# Tuning Connectivity in a Three-Component Assembly of Metal–Organic Cage-Cross-Linked Polymer Networks

## TL;DR

This paper explores how adjusting the connectivity in metal-organic cage-based polymer networks affects their mechanical and self-healing properties.

## Contribution

The study introduces a method to tune network connectivity using metal-organic cages and reveals how this affects material behavior.

## Key findings

- Annealing induces a low-frequency relaxation mode due to cage formation and metal complex interactions.
- Optimal polymer concentration preserves cage integrity and enables a transition to affine network behavior.
- Pd2+ is uniquely effective for stable cage formation, while polymer networks enhance self-healing.

## 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.