancy between the two types of initiation is not observed at higher temperatures where presumably relaxation of solvent-separated ion pairs to contact ion pairs occurs more rapidly. MMA homopolymerization' l4 and copolymerization with STY '' initiated by suspensions of alkaline-earth metals has been shown to involve parallel radical and anionic mechanisms. However initiation of diene copolymerization by these metals seems to occur via a purely anionic route.' l 6 Branching in MMA polymerization occurs by reaction at an ester group on monomer or polymer. A viscometer study has shown that n-butyl-lithium-initiated polymerization results in such branching the frequency of which is independent of conversion up to very high conversion and increases with in-creasing temperature.' ' Polymerizations initiated by lithium t-butoxide give linear polymers ascribed to co-ordination of alkoxide at the growing centre preventing reaction with ester groups.Polymerization by t-butoxide ion is inhibited by p-benzoquinone' at initiator inhibitor ratios < unity suggesting some form of addition reaction of t-butoxide and benzoquinone. When excess initiator is used polymerization proceeds but the rate increases and the polymer stereochemistry changes under the influence of this adduct. In general anionic polymerization of methacrylic esters gives highly syndiotactic polymers,' ' the 'I4 C.Mathis and B. Francois Makromol. Chem. 1972 156 7. 'Is C. Mathis and B. FranGois Makromol. Chem. 1972 156 17. 'I6 P. Maleki and B. Francois Makromol. Chc~nr 1972 156 31. I " J. Trekoval and P. Kratochvil J . Polymer Sci. Part A-1 Polymer Chem. 1972 10, 1391. P. VlEek and J. Trekoval Coll. Czech. Chem. Comm. 1972 37 1918. J. Junquera N. Cardona and J. E. Figueruelo Makromol. Chem. 1972 160 159 150 P . Hyde and A . Ledwith degree of syndiotacticity increasing with the gegen-ion in the order Li+ > Na+ > K+ and also to a small extent with the ester group (methyl x ethyl < n-butyl). Acrylonitrile (AN) is polymerized by triphenylphosphine in DMF at 30 O C . 1 2 0 The initiation mechanism involves zwitterion formation from PPh and AN, followed by hydrogen abstraction from monomer (Scheme 9) ; the resulting anion (28) is the effective initiator.Ph,P + H,C=CHCN -+ Ph,;-CH,-CHCN Polymer C- H,C=eCN + Ph,h-CH,CH,CN Scheme 9 H,C=CHCN monomer (28) Dienes are quite conveniently polymerized by anionic means. l,l'-Bis(tri-fluoromethyl)buta-l,3-diene can be polymerized by butyl-lithium to give a living polymer.121 Although this will not add butadiene to give a block copoly-mer propagating butadiene will add the fluorinated monomer. 1 -Ferrocenylbuta-1,3-diene is also polymerized by butyl-lithium.122" Polydispersion in this system is high and increases with molecular weight suggesting some form of activity transfer to the ferrocenyl nucleus (29) which cannot then propagate the chain. The preparation polymerization and copolymerization of 1-cyanobutadiene have also been reported.' 22b H H I I -C-CH=CH-CH,Li+ --* -C-CH=CH,-CH, Fe Living ends of polymers are found to be associated in hydrocarbon solvents and this may cause complications in the interpretation of kinetic orders of propagation reactions.Light-scattering and viscometric studies have shown that l Z o V. Jaacks C. D. Eisenbach and W. Kern Makromol. Chem. 1972 161 139. l Z 1 M. H. Kaufman J . Polymer Sci. Part A - I Polymer Chem. 1972 10 455. l z 2 ( a ) D. C . Van Landuyt J . Polymer Sci. Part B Polymer Letters 1972 10 125; (6) R. Worley and R. N. Young European Polymer J . 1972 8 1355 Kinetics and Mechanism of Addition Polymerization 151 polystyryl-lithium has an association number 2 in cyclohexane but for the corresponding polybutadienyl and polyisoprenyl salts the value is 4.'23 Pre-viously reported discrepancies in the values for the latter salts are explained by the formation of dimers at low concentration.Use of polar modifiers' 244 in butadiene polymerization affect the polymer microstructure ; suitable choice of modifier concentration and polymerization temperature allows the preparation of polybutadienes with any desired 1,2 microstructure. A similar study has been carried out with 2-~henylbutadiene,'~~' where polar modifiers induce high percentages of pendant vinyl groups in the polymers. Polymer microstructure is also affected by the nature of the interaction between the growing anion and the gegen-ion and has been studied for a number of butadienes.l2' The effect of solvent and gegen-ion in isoprene polymerization is also to influence the rate of polymerization and polymer microstructure.126 Free ions give a polymer of structure 1,4- 25 % 1,2- 33 % 3,4- 42 % independent of the cation.When the solvent favours propagation via contact ion pairs the cation is of extreme importance. Changing from Li+ to Cs' markedly influences the relative amounts of 1,4- (highest for Cs') and 3,4- (highest for Li') structures. This is interpreted (Scheme 10) in terms of different co-ordination of the diene with the small Li' (30) and large Cs+ (31) ions. _+ P Li+ -+ P (30) 3,4-addition cs+ . (31) 1,4-addition Scheme 10 Initiation of isoprene polymerization with l,l'-diphenyl-n-hexyl-lithium127 gives polymers of high 1,4 content predominantly in the cis-form.Determination of the activation energy of propagation gave a value of 19 kcal mol- and the 1 2 3 124 1 2 5 1 2 6 1 2 7 D. J. Worsfold and S. Bywater Macromolecules 1972 5 393. ( a ) T. A. Antkowiak A. E. Oberster A. F. Halasa and D. P. Tate J . Polymer Sci., Part A- I Polymer Chem. 1972 10 1319. (b) R. J. Ambrose and W. L. Hergenrother, Macromolecules 1972 5 275. M. Morton L. A. Falvo and L. J. Fetters J . Polymer Sci. Part A-I Polymer Chem., 1972 10 561. A. Essel and Q. T. Pham J . Polymer Sci. Part A-1 Polymer Chem. 1972 10 2793. J. M . Alvariiio A. Bello and G. M. Guzman European Polymer J . 