Kinetic studies of cationic photopolymerizations of phenyl glycidyl ether: termination/trapping rate constants for iodonium photoinitiators

In this contribution, we have performed a comprehensive investigation of cationic photopolymerizations of phenyl glycidyl ether using two iodonium photoinitiators: diaryliodonium hexafluoroantimonate (IHA), and (tolycumyl) iodonium tetrakis (pentafluorophenyl) borate (IPB). We characterized these reactions using dark-cure experiments in which the polymerization was monitored in the dark after illuminating it for a pre-determined period of time, and obtained profiles of the rate constant for termination/trapping as a function of time. Our studies reveal that though these photoinitiators result in similar reaction kinetics (reaction rate and conversion profiles that are nearly identical) for constant illumination with a Hg(Xe) arc lamp, they lead to very different results in the dark-cure experiments with the iodonium borate salt exhibiting a higher polymerization rate at a given time, and a higher limiting conversion (76%) than observed for the iodonium antimonate salt (62%). These dark-cure trends were explained by the fact that the rate constant for termination/trapping was approximately 50% higher for the iodonium antimonate photoinitiator (0.041 and 0.027 min−1 for the IHA and IBP photoinitiators at 50 °C, respectively). The active center concentrations and propagation rate constants were also characterized. Relative to the IPB, it was found that the IHA initiator leads to a higher active center concentration (due to the higher molar absorptivity of this initiator at the prominent emission wavelengths of the light source) but a lower propagation rate constant. Therefore, these two photoinitiators yield nearly identical kinetic profiles under constant illumination due to the fact that the IPB photoinitiator leads to a lower active center concentration, which is offset by a higher value of the propagation rate constant, and a lower value of the rate constant for termination/trapping.