| Abstract
| - The formation and the reactivity of three selected sulfur-centered radicals formed from mercaptobenzoxazole,mercaptobenzimidazole, and mercaptobenzothiazole toward four double bonds (methyl acrylate, acrylonitrile,vinyl ether, and vinyl acetate) are investigated. The reversibility of the addition/fragmentation reaction inthese widely used photoinitiating systems of radical polymerization was studied, for the first time, throughthe measurement of the corresponding rate constants by time-resolved laser spectroscopy. The combinationof these results with quantum mechanical calculations clearly evidences that, contrary to previous studies onother aryl thiyl radicals, the addition rate constants (ka) are governed here by the polar effects associated withthe very high electrophilic character of these radicals. However, interestingly, the back-fragmentation reaction(k-a) is mainly influenced by the enthalpy effects as supported by the relationship between the rate constantsand the addition reaction enthalpy ΔHR. The addition and fragmentation rate constants calculated from thetransition state theory (TST) are in satisfactory agreement with the experimental ones. Therefore, molecularorbital (MO) calculations offered new opportunities for a better understanding of the sulfur-centered radicalreactivity.
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