1972 8 53 152 P . Hyde and A . Ledwith polymerization is first order in monomer but only of order 0.25 with respect to total polyisoprenyl-lithium indicative of the association of the living anions.Terminally functional polybutadienes have been prepared by the reaction of living polybutadiene and toluene di-isocyanate (TDI),' 28 giving isocyanate-terminated polymers capable of chain extension with caprolactam to give butadiene-nylon 6 block copolymers or with excess TDI and diamines to give butadiene-urea block copolymers. Polybutadienes with hydroxyl end-groups may be made by end-capping with ethylene oxide and subsequent acidic hydro-l y s i ~ . ' ~ ~ Because of its use in nylon manufacture caprolactam polymerization is of basic interest. Metal cations'30 play an important role in the polymerization, owing to complex formation and can strongly retard or even inhibit polymeriza-tion.Viscometric parameters have been given for molecular-weight determina-tions in cresol and H2S0,;'31 comparison with osmometric data indicates that branching is not uncommon. Methods for determining the number of keto-groups in polymers'32 and the content of catalytically active bases'33 have also been given. The sodium alcoholate-initiated polymerization of E-caprolactone' 34 and its ABA-type copolymerization with ethylene oxide' 35 have been reported. Pyrrolidone is similar to caprolactam and may be polymerized using a variety of initiators. '36 Copolymerization with caprolactam has been described ;' the effect of increasing the temperature (70-200 "C) is to decrease the pyrrolidone content of the polymers. Amongst the many other polymerizations reported are those of l-vinyl-pyrene,' 38 isopropenyl-substituted pyridine oxazole and thiazole,' 39 maleic anhydride initiated by pyridine bases or phosphines,' 40 and acryloyl- and methacryloyl-verdazyls (32 ; R = acryloyl or metha~ryloyl).'~' This last case is of great interest since it incorporates a stable free radical into the polymer with consequent antioxidant and/or semiconductor properties.2-Vinylpyridine is polymerizable anionically and conductometric and spectrophotometric studies have shown secondary ionic species (trienyl carb-anions or nitranions) to be significant by-products of the initiation re-12' W. L. Hergenrother and R. J. Ambrose J . Polymer Sci. Part B Polymer Letters, 1972 10 679. S. F. Reed jun. J . Polymer Sci.Part A- I Polymer Chem. 1972 10 1187. 1 3 0 R. Puffr and J. Sebenda European Polymer J . 1972 8 1037. 1 3 ' C. V. Goebel P. Cefelin J. StehliEek and J. Sebenda J . Polymer Sci. Part A - I , Polymer Chem. 1972 10 1411. J. StehliEek P. Cefelin and J. Sebenda Coll. Czech. Chem. Comm. 1972 37 1926. 1 3 3 J. Sebenda and V. Koufil European Polymer J . 1972,8,437. 1 3 4 R. Perret and A. Skoulios Makromol. Chem. 1972 152 291. 1 3 5 R. Perret and A. Skoulios Makromol. Chem. 1972 156 143. 1 3 6 G. Schirawski Makromol. Chem. 1972 161 57. 1 3 ' G. Schirawski Makromol. Chem. 1972 161 69. 1 3 ' J. J. O'Malley J. F. Yanus and J. M. Pearson Macromolecules 1972 5 158. 1 3 9 K. Yagi T. Miyazaki H. Okitsu F. Toda and Y. Iwakura J . Polymer Sci. Par1 A - I , Polymer Chem. 1972 10 1149.H. Zweifel J. Loliger and T. Volker Makromol. Chem. 1972 153 125. Y . Miura M. Kinoshita and M. Imoto Makromol. Chem. 1972 157 51. 14 Kinetics and Mechanism of Addition Polymerization 153 Ph / \ Ph 143 Copolymerization of 2-vinylpyridine with propylene sulphide has been r e p ~ r t e d ; ' ~ ~ ~ ' ~ ~ here initiation by biphenylsodium results in the formation of pyridyl ion-radicals which dimerize ; subsequent polymerization gives 4,4'-bipyridyl units in the polymer chain (cf. ref. 101). The polymerization of vinyl methacrylate and vinyl acrylate initiated by n-butyl-lithium proceeds via the acrylate double bond only to give linear poly-mers of the type fCH2-C(R)(C0,CH=CH2)+,,.'46 Calibration of i.r. pro-cedures with these polymers enables estimates to be made of the extent of cycliza-tion during the free-radical polymerization.Unsaturated aldehydes have been polymerized by the use of anion-radicals of aromatic carbonyl corn pound^.'^^^'^^ The suggested initiation mechanism is one of electron transfer to monomer but analyses for regenerated ketone are low (generally < 50 %) owing to combination of monomer and ketone to give (33) (Scheme 11). 2Ph2c-0- + Me-CH=CH-CHO + Ph2C0 + Me CH 0 \ / \ * * . ; 2 - ' CH ,e'-'.,CH' '**--' I / c-0 / I OH Me /CH2 / CH CH Ph Ph I I /c- 0 Ph I Ph (33) Scheme 11 14' M. Tardi and P. Sigwalt European Polymer J . 1972 8 137. 1 4 3 M. Tardi and P. Sigwalt European Polymer J. 1972 8 151. A. Gourdenne Makromol. Chem. 1972 158 261. 1 4 5 A. Gourdenne Makromol.Chem. 1972 158 271. 146 W. Fukuda M. Nakao K. Okumura and H. Kakiuchi J . Polymer Sci. Part A-1, 14' I. M. Panayotov and I. B. Rashkov Makromol. Chem. 1972 154 129. 1 4 * I . B. Rashkov and I. M. Panayotov Makromol. Chem. 1972 151 275. Polymer Chem. 1372 10 237 154 P. Hyde and A . Ledwith The polymerization of succinonitrile gives polymers whose structure depends on the initiator. 149 Either semiconducting (sodium methanolate) or dielectric (potassium t-butylate) polymers may be obtained. Metal complexes of nitriles or ketones are found to initiate the polymerization of cyclic s ~ l p h i d e s . ' ~ ~ Initi-ation by aromatic nitrile dianions proceeds via electron transfer but initiation by the corresponding anion-radical varies with the monomer ; styrene sulphide (34) undergoes an electron-transfer reaction but ethylene sulphide (35) adds.ArCN- + Ph-CH -CH -+ ArCN + Ph-bH-CH,-S-\S/ (34) - CH -+ ArC=N-CH,-CH,-S-H2C\ / Arc" + S (35) Olefin oxides are polymerizable anionically and recent studies on propylene' and 1,2-butylene oxides'" have shown that in DMSO the order of reaction with respect to initiator is high (1.7-1.8) owing to propagation via both free ions and ion-pairs whereas in THF-DMSO mixtures only ion pairs are involved and the order of reaction for initiator is unity. The effects of substituents'53 on ethylene oxide polymerization and its copolymerization with propylene oxide' 54 have been described. Copolymerization.