J. Chem. SOC., Faraday Trans. I, 1982, 78, 855-868 Alternating Copolymerization of Conjugated Dienes with Methyl Acrylate Part 1 .-Butadiene BY CLEMENT H. BAMFORD* AND XIAO-zu H A N ~ Department of Inorganic, Physical and Industrial Chemistry, University of Liverpool, Liverpool L69 3BX Received 22nd April, 198 1 A study has been made of the alternating copolymerization of methyl acrylate (MA) and butadiene (Bd) in the presence of the Lewis acid ethylaluminium sesquichloride (Al,Et,Cl,), photoinitiated (A = 436 nm) by the system Mn,(CO),, + CCl,. Alternation occurs over a wide range of reactant composition, and the contents of trans-1,4 and 1,2 butadiene units in the alternating copolymers have been estimated as 90% and 10 %, respectively. Measurements at low conversion show that the rate of copolymerization is proportional to [MA- - -all [MAireel0 [Bd]"Yf, where MA- - -a1 represents the methyl-acrylate-Lewis-acid complex and 9 is the rate of initiation.This reaction is accompanied by the Diels-Alder addition yielding methyl cyclohex-3-ene-1-carboxylate (MCC), proceeding at a rate proportional to [MA- - -all [Bd]. At longer reaction times the overall kinetic features reflect the competition between these two processes. Kinetic data on the copolymerization are shown to be consistent with the simplest mechanism of alternation, namely that based on predominating cross-propagation. Propagation between M Bd ' and MA, which is relatively slow in the absence of Lewis acid, is greatly accelerated (ca. 33-fold) by Al,Et,Cl, under the conditions used, and so gives rise to alternation.Chain-transfer to CBr, has been observed and the appropriate kinetic parameters evaluated. Since Hirooka et a1.l first reported that, in the presence of ethylaluminium dichloride, propylene and acrylonitrile yield alternating copolymers, the alternating copolymerization of many other pairs of vinyl monomers has been studied,293 the alternating copolymerization of conjugated dienes and acrylic monomers being of considerable interest. Alternating butadiene-acrylonitrile rubber has been synthesized suc~essfully.~~ Furukawa et al. studied the copolymerization of butadiene and methyl methacrylate in the presence of a Lewis acid (such as Al,Et,Cl,) and a compound of a transition metal of Groups IV or V in the Periodic Table (such as VOC13).6-8 They considered that 1 : 1 and 1 : n complexes of the aluminium compound and methyl methacrylate and the 1 : 1 : 1 ternary complex of the aluminium compound, methyl methacrylate and butadiene participated in the reaction.s* Alternating copolymers of butadiene and methyl acrylate, prepared in a similar way, have been reported.l09 l1 Kuran and his coworkers found that, when benzoyl peroxide was used as initiator in the copolymerization of butadiene and methyl acrylate in the presence of some Lewis acids, the yield of polymer could be increased, while the yield of the Diels-Alder adduct of the monomers was reduced.', The mechanism of alternating copolymerization is still a matter of debate, much argument being centred around the possible involvement in the reaction of complexes containing both monomers.2* In mechanistic investigations it is highly desirable to f Present address : Changchun institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, China.855856 COPOLYMERIZATION OF CONJUGATED DIENES separate the initiation and the regulation processes;13 this was achieved in an earlier study of the alternating copolymerization of methyl acrylate and styrene