Transfer to benzoyl peroxide during the polymerization of p-methoxystyrene

European Polymer Journal Vol. 16, pp 917 to 920

0014-3057/~0 1001-0917502.0Q/0

Pergamon Press Ltd 1980. Printed in Great Britain

TRANSFER TO BENZOYL PEROXIDE DURING THE POLYMERIZATION OF p-METHOXYSTYRENE J. C. BEVINGTONand J. LEECH* Department of Chemistry, The University, Lancaster, England (Received 2 April 1980) Abstract--Transfer to benzoyl peroxide is pronounced in the polymerization of p-methoxystyrene (MOS) at 60°. The process is responsible for the low molecular weights of polymers formed when the peroxide is used as initiator; there is no evidence for a non-radical polymerization of the type found with N-vinylcarbazole and the peroxide. Data on the reactivity of MOS towards the benzoyloxy radical and the copolymerization of MOS with methyl methacrylate are presented.

The polymerizations of N-vinylcarbazole (VCZ) and p-methoxystyrene (MOS) initiated by benzoyl peroxide (BPO) at 60 ° lead to polymers with molecular weights considerably less than those of corresponding polymers made using azoisobutyronitrile (AIBN). A suggestion [1] that transfer to initiator might be responsible for the low molecular weights in the V C Z / B P O system has been rejected in favour of an alternative explanation. Monomeric VCZ and BPO are believed [2] to interact thus VCZ + BPO--+ C6HsCOO.

60° in dilatometers in the absence of air were restricted to about 5% conversion; following Ayrey et al. [4] 12.93~o contraction was taken as corresponding to 100~ polymerization for calculation of rates. Polymers were recovered by precipitation in methanol and purified by dissolution in benzene and reprecipitation in methanol. Average moleculai" weights of polymers were found by GPC. They were based on polystyrene standards; they are to be regarded as approximate and to be used only for comparative purposes. The syntheses of labelled materials and their assay by scintillation counting in solution have been described [5]. All other experimental procedures were similar to those commonly used for work of this type.

+ .CH2.~HX + C6HsCOOwhere X represents the carbazole group. Monomeric VCZ is very reactive in cationic polymerization and the main part of the polymerization initiated by BPO is regarded as involving the positive centres; termination is thought to occur readily by reaction of growing centres with benzoate ions leading to polymers of low molecular weight. It is significant that the decomposition of BPO in solution is promoted by polyVCZ and by N-ethylcarbazole, with profound effects on the yields of the various products [3] ; both additives contain tertiary amine groups of the type present in monomeric VCZ. PolyMOS and p-ethylanisole resemble the carbazole derivatives in their effects on the decomposition of BPO. It seems possible therefore that BPO might react with monomeric MOS in the same way as with VCZ and so promote a cationic polymerization. It is essential however to check whether or not the MOS/ BPO system has the characteristics of a radical polymerization, with pronounced transfer to initiator being responsible for the low molecular weights. The results of various tests are reported and considered in this paper. EXPERIMENTAL

For polymerization of MOS in benzene at 60 ° using either BPO or AIBN as initiator, the kinetic order with respect to initiator was close to 0.5 i.e. the "normal" value for a radical polymerization; in this respect the MOS/BPO and VCZ/BPO systems are quite different. Figure 1 shows that the rate of polymerization of MOS with a mixture of AIBN and BPO

i re"

Monomeric MOS (Koch-Light) was washed repeatedly with 5% aq. NaOH and then with water; after drying with MgSO4, it was distilled under reduced pressure of N2- It was stored at - 3 0 ° under N~ and distilled on the vacuum line before use. Polymerizations and copolymerizations at * Present address: Dow Coming S.A., Pare Industriel, 6198 Scneffe, Belgium. t.rJ. 16/10--^

RESULTS AND DISCUSSION

917

0

/

I

05

I

IO

[ozoigobulyronarde] in g dm"3

Fig. I. Observed and calculated rates of polymerizations at 60° for MOS at 2.2 tool dm -a in benzene with BPO at Ig dm -3 and various cone. of AIBN. O observed; ~ calculated (Rp, + Rp~);• calculated (R~, + R~)L

