J. C.S. FUYU~~Y I, 1980,76,112-117Moderated CopolymerizationPart 5.-Penultimate Unit Effect in Chain-transfer. Systems StyrenelMethylAcrylate/Carbon Tetrabromide and StyrenelMethyl Acrylatell -Butane Thioland Some General ConsiderationsBY CLEMENT H. BAMFORD" AND SULAIMAN N. BASAHELDepartment of Inorganic, Physical and Industrial Chemistry,University of Liverpool, Liverpool L69 3BXReceived 22nd February, 1979Investigation of the transfer constants of styryl radicals in copolymerizations of styrene (St) hasbeen continued by the technique of moderated copolymerization with methyl acrylate (MA) ascomonomer. Using a low value of [styrene]/[methyl acrylate] (8.75 x we fhd that the transferconstant towards carbon tetrabromide is 302 at 60°C; this value, which refers essentially toWMA-St.radicals, approximates to that found previously for WSt-St- radicals (337). Theresults therefore support the view that the low transfer constant found for WMMA-St radicals(MMA = methyl methacrylate) is attributable to steric interference between the a-methyl of thepenultimate MMA unit and the incoming bromine atom. With 1-butane thiol as transfer agent,the transfer constant found for -MA-St- (20.0) is close to that of WMMA-St. (22.4), a resultwhich also strongly supports the hypothesis of local steric interference mentioned above.The general case of copolymerization of monomers A and I3 in which both MA. and -B-radicals are subject to penultimate unit effects is considered and general equations are developed.Finally, the possibility of penultimate unit effects in chain-transfer in homopolymerizations ismentioned.In this paper we report studies of the transfer constant of styryl radicals towardscarbon tetrabromide and 1-butane thiol made with the aid of the moderated co-polymerization technique with methyl acrylate (MA) as moderating comonomer.The general background of this work has been described in Part 4;2 the presentpaper concludes the examination of our hypothesis, advanced in Part 2,3 that thedifference between the reactivities of the styryl radicals (I) and (11) (St = styrene) inchain-transfer to carbon tetrabromide arises predominantly from steric interferencebetween the a-methyl group of the penultimate methyl methacrylate (MMA) unitin (I) and the incoming bromine atom.wMM A-S t (I)-St-St* (11)Preliminary results on the styrenelmethyl acrylate/carbon tetrabromide systemhave already been reported briefly in Part 2.3EXPER1,MENTALMethyl acrylate, styrene, carbon tetrabromide and 1-butane thiol were purified asdescribed in the preceding paper.2Copolymerizations were carried out at 60°C with azo-bis-isobutyronitrile as initiatorand number-average molecular weights were determined with the aid of a Hewlett-Packardmembrane osmometer.Like other workers, we experienced difficulty in obtaining satisfactory precipitation of11C.H. BAMFORD A N D S . N. BASAHEL 113copolymers rich in methyl acrylate. Finally, we adopted the procedure of precipitatinginto petroleum ether (40-60°C) cooled to - 50°C, allowing to stand overnight then recoolingto -50°C before filtrati~n.~.Polymers were dried by prolonged evacuation at roomtemperature. Attempts were also made to remove unreacted monomers by evaporationin vacuum at room temperature, but this appeared to lead to some cross-linking, no doubton account of the presence of residual initiator.RESULTS AND DISCUSSIONSYSTEM STYRENE(B)/METHYL ACRYLATB(A)/CARBON TETRABROMIDE(S)In these experiments the value of u,( = [B],/[A],) was as low as convenientlypossible. Further, carbon tetrabromide concentrations were also kept low, tominimize problems (mentioned above) associated with isolation of the copolymersby precipitation. Values of CB,obs (the observed overall transfer constants for-Be radicals) were computed from the measured E a s described in Part 4,2 takingthe reactivity ratios rA = 0.18, rB = 0.75 and the transfer constant for methylacrylate towards carbon tetrabromide CA = 0.41.Results are presented in table I ,in which the total monomer conversions are denoted by A(A + B).TABLE 1 .