Characterization and photopolymerization of divinyl fumarate
A complete characterization of the electron density distribution of divinyl fumarate and its effect on various properties has been performed by using a combination of UV-vis spectroscopy, cyclic voltammetry, theoretical calculations, and a diagnostic Michael addition reaction involving an aliphatic thiol and the fumarate carbon-carbon double bond. The results show that the presence of the conjugation between the two vinyl ester double bonds and the fumarate carbon-carbon double bond significantly changes the electron density in both; that is, the vinyl ester double bonds of divinyl fumarate are more electron rich and the fumarate double bonds are more electron poor compared to nonconjugated analogues. This electron density distribution greatly influences the copolymerization behavior of divinyl fumarate. Divinyl fumarate also acts as both a monomer and photoinitiator in the photopolymerization of 1,6-hexanediol diacrylate. Because of the larger electron density deficiency of the fumarate group on divinyl fumarate compared to its saturated analogue, diethyl fumarate, there is a reduced propensity of the fumarate group to copolymerize with electron-deficient acrylate groups. Finally, the fundamental photocleavage reaction of vinyl fumarate that leads to initiating radicals was determined by chemical trapping (2,2,6,6-tetramethyl-1-piperidinyloxy free radical, TEMPO) to be the primary alpha-cleavage process between the carbonyl carbon and the vinyl ester oxygen.