Diastereomeric Monomers Enable Anion‐Exchange Membranes With Controlled Local Polymer Backbone Flexibility and High Conductivity

Multiple interdependent properties govern the performance of ion-exchange membranes. Here, we demonstrate a polymer backbone design that allows a constrained local flexibility, suppressing excessive hydration while facilitating high hydroxide conductivity in poly(arylene piperidinium) anion-exchange membranes (AEMs). This structural design builds on a pair of diastereomeric arene monomers with vicinal methyl substitutions that introduce local conformational constraints and enable systematic investigation of how subtle stereochemical variations influence hydration behavior, ion transport, and device performance without altering the overall polymer composition. Two corresponding membrane series are synthesized that differ only in stereochemistry, providing a composition-conserved platform for studies of structure-property relationships. The diastereomeric units in the AEMs facilitate the formation of ion-conducting domains, providing membranes combining high hydroxide conductivity with controlled water uptake. Hence, the syn-enriched membranes consistently exhibit higher water uptake than their anti-enriched counterparts at comparable ion-exchange capacities, resulting in higher hydroxide conductivity and improved anion exchange membrane water electrolysis performance. In comparison, the anti-enriched membranes display improved mechanical robustness and higher hydration efficiency for ion transport. Overall, this work establishes a stereochemistry-based synthetic strategy that enables systematic investigation and control of membrane properties through minimal structural variation, providing a molecular platform for rational AEM design.