Poly(<i><i>N</i>,<i>N</i></i>-diallylazacycloalkane)s for Anion-Exchange Membranes Functionalized with <i>N</i>‑Spirocyclic Quaternary Ammonium Cations

The alkaline stability of organic cations tethered to anion-exchange membranes (AEMs) is essential for the long-term performance of alkaline membrane fuel cells and electrolyzers. Here, we have prepared and studied the thermal and alkaline stability of a series of polyelectrolytes functionalized with <i>N</i>-spirocyclic quaternary ammonium (QA) cations. <i>N</i>,<i>N</i>-Diallylazacycloalkane quaternary salts were readily synthesized by diallylation of pyrrolidine, piperidine, azepane, and morpholine. These monomers were employed in radical-initiated cyclo-polymerizations to obtain the target poly­(<i>N</i>,<i>N</i>-diallyl­azacycloalkane)­s. ¹H NMR spectroscopy revealed that the stability of the polyelectrolytes in 2 M KOD/D₂O solutions critically depended on the ring size and the absence of additional heteroatoms in the ring. Thus, poly­(<i>N</i>,<i>N</i>-diallyl­piperidinium) showed the highest alkaline stability, with only minor signs of degradation at 120 °C after 14 days, while the polyelectrolytes based on the morpholine and azepane rings clearly degraded via both Hofmann elimination and ring-opening substitution already at 90 °C. Cross-linked water nonsoluble AEMs were prepared by copolymerizing <i>N</i>,<i>N</i>-diallyl­piperidinium chloride with methyl­benzyldiallyl­ammonium groups tethered to poly­(phenylene oxide). These transparent and mechanically robust AEMs reached high OH^– conductivities, above 0.1 S cm^–1 at 80 °C. The present work demonstrates the high alkaline stability of suitably configured <i>N</i>-spirocyclic QA cations, which will open up new prospects for readily accessible high performance polyelectrolytes and membranes