Chain length dependent termination in radical cross-linking polymerization

The work presents an evidence in support of chain length dependent termination during cross-linking polymerization. It is based on the behavior of the ratio of the bimolecular termination coefficient ktb to propagation rate coefficient kp during the after-effect of a photo-induced polymerization. The chain-length dependence was manifested by a decrease of the ktb/kp ratio with the increase in dark reaction time faster than that resulting from the conversion increase. Two monomethacrylate/dimethacrylate and one dimethacrylate/dimethacrylate systems were chosen, which enabled to study the chain-length dependence as a function of cross-link density of the polymer being formed and physical properties of the initial composition. The ratios of the polymerization rate coefficients ktb/kp were calculated for various postpolymerization processes as a function of dark reaction conversion using the mixed termination model (concerning the bimolecular and monomolecular termination occurring parallel). A rapid drop of the ktb/kt ratio during the dark reaction was observed at the beginning of the after-effect and in slightly cross-linked systems suggesting a significant chain length dependence, but when the cross-link density increased with conversion or with cross-linking agent concentration in the feed, the drop of the ktb/kt ratio became much slower indicating that the chain length dependence decreased.