-Although anionic formation of block copolymers is well known and still extensively studied comparatively little effort is expended in the study of copolymerization systems which result in the distribution of the mono-mers along the whole length of the chain.Copolymerization of acrolein with aldehydes' gives unsaturated polyacetals since the acrolein is incorporated via its carbonyl double bond i.e. R- + CH,=CH-CH=O + R-CH(CH=CH,)-O- -P polymer These polymers have greater stability to photochemical and oxidative cross-linking than acrolein homopolymer. Anionic copolymerizatior of various alkyl dibromides (RX,) with STY or a-methylstyrene gives regular copolymers of structure +monomer-monomer-R j-, 149 D. Wohrle Makromol. Chem. 1972 160 83. l S o I. M. Panayotov and I . V. Berlinova Makromol. Chem. 1972 154 139. 1 5 1 L.P. Blanchard V. Hornof J. Moinard and F. Tahiani J . Polymer Sci. Part A-1, Polymer Chem. 1972 10 3089. L. P. Blanchard K. T. Dinh J. Moinard and F. Tahiani J . Polymer Sci. Part A - I , Polymer Chem. 1972 10 1353. 1 5 3 V. A. Ponomarenko A. M. Khomutov S. I. Il'chenko and A. V. Ignatenko Polymer Sci. (U.S.S.R.) 1971 13 1735. 1 5 4 G. A. Gladkovskii and Ye. V. Ryzhenkova Polymer Sci. (U.S.S.R.) 1971 13 723. I s 5 J. L. Mateo and R. Sastre Makromol. Chem. 1972 157 141 Kinetics and Mechanism of Addition Polymerization 155 when the feed has 2 1 monomer dibromide molar stoicheiometry,' 56 which become less regular and richer in monomer as the proportion of dihalide de-creases. Because of the reactivity of styrene in anionic polymerization its copolymerization is complicated by formation of homopolymer after consump-tion of the dihalide component when this is present only in small amounts.Similar studies have been reported for butadiene and isoprene.' The copolymerization of cyclic sulphides and aryl or alkyl isothiocyanates (36) is reported to yield a regular alternating copolymer (Scheme l2).ls8 Isothio-NR' S NR' R2 II / \ II I -C-S- + H2C-CH --* -C-S-CH2-CH-S-I (36) R2 R2 R2 NR ' I I II -CH2-CH-S- + R'-N=C=S --* -CH,-CH-S-C-S-(37) Scheme 12 cyanates do not homopolymerize anionically but appear to be reactive enough to scavenge the ring-opened sulphide (37) before it can add a second sulphide monomer unit. 3 Cationic Polymerization The cationic polymerization of monomers initiated by Friedel-Crafts halides has been supposed always to require the presence of small amounts of a co-catalyst (water HCl alkyl halides).Kennedy'"" has recently made a critical evaluation of much of the literature pertinent to this concept and has concluded that there are in fact many systems which will polymerize in a wholly reproducible fashion in the absence of co-catalyst. It was proposed that the self-initiation reaction shown in Scheme 13 occurs. Key factors in this mechanism are basicity I l l I l l I I I I C=C-CH + MX C=C-C+MX,H-Scheme 13 of the Lewis acid MX, facilitating the hydride-abstraction reaction and the stability of the resulting allylic cation. Initiation is therefore not possible when the monomer does not have an allylic hydrogen and in support it is found that a-methylstyrene is polymerized by AlEt,Cl whereas STY is not.A. V. Cunliffe W. J. Hubbert and D. H. Richards Makromol.Chem. 1972 157 23. 15' A. V. Cunliffe W. J. Hubbert and D. H. Richards Makromol. Chem. 1972 157 39. G. Belonovskaya Zh. Tchernova and B. Dolgoplosk European Polymer J. 1972, 8 35. ( a ) J. P. Kennedy J . Macromol. Sci. Chem. 1972 A6 329; (6) N. A. Ghanem and M. Marek European Polymer J . 1972 8 999 156 P. Hyde and A . Ledwith Further experimental support rejecting the necessity for co-catalytic activity comes from a study of the polymerization of isobutene initiated by AlBr or BF under anhydrous conditions;' 5 9 b TiCl, however was found to require the addition of water. The use of tritiated water indicates that no incorporation of protons occurs during the initiation process and it is proposed that the function of any water present is to solvate (and so stabilize ion-pair formatioil ~ .g . "2: TiCI . Studies of the effect of water on the polymerization of styrene in various solvents indicate that the primary effect is to influence the equilibria between contact ion pairs solvated ion pairs and free ions for the initiator.160,'61 Overall rates of polymerization show maxima as solvent polarity and water concentration are changed. n-Alkyl and benzyl halides have been used as co-catalysts in the aluminium iodide-initiated polymerization of isobutene ; 16* their function is not simply co-catalytic however and it appears that a halide-exchange reaction occurs between the n-alkyl halides and aluminium iodide to give catalytically active products.Cationic polymerization may be electro-initiated using various metal salts as electrolytes in polar solutions. The electro-initiated polymerization of NVC using zinc bromide in acetone is found to produce low-molecular-weight polymer having narrow di~tribution.'