Benzophenone−Phenylthioacetic Acid Tetraalkylammonium Salts as Effective Initiators of Free-Radical Photopolymerization of Vinyl Monomers, Mechanistic Studies

Abstract

The mechanism of the photooxidation of tetraalkylammonium salts of phenylthioacetic acid, C6H5−S−CH2−COO-N+R4, was investigated in detail in order to understand why they turned out to be efficient co-initiators of free-radical polymerizations. The photosensitizer was benzophenone (BP), and the alkyl substituents (R) were n-butyl, n-propyl, or ethyl. The techniques used were steady-state and nanosecond flash photolysis. It was shown that electron transfer from the sulfur atom to the benzophenone triplet state was the primary photochemical step followed by a decarboxylation reaction leading to CO2, the C6H5−S−CH2• radical, and a [BP•-···N+R4] ion pair. The latter underwent a Hofmann elimination (unexpected in these mild experimental conditions) leading to alkene-1 and trialkyl amine. The quantum yields of all the observed transients and the stable products were determined. The mechanism of primary and secondary photochemical reactions was quantitatively described, and it was shown to be similar for all of t...