in the presence of ethylaluminium sesquichloride by the use of free-radial initiators based on transition-metal derivative + organic halide The results of this work were consistent with the simplest proposal for alternation, in which cross-propagation plays a predominant part. This and the following paper describe similar investigations of the alternating copolymerizations of methyl acrylate with butadiene and with isoprene.In both cases photoinitiation (A = 436 nm) by Mn,(CO),, + CCl, was employed,15 with Al,Et,Cl, as the Lewis acid. EXPERIMENTAL MATERIALS Butadiene (Bd) was purified by passing through a column packed with molecular sieve 4A, followed by cooling in a dry-ice + methanol bath overnight, then filtering off frozen water and drying over calcium hydride at -78 "C. The butadiene was then distilled on the vacuum line into a graduated vessel, in which a 4 mol dm-, solution in toluene was made for use. Three grades of butadiene were used in our experiments, the initial purities being 99% (B.D.H. Laboratory gas), 99.5 % (Cambrian Gases) and 99.86% (Phillips Petroleum Co.). No significant differences between them were observed.Methyl acrylate (MA) having a purity of 99% was washed several times with an aqueous solution containing 5% sodium hydroxide and 20% sodium chloride. After drying over anhydrous calcium chloride, it was distilled and stored over calcium hydride in a refrigerator. Before using, the monomer was distilled on the vacuum line. Ethylaluminium sesquichloride (25 % toluene solution) was used without further purification. Manganese carbonyl [Mn,(CO),,] was sublimed in vacuum at room temperature. Toluene (A.R.) was dried over sodium wire. APPARATUS AND TECHNIQUE All the experiments were carried out in a laboratory illuminated by inactive (sodium) light. Reactions were initiated by light of wavelength 436 nm, the optical system being the same as described in an earlier paper.Is Reaction vessels consisted of Pyrex glass ampoules with internal diameter 16 mm, which were washed, dried, flamed and filled with nitrogen.Mn,(CO),,, toluene, CCl,,Al,Et,Cl,, MA and Bd were introduced by pipetting required volumes into the reaction vessel in the above order (CCl,,Al,Et,CI, and Bd being in toluene solution). Reaction mixtures, of which the total volume was 10 cm3, were thoroughly degassed by the conventional freeze-thaw technique; when the reaction vessel had been sealed it was illuminated in a thermostat at 25 f 0.1 OC for the required time by filtered radiation from a 250 W high-pressure mercury arc (AEI, type ME/D). The reaction mixture was then poured into 400cm3 methanol containing a small amount of phenyl-/3-naphthylamine as stabilizer for the copolymer.The latter was precipitated, filtered off, dried and weighed; it was a rubbery solid free from gel. Samples for analysis were reprecipitated from toluene solution into methanol. The random copolymer of MA and Bd required for spectral observations was prepared by copolymerizing the two monomers (Bd: MA = 1 : 2.4 mol dm-3) in toluene with benzoyl peroxide as initiator at 40 O C . The conversion was only ca. 2% after 40 h. Rates of initiation (f) were determined by calibration experiments in which the homopoly- merization of methyl methacrylate was photoinitiated by Mn,(CO),, + CCl, with similar light intensities and concentrations of Mn,(CO),, and CCl,.14 This procedure is justified by earlier inve~tigationsl~ which show that in homopolymerizations the rate of photoinitiation by this system is effectively independent of the nature of the monomer.In our experiments 9 was 9.18 x Number-average molecular weights of copolymers were measured osmometrically with a Hewlett Packard 503 high-speed membrane osmometer, with toluene as solvent. mol dm-, s-,, except when stated otherwise.C. H. BAMFORD A N D XIAO-ZU HAN 8 57 80 n 5 L3 60- x 0 a - 4 0 - .