~~ Changing the rate of initiation and electrolyte concentration has little effect on either molecular weight or distribution. Electro-initiation of styrene polymerization yields a polymer suggested as having 'living' character with molecular weight governed by transfer to monomer.'64 Values of k depend on the type of initiation continuous electrolysis gives k z 4.2 1 mol- ' min- whereas polymerization after the current has stopped has k x 1.6 1 mol-' min-'.Electro-initiation of n-butyl vinyl ether'65 and ethyl and isobutyl vinyl ether'66 polymerization results in similar behaviour. Transfer to monomer and solvent govern the molecular weight although here th, systems are not thought to be 'living'. Vinyl ethers are readily polymerizable by cationic mechanisms ; polymerization of alkyl vinyl ethers can be induced by the presence of acceptor molecules whose function is to complex with the ether electron transfer then giving an anion-radical and an initiating cation-radi~a1.I~~ The order of reactivity of a number of alkyl vinyl ethers was found to be t-butyl > isopropyl > ethyl % n-butyl x isobutyl which corresponds to the order of inductive effects of the alkyl groups. Pure tetracyanoethylene (TCNE) a strong acceptor molecule did not induce polymerization preferring instead to add to the vinyl ethers to form cyclobutanes.Samples of TCNE contaminated with tricyanoethanol however were sufficiently acidic to initiate polymerization. 160 M. KuEera J. Svabik and K. Majerova Coll. Czech. Chem. Comm. 1972 37 2004. 1 6 1 M. KuEera J. Svabik and K. Majerova Calf. Czech. Chem. Cornm. 1972,37,2708. 1 6 2 P. Lopour J. Pecka and M. Marek Makrornof. Chern. 1972 151 139. 1 6 3 D. C. Phillips D . H. Davies and J. D. B. Smith Macromolecules 1972,5 674. 164 G . Meqgoli and G . Vidotto European Polymer J . 1972 8 661. G. Mengoli and G. Vidotto European Polymer J. 1972 8 671. 1 6 6 G. Mengoli and G. Vidotto Makromof. Chem. 1972 153 57. 1 6 ' R. F. Tarvin S. Aoki and J. K. Stille Macromolecufes 1972 5 663 Kinetics and Mechanism of Addition Polymerization 157 Anilinium hexafluoroantimonate (PhNH,+ SbF,-) is reported to be a good initiator for the polymerization of ethyl vinyl ether.',* In common with hexa-chloroantimonate salts the anion is large chemically stable and has no reaction with growing cations.Catalysis is very efficient concentrations as low as lo-, mol % producing nearly quantitative yields of polymer. Stereoregular polymers may be prepared from butenyl ethers (EtCHZCHOR) at low temperature.' 69 Crystalline polymers were obtained from the methyl and ethyl ethers the same stereochemistry being obtained using either the cis- or trans-monomer. Positive assignment of the stereochemistry has yet to be made, and a similar situation occurs for poly(methy1 propenyl ether).' 7 0 The polymer-ization of a number of vinyl ethers of oxy-acenaphthenes (38; R' = R2 = OCH=CH,; R' = H R2 = OCH=CH,; R' = OH R2 = OCH=CH,) has been reported.' " These monomers are cationically polymerizable by BF, etherate and are also susceptible to radical polymerization.Episulphides and epoxides although structurally similar show important differences in mode of reaction in ring-opening polymerization.' 72 cis and trans-but-2-ene serve as examples. Whereas the cis-sulphide gives crystalline polymer and the cis-oxide gives amorphous polymer the trans-oxide gives a crystalline polymer and the trans-sulphide does not. The difference in behaviour of the trans-compounds is ascribed to the extra length of the C-S bond which lowers steric hindrance so reducing isomer selection.The difference in the cis-compounds is suggested as resulting from co-ordination of the gegen-ion with the sulphur atom in the last-added unit. Polymerization of propene sulphide initiated by AlEt,-H,O gives amorphous polymer but addition of alcohols to the initiator system induces partial crystallinity.' 7 3 Olefin oxides may also be polymerized with dialkylzinc-tertiary alcohols' 74 or diethylzinc-nitromethane. 75 The latter system requires heat treatment at 100 "C for activation but is consequently 1 6 8 1 6 9 I 7 0 1 7 1 1 7 2 1 7 3 1 7 4 1 7 5 J. J. Harris and S. C. Temin J . Polymer Sci. Part A-1 Polymer Chem. 1972 10 1165. T. Higashimura and M. Hoshino J . Polymer Sci. Part A-1 Polymer Chem.1972, 10 673. T. Higashimura and M. Hoshino J . Polymer Sci. Part B Polymer Letters 1972 10, 269. A. I. Levchenko T. N. Pliev N. I . Shishkina and S. V. Timoshenko Polymer Sci. ( U . S . S . R . ) 1971 13 2560. E. J. Vandenberg J . Polymer Sci Part A-1 Polymer Chem. 1972 10 329. P. Dumas N . Spassky and P. Sigwalt Makromol. Chem. 1972 156 65. M. Nakaniwa I . Kameoka R. Hirai and J. Furukawa Makromol. Chem. 1972, 155 197. M. Nakaniwa I. Kameoka K. Ozaki N. Kawabata and J. Furukawa Makromof. Chem. 1972 155 185 158 P. Hyde and A . Ledwith stable and shows high catalytic activity. The nature of the catalyst is not known precisely but it is not the product of reaction of diethylzinc and nitromethane (zinc methazonate) which is inactive in polymerization.Two independent studies' 76,1 7 7 of the ring-opening polymerization of the bicyclic ether 2-methyl-7-oxabicyclo[2,2,l]heptane (39) have concluded that propagation proceeds oiu nucleophilic monomer attack at C-4 of the derived oxonium ion for both endo- and exo-isomers resulting in single (but different) stereochemistry for the polymer from each isomer (Scheme 14). The polymer H Me = 03 H + + 5 0 exo (41) Scheme 14 from the endo-isomer is thought to exist in the equatorial conformation (40), whereas the polymer from the exo-isomer has an equilibrium between the equatorial (41) and axial (42) conformations. Kinetic studies on these system^'^^,'^^ indicate that k for the endo-isomer is always larger than k for the exo-isomer. Arrhenius plots show that this difference arises chiefly from the value of the A-factor:exo AE,* 15 kcal mol-' A,* 41 x lo9 1 mol- ' s-' ; endo, AE,* 14 kcal mol-' A,* 7 x lo9 1 mol- s- '.Ring-opening polymerization studies have also been performed for oxepan (AEp * 18 kcal mol- ' A * 1.9 x lo9 1 mol- ' s- 2-methyl-2-0xazoline,'~~ and a series of gern-dichloro-bicyclo-[n,l,O]alkane~,'~~ where opening of the ring bridge is accompanied by a simul-1 7 6 J. Kops and H. Spanggard Makromol. Chem. 1972 151 21. 17' T. Saegusa M. Motoi S. Matsumoto and H. Fujii Macromolecules 1972 5 233. 