The alternating copolymerization of phenylmaleimide (PMI) with a pendant sulfonamide acid group (sa-PMI) and n-butyl vinyl ether (BVE) as the aliphatic vinyl monomer afforded proton-conducting polymer electrolytessa-PMI-BVEsand their properties were compared with those of sa-PMI-STs that were synthesized from sa-PMI and styrene. The ion exchange capacities (IECs) can be easily controlled by partly replacing sa-PMI with unsubstituted PMI. sa-PMI-BVE is more flexible than sa-PMI-ST, and therefore, forms thin membranes even at high IECs, while sa-PMI-ST membranes are rigid and brittle. However, sa-PMI-BVE exhibits rather low thermal and oxidative stability. To realize polymer electrolyte membranes with reliable mechanical strength and a high IEC, gel-filled membranes were prepared by polymerization in the presence of a small amount of a crosslinker, divinylbenzene, in porous polytetrafluoroethylene membranes. By using the gel-filled membrane, H2/O2 fuel cells could be operated at 80 degrees C with reasonable performance. (c) 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013, 51, 2233-2242