+ -€ E 20 Infrared absorption spectra of copolymers were recorded by a Perkin-Elmer 5 17 grating spectrophotometer with films cast from chloroform solution. Copolymer composition was determined by elemental analysis and the degree of alternation was estimated from the 60 MHz n.m.r. spectrum in deuterochloroform. The Diels-Alder adduct of Bd and MA in the reaction system was estimated on a Pye 104 gas-liquid chromatograph (g.1.c.) having a 3 m column packed with 3 % OV-17 on Supelcoport (100-120 mesh) at 110 OC.- 0 n W 0 - RESULTS AND DISCUSSION COMPOSITION A N D STRUCTURE OF THE COPOLYMERS Over a wide range of initial concentrations of methyl acrylate [MA], at constant [Bd],, the copolymer was found to maintain a 1 : 1 composition (fig. 1). When [MA], and [Bdl0 were both 0.8 mol dm-3 and polymerization had proceeded for 10 h, the content of MA in the copolymer was still 50%. '0° T 01 1 I 1 I I I 0 0.4 0.8 1.2 1.6 2.0 2.4 [MA], in reaction mixture/mol dm-3 FIG. 1 .-Dependence of copolymer composition on [MA],. Initial concentrations/mol dm-3 : Bd 0.8, Al,Et,Cl, 0.2, Mn,(CO),, 2 x lo-,, CC1, 0.1 ; 25 OC, reaction time 1 h, 1 = 436 nm.The infrared spectra of alternating and random copolymers of Bd and MA are shown in fig. 2. The absorption band at 1730 cm-l is characteristic of the C=O bond in MA units. The bands at 970, 915 and 720 cm-l are characteristic of trans-1,4, 1,2 and cis-1,4 isomers of Bd units, respectively. In the spectrum of the alternating copolymer there is no absorption band at 720 cm-l, implying effectively no cis-1,4 isomer. The contents of trans-1,4 and 1,2 isomer in the alternating copolymer were estimated as 90% and lo%, respectively, according to the method of Haslam et a1.18 The contents of trans-1,4, 1,2 and cis-1,4 isomers in the random copolymer were 74.7%, 9.0% and 16.3%, respectively. The n.m.r. spectra of alternating and random copolymers of Bd and MA are shown in fig.3. The peak near 26 (ppm relative to TMS) in the spectrum of the random copolymer [fig. 3 (a)] arises from the adjacent methylene groups in butadiene-butadiene diads. The absence of this peak from the spectrum of fig. 3 (b) confirms the alternating structure of the copolymer. The peaks (i) at 5.1-5.46 arise from -CH= and CH2=CH- protons in Bd units and the peak (ii) at 3.65 6 from H3CO- protons858 COPOLYMERIZATION OF CONJUGATED DIENES 2000 1800 1600 1400 1200 1000 aoo 600 wavenumberlcm-' FIG. 2.-Infrared spectra of MA-Bd copolymers. (a) Random copolymer prepared by free-radical copolymerization with initial concentrations/mol dm+: MA 6, Bd 2.5, benzoyl peroxide 40 OC, 40 h. (b) Alternating copolymer, prepared with initial concentrations/mol dm-3 : MA, Bd 0.8, A12Et3C13 0.2, Mn,(CO),, 2 x CCl, 0.1 ; 25 OC, 1 h, II = 436 nm.I I I I , 6 5 4 3 2 1 0 6 (PPm) FIG. 3.