The significance of chain length dependent termination in cross-linking polymerizations

Evidence is presented in support of the existence of chain length dependent termination in cross-linking free radical polymerizations. Photopolymerizations of multifunctional methacrylates and acrylates were examined, and the difference in the inherent chain transfer characteristics of these systems provided a means for studying the effect of kinetic chain length on the polymerization behavior. The kinetic chain length was varied in each system by changing the initiation rate (Ri) and via the addition of a chain transfer agent (1-dodecanethiol). Changing Ri in the methacrylate systems led to a non-classical dependence of the rate of polymerization on Ri similar to that observed in linear polymerizations that exhibit chain length dependent termination. Additionally, incorporation of a chain transfer agent into the polymerization system, even at very low concentrations (0.1 wt%), had a dramatic effect on the methacrylate polymerization kinetics. As expected, because of their predisposition to chain transfer, the acrylate systems were minimally affected by changes in Ri and the addition of chain transfer agent.