Termination Rates in Free Radical Copolymerizations

Abstract

The rates of free radical copolymerizations at given rates of initiation can be analyzed ideally in terms of monomer feed concentrations and reactivity ratios, propagation rate constants for homopolymerizations of the particular monomers, and an overall rate constant for termination during copolymerization. This model, which is due to Atherton and North, can account for the effects of initiator concentration and viscosity of the polymerization medium on copolymerization rates. This article reports an empirical formulation for the overall termination rate constant in terms of monomer concentrations and reactivity ratios and a cross-termination factor. The new model accounts for experimental data in the styrene-methyl methacrylate system in which polarity differences between unlike radicals may result in enhanced termination rates. It also predicts observed copolymerization rates of methyl methacrylate-vinyl acetate and styrene-α-methylstyrene mixtures in which polarity effects are absent. The cross-termination factor may be approximated from reactivity ratio data for predictive purposes.