Regulating exchange dynamics in poly(β-hydrazide esters) via push-pull electronic effects towards covalent adaptable networks with fast reprocessing

Poly(β-hydrazide esters), prepared by the aza-Michael addition of a hydrazide to an acrylate moiety, have been used to accelerate the dissociation rates of dynamic aza-Michael adducts via push-pull electronic effects to create covalent adaptable networks (CANs) with faster reprocessing rates. Hydrazide precursors containing aryl groups that can conjugate with the carboxamide functionality can increase the electron density on the terminal amine of the hydrazide (push electronic effect). On the other hand, hydrazide precursors containing alkyl groups that cannot conjugate with the carboxamide will have terminal amines that are more electron-deficient since part of its electron density is donated to the carbonyl group (pull electronic effect). We used small molecule models of the CANs to show that the push electronic effect of the aryl carboxamide group accelerated the dissociation rates of the dynamic aza-Michael adducts and significantly increased the reversibility of the Michael addition. In contrast, the pull electronic effect of the alkyl carboxamide groups reduced the dissociation rates of the dynamic aza-Michael adducts. Likewise, when we examined the reprocessing of the CANs, the poly(β-hydrazide esters) with the push electronic effect had lower stress relaxation times (69–79 s) and reprocessing times than their congeners with the pull electronic effect (182–3760 s). They can be reprocessed at least 10 times when heated to 180 °C for 0.5 h. Regulating the thermal stability of the dynamic aza-Michael adducts can thus reduce the dependence of the reprocessing performance of the CANs on the boiling points of the monomers. In addition, these poly(β-hydrazide esters) show excellent heat resistance (T_g = 113 °C and T_d-5% = 279 °C). Thus, the push-pull electronic effect of the carboxamide groups provides a useful strategy to regulate the reversibility of the dynamic covalent bonds to achieve thermosets that can be reprocessed rapidly.