Flash photolysis investigation on primary processes of the sensitized polymerization of vinyl monomers: 2. Experiments with benzoin and benzoin derivatives

Abstract

Benzene solutions of benzoin (B), benzoin acetate (BA), benzoinmethyl ether (BME) and benzoinisopropyl ether (BIPE) were irradiated with 0.1 or 25 nsec flashes of 347.1 nm light at room temperature. With B and BA optical absorptions due to radical precursors were detected. The first order decay constants of these absorptions are 1.1 × 10 8 sec −1 (B) and 5.3 × 10 7 sec −1 (BA). The radical spectra decayed according to second order laws. In the case of B only one mode of decay was observed which accounts for the major part of the reaction (benzaldehyde formation by disproportionation of unlike radicals). During the experiments with BA, BME and BIPE two modes of radical-radical reactions were detected. Styrene was found to react with the rapidly decaying species with k M = (1.3 ± 0.2) × 10 5 M −1 sec −1, but did not react with the slowly decaying species. By considering the end product analyses of other authors it was concluded that the rapidly decaying mode of the absorption decay is due to the combination of benzoyl radicals forming benzil. The slowly decaying mode was assigned to the combination of alkoxybenzyl radicals (in the case of BME and BIPE) or of acetonbenzyl radicals (in the case of BA) forming pinacol derivatives. The rate constants of the reaction of naphthalene with triplet excited benzoin (7.7 ± 0.8) × 10 9 M −1 sec −1 and benzoin acetate (5.0 ± 0.5) × 10 9 M −1 sec −1 were determined.