Supramolecular metallacyclic hydrogels with tunable strength switched by host–guest interactions

Herein, we present the construction of a new type of supramolecular hydrogel with tunable strength switched by host–guest interactions via a hierarchical self-assembly strategy. Firstly, through coordination-driven self-assembly, a rhomboidal metallacycle H with two crown ether units and two chain transfer agent (CTA) units was constructed. The successful construction of metallacycle H was confirmed via multiple NMR and MS spectra. Subsequently, two poly(N-isopropylacrylamide) (PNIPAM) chains were introduced into the metallacycle through reversible addition–fragmentation chain transfer polymerization of N-isopropylacrylamide (NIPAM) monomers. The obtained PNIPAM chains can form hydrogels upon being mixed with water. With the addition of the ditopic guest G to the system, a new polymer network was formed via host–guest interactions between the crown ether units and the guest. Interestingly, the strength of the hydrogel increased in the presence of the ditopic guest G and decreased with the addition of KPF₆ as a competitive guest.