-SYSTEM STYRENE~METHYL ACRYLATE~CARBON TETRABROMIDE AT 60'C104[S]o 102A(A+ B) ~ O - ~ K 10314 CBpbs0 39.56 62.47 7.428 -0.95 34.97 10.36 7.620 3151.42 33.61 7.434 7.663 3061.89 33.48 5.707 7.668 3102.84 33.24 4.022 7.675 2983.32 33.48 3.663 7.668 279mean : 302[styrene], = 0.091 mol dnr3, [methyl acrylate], = 10.35 mol dm-3, uo = 8 . 7 9 2 ~[azo-bis-isobutyronitrile] = 8.29 x mol dnr3. All concentrations are expressed inrnol dnr3 at 60°C.The mean value of CBYobs in table 1 is comparable with that for radical (11) (CBB),viz.337.2 As a consequence of the low value of uo in these experiments > 99 %of the radicals with terminal styryl units have structure (111)so that we may conclude that CBA = 302, approximately. Comparison with thevalue CBA = 60 (for I)3 (A = methyl methacrylate) reveals the large influence exertedby the a-methyl substituent in the penultimate unit on the transfer reaction. Theelectronic effect arising from this methyl must be small and in any case would tendto increase the transfer constant.' The present results therefore strongly favour theview that steric congestion involving the a-methyl group in (I) is responsible for therelatively low reactivity of this radical towards CBr4.gave CBA = 169.Webelieve that incomplete precipitation of the lower copolymers was responsible forthe smaller values of CBA.wMA--St* (111)Preliminary results for this system reported in Part 114 MODERATED COPOLYMERIZATIONAs mentioned in Part 2,3 transfer constants calculated by the moderated co-polymerization procedure are sensitive to the magnitude of rB in the styenelmethylacrylate system, for which a range of values extending from 0.60 to 1.1 exists in theliterature.8a Recalculation of the present results with V, = 0.60, rA = 0.18 yields amean value of CBA = 378. This figure is also reasonably close to the value CBB = 337for styryl radicals with penultimate styrene units.2SYSTEM STYRENE(B) /METHYL ACRYLATE(A) / I -BUTANE THIOL( S)In computing CB,-,bs the following values of the kinetic parameters were used:The mean value CB,obs = 20.0, which effectively refers to radical (In) (i.e., CBA),is close to the transfer constant previously determined for radical (I) (22.4).Thus,for chain-transfer to 1-butane thiol, the presence of an a-methyl group in thepenultimate unit has little effect. These conclusions are not affected by takingr A = 0.18, rB = 0.60;8a with these values CB,obs (mean) = 25.0.r A = 0.18, r~ = 0.75,6 CA = 1.69.8bTABLE 2.-sYSTEM STYRENEIMETHYL ACRYLATE/l-BUTANE THIOL AT 60°C104[Slo 102A(A+B) 10-3E 103u c30 39.56 62.47 7.428 I1.85 19.54 18.59 8.123 18.43.79 24.78 10.25 7.950 22.55.68 24.58 7.463 7.956 19.919.1 7.50 24.41 5.882 7.962mean : 20.0-[styrenelo = 0.091 mol dm-3, [methyl acrylatel0 = 10.35 mol dm-3, uo = 8.792~[azo-bis-isobutyronitrile] = 8.29 x mol dm-3.All concentrations are expressed inmol dm-3 at 60°C.CONCLUSIONSTable 3 presents transfer constants for styryl radicals with different penultimateunits. These data demonstrate that, in the systems studied, (a) the electronicproperties of the penultimate unit in a styryl radical do not exert a major influenceon the transfer constant and (b) the effect of a penultimate unit becomes very smallwhen either the unit or the transfer agent is such as to eliminate steric congestion ofthe type described. The conclusions evidently support our hypothesis of the stericorigin of the penultimate unit e f f e ~ t .~It is possible that smaller penultimate unit effects, perhaps of different origin,arise in these and other systems, but the experimental results at present available arenot sufficiently precise to establish their existence. We hope, however, that theymay be observable through the development of sufficiently refined procedures basedon moderated copolymerization.COPOLYMERIZATION I N WHICH BOTH A- AND B* RADICALS SHOWPENULTIMATE-UNIT INFLUENCES I N CHAIN TRANSFEREvaluation of CB,obs from the equations already presented [eqn (4) of Part 1,leqn (22), (24) and (27) of Part 3 and eqn (3) of Part 4 2] requires a knowledge oC. H. BAMFORD AND S . N. BASAHEL 115the transfer constant of the moderating comonomer A. Hitherto we have assumeda constant value CA for this quantity, but our results suggest that in some systemsMA. radicals may show penultimate unit effects so that CAA # CAB andCA = CA,obs (which contains contributions from wAA.and wBA.) becomes afunction of [B]/[A]. In the system styrene/methyl methacrylatelcarbon tetrabromideit seems probable that radical (IV) is more reactive than (V) towards CBr,WSt-MMA.wMMA-MMA-Our estimates of CB,obs and hence of CBB have neglected such differences; in thisparticular instance reasonable changes in CA arising in this way do not lead toappreciable errors in CBB, since the terms in C, are relatively small.TABLE 3.