Using changes in initiation and chain transfer rates to probe the kinetics of cross-linking photopolymerizations: Effects of chain length dependent termination

The importance of incorporating chain length dependent termination (CLDT) behavior into the interpretation of the kinetics of cross-linked systems has been examined. Kinetic chain length distributions were varied in a variety of di(meth)acrylate photopolymerizations via the manipulation of the initiation rate and the chain transfer rate. Shifting the kinetic chain lengths toward shorter chains has little visible effect on multiacrylate systems. In contrast, similar changes in the corresponding multimethacrylate polymerizations changed the kinetics significantly. Shorter kinetic chains led to the delayed onset of reaction-diffusion-controlled termination behavior, as well as an increase in the ratio of k(t)/k(p)[M] at all conversions prior to the onset of reaction-diffusion control. Additionally, the magnitude of the kinetic constant ratio in the reaction-diffusion-controlled regime was affected by the kinetics at low conversion in the polymerization of a rubbery system, PEG(600)DMA. This behavior was independent of the method used to alter the kinetic chain length distribution and thus implies that CLDT may potentially impact the network formation in polymerizations occurring above the T-g of the system. These results illustrate that, although counterintuitive, CLDT is an important factor in cross-linking free radical polymerizations.