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

Schiff-base-zinc chloride complexes (ZnCl2/1(R)) thermal-latently catalyze the reaction of glycidyl phenyl ether (2) and I -propoxyethyl-2ethylhexanate (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 (ZnCl2/1(R)). Catalytic activities of (ZnCl2/1(R)), depending on the basicities of the a-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). (ZnCl2/1(R)) bearing the electron-withdrawing chlorine group initiates the reaction above 80 degrees C, whereas (ZnCl2/1(OMe)), bearing the electron-donating methoxy group initiates the reaction above 100 degrees C. The E-a values in the reactions with (ZnCl2/1(Cl)) and (ZnCl2/1(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 x 10(2) and 2.04 x 10(19) L mol(-1) s(-1) for (ZnCl2/1(Cl)) and (ZnCl2/1(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 (ZnCl2/1(R)) was evaluated by H-1 NMR, C-13 NMR, N-15 NMR, and lR spectroscopies to understand the substituent effects. (C) 2007 Elsevier B.V. All rights reserved.