Staged Network Evolution Enables In Situ Strengthening and On‐Demand Recyclability of Polyurethanes via Dynamic Sulfilimine–Urea Bonds

Cross‐linked polymers exhibit exceptional mechanical robustness, dimensional stability, and chemical resistance arising from permanent covalent networks. However, it remains challenging to tune mechanical properties after curing and to decouple on‐demand recyclability from in‐use stability. Herein, we report a staged network evolution strategy based on dynamic sulfilimine–urea bonds to address these demands. The rapid room‐temperature addition of isocyanates to sulfilimines forms dynamic sulfilimine–urea linkages, affording a solvent‐resistant initial polyurethane (SUP‐S1). Subsequent thermal treatment of SUP‐S1 induces dissociation of the sulfilimine–urea linkages, regenerating the sulfilimine motifs and releasing isocyanates that are captured in situ by backbone hydroxyl groups to form thermodynamically stable urethane bonds. The tensile strength and toughness of the obtained polyurethane (SUP‐S2) are significantly enhanced, increasing from 17 to 42 MPa and from 55 to 158 MJ m −3 , respectively, while the Young's modulus is retained. Moreover, the photoclip of sulfilimine motifs facilitates on‐demand recyclability by reducing the cross‐link density. The resulting recyclable polyurethane (SUP‐S3) exhibits excellent multi‐recyclability, retaining over 96% of its mechanical properties after five recycling cycles. This staged network evolution design reconciles the enhancement of mechanical properties with end‐of‐life recyclability within a single material platform, providing a robust pathway for the development of next‐generation sustainable cross‐linked polymeric materials.