Highly Tunable and Robust Dynamic Polymer Networks via Conjugated-Hindered Urea Bonds

Mechanical robustness and network reconstruction have always been paradoxical in dynamically cross-linked polymers. It is highly desirable to consider polymer architecture while studying chemical bond dissociation to acquire reversible cross-linking networks with high performance. Herein, a versatile dynamic covalent bond, e.g., a conjugated and hindered urea bond (CHUB), and its dissociation chemistry are reported. Benefiting from the dual effects of conjugation and steric hindrance, CHUB displayed significant and highly tunable reversibility with respect to the polymer network design. Three kinds of dynamically cross-linked networks with widely tuned properties were developed, including the pristine polyurea network, expanded polyurea network containing epoxy cores, and networks both cross-linked with traditional and dynamic covalent bonds. The aromatic, stiff backbone of CHUB ensured dynamically cross-linked networks with a considerable glass transition temperature and outstanding mechanical properties. By tuning the ratio of CHUBs with respect to conventional covalent cross-linking points, dynamically cross-linked polymers with good creep resistance were also obtained. The CHUB with both a rigid backbone and high reversibility thus effectively expanded the scope of dynamic covalent bonds and provided efficient routes to catalyst-free, easy-to-synthesis, and efficient renewable networks that are beneficial for theoretical study or practical applications.