Postmodification of highly delocalized cations in an azide-based polymer via copper-catalyzed cycloaddition for anion exchange membranes

We synthesized a styrene-based polymer containing fused expanded pyridinium (FEP) moieties as side chains for an anion exchange membrane. FEPs were incorporated into poly(4-azidomethylstyrene) via copper-catalyzed alkyne-azide cycloaddition (click chemistry) to yield an FEP content of 10.5 mol%, which depended on the FEP feed ratio. The FEP polymer membrane containing 10.5 mol% of FEP exhibited high hydroxide conductivity of 123.4 (±13.3) mS cm^–1 with a relatively low ion-exchange capacity of 0.77 (±0.03) at 80 °C under 80% relative humidity. Density functional theory studies revealed that FEP is a considerably soft acid owing to substantial delocalization of the positive charge in the rigid π structure. The high ion conductivity and low ion-exchange capacity can be attributed to weak interactions between the hard base (hydroxide ions) and soft acid (FEP).