1 7 8 T. Saegusa S. Matsumoto M. Motoi and H. Fujii Mucromolecules 1972 5 236. T. Saegusa M. Motoi S. Matsumoto and H. Fujii Macromolecules 1972 5 815. T. Saegusa T. Shiota S. Matsumoto and H. Fujii Macromolecules 1972 5 34.T. Saegusa H. Ikeda and H. Fujii Mucromolecules 1972 5 359. l s 2 Chr. P. Pinazzi A. Pleurdeau J. C. Brosse and J. Brossas Mukromol. Chem. 1972, 156 173 Kinetics and Mechanism of Addition Polymerization 159 taneous dehydrochlorination. Ring-opening polymerization of 3,3-dimethyl-selenetan (43) proceeds to give polymers with selenium in the main chain but only of low molecular weight due to transfer reactions to p01ymer.l~~ Anionic polymerization however gives high-molecular-weight material. The polymerization of indene (44) by methyl ethyl ketone peroxide in liquid SO has some of the characteristics of a cationic polymerization,' 84 suggested as arising from a charge-transfer interaction between an indene-SO complex and MEK peroxide but the mechanism needs further clarification.The polymer-ization of styrene initiated by metal halides and chlorosilanes is suggested as being initiated by a siliconium cation arising from charge-transfer interaction with the metal halide^.'^ Cationic polymerization of fluoral yields polymers which are thermally unstable;' 86 end-capping with a variety of reagents improves the thermal stability and allows calculations of wn by i.r. estimation of end groups. Copolymerization.-The effect of diols and triols as co-catalysts in the copolymer-ization of THF and propene oxide has been extensively studied.' 87-1 89 Although the SbC15-propane-l,2-diol system cannot initiate homopolymerization of THF, it will initiate ethylene oxide homopolymerization and copolymerization with THF.The copolymer reactivity ratios are r l (propene oxide) 1.15 and r (THF) 0.7 suggesting that THF reacts to some extent with its own active centre. Chain transfer is extensive in this system and keeps molecular weights very low. Varia-tion of diol type and concentration affects the rate of monomer consumption, explained in terms of the solvation properties of the alcohols. A study of the cationic copolymerization of anethole [ l-methyl-2-(4-methoxy-phenyl)ethylene]'90 has shown that in additions to growing cations with little 1 8 3 184 1 8 5 1 8 6 1 8 7 I 8 8 189 190 E. J. Goethals E. Schacht and D. Tack J . Polymer Sci. Part A - I Polymer Chem., 1972 10 533. A. De Souza Gomes 0. Do Conto Filho J . Polymer Sci. Part B Polymer Letters, 1972 10 725.Y . Minoura and H. Toshima J . Polymer Sci. Part A-1 Polymer Chem. 1972 10, 1109. W. K. Busfield and I. J. McEwen European Polymer J. 1972 8 789. L. P. Blanchard S. Kondo J. Moinard J. F. Pierson and F. Tahiani J . Polymer Sci., Part A - I Polymer Chem. 1972 10 399. E. J. Alvarez V. Hornof and L. P. Blanchard J . Polymer Sci. Part A- I Polymer Chem., 1972 10 1895. E. J. Alvarez V. Hornof and L. P. Blanchard J . Polymer Sci. Part A-1 Polymer Chem., 1972 10,2237. T. Higashimura K. Kawamura and T. Masuda J . Polymer Sci. Part A-1 Polymer Chem. 1972,10,85 160 P. Hyde and A . Ledwith steric hindrance [P(STY)+ P(p-OMe-STY)'] the trans-isomer is the more reactive whereas in the mutual copolymerization of cis- and trans-anethole, where the attacking cation is always the same i.e.a P-methyl-p-methoxystyryl cation the steric effect of the methyl group makes the cis-isomer more reactive. This behaviour is in contrast to vinyl ethers where the cis-isomer is always the more reactive. The copolymerization of 1,3-dioxolan with diketen is found to procede 70-80 % by addition of the dioxolan cation to the C=C bond of the diketen,'" and polymers have random arrangements of the monomer units. The copolymer-izations of dioxolan with t e t r ~ x a n ' ~ ~ (which is complicated by the high reactivity and consequent early consumption of tetroxan) and with t r i ~ x a n ' ~ ~ have also been described. Cyclopolymerization is most commonly observed in free-radical or conden-sation systems.' 94 Of interest therefore is the cationic copolymerization of benzaldehyde and divinyl ether to yield polymers with large numbers of the ring structures (45) and (46).'95 \ O-CH-I Ph 4 Polymerization by Co-ordination Catalysts Because of difficulties concerning the nature and number of active sites studies of Ziegler-Natta and related polymerizations place the emphasis on the type of catalyst used and its effect on the stereochemistry of the resulting polymer, rather than on the kinetics of the polymerization process.However a general kinetic scheme has been suggested for Ziegler-Natta polymerization occurring between adsorbed monomer and an active centre formed by reaction of a metal alkyl and a transition-metal halide,lg6 and has been shown to be valid for the system 4-methylpent-l-ene-VCl,-A1R3 .lg7 AEG is found to vary between 16.6 kcal mol- ' (benzene) and 13.7 kcal mol- ' (heptane; a poorer solvent than benzene).Determination of the number of active centres using tritiated quenchers ''I M. Okada Y. Yokoyama and H. Sumitomo Makromol. Chem. 1972 162 31. 19' Y. Yamashita T. Inoue G. Hattori and K. Ito Makromol. Chem. 1972 151 91. 1 9 3 D. Fleischer R. C. Schulz and B. Turcsanyi Makromol. Chem. 1972 152 305. For a recent review see G. C. Corfield Chem. SOC. Rev. 1972 1 523. ''' C. Aso and S. Tagami J. Polymer Sci. Part A - I Polymer Chem. 1972 10 1851. l g 6 D. R. Burfield I. D. McKenzie and P. J. T. Tait Polymer 1972 13 302. l g 7 I. D. McKenzie P. J. T. Tait and D. R. Burfield Polymer 1972 13 307 Kinetics and Mechanism of Addition Polymerization 161 gives a value of 2.3-6.1 x mol per mol VC1 chain-transfer constants for a number of aluminium alkylslg9 and monomers2oo have been estimated.Catalyst systems based on TiC1 are the most commonly studied and reports have been published concerning the polymerizations of heptenes and 0ct-2-ene,~” butadiene and ethylene,202 pr~pene,~’ and butene and substituted ethylenes.’04 The effect of hydrogen on the polymerization of 4-methylpent-1-ene catalysed by y-TiCl,-AlEt,Cl is to increase the rate of polymerization and decrease the rate of deactivation of the cataly~t.