Poly(N,N-diallylazacycloalkane)s for Anion-Exchange Membranes Functionalized with N-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 N -spirocyclic quaternary ammonium (QA) cations. N, N -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( N, N -diallylazacycloalkane)s. 1 H NMR spectroscopy revealed that the stability of the polyelectrolytes in 2 M KOD/D 2 O solutions critically depended on the ring size and the absence of additional heteroatoms in the ring. Thus, poly( N, N -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 N, N -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 N -spirocyclic QA cations, which will open up new prospects for readily accessible high performance polyelectrolytes and membranes.