Thermally latent reaction of hemiacetal ester with epoxide controlled by Schiff-base–zinc chloride complexes with tunable catalytic activity

Abstract

Schiff-base–zinc chloride complexes (ZnCl 2 /1 R) thermal-latently catalyze the reaction of glycidyl phenyl ether ( 2) and 1-propoxyethyl-2-ethylhexanate ( 3) that proceeds at appropriate temperatures for latent curing. This reaction proceeds via the nucleophilic addition of carboxylic acid generated from the thermal dissociation of 3 to 2, which takes place faster than the reaction without ZnCl 2 /1 R. Catalytic activities of ZnCl 2/ 1 R, depending on the basicities of the α-diimine ligands controllable by the substituents on the aromatic rings, were evaluated by kinetic parameters; namely the reaction rate constants ( k), the activation energies ( E a), and the frequency factors ( A). ZnCl 2/ 1 Cl bearing the electron-withdrawing chlorine group initiates the reaction above 80 °C, whereas ZnCl 2/ 1 OMe bearing the electron-donating methoxy group initiates the reaction above 100 °C. The E a values in the reactions with ZnCl 2/ 1 Cl and ZnCl 2/ 1 OMe were estimated to be 52.2 and 177 kJ mol −1, respectively, which agree with the latencies at ambient temperatures. The A values also differ with the catalysts (6.46 × 10 2 and 2.04 × 10 19 L mol −1 s −1 for ZnCl 2/ 1 Cl and ZnCl 2/ 1 OMe, respectively). The very high A values for the catalysts with electron-donating groups manifest the very good latencies under ambient conditions, in spite of the high activities at elevated temperatures. The coordination behavior of ZnCl 2 /1 R was evaluated by 1H NMR, 13C NMR, 15N NMR, and IR spectroscopies to understand the substituent effects.