| Abstract
| - The effect of triplet sensitizers on the photoinitiated polymerization (cure) of a model acrylate monomer,isobornyl acrylate (IBOA), has been investigated. Time-resolved electron paramagnetic resonance (TR EPR)spectroscopy was employed to investigate the initiation of polymerization. Cure monitoring and photodifferentialscanning calorimetry (photoDSC) were employed to follow the course of the polymerization. Thioxanthen-9-one (TX) and 2-isopropylthioxanthen-9-one (ITX) were found to be effective sensitizers of the photopolymerization, which was initiated by radicals produced from (2,4,6-trimethylbenzoyl)diphenylphosphine oxide(TMDPO) and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO). TR EPR experiments demonstratedthat the mechanism of sensitization involves T−T energy transfer from TX (or ITX) to TMDPO or BAPOfollowed by formation of radicals by α-cleavage of the photoinitiators. Direct photolysis of TMDPO andBAPO results in an absorptive chemically induced dynamic electron polarization (CIDEP) pattern due to thetriplet mechanism (TM) of polarization of the substituted benzoyl and P-centered radicals produced byα-cleavage of the photoinitiators. TR EPR demonstrates that the same radicals were produced during directand sensitized photolysis. However, a different CIDEP pattern is produced by photosensitization, namely anemissive/absorptive (E*/A) pattern. A TR EPR study of solutions containing phosphine oxide initiators andIBOA under direct and photosensitized conditions demonstrated that the polarized primary P-centered radicalsadd to the double bond of IBOA with the formation of polarized secondary radical adducts. Both primaryand secondary radicals exhibit the same polarization pattern as the primary radical precursors, i.e., A in directphotolysis and E*/A in the presence of a sensitizer. The rate of polymerization of neat IBOA was followedby cure monitoring. In the presence of ITX the rate of cure increases significantly compared to direct photolysisof same. The heat evolved in the polymerization of IBOA photoinitiated (direct and sensitized) with TMDPOwas monitored by photoDSC, and at early times was found to be higher in the sensitized photopolymerization.Time-intermittent UV irradiation allowed an estimation of the ratio of termination to propagation rate constants(kt/kp) during dark periods of polymerization. The observed decrease of kt/kp with the progress of polymerizationis discussed. The results suggest that photosensitization may provide a means of manipulating and controllingthe parameters of photocuring of acrylates.
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