Search

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. [ABSTRACT FROM AUTHOR]

Recently, fluorine neighboring group activation emerged as a new way of promoting acid–epoxy reaction and transesterification in vitrimer materials. Pursuing this idea, the effect of a CF3 group on aza-Michael addition and aza-Michael exchange was examined and confirmed on model molecules. Following these positive results, a CAN incorporating aza-Michael bonds and CF3-activated ester functions was synthesized and compared to analogous materials deprived of any CF3 groups on the one hand, or of any hydroxyl groups on the other hand. The study of the mechanical properties of these materials highlighted the synergistic effect of the two exchange reactions and the accelerating effect of the fluorinated group. The fluorinated and hydroxylated material was shown to be reprocessable at 100 °C in 1 h under 3 tons whereas the fluorinated only and the hydroxylated only materials were respectively reprocessed at 120 °C and 150 °C. These catalyst-free CANs were synthesized from natural resources further enhancing the sustainability of these materials. This study demonstrates the potential of biobased CANs featuring fluorinated esters as low environmental impact and easily reprocessable materials. [ABSTRACT FROM AUTHOR]

Thia-Michael additions (1,4-additions of a thiol to a Michael acceptor) are generally catalyzed by an external Brønsted or Lewis base. A spontaneous (uncatalyzed) Michael addition of thiols to α-trifluoromethyl acrylates is described, as well as its application to the very efficient preparation of a thermoset. A thorough mechanistic investigation, based on an experimental kinetic study and on DFT calculations, is presented for the addition of arene- and alkanethiols to tert-butyl trifluoromethyl acrylate in polar aprotic solvents, unveiling a probable solvent-assisted proton transfer in the rate-determining step and a considerable lowering of the energy barrier induced by the CF 3 group., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Vitrimers are 3D "covalent adaptable networks" (CANs) with flow properties thanks to thermally activated associative exchange reactions. This contribution introduces coordination adaptable networks, or CooANs, that are topologically related to metal-organic frameworks with octahedral Zr 6 clusters as secondary building units in a carboxylic acid-functionalized acrylate-methacrylate copolymer matrix. A series of Zr-CooAN-x materials (x=percent of Zr 6 loading relative to maximum capacity) was synthesized with x=5, 10, 15, 20, 25, 50 and 100. The mechanical and rheological investigations demonstrate vitrimer-like properties for x up to 50, the crosslink migration being ensured by carboxylate ligand exchange, with relaxation becoming slower as the Zr 6 content is increased. The flow activation energy of Zr-CooAN-10 is 92.9±3.6 kJ mol -1 . Rapid (30 min) hot-press reshaping occurs at temperatures in the 50-100 °C range under a 3-ton pressure and does not significantly alter the material properties., (© 2022 Wiley-VCH GmbH.)

Vitrimers are a third class of polymers gathering the mechanical properties and solvent resistance of 3D thermosets and the reprocessability of thermoplastics. This unique behaviour is due to the triggering of certain covalent exchange reactions that allow the network to rearrange upon application of a stimulus. The constitutive feature of vitrimers is the adoption of a glass-like viscosity during the rearrangement of the network, often due to an associative mechanism for the exchange reaction. Transesterification networks are one of the most studied type of vitrimers that usually require the incorporation of a catalyst, implying the associated drawbacks. Following up on a recent report on catalyst-free transesterification vitrimers in which the ester functions are particularly reactive thanks to the presence of fluorine atoms in α- or β-position, parallel DFT calculations and an experimental kinetic study on model molecules are presented in order to quantitatively assess the effect of neighbouring fluorinated groups on the transesterification reaction rate., (© 2022 Wiley-VCH GmbH.)