Modeling primary radical termination and its effects on autoacceleration in photopolymerization kinetics

In industrial photopolymerization processes, high light; intensities are used to achieve fully cured products in short exposure times. Under these conditions, a high concentration of primary radicals-those radicals derived directly from the photocleavage of the initiator-is established. These high concentrations can lead to a phenomenon known as primary radical termination, in which growing macroradicals are terminated by primary radicals instead of other macroradicals. This mechanism violates the pseudo-steady-state assumption used in most radical polymerization modeling. While previous research has demonstrated the effects of primary radical termination on low conversion, linear polymerizations, little has been done to describe its effects on diffusion-controlled polymerizations and high conversion systems. To improve our understanding of primary radical termination on photopolymerization kinetic behavior, a previously developed kinetic model is extended to include the primary radical termination mechanism. In the initial simulations, the primary radical termination kinetic constant (k(tp0)) is varied to determine its effects on the polymerization rate, the macroradical and primary radical concentrations, and the dominant mode of termination. The effect of primary radical termination on autoacceleration is addressed in particular. In a second series of predictions, k(tp0) will be held constant as the light intensity is varied to demonstrate its effects on the polymerization rate profile. Last, the phenomenon of rate saturation is reproduced and explained using the kinetic model.