Thermal degradation of visible-light-cured thiol-methacrylate networks photoactivated with Camphorquinone

This report describes the yellowing of visible light-cured thiol-ene networks formulated with Camphorquinone (CQ) as photoactivator. CQ displays an intense dark yellow color due to the presence of a conjugated diketone chromophore that absorbs at 470 nm. Irradiation of CQ with visible light produces an excited triplet state which interacts with oxidizable species, thus decomposing into colorless products. During the course of this study it was found that the color of thiol-ene networks changes from colorless to yellow after 4 months storage at 20 °C. This investigation was carried out to provide clarification of this issue. The yellowing in initially photobleached thiol-methacrylate networks was studied by UV-visible spectroscopy. The yellowing of the networks was apparent after 4 h at 50 °C, 1.5 h at 70 °C or 20 min at 110 °C. However, color changes were perceptible after 40 days at 20 °C. The change in color observed in the studied polymer networks is explained in terms of the regeneration of CQ through hydrogen transfer reactions between thiyl radicals and ketyl radicals. Results obtained in this research demonstrate that the yellowing displayed by thiol-methacrylate systems photoactivated with CQ jeopardizes the long-term stability of the derived films and bulk materials. Visible light photopolymerization offers many advantages over UV photopolymerization. However, in the particular case of thiol-ene resins photoactivated with CQ, the use of visible light is accompanied by undesired yellowing of the resulting polymers.