Three-component photoinitiator systems generally consist of a photosensitizer, which is usually a dye, an electron donor such as amine and a third component, which is typically iodonium salt. For efficient visible-light-induced polymerizations, a series of fundamental and systematic design criteria of three-component photoinitiator systems must be established. These criteria include general criteria, thermodynamic considerations and kinetic considerations. A series of experiments were performed to identify a systematic list of criteria that the photosensitizers, electron donors and third components must meet.
The photo differential scanning calorimetry (Photo-DSC) was used to characterize fundamental principles of polymerization rate using a series of electron donors with systematically varying structure. Based upon the experimental results, it was characterized that Rhem-Weller equation allows the thermodynamic feasibility of photo-induced electron transfer reaction to be established between dye and electron donor. For the kinetic consideration, three-component initiator systems containing methylene blue (MB) as a photosensitizer, a systematic series of electron donors and diphenyliodonium salt as a third component was used. In this study, we have presented the experimental evidence of secondary reaction step and the role of iodonium salt (DPI) in three-component initiator systems. It was concluded that the DPI enhances the photopolymerization kinetics in two ways: (1) it consumes an inactive MB neutral radical and produces an active phenyl radical thereby regenerating the original methylene blue, and (2) it reduces the recombination reaction of MB neutral radical and amine radical/cation. In addition, we observed that high separation yield of radical pair increases the MB neutral radical concentration, thereby enhancing efficiency of secondary reaction step. Therefore, steric hindrance effects increase separation distance and decrease back electron transfer thereby increasing separation yield of radical pair and increasing rate of polymerizations.
Finally, a series of experiments were performed to explore the effect of proton transfer efficiency and radical/cation persistence of electron donor on the shelf life. Based upon the experimental results, it was concluded that electron donors containing high radical/cation persistence or no ability for proton transfer provided good shelf life and high thermal stability of initiator systems.