The Role of Polymer Backbone Chemistry on Ionic Aggregation and Conductivity in Metal-Ligand Coordinating Polymers

Polymeric ionic liquids capable of conducting multivalent ions are based on metal-ligand coordination interactions. This work explores the roles of polymer backbone chemistry, metal-ligand coordination, and ion concentration on ionic conductivity. In these systems, multivalent ion transport is facilitated via ligands pendant to the polymer backbone. While previous results suggest the matrix dielectric constant affects ion dissociation and aggregate structure, this has been studied only in systems where ions interact directly with the polymer backbone. In the case of pendant ligands, it is not clear immediately that the dielectric constant of the backbone would be relevant in determining ion transport. We show for polymers with pendant imidazole ligands, the local dielectric properties of the polymer backbone, based on either poly(ethylene oxide) (PEO) or poly(butadiene) (PBD), still affect ion aggregation and ionic conductivity. While no aggregation is observed at low salt concentration for the PEO-based polymer, Li⁺, Zn²⁺ and Cu²⁺ show aggregation in the PBD polymer resulting in reduced ionic conductivity. This highlights the importance of backbone dielectric properties even in polymer electrolytes with tethered solvating groups.