Ethoxycarbonyl-Stabilized <i>N,S</i> -Acetals as a Fast Exchange Platform for Catalyst-Free Covalent Adaptable Networks

This work presents the development of catalyst-free covalent adaptable networks (CANs) based on ethoxycarbonyl-stabilized N,S-acetal exchangeable groups. The syntheses of ethyl glyoxylate-derived aliphatic and aromatic bis-imines monomers are described, allowing to tune the thermo-mechanical properties (Td5%, Tg, relaxation times τ, Young’s moduli E) of the resulting materials. Compared to their fluorine-stabilized counterparts, these networks exhibit much shorter relaxation times (5 times shorter for the aliphatic structure and 2 times shorter for the aromatic structure, respectively, at 100 °C) and milder reprocessing conditions (80 °C for aliphatic structure). At higher temperature (T > 150 °C), these ethoxycarbonyl-stabilized N,S-acetal CANs undergo a thioesterification side reaction. A detailed study was performed to determine the threshold temperatures allowing to avoid this phenomenon and preserve the dynamic network properties, and synthetic strategies were explored to take advantage of this particular reactivity. Finally, hydrolysis and solvolysis experiments have highlighted the hydrolytic stability of such networks in aqueous media from pH 1 to 14 at room temperature, and identified several conditions of degradation at 100 °C (in HCl 1 M, NaOH 1 M and 1-dodecanethiol).