Proton conducting electrolyte membranes derived from novel branched sulfonated poly(ether ether ketone)s with benzimidazole sulfonic acid pendants via thiol-ene click chemistry

• A series of BSPEEK containing propenyl groups were synthesized.
• Benzimidazole sulfonic acid groups have been grafted onto the polymer backbone via thiol-ene click reaction.
• The membranes exhibited enhanced proton conductivity, relative lower water uptake and methanol permeability.

A series of novel branched sulfonated poly(ether ether ketone)s containing propenyl groups have been synthesized using a nucleophilic polycondensation reaction. Subsequently, a thiol-ene click chemistry reaction between propenyl and thiol groups results in a new series of copolymers containing benzimidazole sulfonic acid groups. The expected structures of the copolymers are confirmed by 1H NMR and Fourier transform infrared spectroscopy. The introduction of benzimidazole sulfonic acid groups into the copolymer improves proton conductivity obviously. Compared with the linear sulfonated poly(ether ether ketone)s with the same sulfonation degree (DS), the BSPEEK-BIS membranes show lower water uptake and methanol permeability. The branching structure and the interaction between sulfonic acid and benzimidazole groups make the contribution to the improvement of the membrane's properties. Therefore, the high proton conduction, low methanol permeation, and low water uptake property of the as-prepared membranes are of significant interest for DMFCs.