~” It is suggested that hydrogen prevents the occurrence of catalyst deactivation from monomer-polymer complexing by reacting to give (47) (Scheme 15). In the hydrogen-containing system the catalyst (47) catalytically active 0 = vacancy Scheme 15 is deactivated slowly possibly owing to the radical-forming reaction shown.The copolymerization behaviour of 4-methylpent-1-ene with STY raises certain problems.206 Copolymer analysis gives rlr2 x 4 indicating a tendency for blocking but i.r. spectra indicate a more random arrangement of monomer units since r1r2 determined by this method is approximately unity. The discrepancy is tentatively suggested as arising from the sites on the catalyst [TiCl,-Al(Bu’),] having different reactivities towards the monomers implying that overall copolymer composition is but an average of the various processes occurring. Soluble titanium catalysts of the type (n-C,H5)2Ti(R1)Cl-AlR2C12 are found to produce low-dispersion p~lyethylene.~” One polymer molecule is formed per 1 9 * D.R. Burfield and P. J. T. Tait Polymer 1972 13 315. 199 D. R. Burfield P. J. T. Tait and I. D. McKenzie Polymer 1972 13 321. 2 o o I. D. McKenzie and P. J. T. Tait Polymer 1972 13 510. 2 0 1 T. Otsu H. Nagahama and E. Endo J . Polymer Sci. Part B Polymer Letters 1972, 10 601. 2 0 2 V. A. Khodzhemirov V. Ye. Ostrovskii Ye. V. Zabolotskaya A. Ya. Gantmakher, and S. S. Medvedev Polymer Sci. (U.S.S.R.) 1971 13 2079. ’03 J. Mejzlik S . Petfik and B. Kokta Coll. Czech. Chem. Comm. 1972 37 2920. 2 0 4 K. A. Jung and H. Schnecko Makromol. Chem. 1972 154 227. 20s E. M. J. Pijpers and B. C. Roest European Polymer J . 1972 8 1 15 1 . * 0 6 Yu. V. Kissin Yu. Ya. Goldfarb B. A. Krentsel and H. Uyliem European Polymer J ., ’07 J. A. Waters and G. A. Mortimer J . Polymer Sci. Part A-I Polymer Chem. 1972, 1972 8 487. 10 895 162 P . Hyde and A . Ledwith titanium atom and the system is free of transfer reactions. When R’ is methyl or phenyl first insertion of ethylene is difficult but subsequent polymerization is similar to the case of R’ = ethyl or higher alkyl.’’* More complex titanium catalysts are used in the ring-opening polymerization of methylenecyclobutane~09 and ring-opening studies on cyclo-olefins have also been carried out using n-allylic transition-metal catalysts’ ’ ’ and more conventional Ziegler cata-lysts.” l a b ~ c Soluble vanadium catalysts”’ are able to polymerize ethylene in homogeneous solution at 120 “C. Addition of halogenated promoters reactivates catalytic sites giving polymer-chain vanadium-atom ratios greater than 250 and the system produces low-dispersion polymer (MJM, - 2).A study of the copolymerization of ethylene and propene catalysed by vanadium compounds has confirmed that the propene inserts into the metal-carbon bond via a ‘secondary’ insertion i.e. with the methyl-substituted carbon bonding to the metal (Scheme 16).’13 Me Me Me I I AH=CH2 M-CH -CH2 -CH -CH2- ___* Me Me Me I I I M-CH-CH2-CH-CH2 -CH-CH2-Scheme 16 Soluble catalyst systems based on VOCl are efficient in ethylene-propene copolymerization yielding more than one polymer chain per active site.214 The polymerization of polar monomers with this system shows quite complex behaviour ; polymerization of vinyl chloride in the presence of complexing agents” seems to occur via a co-ordination mechanism whereas MMA polymer-ization and copolymerization with STY proceeds via a radical mechanism.’ l 6 Dienes are readily polymerized by a variety of co-ordination catalysts.Buta-diene is polymerized by a ternary nickel system,2’7-219 by nickel perfluoro-2 0 8 J. A. Waters and G . A. Mortimer J . Polymer Sci. Part A-I Polymer Chem. 1972, 209 R. Rossi P. Diversi and L. Porri Macromolecules 1972 5 247. 10 1827. l o V. A. Kormer I. A. Poletayeva and T. L. Yufa J . Polymer Sci. Part A- I Polymer Chem. 1972 10 251. 2 1 1 ( a ) G . Dall’asta G . Motroni and L. Motta J . Polymer Sci. Part A - I Polymer Chem., 1972 10 1601; (6) G. Dall’asta Makromol. Chem. 1972 154 1; ( c ) E. A. Ofstead and N.Calderon Makromol. Chem. 1972 154 21. A. Zambelli C. Tosi and C. Sacchi Macromolecules 1972,5 649. l 4 G . I. Keim D. L. Christman L. R. Kangas and S. K. Keahey Macromolecules 1972, 5 217. A. Akimoto and T. Yoshida J . Polymer Sci. Part A - I Polymer Chem. 1972 10 993. ’12 D. L. Christman J . Polymer Sci. Part A - I Polymer Chem. 1972 10,471. 2 1 6 A. Akimoto J . Polymer Sci. Part A - I Polymer Chem. 1972 10 31 13. 2 1 7 A. TkaE and A. StaSko CON. Czech. Chem. Comm. 1972,37 573. 2 1 8 A. TkaE and A. StaSko Coll. Czech. Chem. Comm. 1972,37 1006. ’19 R. Pfikryl A. TkaE and A. StaSko Coll. Czech. Chem. Comm. 1972 37 1295 Kinetics and Mechanism of Addition Polymerization 163 carboxylates,220a and other complex nickel systems220b to give high cis-1,4-polymers by cobalt complexes,22' and by rhodium trichloride in emulsion or rhodium complexes in solution.222 In this latter case propagation is envisaged as occurring through a n-ally1 intermediate.Butadiene polymerization catalysed by various n-allylnickel complexes gives principally cis- 1,4-polymers although certain combinations of allylic substituents and solvents increase the trans-l,4 content.223 Studies of the polymerization of butadiene by n-crotylnickel iodide ~ a t a l y s i s ~ ~ ~ * ~ ~ ~ show that the monomer order is unity at low concentrations but decreases to 0.5 or lower at high concentrations. It is thought that (48) is the effective initiator species (Scheme 17). Me Me Me I CH I I CH I CH y.