-N.m.r. spectra of MA+Bd copolymers in CDCl,: (a) random, (b) alternating, prepared as described for fig. 2.C. H. BAMFORD AND XIAO-ZU HAN 859 in MA units. The observed ratio of peaks (i)/(ii) determined from the integral line in fig. 3 (b) is ca. 2/3, while the predicted ratio from the 1 : 1 composition and the trans-l,4 and 1,2 isomer contents derived from the i.r. observations is 2.1/3. The spectra of copolymers of Bd and MA are similar to those of Bd and methyl methacrylate.s~19 KINETICS In general, the concentrations of MA (A) and Bd (B) were both 0.8 mol drn-,, and the concentrations of Al,Et,Cl,, Mn,(CO),, and CCl, were 0.2, 2 x lo-, and 0.1 mol drn-,, respectively. To determine the rate of copolymerization we used the gravimetric method, which involved weighing the copolymer formed at low conversion : d([MA]+[Bd]) - W 1000 dt 70.09 10t - X- rate of copolymerization co = where W is the copolymer weight (in g), t the reaction time (in s) and 70.09 is the average molecular weight of MA and Bd monomers.Previo~slyl~ both Mn,(CO),, + CCl, and Ni(CO), {P(OPh),}, + CCl, systems were found to be very effective initiators for the alternating copolymerization of MA and styrene. In the present work only the former was effective; it is likely that the nickel complex forms a n-ally1 derivative with Bd and so does not initiate.,O The concentration of CCl, (0.1 mol drn-,) is in the ‘plateau’ range’, over which the rate of initiation is independent of [CCl,].The rate of copolymerization was found to depend linearly on [Mn,(CO),,]~ up to [Mn,(CO),,] = 8 x lo-, mol dm-, (fig. 4), showing that the reaction proceeds by a free-radical mechanism. 3.2 - t m 2.4 E -0 - 1.6 t 2 3 Oa \ 0 0 1 2 3 4 5 [Mn,(CO),,l$l 0-2 moh dm -* FIG. 4.-Dependence of rate of copolymerization o of MA and Bd on [Mn,(CO),,]fo. Initial concentrations/mol dm-3: MA, Bd 0.8, Al,Et3C13 0.2, CCl, 0.1 ; 25 OC, 1 = 436 nm. Furukawa et aLs considered A1,Et,Cl3 to behave both as an initiator and a regulator in the copolymerization of Bd and MMA, the rate of copolymerization being proportional to [Al,Et,Cl,]% In our system the rate of reaction was negligible in the absence of Mn,(CO),, or light and Al,Et,Cl, was only a regulator; the effect of its concentration on o is shown in fig.5 . The presence of Al,Et,Cl, leads to greatly enhanced rates and without Al,Et,Cl, no polymer was obtained even at very long reaction times. The linear relation (fig. 5 ) is observed when the concentration of Al,Et,Cl, exceeds 0.05 ml drn-,, up to the highest value studied (0.4 mol dm-,).860 COPOLYMERIZATION OF CONJUGATED DIENES 2.4 * I m m E a 1.6 - z 4 0.8 --- 3 0 0 01 0.2 03 0.4 0.5 [Al,Et3C13]o/mol dm-3 FIG. %-Dependence of rate of copolymerization w of MA and Bd on [Al,Et3Cl,],. Initial concentrations/mol dm-3: MA, Bd 0.8, Mn,(CO),, 2 x lo-,, CCl, 0.1; 25 OC, 1 = 436 nm. 3 0 0.8 1.6 2.4 3.2 [MA],/mol dm- FIG. 6.-Dependence of rate of copolymerization w of MA and Bd on [MA],.Initial concentrations/mol drn-,: Bd 0.8, Al,Et,Cl, 0.2, Mn,(CO),, 2 x CCI, 0.1 ; 25 OC, 1 = 436 nm. 0 0.8 1.6 2.4 3.2 [ Bd],/mol dm-3 FIG. 7.-Dependence of rate of copolymerization w of MA and Bd on [Bd],. Initial concentrations/mol drn-,: MA 0.8, Al,Et,Cl, 0.2, Mn,(CO),, 2 x lop4, CC1, 0.1; 25 OC, 1 = 436 nm. When [MA], was lower than 0.4 mol dmm3, the reaction mixture became hetero- geneous after reaction for a few minutes. The dependence of cc) on the concentrations of monomers is shown in fig. 6 and 7, from which it can be seen that w is effectively independent of [MA], from 0.6 to 2.4 mol dm-3, and [Bd],'from 0.4 to 2.0 mol dm-3. Rates of copolymerisation were determined for short reaction times (15 min) in the above experiments.Data for longer reaction times are shown in table 1 and fig. 8 andC. H. BAMFORD A N D XIAO-ZU HAN 86 1 TABLE EFFECT OF REACTION TIME ON THE COPOLYMERIZATION OF MA AND Bd IN THE PRESENCE OF Al,Et,Cl, Initial concentrations/mol dm-3: Bd 0.8, Al,Et,Cl, 0.2, Mn,(CO),, 2 x lo-,, CCl, 0.1 ; 25 OC, A = 436 nm. reaction conversion of Bd [MA]/mol dm-, time/h into copolymer (%) lo-, M, 0.6 0.8 1.6 2.4 0.25 1 3 5 10 0.25 1 3 5 10 0.25 1 3 5 10 16 0.25 1 3 5 10 6.1 13.7 17.8 21.1 20.7 6.3 17.0 25.8 29.7 33.2 6.1 17.7 32.2 41 .O 53.2 53.4 6.9 18.2 35.1 45.0 57.9 70 81 112 152 65 91 110 119 64 102 132 - - - 0 2 4 6 8 10 reaction time/h FIG. 8.