Kinetic Pathway Investigations of Three-Component Photoinitiator Systems for Visible-Light Activated Free Radical Polymerizations

Three-component photoinitiator systems generally include a light-absorbing photosensitizer (PS), an electron donor, and an electron acceptor. To investigate the key factors involved with visible-light activated free radical polymerizations involving three-component photoinitiators and 2-hydroxyethyl methacrylate, we used thermodynamic feasibility and kinetic considerations to study photopolymerizations initiated with either rose bengal or fluorescein as the PS. The Rehm-Weller equation was used to verify the thermodynamic feasibility for the photo-induced electron transfer reaction. It was concluded that key kinetic factors for efficient visible-light activated initiation process are summarized in two ways: (1) to retard back electron transfer and recombination reaction steps and (2) to use a secondary reaction step for consuming dye-based radical and regenerating the original PS (dye). Using the thermodynamic feasibility and kinetic data, we suggest three different kinetic mechanisms, which are (i) photo-reducible series mechanism, (ii) photo-oxidizable series mechanism, and (ii) parallel-series mechanism. Because the photo-oxidizable series mechanisms most efficiently allow the key kinetic factors, this kinetic pathway showed the highest conversion and rate of polymerization. The kinetic data measured by near-IR and photo-differential scanning calorimeter verified that the photooxidizable series mechanism provides the most efficient kinetic pathway in the visible-light activated free radical polymerizations. (C) 2008 Wiley Periodicals, Inc. i Polym Sci Part A: Polym Chem 47: 887-898, 2009