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 (iii) 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 photo‐oxidizable series mechanism provides the most efficient kinetic pathway in the visible‐light activated free radical polymerizations.