-Dependence of polymer yield on reaction time for various values of [MA],. Initial concentrations/mol dm-3: Bd 0.8, A1,Et3C1, 0.2, Mn,(CO),, 2 x CCl, 0.1; 25 OC, I = 436 nm.[MA],/m~ldrn-~: 0, 2.4; 0, 1.6; 0, 0.8; 0, 0.6.862 COPOLYMERIZATION OF CONJUGATED DIENES 0 2 4 6 8 10 reaction time/h FIG. 9.-Dependence of polymer yield on reaction time for various values of [Bd],. initial concentrations/mol drn-,: MA 0.8, A12Et,Cl, 0.2, Mn,(CO),, 2 x lo-,, CCl, 0.1; 25 OC, d = 436 nm. [Bd],/mol dm-a: 0, 1.6; 0, 0.8; 0, 0.4. 9. It is obvious that, when the reaction time is short, the yields of copolymer are almost independent of [MA], and [Bd],; when the reaction time is > 1 h, the yields increase with increasing [MA], but decrease with increasing [Bd],. It is well-known12* 21 that, in the presence of a Lewis acid, Diels-Alder addition can occur between methyl acrylate and butadiene to form methyl cyclohex-3-ene-l- carboxylate (MCC) : O\,,,M, Lewis acid (= + CH?=CH-COOMe -P M CC The yield of MCC is proportional to the concentrations of complex of MA with Lewis acid (MA- - -al, see below) and Bd, but is independent of the concentration of free MA : -- - k,[MA- - -all [Bd].d[MCC] dt 0 1 2 3 4 5 reaction time/h FIG. 10.-Dependence of yield of MCC on reaction time for various values of [Bd],. The conditions were the same as in fig. 9.C. H. BAMFORD AND XIAO-ZU HAN 863 In our system the formation of MCC occurs simultaneously and competes with copolymerization.8* l2 Concentrations of MCC in the reaction mixture for a series of reaction times and a range of [Bdl0 are shown in fig. 10. 1 3 Kb+A+A' Rb+B-+B' B' +A -+ A' kba A'+B+B' k,, TABLE 2.-EFFECTS OF SOME CONDITIONS ON THE YIELDS OF COPOLYMER AND ADDUCT MCC Initial concentrations/mol dm-3: MA, Bd 0.8, CCl, 0.1 ; 25 OC, 1 h.B' +S + P+S' kfb S'+A+A' kra S' +B -+ B' krb A'+A'-+P ktaa A'+B' -+P ktab 0.2 2 x 10-4 dark 0.2 2~ 10-4 light 0.2 2 x 10-3 light 0.1 2 x 10-3 light 0.29 0.25 0.22 0.12 0.02 0.13 0.30 0.15 From fig. 9 and 10 it is clear that the higher [Bdl0 the greater is the ratio adduct : polymer for long reactions times. The effects of other conditions on the yields of copolymer and adduct are shown in table 2. The concentration of Al,Et,Cl, affects the yields of both copolymer and adduct. Irradiation and [Mn,(CO),J have no signifiant effect in the Diels-Alder reaction but of course greatly influence the yield of copolymer.MECHANISM (3)864 COPOLYMERIZATION OF CONJUGATED DIENES Homopropagations are not included. Initiating radicals R, are formed from the initiator C and P represents the dead polymer. Termination by combination is assumed, this seeming most appropriate for the two monomers under consideration; our general conclusions are not affected by this assumption. Conventional stationary-state treatment leads to relations (4) and ( 5 ) for long chains : O = 2kba[A] (t)! (4) In deriving these equations the total rate of termination has been written as k,[B'I2, so that k, contains contributions from all three termination steps in reaction (3) and is given by