Free-radical vinyl polymerization in bulk: Conversion dependence of initiation efficiency

The systematic array of data concerning the conversion C dependence of initiation rate constants ki has been collected for the first time via direct experimental measurements (via the inhibitor method) of the rate of initiation in the course of free-radical polymerization carried out at various temperatures (50, 60, and 70°C) for three most representative monomers (styrene, methyl methacrylate, and vinyl acetate) in the presence of AIBN as a typical initiator. The ki = f(C) dependence demonstrates an exponential behavior with a sharp change in the exponent at a certain critical point Ccr whose position on the conversions scale correlates with the kinetic features of polymerization for each of the monomers. The results of this study are necessary for correction of approaches useful for development of a new generation of mathematical models of free-radical polymerization processes, since, in terms of previously advanced models, the values of ki are assumed as conversion-independent. On the basis of the experimental data, the mechanisms of some physical processes that occur in monomerpolymer systems and determine diffusion-controlled chemical transformations are discussed. Specifically, factors controlling the diffusion exit of initiator radicals from the Franck-Rabinowitch cage and factors governing the quadratic termination of growing polymers chains are compared.