-TRANSFER CONSTANTS AT 60°Ctransferradical ‘F. CBr4 1 -BUSHwst-st. 337 22WMMA-St. 60 2220 MMA-S t 302In general, if penultimate unit effects are significant for both MA- and M B ~radicals, eqn (3) of Part 1,l applicable at infinitesimal conversions, must be replaced byIn the styrenelmethyl methacrylate system CA,obs and CB,obs include contributionsfrom radicals of types (IV), (V) and (I), (11).Both CA,obs and CB,obs may be functionsof [B]/[A] of the form shown in eqn (2) [cf. eqn (6) of Part 111CA,obs = I___ (rACAA/uO f CAB),$-rA/uOCombination of eqn (1) and (2) gives eqn (3) :In principle, eqn (3) permits calculation of CAA, CABy CBA, CBB from copolymerizationexperiments carried out over a range of [A] and [B]11 6 MODERATED COPOLYMERIZATIONWith finite (but small) conversions, it is appropriate to use eqn (14) and (22) ofPart 3.9 We write the latter relation in the formin whicha =P =( 5 )[It is assumed that CA,obs and CB,obs remain effectively constant in the compositionrange between uo and u.This is the reason for the restriction to small conversionsstated above; otherwise, eqn (14) and (22) of Part 3 are valid for all conversions.]Combination of eqn (Z), (4) and (5) yieldsCBBarA au0 PCAB+ l+r,uo CAA + -uO + rA uO + rAIn eqn (6), a and p axe calculable from eqn (5) for given values of uo and u, so thatby carrying out a series of (modified) copolymerization experiments over a range ofvalues of uo a set of simultaneous equations in CAA, CAB, CBA, CBB may be obtainedwith the aid of this equation.Determination of these four quantities would reveal penultimate unit influenceson both MA* and -Be radicals.Direct measurement of CAA by studies of thehomopolymerization of the moderating comonomer A would normally be possibleand so facilitate application of eqn (3) or (6). Clearly, great experimental precisionis essential if significant values are to be obtained in this way.PENULTIMATE UNIT INFLUENCES I N CHAIN-TRANSFER INHOMOPOLYMERIZATIONThere is no reason why penultimate unit effects of the type described should beconfined to copolymerizations; on the contray, our data imply that they shouldalso exist in some homopolymerizations. For example, we might expect that, inthe homopolymerization of methyl methacrylate, the rate of transfer with carbontetrabromide is not determined solely by the properties (reactivity) of the terminalunit, but is reduced by the presence of the a-methyl in the penultimate residue.Evidently this possibility must be borne in mind when assessing reactivities ofpropagating chains from chain-transfer data.'C. H. Bamford, J.C.S. Faraday I, 1976,72,2805.C. H. Bamford and S. N. Basahel, J.C.S. Faraday 4, 1979,75, 107.C. H. Bamford and S. N. Basahel, J.C.S. Faraday I, 1978,74, 1020.H. Fuhrman and P. B. Mesrobian, J. Amer. Chem. SOC., 1954,76, 3281.T. Huff and E. Perry, J. Polymer Sci. A, 1963, 1553.F. M. Lewis, C. Walling, W. Cummings, E. R. Briggs and F. R. Mayo, J. Amer. Chem. SoC.,1948,70,1519C . H. BAMFORD AND S . N. BASAHEL 117' C. H. Bamford, A. D. Jenkins and R. Johnston, Trans. Faraday SOC., 1958, 55, 418; C. H.Bamford and A. D. Jenkins, J. Polymer Sci., 1961, 53, 149 ; Trans. Favaday SOC., 1963, 59,530 ; N. C. Billingham and A. D. Jenkins, in Comprehensive ChemicaE Kinetics, ed. C . H. Bam-ford and C. F. H. Tipper (Elsevier, 1976), vol 14A, chap. 5. * G. C. Eastmond, in Comprehensive Chemical Kinetics, ed. C . H. Bamford and C. F. H. Tipper(Elsevier, Amsterdam, 1976), vol. 14A, (a) p. 379; (b) p. 226.C. H. Bamford and S . N. Basahel, Polymer, 1978, 19,943.(PAPER 9/285