Kinetic studies of cationic photopolymerizations of epoxide monomers

The selection of monomers and photoinitiators for cationic polymerizations is reasonably broad; however, the fundamental characterization of kinetics of these reactions has received relatively little attention. The broad objective of this research is to address this concern by characterizing the factors that affect the rate constants for termination and propagation. These polymerizations were characterized using dark-cure experiments in which active centers are produced photochemically for a specified period of time until the initiating light is shuttered off, and then the polymerization rate is monitored in the dark. Analysis of the dark-cure polymerization rate as a function of time provided profiles of the instantaneous effective propagation and termination rate constants.; For bulk photopolymerizations of phenyl glycidyl ether initiated using iodonium photoinitiators, the values of the effective rate constants for termination/trapping (kt/t) and propagation (kp) were found to remain essentially invariant for conversions up to 50%. For the iodonium antimonate photoinitiator the values of kt/t and kp at 50°C were found to be 0.041 min-1 and 0.4 L mol-1 s -1. At higher conversion the kp profile exhibits a monotonic increase which was attributed to an increase in the average separation distance between the active center and the counterion. The kp profiles were further investigated as a function of temperature, photoinitiator anion, and photoinitiator concentration.; Effects of monomer functionality and water on the kinetic rate constants were also characterized. The diepoxide monomers possess a second epoxide ring thereby leading to crosslinking in the polymerization. When a propagating active center reacts with the first ring of a 'free' diepoxide monomer, the second ring becomes a pendant epoxide ring attached to the polymer chain. Due to mobility and accessibility considerations of the 'free' and the pendant epoxide rings, the propagation rate constants of reactions involving these two species are different and the weighted average propagation rate constants for the diepoxide monomers were characterized. The effect of water in the cationic photopolymerization of epoxide monomer was also studied and it was found that water in trace amounts increases the reaction rate. Finally, dark-cure technique was used to investigate the hybrid photopolymerizations of acrylate/epoxide monomer mixture.