Photocuring of a thiol-ene system based on an unsaturated polyester

To produce a photocurable thiol-ene system, unsaturated polyester was prepared from the condensation reaction of ethylene glycol, diethylene glycol, and fumaric acid. Diallyl groups were introduced into the ends of the unsaturated polyester by a sequential condensation reaction. The coating formulation studied contained an equimolar ratio of thiol and vinyl groups of the prepared unsaturated polyester, including 1 wt% Irgacure 184. The curing behaviors of the unsaturated polyester with multifunctional thiols were investigated using real-time FTIR spectroscopy. The rates of disappearance of thiol and vinyl groups of the unsaturated polyester were similar, demonstrating that there was little free-radical homopolymerization of the internal fumaric group or the end-capped vinyl ether group during the photocuring process and that the thiol-ene reaction is the dominant process. The kinetics of the model compounds demonstrated that the reaction of the terminal allyl double bond with the thiyl radical is faster than that of the internal fumaric double bond in the UV curing of the unsaturated polyester. The storage stability of the thiol-ene system based on unsaturated polyester was effectively increased by the addition of N-PAL. The Raman spectra revealed that the presence of a multifunctional thiol (penta 3-MP4) in the coating formulation increased the degree of surface curing due to the chain-transfer ability of the thiyl radical. (C) 2004 Wiley Periodicals, Inc.