Studies in chain transfer II. Catalyzed polymerization of methyl methacrylate

Abstract

The chain-transfer reaction between growing methyl methacrylate ( M ) polymer radical and toluene ( S ) when catalyzed by varying quantities of benzoyl peroxide ( B ) at 80 and 60°C and p -nitrobenzoyl peroxide at 80°C has been studied. It has been established that the transfer constant calculated from the slope of 1/ P̄ against S / M curve at constant B / M is not affected by the presence of low concentrations of benzoyl peroxide ( B ), which opens up the possibility of using catalysts for chain-transfer studies. At high concentrations of the catalyst, p -nitrobenzoyl peroxide, the calculated transfer constant has been found to be appreciably altered due to transfer with the catalyst itself. A crucial experimental test of whether the initiation mechanism is unimolecular or bimolecular has been made. In conformity with the deduction from the latter mechanism it has been found that at a constant monomer concentration 1/ P̄ against √( B / M ) plot is linear, and the slope of this line remains unaffected by a change of monomer concentration. On the contrary, the alternative deduction based on unimolecular initiation that 1/ P̄ against √( B / M ) plot should be linear and of equal slope at all monomer concentrations is found inconsistent with experimental data. This strongly supports bimolecular initiation and definitely dis­counts the idea of monomolecular initiation by the catalyst. Four probable mechanisms from literature which are equivalent to a bimolecular mechanism are discussed. A critical test of the whole scheme has been made by calculating D. P. values in bulk polymerization by extrapolation from our data in solution. These extrapolated values are used to construct 1/ P̄ against √( B / M ) curves, and these latter curves are compared with similar curves based on experimentally determined data. A fair degree of concordance between the predicted and the observed values, the predicted values being somewhat lower, is obtained, confirming the essential correctness of the assumed mechanisms of polymeriza­tion and chain transfer.