A series of block copoly(arylene ether)s containing pendant superacid groups were synthesized, and their properties were investigated for fuel cell applications. Two series of telechelic oligomers, iodo-substituted oligo(arylene ether ketone)s and oligo(arylene ether sulfone)s, were synthesized. The degree of oligomerization and the end groups were controlled by changing the feed ratio of the monomers. The nucleophilic substitution polymerization of the two oligomers provided iodo-substituted precursor block copolymers. The iodo groups were converted to perfluorosulfonic acid groups via the Ullmann coupling reaction. The high degree of perfluorosulfonation (up to 83%) was achieved by optimizing the reaction conditions. Tough and bendable membranes were prepared by solution casting. The ionomer membranes exhibited characteristic hydrophilic/hydrophobic phase separation with large hydrophilic clusters (ca. 10 nm), which were different from that of our previous random copolymers with similar molecular structure. The block copolymer structure was found to be effective in improving the proton-conducting behavior of the superacid-modified poly(arylene ether) ionomer membranes without increasing the ion exchange capacity (IEC). The highest proton conductivity was 0.13 S/cm at 80 degrees C, 90% relative humidity, for the block copolymer ionomer membrane with IEC = 1.29 mequiv/g. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 452-464, 2011