Ii

918

J.C. BEVINGTONand J. LEECH Polymerizations were performed with monomeric M O S and a labelled initiator at fixed concentrations and a second initiator in an unlabelled form at various concentrations. From overall rates of polymerization (Rp) and specific activities of polymers and initiators, rates of incorporation (R~,c) in polymer of labelled fragments from the initiator were calculated. A typical set of results is shown in Table 1 which also contains information to be considered in a later section of this paper. The relationship I7]

4C

% ~;

Rin¢

2c

o

I I io 20 IO(Rate ofl:xWmerizolio~)in rnohdrri~ sec"

]

Fig. 2. Values of M . for polymers of MOS made with various initiator~. • A I B N ; O BPO; (9 BPO at I g dm -3 with various conc. of AIBN. [ M O S ] = 2.2 mol dm -3.

was close to (R 2, + R22)* where Rp, and Rp= are the rates found with the separate initiators; in the case of VCZ [6] however the mixture of initiators gave a rate close to (Rp, + Rp,). This result further indicates that polymerizations of M O S promoted by A I B N and B P O proceed by the same mechanism involving termination by interaction of pairs of centres. Additional confirmation is provided by the results of fractionation by G P C of p o l y M O S produced with a mixture of initiators; there was a single peak in the distribution of molecular weights whereas there was a bimodal distribution in a sample of polyVCZ prepared similarly [6]. _ Figure 2 shows values of M , for samples of polyMOS prepared at 60 ° from monomer at 2.2 mol d m - 3 in benzene using BPO, AIBN and their mixtures as initiator. It is evident that, for a given rate of polymerization, the molecular weights of polymers prepared in the presence of the peroxide were much lower than those prepared with A I B N as sole initiator. If the initiators function only as sources of radicals to initiate polymerization, this effect should not arise; it can be explained by the occurrence of pronounced transfer to BPO.

2kx[r]

=

Ri + ~

Rp

where R~ = rate of initiation by the labelled initiator, [I] and [M] = concentrations of labelled initiator and m o n o m e r respectively, kx and kp = velocity constants for transfer to labelled initiator and propagation respectively, was applied. Figure 3 shows that transfer to AIBN is negligible in the polymerization of M O S at 60 ° but that the transfer constant, k,,/kt,, for BPO has the high value of 5.3. Polymers prepared from M O S at various concentrations in benzene using 3H-BPO were subjected to hydrolysis under conditions known to lead to detachment of all benzoyloxy end-groups. Examination by G P C of polymers before and after hydrolysis revealed no changes in average molecular weights or molecular weight distributions; it was legitimate therefore to -~$ 4 o

-o o

o

j; ~

z

"6 ,, •

.

.

.

.

oo~o~o~

~o -

I o

4

8

~2

IO x(Roteof t:x~l~rtz~ic~"~)in rnol.dn'~~. sec"~

Fig. 3. Plots of (rate of incorporation of initiator fragments in polymer) vs (rate of polymerization) for polymerizations of MOS at 2.2 mol din- 3. O__3H.BPO at 1 g dm- 3 with various conc. of AIBN; ~---I"C-AIBN at 1 g dm -3 with various conc. of BPO.

Table I. Polymerizations of p-methoxystyrene initiated with 3H-benzoyl peroxide and unlabelled azoisobutyronitrile Counting rate for polymer (counts sec- ~ mg- ') Conc. AIBN (g dm - 3)

Rp x 106 (mol dm - ~ sec - I )

Before hydrolysis

After hydrolysis

R~,c x 10~ (mol dm - 3 s¢c- 1)

Fraction x

0.10 0.30 0.75 1.00 1.50

6.40 8.95 10.66 13.77 14.81

! 1.97 11.63 10.64 11.87 10.71

! .05 1.12 0.83 0.89 1.16

1.63 2.20 2.38 3.49 3.35

0.91 0.90 0.92 0.92 0.89

[MOS] = 2.2 mol dm- 3 ; [3H.BPO] = !.0 g dm- 3 ; counting rate for peroxide = 550 counts s¢¢- a mg- t.

