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

Schiff-base–zinc chloride complexes (ZnCl2/1R) 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 ZnCl2/1R. Catalytic activities of ZnCl2/1R, 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 (Ea), and the frequency factors (A). ZnCl2/1Cl bearing the electron-withdrawing chlorine group initiates the reaction above 80 °C, whereas ZnCl2/1OMe bearing the electron-donating methoxy group initiates the reaction above 100 °C. The Ea values in the reactions with ZnCl2/1Cl and ZnCl2/1OMe 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 × 102 and 2.04 × 1019 L mol−1 s−1 for ZnCl2/1Cl and ZnCl2/1OMe, 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 ZnCl2/1R was evaluated by 1H NMR, 13C NMR, 15N NMR, and IR spectroscopies to understand the substituent effects.

Graphical abstractWe describe a model reaction for a cross-linking system with high activity and latency catalyzed by a Schiff-base–zinc chloride complexes. The Schiff-base–zinc chloride complexes thermally–latently catalyze the addition of a hemiacetal ester and an epoxide through the thermal dissociation of the hemiacetal ester to a carboxylic acid and a vinyl ether, which takes place only at appropriate temperatures for cross-linking. This latent reaction shows both excellent latency under ambient conditions and high activity only at specific temperatures.Figure optionsDownload full-size imageDownload as PowerPoint slide