Structurally Switchable Hydrogels with Multifunctions Enabled by Controlled Catenated Junctions

In the present work, the topological hydrogels are prepared through cross-linking Tetra-PEG chains using catenated junctions of bis(phenanthroline) complexed by metal ions. The catenated junctions are either interlocked through metal–ligand coordination, unlocked through demetalation, or relocked through remetalation. Once unlocked, the cross-links become slidable, thereby leading to a reduction of the elasticity, and the degree of reduction depends on the detailed topological structure, particularly the number of network defects. The mechanical properties of the prepared Tetra-PEG can be tuned by changing either the species of the metal ions or the pH values. In comparison, for the linear PEG precursor, the gel-to-sol transition occurs upon demetalation that leads to the formation of linear chains (i.e., precursor chains connected by the catenated junctions). The paradigm described in this work offers a synthetic approach to the hydrogels with varied cross-links.