Tuning connectivity in a three-component assembly of metal–organic cage-cross-linked polymer networks

Abstract

Network connectivity strongly influences the dynamics and mechanical properties of materials such as natural tissues and hydrogels, which are known for their adaptability and self-healing. Metal–organic cages (MOCs), with modular structures and reversible coordination, provide a versatile platform to engineer connectivity in polymer networks. Here, we use an octahedral MOC to form transient poly(ethylene glycol) (PEG)-based hydrogels and investigate their viscoelastic behavior by varying the junction functionality and polymer architecture. A distinct low-frequency relaxation mode emerges after annealing, reflecting the interplay between cage formation and a mixture of homo- and heteroleptic metal complexes. Cage formation is undermined at both high and low polymer concentrations due to steric hindrance and chain overstretching, respectively. At an optimal polymer concentration, reducing cage content preserves the cage integrity and reveals a transition from phantom to affine network behavior. In contrast, replacing polymeric ligands with small-molecule equivalents results in misconnectivity and a lower modulus. Kinetics analysis at the microscale using Fluorescence Resonance Energy Transfer (FRET) shows that incorporation into polymer networks destabilizes the cage, likely due to chain dynamics. DFT calculations further reveal that only Pd2+, among several tested transition metal ions, provides the appropriate coordination environment and bond stability for robust cage formation. Despite this, the high junction functionality enables rapid and efficient self-healing. This work examines how tuning connectivity in transient networks can guide the design of materials with tailored properties such as recyclability, self-healing, and stimuli-responsiveness.