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Alkaline Stability Evaluation of Polymerizable Hexyl-tethered Ammonium Cations.

One of the important challenges in designing robust alkaline anion exchange membranes is the difficulty associated with the chemical stability of covalently bound cationic units. Here, a systematic study exploring alkaline stabilities of polymerizable hexyltrimethylammonium cations is presented, where the hexyl spacer chain is linked to a phenyl ring through a direct carbon-carbon, phenyl ether, or benzyl ether functionality. For this work, small molecule model compounds, styrenic monomer analogues, and their homopolymers were synthesized. Alkaline stabilities of the small molecule cations and their homopolymers were compared to alkaline stability of benzyltrimethylammonium (BTMA) cation and its homopolymer poly(BTMA), respectively. All the hexyl tethered cations and their homopolymers were significantly more stable under strongly alkaline conditions (2M KOD at 80 °C). Moreover, ether linked cations showed comparable, if not better, stability compared to the direct carbon-carbon linked cation. Via 1 H NMR analyses, possible degradation mechanisms were investigated for each small molecule cation. Findings of this study strongly suggested that the alkaline stability is dictated by the steric hindrance around the β-hydrogen. This study expands beyond the limits of general knowledge on alkaline stability of spacer modified ammonium cations via the Hofmann elimination route, highlights important design parameters for stable ammonium cations, and demonstrates accessible directly polymerizable alkaline stable ammonium cations . This article is protected by copyright. All rights reserved.