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 (R-i) and via the addition of a chain transfer agent (1-dodecanethiol), Changing R-i 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 R-i and the addition of chain transfer agent. (C) 2001 Elsevier Science Ltd. All rights reserved.