Synthesis and properties of mono- and di-sulfonic acid polymers with precisely controlled substitutional positions

Sulfonated polystyrenes have attracted significant attention as polymer electrolytes owing to their high proton conductivity when doped with protic media. These polymers are typically synthesized via post-sulfonation of polystyrene; however, uncertainties in local acid-group concentration and substitution positions hinder a fundamental understanding of proton transport mechanisms in such electrolytes. In this study, we synthesized mono- and di-sulfonic acid polymers through controlled polymerization of monomers that precisely define both the number (mono-, di-) and the substitution position (ortho-, meta-, para-) of sulfonic acid groups. In di-sulfonic polymers, the spatial proximity of sulfonic groups dictates the formation of intramolecular hydrogen bonds, thereby impacting their acidity. Interestingly, polystyrenes bearing di-sulfonic acid groups at meta and para positions exhibited hydrophobic characteristics, attributed to dominant intramonomeric hydrogen-bonding interactions. Upon incorporation of ionic liquids, electrostatic interactions with sulfonic acid groups were strongly dependent on the positional arrangement of the tethered acid groups, leading to distinct ion-clustering behaviors and variations in proton conductivity and mechanical properties. Our study provides new insights into how the substitution positions of acid functional groups in polymers govern their electrolyte properties.