Quaternary ammonium salts of phenylglyoxylic acid as photobase generators for thiol-promoted epoxide photopolymerization
A novel series of photobase generators (PBGs), which are quaternary ammonium salts of phenylglyoxylic acid (PA) having the ability to generate strong bases such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) or 1,1,3,3-tetramethylguanidine (TMG), was synthesized. The photochemistry of these PBGs was studied by photolysis, pH measurements, and electron spin resonance-spin trapping (ESR-ST) experiments. It was shown that the PBGs underwent a photodecarboxylation process leading to the release of the base under irradiation. These photobase generators were successfully applied to the crosslinking of an epoxide monomer without any post-thermal treatment. By adding thiols in the photocatalyst system, an improvement on the polymerization efficiency was observed due to the formation of a highly reactive initiating thiolate anion.