Use of pyridine derivatives as inhibitor/retarding agent for photoinduced cationic polymerization of epoxides

In this study, the cationic photopolymerization of epoxy controlled by pyridine derivative was investigated. Two pyridines were selected in order to explore the impact of the associated pKa and the amine steric hindrance on the polymerization. The inhibition, the polymerization rate, the heat flow and the conversion during dark polymerization were found to be affected depending of the pyridine derivative structure. Two mechanisms seem to be involved. 1) During the initiation step, the proton is trapped by pyridine derivative, a fact which delays the polymerization reaction, because of less growing chains and lower exothermicity. 2) During the propagation reaction, an interaction between the oxonium ion and the pyridine derivative takes place, which results in a decrease of the rate of polymerization. These two mechanisms depend on the concentration, but also on the associated pKa and on the steric hindrance of the amine, which allows a tailorable control over the cationic polymerization kinetics.