An investigation of Poly(imidazolium) end groups under caustic conditions

Fluorine-free anion exchange polymers are critical components of alkaline energy storage and conversion devices. Numerous studies have been conducted to investigate and improve the alkaline durability of these polymers. Poly(imidazolium)-based polymers, with steric protection around the electrophilic C2 position, have emerged as an alternative to fluorinated polymers due to the high ionic conductivity and high durability with respect to hydroxide attack. However, whilst many studies have focused on improving the durability of the main chain cationic repeat unit, the stability of carbonyl end groups and its effect on poly(imidazolium) backbone stability has not been explored. In this work, we investigate the stability of the diketone and aldehyde end groups of a purposely designed low molecular weight oligo(imidazolium) (DMP-PPI-AD). By comparing the behaviour of trimethylbenzaldehyde and benzil small molecules - identical to the poly(imidazolium) end groups - we were able to observe chemical rearrangements of the end groups occurring under high pH conditions. Building from this, oligo(imidazolium)s were synthesised with capped carbonyl end groups to yield imidazolium end groups, which showed no observable rearrangement or degradation under caustic conditions. Despite evidence of oligo(imidazolium)carbonyl end group rearrangement under caustic conditions, such reactions had no observable impact on the chemical structure of the imiadazolium main chain under similar high pH conditions. We conclude that degradation of end groups has no impact on the stability of poly- and oligo-(imidazoliums) under caustic conditions. • Investigation of hydroxide conducting poly(imidazolium) carbonyl end group stability. • Rearrangement of carbonyl end groups in caustic conditions do not affect do not affect poly(imidazolium) backbone. • Capping of carbonyl end groups prevents rearrangements. • Precise end-group control provides platform for optimization of alkaline-stable poly(imidazolium) properties.