Poly(arylene alkylene)s with pendent benzyl-tethered ammonium cations for anion exchange membranes

Polymeric anion-exchange membranes (AEMs) with alkali-stable quaternary ammonium (QA) cations are essential components for the development of alkaline membrane fuel cells and water electrolyzers. Ionic loss by β-elimination reactions typically accelerates at low water contents, i.e., high alkali concentrations, which makes QA cations attached to polymer backbones via benzylic sites interesting alternatives. In the present study, we synthesized and studied a series of ether-free poly(biphenyl alkylene)s (PB) and poly(p-terphenyl alkylene)s (PT) functionalized with different mono- or di-QA groups placed in benzylic positions. By employing different synthetic strategies, we systematically varied both the polymer backbone and the cationic structure to investigate the effect on morphology, alkaline stability and hydroxide conductivity. Two precursor polymers were first synthesized via superacid-mediated polyhydroxyalkylations involving 4′-methyl-2,2,2-trifluoroacetophenone, and biphenyl and p-terphenyl, respectively. Next, these polymers were benzylbrominated to allow the introduction of trimetylammonium (TMA), quinuclidinium (Qui), piperidinium (Pip), and bis-piperidinium (bisPip) cations, respectively, through Menshutkin reactions. The ionic content was conveniently controlled by adjusting the degree of bromination through the efficient Wohl-Ziegler reaction. AEMs functionalized with bisPip groups efficiently formed ionic clusters to reach high hydroxide ion conductivities, up to 78 and 133 mS cm−1 at 20 and 80 °C, respectively, and only decomposed above 262 °C. After storage in 1 M aq. NaOH at 80 °C, AEMs functionalized with Qui, and bisPip cations showed less ionic losses in comparison to those carrying Pip and TMA cations, which may be due to the bulky structure of the cage-like Qui cation. Careful 1H NMR analysis indicated that at low alkaline concentration, loss by nucleophilic substitution at benzylic positions dominated, while ring opening by Hofmann β-elimination of the alicyclic QAs accelerated at higher alkali concentrations. The findings of the present study provided valuable insights into the influence of structure and position of QAs on the stability and degradation mechanisms of benzylic QA cations at different alkali concentrations.