Anion-conducting polysulfone membranes containing hexa-imidazolium functionalized biphenyl units

• Polysulfones containing units with exactly six imidazolium ions along the backbone.
• The dense functionalization promotes efficient phase separation in the membranes.
• Reach same ion conductivity as randomly functionalized polymer with 70% less water.
• 1-Methylimidazolium ions give higher conductivity than tetramethylimidazoliums.
• Anion-exchange membranes potentially useful in various electrochemical applications.

Poly(arylene ether sulfone)s containing randomly distributed biphenyl units tethered with precisely six imidazolium cations are designed and prepared with the aim to facilitate ionic clustering and conductivity of anion exchange membranes (AEMs). A series of statistical copolymers with different cationic contents are synthesized via K2CO3-mediated polycondensations of 2,2′,3,3′,5,5′-hexamethyl-4,4′-dihydroxybiphenyl, bisphenol-A and 4,4′-dichlorodiphenylsulfone. After near quantitative benzylic brominations, the copolymers are functionalized with N-methylimidazolium (NIM), 1,2,4,5-tetramethylimidazolium (4IM) and trimethylammonium (QA) cations, respectively. Small angle X-ray scattering of AEMs cast from solution shows that all the different hexa-functionalized moieties induce distinct phase separation. This is especially efficient in the NIM materials, which may be because the less bulky nature of this cation in comparison with 4IM. Thus, at a given water uptake the AEMs containing NIM reach a significantly higher conductivity than those with 4IM ions. In addition, AEMs containing any of the two hexa-imidazolium moieties reach higher conductivities than corresponding materials with hexa-QA moieties, which probably results from the delocalized charge of the former cations which promotes ionic dissociation despite the very high local ionic concentrations. Introducing biphenyl units tethered with precisely six imidazolium cations along a copolymer backbone may be a viable synthetic strategy towards efficient AEMs for different electrochemical energy applications.

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