Catalyst-Free Dynamic Covalent Knoevenagel/Hydrazide Condensation for Polyacylhydrazones and Covalent Adaptable...

The rapid advancement of dynamic covalent chemistry (DCvC) has significantly impacted both chemistry and materials science. There is an increasing need for exploring catalyst-free dynamic covalent reactions with large equilibrium constant (K_eq) ranges to provide new avenues for the tailored design of dynamic polymers. Here, we report a catalyst-free dynamic covalent condensation reaction between Knoevenagel derivatives (Kn) and hydrazides to generate acylhydrazones. Systematic small-molecule studies validate a significant substituent effect on the Kn reactant, resulting in a wide K_eq range covering nearly four orders of magnitude (0.1-719). The high K_eq values (>500) achieved in polar aprotic solvents enable the catalyst-free synthesis of high-molar-mass (-180 kDa) polyacylhydrazones. The retention of by-products during polycondensation leads to concentration-dependent topology switching between polymeric and macrocyclic acylhydrazones. By leveraging this reaction, we developed a novel covalent adaptable network (CAN) that exhibits remarkable stress relaxation properties (38 s at 160 °C), facilitating efficient thermal reprocessing while maintaining high mechanical performance. This condensation reaction enriches the dynamic covalent toolbox and offers a versatile approach for the design and fabrication of dynamic polymers with tailored mechanical and dynamic behavior.