Toward a Molecular Theory of the Trommsdorff Effect

A theory for the kinetics of high-conversion autoaccelerating free radical polymerization, incorporating the reptation model of de Gennes for polymer diffusion, is presented. Earlier models of various aspects of the Trommsdorff effect are reviewed and several weaknesses pointed out. These approaches have been empirical in the sense that diffusion behavior has usually been put in in an ad hoc manner flexible enough so that it could be adjusted to fit experimental data. The present model uses classical diffusion-controlled reaction kinetics (the Smoluchowski equation) to relate the termination rate constant to the diffusion constant of the growing radical. The best current theory for polymer diffusion is then included in the kinetic model. The model is shown to fit data on conversion vs. time and molecular weight vs. conversion up to 70 or 75% conversion. Examination of some data due to Brooks shows that the dependence of the termination rate constant on molecular weight is consistent, at high molecular weight, with the inverse square chain length dependence expected from this reptation-based kinetic model. The model suggests several new experimental approaches to a better understanding of the molecular dynamics of polymerizing media.