Photopolymerization and photochemistry of novel photoinitiators based on p-benzoylbenzophenone

Six novel derivatives based on the p-benzoylbenzophenone chromophore have been examined in terms of their photochemical and photoinitiator properties. All the molecules exhibit high extinction coefficients in the u.v. region together with high photoinitiator activity for the polymerization of methyl methacrylate. The 2-substituted methyl and isopropyl derivatives exhibited the lowest activity, consistent with lower phosphorescence quantum yields and ketyl radical formation on microsecond flash photolysis compared with the corresponding 4-substituted derivatives. In the case of 2-substitution with an isopropyl group, ketyl radical formation was not quenched by oxygen indicating that a process of intra-molecular hydrogen atom abstraction predominates. This derivative was also less effective than benzophenone as a photoinitiator. Analysis of the photoproducts in 2-propanol confirmed the involvement of a single step reaction to give a corresponding pinnacol type compound.