Heteroaromatic Thiols as Co-initiators for Type II Photoinitiating Systems Based on Camphorquinone and Isopropylthioxanthone

The heteroaromatic thiols (imidazole, oxazole, and thiazole derivatives) were investigated in regard to their abilities to function as co-initiators in free-radical photopolymerizations induced by camphorquinone and isopropylthioxanthone. As shown by the kinetic data, these heteroaromatic thiols are efficient co-initiators with activities comparable to aromatic amines. They quenched the triplet states of isopropylthioxanthone and camphorquinone with rate constants determined to be on the order of 109 and 108 M-1 s-1, respectively. The results suggested that rates of polymerization of a low-viscosity monomer (triethylene glycol dimethacrylate) in an inert atmosphere are dependent on the quantum yields of formation of primary thiyl radicals Φ. However, this effect was also moderated by the reactivities of the thiyl radicals that were generated in the photosensitization stage. The observed activities of the initiator/thiol systems also depended on the possibility of initiator photoreduction by the monomer and, in air, on the ability of the co-initiators to reduce oxygen inhibition.