IADR Abstract Archives
Visible Light Photopolymerization of Thiol-Ene Resins
Objectives: This study was to determine if hexaarylbiimidazoles (HABIs) are efficient, visible light-active photoinitiators for thiol–ene systems. We hypothesized that, owing to the reactivity of lophyl radicals with thiols and the necessarily high concentration of thiol in thiol–ene formulations, HABIs would effectively initiate thiol–ene polymerization upon visible light irradiation.
Methods: UV-vis absorption spectra of HABI photoinitiator solutions were obtained using UV-vis spectroscopy, while EPR spectroscopy was used to confirm the generation of radical species upon HABI photolysis. Functional group conversions during photopolymerization under various irradiation conditions were monitored using FTIR spectroscopy. Additionally, thermomechanical properties, including glass transition temperatures, were determined using dynamic mechanical analysis.
Results: UV-vis absorption spectra of HABI photoinitiator solutions demonstrated that, although the compounds investigated exhibited less absortivity than camphorquinone at 469 nm, they did afford increased sensitivity at this wavelength when compared with bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide. EPR spectroscopy revealed that, upon visible light irradiation, the investigated HABIs afforded long-lived lophyl radicals. In the absence of a thiol co-initiator, the investigated HABIs proved ineffective as (meth)acrylate photoinitiators; however, thiol–ene resins formulated with HABI photoinitiators polymerized rapidly upon both UV and visible light irradiation up to 469 nm.
Conclusions: Although thiol–enes show promise as the continuous polymeric phase for composite dental restorative materials, they show poor reactivity with the conventional camphorquinone/tertiary amine photoinitiation system. Conversely, despite their relatively low visible light absorptivity, HABI photoinitiators afford rapid thiol–ene photopolymerization rates. Moreover, minor structural modifications suggest facile pathways for improved HABI solubility and visible light absorption.
Methods: UV-vis absorption spectra of HABI photoinitiator solutions were obtained using UV-vis spectroscopy, while EPR spectroscopy was used to confirm the generation of radical species upon HABI photolysis. Functional group conversions during photopolymerization under various irradiation conditions were monitored using FTIR spectroscopy. Additionally, thermomechanical properties, including glass transition temperatures, were determined using dynamic mechanical analysis.
Results: UV-vis absorption spectra of HABI photoinitiator solutions demonstrated that, although the compounds investigated exhibited less absortivity than camphorquinone at 469 nm, they did afford increased sensitivity at this wavelength when compared with bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide. EPR spectroscopy revealed that, upon visible light irradiation, the investigated HABIs afforded long-lived lophyl radicals. In the absence of a thiol co-initiator, the investigated HABIs proved ineffective as (meth)acrylate photoinitiators; however, thiol–ene resins formulated with HABI photoinitiators polymerized rapidly upon both UV and visible light irradiation up to 469 nm.
Conclusions: Although thiol–enes show promise as the continuous polymeric phase for composite dental restorative materials, they show poor reactivity with the conventional camphorquinone/tertiary amine photoinitiation system. Conversely, despite their relatively low visible light absorptivity, HABI photoinitiators afford rapid thiol–ene photopolymerization rates. Moreover, minor structural modifications suggest facile pathways for improved HABI solubility and visible light absorption.