Enhancement of Dark Polymerization by Oxygen Quenching during Network Formation in Ultraviolet-Light-Induced Radical Polymerization of Multifunctional Monomers and Reactive Polymer

This study investigated the impact of oxygen quenching on dark polymerization by changing the atmosphere from nitrogen to air after stopping UV light irradiation. Real-time FT-IR measurements showed that C=C bond conversions of multifunctional monomers (diurethane dimethacrylate, dipentaerythritol penta-/hexa-acrylate) and a reactive polymer in which the functional group was acrylate were 0.01–0.04, greater than that in an environment of continuous nitrogen purging, when the UV irradiation dose larger than 3 mJ/cm2 with a photoinitiator 1-[4-(phenylthio)phenyl]-1,2-octanedione 2-(O-benzoyloxime) (OXE01). Kinetic analysis of the dark polymerization elucidated that it was promoted when the polymerization and termination reaction rates were controlled by the diffusion of the C=C bond groups. Exposure to oxygen caused a portion of dangling radicals to be quenched, stopping the reaction and forming a dense network structure. The remaining dangling radicals could continue the polymerization, and more C=C bonds were converted than that in the continuing nitrogen atmosphere. This effect was not observed on using another photoinitiator, 1-hydroxycyclohexyl phenyl ketone (HCK), and diurethane dimethacrylate. The promotion of dark polymerization thus depends on the photoinitiator. Promotion of dark polymerization was observed in the case of OXE01 as it is less susceptible to the radical quenching of oxygen.