N-Spirocyclic quaternary ammonium ionene membranes prepared by polyhydroxyalkylation

Anion exchange membranes (AEMs) carrying N -spirocyclic quaternary ammonium cations generally have a high alkaline stability because of the unstrained ring and constrained conformations of this type of bicyclic cation. Here, we have prepared a series of N -spirocyclic quaternary ammonium ionenes (spiro-ionenes) using a diphenyl-functionalized monomer with a di- N -spiro-isoindolium moiety containing two N -spiro cations. This di-cationic monomer was directly employed in superacid mediated polyhydroxyalkylations with isatin (or alternatively N -methylisatin) and p -terphenyl to obtain ether-free oxindole-containing spiro-ionenes with different ion exchange capacities. AEMs based on the spiro-ionenes reached a high ion conductivity, up to 152 mS cm −1 , with no detectable degradation of the N -spiro cations after storage in 1 M aq. NaOH for 720 h at 80 °C. Degradation was however detected after storage in 2 M aq. NaOH at 90 °C. Still, the most stable spiro-ionene AEM displayed an ion loss of merely 3 % after 720 h. In conclusion, we demonstrate that highly conductive and alkali-stable spiro-ionenes for AEMs can efficiently be prepared by employing N -spirocyclic monomers in polyhydroxyalkylations. • Diphenylated di- N -spirocyclic quaternary ammonium monomer (DSp) synthesized. • Polyhydroxyalkylations optimized to prepare high molecular weight N -spiro-ionenes. • Series of copolymer ionenes produced using DSp, p -terphenyl and isatin. • Anion exchange membranes highly alkali-stabile and efficient OH − conductors.