Synthesis, photopolymerization and photochemistry of novel polysiloxane photoinitiators

Abstract

Three novel polysiloxane photoinitiators have been synthesized and their photochemistry and photopolymerization activities related to their structure using absorption and luminescence spectroscopy and conventional microsecond and nanosecond laser flash photolysis techniques. A comparison with the properties of a model photoinitiator 2,2-diethoxyacetophenone (DEAP) has also been made. Extinction coefficients for the polysiloxanes at the absorption maxima are generally higher than those of the model initiator while the phosphorescence spectra exhibit typical nπ* carbonyl symmetry similar to that of the model but with longer phosphorescence lifetimes in the millisecond region. Photopolymerization rates for methyl methacrylate in ethyl acetate using the polysiloxanes as initiators are higher than those of the model and dependent on structure and molecular weight. Photopolymerization rates for both the model and the polysiloxanes correlate with their absorption maxima and phosphorescence quantum yields while the presence of a tertiary amine ( N-methyldiethanolamine) had a variable effect. End-of-pulse transient absorption spectra of the polysiloxanes on the microsecond time scale using conventional flash photolysis are assigned to the benzoyl radical produced on direct photolysis of the acetophenone chromophores and closely match those of the model. The transient absorption spectra also correlate with the photopolymerization rates of methyl methacrylate, indicating that the benzoyl radical is the key initiating species. This is confirmed by the formation of benzaldehyde in the photolysis products of repetitive flash photolysis in tetrahydrofuran produced by the benzoyl radical abstracting a hydrogen atom from the solvent system. Second-order derivative absorption spectroscopy confirms the presence of acetophenone end groups in the poly(methyl methacrylate) samples. On the nanosecond time scale laser flash photolysis revealed the formation of long-lived transient spectra which are also assigned to the benzoyl radical for both the polysiloxanes and the model. No triplet precursor is observed since in the presence of oxygen no quenching occurs. In fact, the transient absorption is enhanced followed by its decay to the same spectrum as that observed under nitrogen. Possible complexation between the initiator and oxygen is considered to be responsible for this observation. The significance of these results in terms of the structure and activity relationship of these novel polysiloxanes is discussed.