Directing Polymer-Metal Binding Interactions by Modifying Polymer and Solvation Structure

Abstract

The rare-earth elements (REEs: La-Lu, Y, Sc) are a subset of critical minerals used in a variety of essential technologies, particularly in green energy. However, their chemical similarities and co-occurrence in ores make them difficult to access as they first must be separated from their ores, then from one another. Current industrial methods to accomplish these separations rely on liquid-liquid extraction and other hydrometallurgical techniques that are energy intensive, require large pH swings, and use large quantities of organic solvents. Metal-chelating polymers are a promising class of materials to improve or supplant these existing technologies due to their low cost and high tuneability. Much research has focused on designing specific ligands to bind REEs selectively and occasionally attaching these ligands to polymer backbones; however, nature's approach uses simple carboxylate ligands in proteins and can achieve high selectivity through controlling specific changes in solvation and conformation. This work takes inspiration from nature to study how modifying the polymer and/or solvation structure can direct polymer-metal interactions. The toolbox of synthetic polymer chemistry provides many strategies that we used to alter the polymer structure. To gain molecular-level insight into how these structural changes affected metal-chelation we turned to isothermal titration calorimetry (ITC) to measure the solution thermodynamics of these interactions directly. We studied the effect of the broader solution environment by changing the solution composition and solvent to find that both impacted the thermodynamics of these entropically-driven interactions. We also developed and used new modular polymer synthetic methods to establish structure-property relationships between polymer structure and REE-binding efficacy and applied ITC to study calcium binding in biologically relevant systems. In total, this work developed new materials design principles that will guide polymer-metal interactions by modifications beyond the chelation site.