Rational Design for Switchable Toughness under Constant Hardness via Cu(I)–Bipyridine Coordination in Polymer Networks

Metal–ligand coordination enhances the toughness of polymer networks through sacrificial bond-mediated energy dissipation. Moreover, this toughness can be made switchable on demand via the association and dissociation of metal complexes. However, in conventional coordination networks, such switching unavoidably induces simultaneous changes in material hardness and macroscopic shape because it alters the effective elastic chain density. Here we report a new covalent network embedding 2,2′-bipyridine ligands on the cross-links reversibly converted into a coordination network simply by metal complexation with Cu(I) ions in gels and elastomers. This process increases fracture energy while maintaining a constant effective elastic chain density, as predicted by affine network theory. Consequently, our strategy decouples toughness from hardness during the switching of polymer networks. Furthermore, the elastomer provides partial protection through the spatially selective introduction of metal, resulting in preferential fracture in the nonmetalated region under uniform elongation.