Dependence of Propagation Rate Coefficients in Radical Polymerization on Solution Properties

The effect of solvents and monomer concentration on rate coefficients of propagation in radical polymerization has received tremendous interest during recent years, but a quantitative explanation and method of prediction of the medium effect is still missing. In this paper, it is shown that the thermodynamic formulation of the transition state theory is able to explain such effects almost quantitatively for a wide variety of monomers and solvents, including not only organic solvents but also less common solvents like ionic liquids or water, and monomers ranging from styrene to water‐soluble monomers like acrylic acid or N‐vinylformamide and also bulky monomers like isobornyl methacrylate. The medium effect is dictated by the activity coefficient of the monomer as well as by the ratio of the activity coefficients of the growing chain and the transition state. The solvent and concentration effects are, thus, a consequence of the degree of nonideality of the system. This interpretation based on the thermodynamics of real solutions offers for the first time a straightforward method to estimate the propagation rate coefficient of a given monomer in various solvents once the corresponding coefficient in bulk monomer is known. The transition state theory has successfully been applied to quantitatively explain the solvent effect on the propagation rate coefficients in radical polymerization. The effect can entirely be explained by the degree of nonideality of the system and the thermodynamic activity of all species involved in the transition state.