Metal–Polymer Soft Framework Ion Gels: Linking Coordination Chemistry to Macroscopic Elasticity

Abstract Linking molecular interactions to the macroscopic mechanical properties of soft materials remains a major challenge in soft-matter science. Here, we develop metal–polymer soft framework (MPF) ion gels, in which metal–ligand coordination complexes function as network-forming interchain bridges within a structurally homogeneous polymer network. Terpyridine-terminated TetraPEG prepolymers were cross-linked with divalent metal ions in an ionic liquid to form uniform coordination-bonded gel networks. The elastic modulus (G′) increased in the order Mg2+ < Zn2+ < Co2+ < Ni2+, and this trend was quantitatively reproduced by modeling the equilibrium distribution of bis-terpyridine complexes, M(tpy)22+, acting as network-forming interchain bridges in the polymer network. By integrating coordination thermodynamics with classical rubber elasticity, we established a quantitative framework that determines the stepwise equilibrium constants (K1 and K2) for mono- and bis-terpyridine complex formation and accurately predicts the nonmonotonic dependence of G′ on metal-ion concentration. Furthermore, the stress relaxation time (τ) was found to increase with increasing coordination bond strength, indicating that the dynamic mechanical response is correlated with the strength of metal–ligand interactions. These results establish a quantitative framework linking coordination thermodynamics with macroscopic gel elasticity and further suggest that coordination interactions also play an important role in controlling the dynamic mechanical response, providing a basis for tuning the properties of coordination-bonded soft materials.