A series of sterically-encumbered, sulfonated, poly (arylene ether) copolymers were synthesized and their proton conductivity examined. The series was prepared by copolymerizing a novel monomer, 2 '',3 '',5 '',6 ''-tetraphenyl-[1,1': 4', 1 '': 4 '',1 ''': 4 ''', 1 ''''-quinquephenyl]-4,4 ''''-diol, with 4,4'-difluorobenzophenone and bisphenol A. Subsequent sulfonation and solution casting provided membranes possessing ion exchange capacities of 1.9 to 2.7 mmol/g and excellent mechanical properties (Young's modulus, 0.2-1.2 GPa; tensile strength, 35-70 MPa; elongation at break, 62-231%). Water uptake ranged from 34 to 98 wt% at 80 degrees C/100% RH. Proton conductivities ranged between 0.24 to 16 mS/cm at 80 degrees C/60% RH, and 3 to 167 mS/cm at 80 degrees C/95% RH. TEM analysis of the polymers, in the dehydrated state, revealed isolated spherical aggregates of ions, which presumably coalesce when hydrated to provide highly conductive pathways. The strategy of using highly-encumbered polymer frameworks for the design of mechanically-robust and dimensionally-stable proton conducting membranes is demonstrated. (C) 2014 Wiley Periodicals, Inc.