Minimal N-hydroxyphthalimide-urethane bonds enable superior thermomechanical stability for covalent adaptable networks

Compared to classic thermosets, the reprocessability of covalent adaptable networks (CANs) endowed by dynamic covalent bonds (DCBs) often comes at the expense of mechanical performance and thermomechanical stability. Herein, we report a "High Activity & Low Content" strategy for CANs to achieve superior thermomechanical stability, which is enabled by dynamic N-hydroxyphthalimide-urethane bonds (NUBs). The catalyst-free addition reaction between N-hydroxyphthalimides and isocyanates proceeds to a near-quantitative conversion within 2 h at room temperature in dimethyl sulfoxide, while the formed bonds dissociate even up to ~ 28% at 120 °C. The dual high activity, characterized by a high degree of dissociation and fast association kinetics, allows for an effective reduction in DCB content within CANs while preserving their dynamic characteristics. We incorporate only 5 mol% of dynamic units to develop poly(N-hydroxyphthalimide-urethanes) (PNU) networks. The "High Activity & Low Content" design endows PNUs with superior mechanical properties, exceptional crack tolerance, and remarkable mechanical stability at high temperatures. Furthermore, even with minimal DCB participation, the PNUs exhibit excellent reprocessability and mild degradability in neutral aqueous conditions. This study proposes a compelling strategy that enables CANs to achieve excellent reprocessability while retaining their mechanical strength and thermomechanical robustness-overcoming the traditional trade-off between these properties.