Radical polymerization of vinyl chloride: Kinetics and mechanism of bulk and emulsion polymerization

Abstract

Bulk and emulsion polymerization have as a common feature that there are already formed, from the start, particles which are one-phase systems consisting of polymers swollen with monomer. The composition of the particles is the same in the two cases and is constant up to about 70 per cent conversion at 50°C. Bulk and emulsion polymerization differ in that in the former case the outer phase is practically pure monomer while in the latter it consists of a dilute aqueous solution of vinyl chloride (~0.1 mole/1. at 50°C). Mechanisms of bulk and emulsion polymerization are discussed. A mechanism involving desorption and reabsorption of radicals into the particles is treated in more detail. The result of the calculations seems to indicate that the bulk and emulsion polymerization are similar in that already at low conversion the main reactions, both polymerization and termination, take place in the particles. Even for bulk polymerization the number of radicals in the outer phase is, under ordinary conditions, practically independent of the rate of radical termination in that phase. The present treatment does not involve any assumption of an established thermodynamically determined equilibrium distribution of radicals in the two phases. In both these respects the present theory differs from previously advanced theories for bulk polymerization. The subdivision of the reaction zone in discrete particles has a similar effect on the bulk polymerization as on emulsion polymerization. It increases the rate of polymerization compared to that which would be obtained if the particles formed a continuous phase. The present theory may readily explain the experimentally observed drop in rate at precipitation, as well as the autocatalytic course of the reaction. The increase in the initial rate of bulk polymerization by addition of chain transfer agents results from an increase in the effective rate constant for desorption. Bulk and emulsion polymerization will respond differently to a change in the rate of desorption and absorption of radicals in the particles. This is due to the difference in the concentration of monomer in the outer phase in the two cases. In the case of bulk polymerization an increase in the rate of desorption and a decrease in the rate of absorption of radicals into the particles will tend to increase the rate of reaction whereas the opposite is expected for emulsion polymerization. New experimental evidence concerning emulsion polymerization with mixed emulsifiers consisting of sodium dodecylsulphate and different n-fatty alcohols is presented. A marked decrease in the rate is obtained by use of n-hexadecanol. The mixed emulsifier is compared with the pure anionic emulsifier at seed polymerization with a given amount of seed and number of particles. With the pure anionic emulsifier the rate of polymerization is independent of the degreeof coverage with emulsifier. A complete coverage of the particles with a mixture of sodium dodecylsulphate and n-hexadecanol in a certain ratio leads to a marked decrease in rate. It is suggested that these results may be due to the mixed emulsifier forming a condensed layer on the surface of the particles which reduces the effective absorption constant. © 1971, Walter de Gruyter. All rights reserved.