Photocurable acylhydrazone covalent adaptable networks with fast dynamic exchange and tunable viscoelastic properties

Abstract

Acylhydrazone linkages are appealing for the design of covalent adaptable networks (CANs) due to their reversible nature and ease of formation from readily available starting materials. However, acylhydrazone CANs typically exhibit dynamic behavior within a limited temperature range due to their slow exchange rate. Here, vanillin was modified by a two-step reaction involving allylation and imination, followed by photo-curing with a four-armed thiol to fabricate acylhydrazone CANs. The dynamic behavior of these CANs could be controlled by adjusting the amount of p-anisidine (AD) organocatalyst. A model compound reaction revealed that the addition of a catalyst transforms the mechanism of acylhydrazone exchange from a 2 + 2 cycloaddition reaction to a nucleophilic addition reaction, effectively lowering the energy barrier of the intermediate state. CANs with higher amounts of AD catalyst exhibited shorter relaxation times and lower activation energies, thereby expanding the reprocessing temperature window and enhancing the recovery ratio of tensile stress after reprocessing. All the fabricated CANs demonstrated ready chemical recyclability in acidic or AD/DMSO solutions at 80 °C, following two different mechanisms depending on the reaction medium. These findings pave the way to photo-curable renewable acylhydrazone CANs with adjustable viscoelastic properties and mechanical and chemical recyclability enabled by rapid dynamic exchange reactions.