A Breakthrough toward Long Wavelength Cationic Photopolymerization: Initiating Systems Based on Violanthrone Derivatives and Silyl Radicals

The silyl radical chemistry is used here for the free radical promoted cationic polymerization (FRPCP) of epoxy monomers ((3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate EPOX and limonene dioxide LDO) and the free radical photopolymerization FRP of an acrylate monomer (trimethylolpropane triacrylate TMPTA). The new three-component photoinitiating systems are based on violanthrone derivatives, diphenyl iodonium salt and a silane. The use of very soft irradiations (household fluorescent and LED bulbs, sunlight and xenon lamp as visible light sources) is described. Excellent polymerization profiles are obtained. Addition of ruthenium tris(bipyridine) still enhances the performance. A sunlight irradiation of 2 min leads to a conversion similar to 70% of EPOX The specific role of the silyl radicals generated in this system is investigated and discussed. Comparative assessments with existing initiating systems are also provided evidencing the very high efficiency of the new proposed approach.