Hafnium coordination vitrimer based on carboxylate exchange: synthesis, properties, and mechanistic investigations on the [Hf6O4(OH)4(O2CMe)12]2 model compound

Abstract

A hafnium-based coordination vitrimer (Hf−CooAN-10) has been prepared by carboxylate ligand exchange from [Hf6O4(OH)4(O2CMe)12]2 and poly-((ethylhexyl methacylate)-co-(carboxyethyl acrylate)) (Mn = 14.4 kg mol−1, Đ = 2.2, 90 : 10 comonomer molar ratio), using a 10 % molar Hf loading with respect to the polymer maximum crosslinking capacity. The material shows high insoluble fraction and expected properties for a 3D network and has been reshaped three times at 50 °C under a 3-ton pressure without significant property alteration. The Hf−CooAN-10 thermomechanical and rheological properties are very similar to those previously reported for the analogous Zr−CooAN-10. The associative nature of carboxylate exchange between the [Hf6O4(OH)4(O2CR)12] crosslinks and the polymer matrix free −COOH functions is suggested by parallel 1H NMR line-broadening kinetic investigations of acetate exchange in the [Hf6O4(OH)4(O2CMe)12]2 precursor with free MeCOOH, the rate of which has a zero-order dependence on [MeCOOH], and by DFT calculations on a [Hf(μ-O2CMe)(O2CMe)2(OH)(H2O)]2 model. The results of these investigations, including the activation parameters for the exchange of both chelating and bridging ligands, are very similar to those of the previously investigated zirconium system and suggest partial acetate dissociation assisted by H-bond formation with a (μ3-OH) ligand as the rate-determining step, followed by acid coordination and intramolecular proton transfer.