Secondary Metal Coordination Tunes Thermomechanical Behavior in Ferrocene-Containing Polyimine Vitrimers

Abstract

While coordination chemistry has been explored in dynamic covalent adaptable networks, its integration into vitrimer networks where metal centers directly coordinate with imine motifs remains largely unexplored, particularly in bimetallic systems. Here, we report ferrocene-containing polyimine vitrimers that combine a low content of covalently incorporated organometallic ferrocene units with secondary metal–ligand coordination within a dynamic polyimine network, providing a design strategy to modulate thermomechanical behavior. The secondary metal–ligand coordination enhances the thermal properties of the vitrimer networks, increasing the glass transition temperature (Tg) from 85 °C for the pristine ferrocene-containing network to 103, 118, and 122 °C for the Fe3+-, Cu2+-, and Co2+-coordinated networks, respectively. In contrast, physical incorporation of oxovanadium-salophen results in the lowest Tg (79 °C) and reduced thermal stability. Mechanical characterization reveals metal- and temperature-dependent viscoelastic behavior, with metal-coordinated networks exhibiting increased stiffness and altered stress relaxation behavior. Arrhenius analysis of stress relaxation reveals tunable apparent activation energies, increasing from 27.7 kJ mol–1 for the pristine network to 56.2, 77.4, and 99.6 kJ mol–1 for the Cu2+-, Fe3+-, and Co2+-coordinated networks, respectively, while physical incorporation of oxovanadium-salophen deviates from simple Arrhenius behavior. The DFT calculations indicate a preferred distorted octahedral coordination geometry involving the TREN-derived NNN donor set and chloride ligands, supporting the proposed metal–ligand coordination within the polyimine network. Together, these results establish secondary metal–ligand coordination as an effective strategy for tuning the thermomechanical properties and relaxation behavior of polyimine vitrimer networks while retaining network integrity, solvent resistance, acid-triggered degradability, and thermal reprocessability.