- \ / ~ *'/ CH2 I CH2 / I A,- / \ -\,\ 2C,H A' HC ( Ni Ni CH HC Ni.(48) Scheme 17 /CH2-cH2 I CH CH 'CH I Binodal molecular-weight distributions observed in this system are explained in terms of the three polymer types which may occur (i) growing chains which have not transferred (ii) dead chains and (iii) reactivated chains. This is analogous to the behaviour of free-radical polymerization before the equilibrium molecular weight is reached (less than 0.1 s) but occurs over a much larger time scale. Rhodium catalysts for the polymerization of ~ e n t a - l 3 - d i e n e ~ ~ ~ and propa-diene227 have been described. New catalysts based on Cr(OBu') are found to 2 2 0 2 2 1 2 2 2 2 2 3 2 2 4 2 2 5 2 2 6 2 2 7 ( a ) F. Dawans J. P. Durand and P. Teyssie J . Polymer Sci. Part B Polymer Letters, 1972 10 493; (b) C.Dixon E. W. Duck and D. K. Jenkins European Polymer J . , 1972 8 13. S. S. Medvedev L. A. Volkov V. S. Byrikhin and G. V. Timofeyeva Polymer Sci. (U.S.S.R.) 1971 13 1561. J. Zachoval F. Mikeg J. Kfepelka 0. Prouzova and 0. Pfadova European Polymer J., 1972 8 397. P. Bourdauducq and F. Dawans J . Polymer Sci. Part A-I Polymer Chem. 1972 10, 2527. J. F. Harrod and L. R. Wallace Macromolecules 1972,5,682. J. F. Harrod and L. R. Wallace Macromolecules 1972 5 685. J. Zachoval J. Kfepelka and M. Klimova CON. Czech. Chem. Comm. 1972,37 3271. J. P. Scholten and H. J. van der Ploeg J . Polymer Sci. Part A - I Polymer Chem., 1972 10 3067 164 P . Hyde and A . Ledwith give highly alternating copolymers of isoprene and butadiene with acrylonitrile?28 Systems producing alternating butadiene-propene 29*2 30 and butadiene-ethylene copolymers23 ' have also been reported.Various initiator systems are able to induce the copolymerization of CO with ethylene and vinyl monomers.232 The use of diethylzinc-water for the copolymer-ization of CO and propene oxide results in the formation of the polycarbonates (49) (Scheme 18). **-ZnOR + CO - .*.ZnOCO,R Me Me ZnOC0,R + CH-CH2 --+ Zn-0-CH-CH,-CO,R I I '0' (49) Scheme 18 AlEt,-CuC1,-CCl will polymerize vinyl chloride (VC) to a polymer containing nearly equal amounts of isotactic and syndiotactic Copolymerization of vinyl chloride and CO with this system gives a polymer richer in CO than that produced by conventional radical catalysts [rl(VC) 0.4; r 0.011 and a co-ordinated radical mechanism is suggested (Scheme 19).c=o Scheme 19 The activity of bis(triphenylsily1) chromate in ethylene polymerization is markedly increased by deposition on a silica-alumina support.23 Addition of aluminium alkyls further increases the activity. The active site is thought to be a chromium alkyl bound to the support polymerization occurring by insertion into the chromium-carbon bond. Chromocene is similarly activated by 2 2 8 Y. Koma K. Iimura and M. Takeda. J. Polymer Sci. Part A-1 Polymer Chem., 2 2 9 J. Furukawa and R. Hirai J. Polymer Sci. Part A-I Polymer Chem. 1972 10 2139. 230 J. Furukawa H. Amano and R. Hirai J. Polymer Sci. Part A-1 Polymer Chem., 2 3 1 J. Furukawa and R. Hirai J. Polymer Sci. Part A - I Polymer Chem.1972,10 3027. 232 H. Hoyer and H.-G. Fltzky Makromol. Chem. 1972 161 49. 233 S. Inoue M. Kobayashi H. Koinuma and T. Tsuruta Makromol. Chem 1972 155, 234 W. Kawai and T. Ichihashi J. Polymer Sci. Part A-1 Polymer Chem. 1972 10 1709. 235 W. L. Carrick R. J. Turbett F. J. Karol G. L. Karapinka A. S. Fox and R. N. 23b F. J. Karol G. L. Karapinka C. Wu A. W. Dow R. N. Johnson and W. L. Carrick, 1972 10 2983. 1972 10 68 1. 61. Johnson J. Polymer Sci. Part A - I Polymer Chem. 1972 10 2609. J. Polymer Sci. Part A-I Polymer Chem. 1972 10 2621 Kinetics and Mechanism of Addition Polymerization 165 and produces low-dispersion polyethylene. Copolymerizations using this catalyst system gives a high value of 72 for the ethylene propene reactivity ratio. Many transition-metal catalysts are available for stereoselective polymeriza-tion i.e.the preferential polymerization of one of the enantiomers in a racemic monomer. The extent to which stereoselection proceeds depends on both catalyst and monomer. A zinc-based catalyst system is able to produce monomers of up to 90% optical purity from racemic propene s ~ l p h i d e . ~ ~ ’ TiCl systems have been used in the polymerization of racemic 2-olefins. but neither these nor catalysts based on bis-[(S)-2-methylbutyl]zinc gave a degree of stereoselectivi t y greater than 5 Radiation-induced Polymerization A kinetic scheme has been proposed which takes into account the contributions from free-radical cation-radical and cationic species in 11-ray-induced polymer-i ~ a t i o n . ~ ~ ~ Application of this scheme to STY p~lymerization~~’ meets with success in explaining both the variation in rates and the molecular-weight distribu-tion observed to be dependent on the purity (wetness) of the monomer.Polymeri-zation of styrene in the solid phase proceeds to give a polymer with (basically) a binodal molecular-weight distribution whose exact form depends on the degree of post-polymerization allowed.241 From a comparison with the solution behaviour of STY parallel radical and cationic mechanisms are suggested. E.s.r. n.m.r. and thermal analysis techniques have been applied to the solid-state polymerization of acrylamideZ4’ and acrylic The activation energies of propagation are found to be 19 & 1 and 18.6 & 2 kcal mo1- ’ respec-tively and other parameters (local radical concentration radical decay with tem-perature annealing) are very similar for the two monomers.Post-polymerization of crystalline methacrylic acid gives a narrow molecular-weight distribution atactic polymer suggesting that chain-transfer reactions are unimportant.244 y-Radiation-initiated emulsion polymerization is more efficient for n-butyl acrylate than MMA.245 Polymerization of the former proceeds with an overall activation energy near zero whereas MMA has AE& z-5 kcal mol- ; results are in agreement with those from conventionally initiated systems. The polymerization of ethyl methacrylate has been studied by a calorimetric method.246 Scavenging 2 3 7 M. Sepulchre N. Spassky and P. Sigwalt Macromolecules 1972 5 92. 