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 degreesC, 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. (C) 2003 Elsevier Science B.V. All rights reserved.