Synthesis of Ether‐Free Polymer Containing Sterically Demanding Triarylsulfonium Cation in the Main Chain and Its Alkaline Stability

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To develop alkaline‐ and oxidation‐tolerant polymer materials for anion exchange membranes (AEMs) in electrochemical energy conversion systems such as fuel cells and water electrolyzers, the synthesis of a polymer containing highly alkaline‐stable triarylsulfonium units is described. The Suzuki–Miyaura cross‐coupling polymerization of an arylene dibromide possessing a sulfoxide moiety, bis(4‐bromo‐2,5‐dimethylphenyl) sulfoxide, with an arylene diboronic ester, 2,7‐bis(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)‐9,9‐dioctylfluorene, was carried out to produce a precursor polymer bearing a sulfoxide repeating unit in 82% yield, with a weight‐average molecular weight ( M w ) and polydispersity index (PDI) of 46,200 and 2.94, respectively. Its subsequent chemical transformation was carried out by the Friedel–Crafts S E Ar reaction using mesitylene to produce a polymer containing triarylsulfonium units, which corresponds to the alkaline‐stable bis(2,5‐dimethylphenyl)mesitylsulfonium ( TAS1 ). The transformation efficiency from the sulfoxide to sulfonium moiety was found to be quantitative, as confirmed by the NMR analyses. The alkaline stability of the resulting polymer was evaluated by the time‐course NMR analyses in a KOH methanol solution at 80°C; however, the polymer exhibited an unexpectedly faster alkaline degradation than TAS1 .