The mechanism and kinetics of the heterogeneous polymerization of vinyl monomers I. The benzoyl peroxide catalyzed polymerization of vinyl chloride

Abstract

A study has been made of the polymerization of vinyl chloride, with varying amounts of benzoyl peroxide at temperatures from 33 to 75° C. With from 0.025 to 1.0 mol. % benzoyl peroxide and over the above-mentioned temperature range, the rate of polymerization gradually accelerates over the first 30-40 % reaction. In the presence of a solvent for the polymer, such as dibutyl phthalate or tetrahydrofuran, however, the polymerization proceeds at a constant rate which decreases at later stages in the reaction. ‘Dead’ polyvinylchloride has been shown to act as a co-catalyst with benzoyl peroxide in the polymerization of vinyl chloride; in the absence of benzoyl peroxide, it does not catalyze the polymerization. Polyvinylchloride therefore exerts its catalytic effect when (a) the polymer is present in an undissolved state, and (b) benzoyl peroxide or another source of free radicals is also present in the reacting system. It is suggested that the co-catalytic effect of the ‘dead’ polymer is caused by an increase in the number of centres of polymer growth in the reacting system arising from chain transfer reactions between growing polymer chains and molecules of 'dead’ polymer resulting in the accumulation of stabilized centres of polymer growth on the surface of the solid polymer; the revivified polymer grows by addition of monomer until it is finally terminated by chain transfer with monomer, with the production of a mobile free radical. Relative values of the average molecular weight of polymer formed (a) at different temperatures, (b) at different stages in the reaction, (c) with different concentrations of benzoyl peroxide, and (d) in the presence of dibutyl phthalate, have been obtained. The results support the suggestion that chain transfer takes place to a considerable extent in the polymerization of vinyl chloride.