238 C. Carlini H.Bano and E. Chiellini J . Polymer Sci. Part A - I Polymer Chem. 1972, 239 J . F. Westlake and R. Y. Huang J . Polymer Sci. Part A - I Polymer Chem. 1972, 240 J . F. Westlake and R. Y. Huang J . Polymer Sci. Part A - I Polymer Chem. 1972, 24’ J. F. Westlake and R. Y. Huang J. Polymer Sci. Part A - I Polymer Chem. 1972, 242 C . Chachaty and A. Forchioni J . Polymer Sci. Part A - I Polymer Chem. 1972, 243 A. Forchioni and C. Chachaty J. Polymer Sci. Part A - I Polymer Chem. 1972, 2 4 4 J. B. Lando and J. Semen J. Polymer Sci. Part A-1 Polymer Chem. 1972 10 3003. 2 4 5 T. O’Neill and J. Hoigne J. Polymer Sci. Part A - I Polymer Chem. 1972 10 581. 246 L. Busulini S. Lora G. Palma and G. Lunardon European Polymer J. 1972 8 465. 10 2803. 10 1429. 10 1443. 10 2149.10 1905. 10 1923 I66 P . Hyde and A . Ledwith experiments with DPPH lead to a value of 3.2 x 1* mol* s% for k#:) and propagation rate constants are similar to those for MMA. Polymerization of barium methacrylate proceeds efficiently only if the monohydrate form is used ;247 e.s.r. spectra indicate that at least 75% of the initiating radicals are formed by addition of a hydrogen atom from the water of crystallization to the C=C bond. Analysis of kinetic curves for the polymerization of ethylene in various volumes of t-butyl alcohol-water shows that the rate is proportional to the first power of the fugacity of the monomer indicating that propagation occurs via addition of monomeric ethylene to the growing chain and not excited dimer as previously s~ggested.’~~ Termination is principally a first-order process second-order termination by radical combination only occurring at high volumes of butanol.Activation volumes of the elementary reactions occurring in this system have been obtained from pressure studies249 and give the high value of - 75 ml mol- ’ for the second-order termination reaction. If ethanol is used instead of t-butyl alcohol, chain transfer is found to occur and the first-order termination rate constant increases.’ O Radiation-induced copolymerization of tetrafluoroethylene (TFE) with vinyl ethers yields alternating copolymers over a wide range of feed comp~sition.~~’ Reactivity ratios determined for the TFE (monomer 1)-n-butyl vinyl ether system are r l 0.005 r 2 0.0015 indicative of the high degree of alternation.Solid maleimide produces crystalline polymers with U.V. irradiation but only amorphous polymers result from y-irradiation 2 5 2 copolymerizations with succinimide and maleic anhydride are also described. Copolymerization of trioxan and dioxolan can be initiated by conventional cationic mechanisms ;lg3 solid-state y-ray-initiated cationic copolymerization of these monomers gives polymers of high thermal stability.253 N.m.r. g.l.c. and thermal studies on this system show that the dioxolan is rapidly consumed resulting in a heterogeneous distribution of ethylene oxide units in the chains.254 6 Grafting Reactions Grafting of branches on to polymers which are chemically inactive is easily accomplished by y-irradiation techniques. The grafting of styrene on to poly-2 4 7 M.J. Bowden J. H. O’Donnell and R. D. Sothman Macromolecules 1972 5 269. 2 4 8 T. Wada T. Watanabe and M. Takehisa J . Polymer Sci. Part A-1 Polymer Chem., 1972 10 1655. 249 T. Wada T. Watanabe and M. Takehisa J . Polymer Sci. Part A-1 Polymer Chem., 1972 10 2639. 2 5 0 T. Wada T. Watanabe and M. Takehisa J . Polymer Sci. Part A-1 Polymer Chem., 1972 10 3039. 2 5 1 T. Hikita Y . Tabata K. Oshima and K. Ishiguri J . Polymer Sci. Part A- 1 Polymer Chem. 1972 10 2941. 2 5 2 K. Hayakawa H. Yamakita and K. Kawase J . Polymer Sci. Part A-1 Polymer Chem. 1972 10 1363. 253 I. Ishigaki A. Ito and K. Hayashi J . Polymer Sci. Part A-1 Polymer Chem. 1972, 10 751. 2 5 4 I. Ishigaki A. Ito T. Iwai and K. Hayashi J . Polymer Sci. Part A-1 Polymer Chem., 1972 10 1883 Kinetics and Mechanism of Addition Polymerization 167 ethylene255.256 and cellulose derivative^,"^ acrylonitrile on to p~lyethylene,'~~ and acrylic acid on to radiation-peroxidized propylenezS9 and on to nyl0n-6~~' serve as examples.Chemical methods of grafting are also used for suitable polymers. Several mechanisms are available according to the nature of the polymer and monomer(s) to be added. Anionic grafting of STY and butadiene on to ethylene-propene copolymers,261 cationic grafting of butadiene or isobutene on to PVC,262 and various free-radical grafting h ave been reported. Graft co-polymers with cellulose are of particular interest with respect to the modification of fibre properties. Grafting of methacrylonitrile is very efficient266 and is thought to be aided by monomer-cellulose complex formation.Branches formed of alternating STY-acrylonitrile units can be grafted on to cellulose without the necessity for a metal halide to be present to induce alternati~n,'~~ a factor again attributable to cellulose-monomer interactions. Grafting of short, alternating STY-maleic anhydride chains on to the existing branch points of polyethylene has been described.268 "' I. Kamel S. Machi and J. Silverman J. Polymer Sci. Part A-1 Polymer Chem., 2 5 6 T. Yasukawa T. Takahashi K. Murakami K. Araki T. Sasaga and H. Ohmichi, 2 5 7 J. T. Guthrie M. B. Huglin and G. 0. Phillips European Polymer J. 1972 8 747. 1972 10 1019. J . Polymer Sci. Part A-1 Polymer Chem. 1972 10 259. A. Chapiro A.-M. Jendrychowska-Bonamour and J. P. Leca European Polymer J., 1972 8 1301. 2 5 9 T. 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