Functionalized Poly(benzimidazole)s as Membrane Materials for Fuel Cells

New polymer electrolyte membranes for fuel cell applications were synthesized via covalent bonding of phosphonic acid (PA) onto poly(benzimidazole) (PBI). PBI was functionalized via N‐alkylation with an appropriate phosphonate, followed by hydrolysis of the grafted groups to the desired PA functions. Alternatively, polymer networks based on PBI and vinyl phosphonic acid (VPA) were successfully synthesized. In this second approach, PBI was first functionalized in a polymer analogous modification with polymerizable or radical‐forming groups. Thermally induced ‘grafting‐through’ or ‘grafting‐from’ polymerization of VPA led to the corresponding PBI/PVPA networks. The structure‐property relationships with respect to proton conducting properties of the membrane materials are discussed.