919

Transfer to benzoyl peroxide 6

I

!, "6

=E

I

io 20 (Conch of MOS in rnol.drn~) "~

Fig. 4. Plot of (sum of nos of C6HsCOO and C6H s endgroups)/(no, of C6H~COO end-groups) vs [monomer] -I for polymers of MOS made at 60 °.

compare the numbers of benzoyloxy and phenyl endgroups in the original polymers by considering the specific activities before and after hydrolysis, a~ and a2 respectively, according to the expression

transfer with BPO. With increasing proportion of M O S in the feed, there is a steady increase in the proportion of growing radicals with M O S units at their reactive ends so that reaction with BPO becomes more likely. Comparisons of the numbers of C 6 H ~ C O O and C6H s end-groups were made for copolymers of MOS and M M A prepared using BPO as initiator. It was not possible to apply the method used for homopolymers of M O S because of modifications of the M M A units during hydrolysis, to an extent depending on the composition of the copolymer; the alternative procedure involving doubly-labelled peroxide was adopted. In the copolymerization, it is necessary to consider reactions (2') and (3') C6Hs.COO. + M'--* C 6 H s C O O . M '.

(2')

C6H ~. + M'----* C6Hs.M '.

(Y)

in addition to reactions (1), (2) and (3). The relationship (No. of C 6 H ~ C O O groups)

k2

k~[M']

(No. o f C 6 H s groups)[M]

kl

kl[M]

No. of C 6 H s C O O groups Sum of nos of C 6 H s C O O and C6Hs groups

Figure 4 shows that x depends upon the concentration [M] of monomer during polymerization, apparently satisfying the usual relationship [7] x

--

X.

is satisfied for various copolymerizations initiated by BPO. Figure 6 shows that the relationship applies also to the M O S / M M A / B P O system and leads t o , k2 - - = 3.95 m o l - 1 dm 3 for M O S kl

i = 1 + kl/k2[M]

where k~ and k 2 are the velocity constants for the reactions C6HsCOO.----, C6H 5. + C O 2

(1)

C6H 5. + M ---, C6HsCOO.M.

(2)

reaction (l) being followed by the initiation process C6H 5. + M --* C6Hs.M.

ar I -- a 2 al

and

k~

- - = 0.39 m o l - ~ dm 3 kl for M M A in fair agreement with the results from homopolymerizations.

(3)

to give phenyl end-groups in the polymer. The derived value of k 2 / k : is 4.0tool -1 dm 3 but it will be shown later that correction may be needed. Low conversion copolymerizations of M O S (monomer-l) with methyl methacrylate (MMA) were performed at a total concentration of monomer of 1.5 mol d m - 3 using either AIBN or B P O at 1 g d m - 3 The compositions of the copolymers were determined by ~H-NMR spectroscopy. M o n o m e r reactivity ratios, found using the procedure of Vezrielev e t al. [8] were using AIBN, r~ = 0.41 _+ 0.03; r 2 = 0.95 + 0.05 using BPO, r~ = 0.35 +_ 0.03; r 2 = !.01 + 0.02. The differences between the results for the two initiators are not significant so that it is most improbable that reactions of different types are involved. Figure 5 shows that values of M , for copolymers prepared using B P O fell markedly as the feed was made richer in M O S but that this effect did not arise when AIBN was used. This difference is consistent with the view that the polyMOS radicals readily undergo

0

\

\

0

\ J 0 2

I

I

1

04

06

O8

I J I0

rr~l f r ~ t ~ of MOSmfeed Fig. 5. Dependence of M, of copolymer upon composition of feed in copolymerization at 60 ° of MMA with MOS. Total conc. of monomer = 1.5 tool dm - 3. O---AIBN as initiator at I g din-3; O--BPO as initiator at 1 g drn-~

