Entropy-driven toughening and closed-loop recycling of polymers via divergent metal-pyrazole interactions

Developing dynamic polymeric materials that possess closed-loop recyclability under mild conditions and outstanding mechanical properties is a highly desirable but challenging pursuit. This study presents a entropy-driven toughening strategy through divergent metal-pyrazole (DiMP) coordinations, achieving a mechanically robust and recyclable polyurethane (PU) elastomer. DiMP interactions feature multiple discrete complexes (i.e., CuL₂, Cu₂L₂, and CuL₃, where L = dipyrazole ligand) to minimize the entropy-gain mediated compensatory effect during bond dissociation and accordingly increase energy dissipation upon polymer deformation. The DiMP-crosslinked PUs (DiMPUs) exhibit an unprecedented 11-fold improvement in toughness (310.8 MJ m^-3), a tensile strength of 59.0 MPa, and exceptional elastic recovery (94%), attributed to gradient energy dissipation mechanisms involving dynamic DiMP coordinations and synergistically hierarchical hydrogen bonds. Crucially, the selective acidolysis of pyrazole-urea bonds, together with the inherently weak protonation affinity of pyrazole relative to amine, allows monomers to be efficiently recovered and reengineered into virgin materials, manifesting the closed-loop recyclability. This work provides a sustainable blueprint for dynamic polymers balancing mechanical robustness and environmental circularity.