On the Onset of the Trommsdorff Effect

A new time scaling procedure is applied to data on kinetics of free radical polymerization. Superposition of data at different temperatures and initiator concentrations and from different laboratories is achieved at low conversion with this scaling. Autoacceleration due to the Trommsdorff effect produces deviations from this master curve. The point in scaled time where this deviation occurs depends upon one parameter only: the mean molecular weight of the polymer produced in the reaction mixture up to that point. These scaled kinetic data thus provide the best context for examining the dependence of the onset of autoacceleration on the molecular weight of the polymer. Examination of several sets of data in the literature shows that the transition from normal to accelerating kinetic behavior is sharp when the molecular weight of the polymer being produced is high and more diffuse when the mean molecular weight in the polymerizing medium is lower. Parallels are found between this observation and studies of the concentration dependence of solution viscosity and mutual diffusion coefficient at various molecular weights. From several sets of scaled data we extract the molecular weight dependence of the critical concentration, c**, for the onset of the Trommsdorff effect. We find c** - N^−0.24. This is different from either the molecular weight dependence of the critical concentration for molecular overlap [c* - N^−0.5 to c* - N^−0.8] or the molecular weight dependence of the critical concentration for entanglement [c** - N^−1.0]. An explanation for the discrepancy is suggested.