Efficient Preparation of Styrene Block Copolymer Anion Exchange Membranes via One-Step Friedel–Crafts Bromoalkylation with Alkenes

Introduction of a bromoalkyl group to aromatic polymers is widely applied for the preparation of anion exchange membranes (AEMs), as the tethered bromide moiety can be conveniently converted to various ionic groups. We recently reported a polymer modification method by Friedel–Crafts alkylation using bromoalkylated tertiary alcohols and triflic acid as a catalyst. Although the synthetic method allowed convenient control of the tether length of the ion group and the degree of functionalization by changing the structure and the molar ratio of the bromoalkylated tertiary alcohol, the reaction requires more than 1 equiv of acid catalyst relative to the alcohol because byproduct water reduces the reactivity of the acid catalyst. In this study, alkene substrates instead of tertiary alcohols were employed for the Friedel–Crafts bromoalkylation for efficient incorporation of bromoalkyl groups onto aromatic rings of polymers. Since the reaction does not produce water as a byproduct, a catalytic amount of acid (0.3 equiv) is sufficient for full conversion of the reaction. The polymer functionalization was easily scalable to 44 g in the laboratory. The AEMs prepared by the alkene route showed membrane properties comparable to those prepared by the tertiary alcohol route in terms of ion conductivity, water uptake, and chemical stability. Since the alkyl bromide functional group can be converted to a wide variety of quaternary ammonium bromides by simple nucleophilic reactions, the alkene process in this report can offer a more efficient method for the preparation of AEMs.