Relations (4)-(6) are quite general and involve no assumptions about specific termination mechanisms. Two limiting cases are of interest.If [B] is sufficiently large, kt = ktbb, i.e. termination occurs predominantly between B' radicals, so that from eqn (4) the rate of copolymerization is given by eqn (7a) W = 2kba[A] (L)t. (7 a) ktbb On the other hand, for sufficiently small [B] the third term on the right-hand side of eqn (6) (termination between A' radicals) is large and k, approximates to ktaa(kba[A]/kab[B])2; thus the rate of copolymerization follows eqn (7b) In our system (A = MA, B = Bd) under present conditions the former alternative appears to hold and the rate of copolymerization given by eqn (7a) is almost independent of butadiene concentration for short reaction times (fig. 7 ) . HirookaZ2 showed that complexing between methyl acrylate and ethyl aluminium sesquichloride is very strong and that the complex may be represented by MA---AIEtl~,Cll~, or MA---al: Krn MA + a1 + MA- - -al.Thus in the general kinetic scheme (3) A must be understood to refer to this complex and [A] = [all = 2 [Al,Et,Cl,] so long as [all < [MA] (approximately). Under these conditions eqn (7a) predicts that w cc [all; according to fig. 5 this proportionality is found except at low values of [all. A similar observation was reported for the methyl acrylate + styrene system and evidence was adduced showing that at low [all primary radical termination occurs and interferes with the simple proportionality.'* It seems likely that such an explanation may hold in the present case. Eqn ( 5 ) does not allow explicitly for chain-transfer to species other than the added transfer agent S .Since under our conditions k, = ktb, eqn ( 5 ) predicts that, at constant [all and 9, Pn should be independent of the concentrations of methyl acrylate (> [all) and butadiene (since [A] = [all), provided no significant transfer to these monomers occurs. This is consistent with the data for short reaction times in table 1. The increase (8)C. H. BAMFORD A N D XIAO-ZU H A N 865 in molecular weight with increasing reaction time reflects the consumption of the photoinitiator Mn,(CO),, and consequent decline in the rate of initiation. For short reaction times, the yields of adduct increase with [Bd], as expected (fig. lo), although as explained above the yield of copolymer remains constant. By virtue of the Diels-Alder reaction, the rate of consumption of MA becomes a function of [Bd], increasing with the latter [cf. eqn (2)].For sufficiently long reaction times [MA] eventually falls below [all, and [MA- - -a11 (and hence the rate of copolymerization) becomes dependent on [MA]. Other things being equal, this situation will be reached more rapidly with higher initial butadiene concentrations, so that at long reaction times the mean rate of copolymerization will depend on the initial Bd concentration, [Bd],, decreasing with increase in the latter. These considerations are supported by the data in fig. 9, which show that, for constant [MA],, [all, and initiation conditions, the polymer yield decreases with increasing [Bd],, except for short reaction times. Similar reasoning enables us to understand why the polymer yield for constant [Bd],, [all, and initiation conditions increases with increasing [MA],, except for short reaction times (fig.8). As would be expected, the polymer yield is not much affected by [MA], when the latter is sufficiently high (cf. the curves for [MA], = 1.6 and 2.4 mol dmM3 in fig. 8). When