Post-Functionalized Vinyl-Addition Polynorbornenes as Modular Platforms for Anion Exchange Membranes: Structure–Property Relationships

Fully hydrocarbon-based anion exchange membranes (AEMs) were developed through a facile post-polymerization strategy using vinyl-addition polynorbornene (VA-PNB) backbones. A high-molecular-weight poly(5-vinyl-2-norbornene) (PVNB) precursor was synthesized via chemoselective vinyl-addition polymerization, followed by hydrobromination to introduce tunable brominated functionalities. Subsequent thermal treatment enabled crosslinking through residual vinyl groups without additional crosslinkers. By systematically varying the brominated norbornene content (30–90 mol %), the relationships among ionic composition, crosslinking structure, hydration behavior, mechanical properties, and electrochemical performance were investigated. SAXS and TEM analyses revealed phase-separated ionic aggregates with characteristic d-spacings of approximately 1 nm. Increasing ionic-site density enhanced hydroxide transport but also induced excessive swelling and deterioration of mechanical properties under hydrated conditions. Among the membranes investigated, P(VNB-r-NBQA)50 exhibited a balanced combination of dimensional stability, mechanical properties, and electrochemical performance, showing a low area-specific resistance (0.060 Ω·cm2 at 50 °C) and superior alkaline water electrolysis single-cell performance compared with a commercial FAA membrane. The membrane achieved a current density of 3.06 A/cm2 at 2.0 V and 60 °C, which is approximately 15% higher than that of the commercial FAA membrane (2.66 A/cm2) under identical conditions. Although excessive hydration limited long-term operational stability, this study demonstrates that post-functionalized VA-PNBs provide a versatile platform for designing hydrocarbon-based AEMs and offers important insights into balancing ionic conductivity, swelling behavior, and mechanical stability.