Poly(arylene alkylene)s with pendant N-spirocyclic quaternary ammonium cations for anion exchange membranes

Aromatic polymers functionalised with cycloaliphatic quaternary ammonium (QA) cations are currently emerging as base stable anion exchange membranes (AEMs) for use in alkaline fuel cells and water electrolyzers. In the present work we have first prepared poly(biphenyl piperidine)s in superacid-mediated polycondensations, and then introduced different N-spirocyclic QA cations via cyclo-quaternisations of the piperidine rings. The resulting polymers and AEMs were free of diaryl ether linkages and benzylic C-H bonds, and showed very high thermal stability and hydroxide ion conductivity. Alkaline testing up to 120 °C implied that the alkaline stability of the spirocyclic cations was limited by distortions of the ring conformations by the rigid polymer backbone. As a consequence, the ring directly attached to the backbone degraded significantly faster by Hofmann β-elimination than the pendant ring in the spirocyclic cations. The results provide valuable insights towards the molecular design of highly thermochemically stable AEMs functionalised with N-spirocyclic QA cations