Hypersulfonated polyelectrolytes: preparation, stability and conductivity
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Specially tailored polyelectrolytes are becoming important as energy-related materials. Here we explore a synthetic strategy to prepare fully aromatic polymers containing single phenylene rings in the backbone functionalized with four sulfonic acid groups. Thioether bridges of semifluorinated poly(arylene thioether)s were oxidized to sulfone bridges, followed by substitution of all fluorines by NaSH and quantitative oxidation of the resulting thiol groups. This gave poly(arylene sulfone)s containing octasulfonated biphenyl units, reaching ion exchange capacities up to 8 meq. g-1 and unprecedented high local sulfonic acid concentrations. These polyelectrolytes are stable up to 300 °C under air and achieve proton conductivities of up to 90 mS cm-1 at 120 °C and 50% relative humidity. Despite the excellent performance of this unique new class of hypersulfonated polymers, our data suggests that incomplete proton dissociation may ultimately limit the conductivity of highly sulfonated polymers.