920

J.C. BEVINGTONand J. LEECH

as (62.5x, + 37.5)/100. If less than 25% of the initiator fragments correspond to "true" initiation, as is the case if transfer to initiator is made more important by use of a second initiator, the discrepancy between the "observed" and "true" values of x must be larger. The values of x found for the polymers referred to in o 5 Table 1 are just over 0.9 not obviously increasing as the concentration of the second initiator is raised l It is possible that small changes in x are obscured by experimental errors. The "observed" value of x are con3 I I I I I I sistent with x, being about 0.85. Use of this value, 0 2 4 6 8 i0 with the fact that x must be unity when I-MOS]-1 is [ M M A ] / [ MOS] zero, leads to a value of 2.67 tool- 1 dm 3 for k2/k~ to be compared with 2.50mo1-1 dm 3 for the correFig. 6. Plot of (no. of C6HsCOO groups)/(no, of C6Hs sponding quantity for styrene. On this basis, it groups)[MOS] vs [ M M A ] / [ M O S ] in copolymerizations. appears that MOS is slightly more reactive than styrene towards the C6HsCOO. radical. On the Q and e scheme, the values of e for MOS and styrene are - 1.1 Transfer to BPO may be represented by the equa- and -0.8 respectively; the reactivities of these tion monomers towards the C6HsCOO. radical satisfy a previous correlation between the value of e for a P,. + (C6H5COO)2 ---, P,.O.CO.C6Hs + C6H5COO. monomer and its reactivity towards this reference followed by re-initiation by the resulting C6H5COO. radical [9-1. radical or C6H5. radical derived from it by decarboxAll the results reported here support the view that ylation. According to this scheme, the transfer leads transfer to BPO is pronounced in the polymerization to incorporation of some C6F15COO groups without of MOS at 60°. There is no indication that the peroxide gives rise to a non-radical polymerization; in any accompanying C6H5 groups; thus the proportion this respect, the monomer is quite different from VCZ. of C6H5COO groups in a polymer exceeds that to be expected on the basis of reactions (2) and (3) being the only processes leading to incorporation of initiator Acknowledgement--The work described in this paper was fragments. The value of the fraction x is therefore performed while J.L. held a Research Studentship awarded expected to rise if transfer to BPO is made more im- by the Science Research Council. portant by addition of a second initiator, as in the experiments referred to in Table 1. The argument is unaffected if some C6H5. radicals engage in primary radical termination by combination; the value of x REFERENCES would be increased if this type of termination occurs 1. T. Sato, M. Abe and T. Otsu, Makromolek. Chem. 178, for the "C6H5COO. radical but the effect is normally 1259 (1977). small. 2. J. C. Bevington, C. J. Dyball and J. Leech, MakroAccording to Fig. 3, about 25~o of the incorporated molek. Chem. 180, 657 (1979). initiator fragments result from true initiation (i.e. 3. J. C. Bevington, C. J. Dyball, B. J. Hunt and J. Leech, reactions involving radicals formed by thermolysis of Polymer 19, 990 0978). the peroxide) when [MOS] is 2.2moldm -3 with 4. G. Ayrey, F. G. Levitt and R. J. Mazza, Polymer 6, 157 (1965). BPO at l g d m - 3 and without added AIBN. If the 5. J. C. Bevington and J. A. L. Jemmett, J.C.S. Faraday "true" value of x for these conditions is x,, the relative Trans. I 69, 1866 (1973). numbers of C6HsCOO and C6H s groups in the 6. J. C. Bevington and C. J. Dyball, J. Polym. ScL Chem. polymer can be calculated. Suppose that the total Ed. 14, 1819 0976). number of incorporated fragments is 100. "True" ini7. J. C. Bevington and T. D. Lewis, Polymer I, l (1960). tiation leads to 25x, C6H~COO and 25(1-x,) C6H5 8. A. I. Yezrielev, E. L Brokhina and Ye. S. Roskin, groups, transfer to 37.5 C6HsCOO and no C6Hs, and Polym. Sci. USSR II, 1894 0969). re-initiation to 37.5xt C6H5COO and 37.5 ( I - x t ) 9. J. C. Bevington, D. O. Harris and M. Johnson, Eur. Polym. J. I, 235 (1965). C6Hs; the "observed" value of x is therefore predicted b