A Photo-Oxidizable Kinetic Pathway of Three-Component Photoinitiator Systems Containing Porphrin Dye (Zn-tpp), an Electron Donor and Diphenyl Iodonium Salt

A series of kinetic experiments were conducted involving visible-light activated free radical polymerizations with three-component photoinitiators and 2-hydroxyethyl methacrylate (HEMA). Three-component photoinitiator systems generally include a light-absorbing photosensitizer (PS), an electron donor and an electron acceptor. To compare kinetic efficiency, we used thermodynamic feasibility and measured kinetic data. For this study, 5,10,15,20-tetraphenyl-21H,23H-porphyrin zinc (Zn-tpp) and camphorquinone (CQ) were used as the PSs. The Rehm-Weller equation was used to verify the thermodynamic feasibility for the photo-induced electron transfer reaction. Using the thermodynamic feasibility, we suggest two different kinetic mechanisms, which are (i) photo-reducible series mechanism of CQ and (ii) photo-oxidizable series mechanism of Zn-tpp. Kinetic data were measured by near-IR spectroscopy and photo-differential scanning calorimetry based on an equivalent concentration of excited state PS. We report that the photo-oxidizable series mechanism using Zn-tpp produced dramatically enhanced conversions and rates of polymerizations compared with those associated with the photo-reducible series mechanism using CQ. It was concluded from the kinetic results that the photo-oxidizable series mechanism efficiently retards back electron transfer and the recombination reaction step. In addition, the photo-oxidizable series mechanism provides an efficient secondary reaction step that involves consumption of the dye-based radical and regeneration of the original PS. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3131-3141, 2009