equimolar concentrations of MA and Bd are used, the total conversion to polymer and MCC is very high at long reaction times. It must be stressed that not only methyl acrylate but also the copolymer and MCC can form complexes with the aluminium derivatives, so that, in addition to equilibrium (8), there exist the equilibria KP P + a1 + P-- -a1 KlL MCC + a1 e MCC- - -al. (9) Thus the aluminium is distributed between MA, P and MCC in proportions determined by the equilibrium constants Km, Kp and Ka.Quantitative treatment of the data in fig. 8-10 requires a knowledge of these constants. Unfortunately there is little information about the magnitudes of Kp and K, except that these are probably much less than Km. In principle, therefore, as the reaction proceeds a decreasing fraction of aluminium is available for complexation with MA, and a progressive (probably small) decrease in the overall rate would be expected for this reason. CHAIN TRANSFER TO CARBON TETRABROMIDE: REACTION PARAMETERS Chain transfer occurs in alternating copolymerizations, but usually to a relatively small extent. Thus in the alternating copolymerization of methyl acrylate and styrene in the presence of Al,Et3C13 the rate coefficient for the transfer reaction between the radical M MA-St and CBr, is ca.30 times smaller than that for the reaction without complexing agent. Possible reasons for this, based on complexing of the radical with Lewis acid, have been advanced.13 Corresponding transfer data for the MA + Bd system with short reaction times (10 min) are presented in table 3. The presence of CBr, leads to copolymers of lower molecular weight but has little effect on the rate of copol ymeriza tion. The plot of l / e against [CBr,] presented in fig. 11 is satisfactorily linear with slope 0.545 mob1 dm3. According to eqn (5) the slope is866 COPOLYMERIZATION OF CONJUGATED DIENES TABLE 3.-cHAIN-TRANSFER TO CBr, IN THE ALTERNATING COPOLYMERIZATION OF MA AND Bd AT 25 OC Initial concentrations/mol dm-3: MA, Bd 0.8, A1,Et3C13 0.2, Mn,(CO),, 2 x lo-*, CC1, 0.1 ; 9 = 3.2 x lo-' mol dmP3 s-l.[CBr,]/ 1 O-, mol dm-3 o/ lo-, mol dm-3 s-l 0 2 5 10 1.82 1.80 1.88 1.84 1244 1028 939 746 14 1 2 E '5 2 u 8 6 0 2 4 6 8 10 [CBr,]/104 mol dm-3 FIG. 11 .-Dependence of 8 on CBr,; plot according to eqn (5). Initial concentrations/mol drn-,: MA, Bd 0.8, AI,Et,CI, 0.2, Mn,(CO),, 2 x CCI, 0.1 ; 25 OC, L = 436 nm, 9 = 3.2 x lo-' mol dm-3 s-*. For systems without Lewis acids, the first term in expression (10) is equal to rA CA/2[B], rA and CA denoting reactivity ratio and transfer constant, respectively. If we assume rA = 0.0524 and CA = 0.325 we obtain kfa = 0.0094 mol-l dm3. 2kab[Bl This is only a small contribution towards the observed slope. In the presence of A12Et3Cl,, C , is probably greatly reduced so that eqn (1 1) is an overestimate; however, its use will not introduce significant error.From eqn (10) and (1 1) we find k,, = 0.43. (12) kba The rate data in table 3 permit the.evaluation of kbak& from eqn (7a); we find that kbakcib = 0.40 mOl4 dmi S b . kfb k;&, = 0.17 m o l t dm; S-1 (1 3) (14) Combination of eqn (12) and (13) shows thatC. H. BAMFORD AND XIAO-ZU HAN 867 a value very close to that of the same parameter in the methyl acrylate + styrene system (0.18 mol-g dmi s t ) (styrene = B).14 No experimental determinations of kt,, have been reported; however, it seems unlikely that this coefficient is less than ktbb for the MA+St system,13 uiz. 6 x lo6 mol-l dm3 s-l. With this value we find from eqn (13) that k,, = 980 mol-l dm3 s-l.According to Walling and D a ~ i s o n ~ ~ the reactivity ratios for the system in the absence of Lewis acids are rA = 0.05 and rB = 0.76 at 5 O C ; no determinations are recorded at other temperatures. Assuming the values at 25 OC are not very different, we find, with the aid of the propagation coefficient in the homopolymerization of butadiene (19 mol-1 dm3 s-l) determined by Morton et a1.,26 that in the simple free-radical copolymerization k,, x 25 mol-l dm3 s-l. Then the presence of Lewis acid brings about a 39-fold increase in this coefficient. The other cross-propagation coefficient, k,,, estimated similarly from the homopropagation coefficient of methyl a ~ r y l a t e ~ ~ (592 mol-1 dm3 s-l) turns out to be 11 840 mol-l dm3 s-l in the absence of Lewis acids.The influence of A12Et3C13 on k,, is not yet known but analogy with the MA+ St system2* suggests that presence of the Lewis acid would produce an increase in k,,. The high estimates of kba and k,,, compared with values of homopropagation rate coefficients in the system, are clearly consistent with the proposed mechanism of alternation [eqn (3)]. With the aid of the value of kt,, assumed above we find from eqn (14) that k,, z 4 16 mo1-l dm3 s-' ; unfortunately no data on this rate coefficient for systems free from Lewis acids are available for comparison. To summarise, we believe that the detailed kinetic data strongly support the cross-propagation mechanism of alternation. Further, the results presented indicate that the conditions favouring the copolymerization reaction over the Diels-Alder addition are high [MA],, low [Bd],, high light intensity and high initiator (Mn,(CO),,) concentration; the findings of Kuran et a1.12 are consistent with this conclusion.M. Hirooka, H. Yabuuchi, S. Morita, S. Kawasumi and K. Nakaguchi, J. Polym. Sci., Part B, 1967, 5, 47. M. Hirooka, Doctoral Thesis (Kyoto University, 1971). For reviews see C. H. Bamford in Molecular Behaviour and the Development of Polymeric Materials, ed. A. Ledwith and A. M. North (Chapman and Hall, London, 1975), chap. 2; H. Hirai, J. Polym. Sci., Macromol. Rev., 1976, 11, 47. J. Furukawa and Y. Iseda, J. Polym. Sci., Part B, 1969, 7, 47. N. G. Gaylord and A. Takahashi, J. Polym. Sci., Part B, 1969, 7, 443. J. Furukawa, E.Kobayashi, Y. Iseda and Y. Arai, Polym. J., 1970, 1, 442. J. Furukawa, Y. Arai and E. Kobayashi, J. Polym. Sci., Part A , 1974, 8, 417. J. Furukawa, Y. Iseda and E. Kobayashi, Polym. J., 1971, 2, 337. a J. Furukawa, E. Kobayashi, Y. Iseda and Y. Arai, J. Polym. Sci., Part B, 1971, 9, 179. lo British Patent, 1,186,461, 1968 (to Bridgestone Tyre Co). l1 Japanese Patent, 23,181, 1972 (to J. Furukawa, Y. Iseda, Y. Kazuo and K. Nobuyaki, Japan). l2 W. Kuran, S. Pasynkiewicz, R. Nadir and B. Kowaleweska, Macromol. Chem., 1976, 177, 1291. l 3 C. H. Bamford and P. J. Malley, J . Polym. Sci., Polym. Lett. Ed., 1981, 19, 239. l4 C. H. Bamford, S. N. Basahel and P. J. Malley, Pure Appl. Chem., 1980, 52, 1837. l5 C. H. Bamford in Reactivity, Mechanism and Structure in Polymer Chemistry, ed. A. D. Jenkins and A. Ledwith (John Wiley, London, 1974), chap. 3. C. H. Bamford and S. U. Mullik, Polymer, 1973, 14, 38. l7 C. H. Bamford, R. W. Dyson and G. C. Eastmond, Polymer, 1969, 10, 885. la J. Haslam, H. A. Willis and D. C. M. Squirrell, ZdentiJication and Analysis of Plastics (Iliffe Books, l9 J. R. Ebdon, J. Macromol. Sci., Chem., 1974, 18, 417. *O J. 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