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

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 dynamic cross-linking points within a structurally homogeneous polymer network. Terpyridine-terminated TetraPEG prepolymers were cross-linked with first-row transition-metal ions in an ionic liquid to form uniform coordination-bonded gel networks. The elastic modulus ( G ′) increased systematically in the order Zn 2+ < Co 2+ < Ni 2+ , and this trend was quantitatively reproduced by modeling the equilibrium distribution of bis-terpyridine complexes, M(tpy) 2 2+ , acting as cross-linking points. By integrating coordination thermodynamics with classical rubber elasticity, we established a quantitative framework that determines the stepwise equilibrium constants ( K 1 and K 2 ) for mono- and bis-terpyridine complex formation and accurately predicts the non-monotonic dependence of G ′ on metal-ion concentration. Furthermore, the stress relaxation time ( τ ) exhibited an exponential dependence on coordination bond strength. An Arrhenius-type analysis revealed that macroscopic stress relaxation is governed by a bond-activation process associated with M–tpy bond dissociation. These results establish a quantitative molecular framework connecting coordination thermodynamics and kinetics with macroscopic gel elasticity, providing design principles for coordination-bonded soft materials with programmable elasticity.