Long wavelength photoinitiated free radical polymerization using conjugated thiophene derivatives in the presence of onium salts

Long wavelength free radical photopolymerization of (meth)acrylic monomers using a conjugated thiophene derivative, namely 3,5-diphenyldithieno[3,2-b:2', 3'-d]-thiophene (DDT) and one of the following compounds, namely diphenyliodonium hexafluorophosphate (Ph₂I⁺PF ₆^-), triphenylsulfonium hexafluorophosphate (Ph ₃S⁺PF₆^-) and N-ethoxy-2- methylpyridinium hexafluorophosphate (EMP⁺PF₆ ^-), as photosensitizer and coinitiator, respectively, was described. Polymerization occurred only when DDT, in the presence of the onium salts, was irradiated at wavelengths where the salts have no absorption. A feasible initiation mechanism involves intermolecular electron transfer from photoexcited thiophene compound to the onium salt. The relative initiation efficiencies of the salts were investigated and evaluated in terms of their redox potentials.