A Three-component Photoinitiator System with Extensive Dark Cure Potential

Radical-based photopolymerization is widely used for in-situ formation of polymers in numerous dental materials applications. Since biradical termination reactions effectively compete with polymer propagation, continued illumination is generally required to achieve full conversion. Objective: This study evaluates the thermal stability of a three-component photoinitiator based on a photosensitizer (methylene blue, MB), electron donor (N,N-diisopropylethylamine, DIPEA) and an electron acceptor (diphenyliodonium chloride, DPI-Cl) and examines the photo-activated dark curing behavior of this unique initiating system. Methods: 2-Hydroxyethyl methacrylate (HEMA) was used as the monomer throughout. Polymerization reaction kinetics were followed by near-infrared spectroscopy and photo-differential scanning calorimetry (photo-DSC) while ground-state interactions were probed by UV/Vis spectroscopy. Results: While MB with either DIPEA or DPI-Cl can be used as two-component visible light initiators, their combination in a three-component mixture provides significantly more efficient photocuring but leads to limited shelf-life storage with approximately 5% conversion occurring over 24h in the dark. However, the combination of DIPEA and DPI-Cl in HEMA without MB lead to a spontaneous redox polymerization with conversion reaching approximately 40% over three hours. In a brief partial-photocure of the full HEMA/three–component initiator system, the visible curing light was extinguished at less than 5% conversion but dark cure continued to advance conversion to over 80% during several hours. Thermal stability studies of various combinations of initiator components in HEMA indicated that MB effectively inhibits the ground-state redox initiation promoted by DIPEA and DPI-Cl. Rate maxima (and times) for DSC-based thermal-induced polymerizations in the dark at 60, 70 and 80 °C were 0.003mol/L*s (25.2min), 0.017 mol/L*s (9.8min) and 0.026mol/L*s (6.5min), respectively. Conclusions: It appears MB acts as a redox inhibitor that is consumed rapidly during a brief visible light exposure and then promotes an extended redox chemical cure process that retains the spatial control of photopolymerization. Supported by: NIDCR-R01DE014227