PRIMARY PROCESSES IN THE PHOTOSENSITIZED POLYMERIZATION OF CATIONIC MONOMERS

A detailed time-resolved laser spectroscopy investigation has been carried out on the electron transfer reactions of substituted thioxanthone derivatives with diphenyliodonium (Ph2I+) salts having different metal halide counterions (MX(n-). Quenching of thioxanthones' triplet state has been followed under various conditions, by changing the number and nature of substituents on the thioxanthone skeleton, using anions with different nucleophilicity and employing different solvents, namely methanol and acetonitrile. A photosensitization mechanism is proposed involving an electron transfer from thioxanthone to diphenylio-donium salt. The absorption spectra of the thioxanthone's excited state and the formed new transient are recorded and the rate constants of the excited state processes are measured. The triplet state of thioxanthone derivatives has been quenched by cationically polymerizable monomers and the quantum yield of the major processes has been evaluated. Photolytic experiments have been performed to measure the extent of acid formation. From photopolymerization experiments using different photoinitiating systems, the rate of polymerization and percentage of monomer conversion have been determined. Both the reactivity in the excited states and the nucleophilicity of the anions affect the efficiency of the photopolymerization reaction.