The synthesis and properties of the first examples of dendritic multiblock coPESs, bearing sulfonated dendritic clusters, that form strong membranes are described. End-capped dendritic multiblock coPESs with various average block lengths (n = 50-80) were synthesized by two-step reactions. The synthesis of dendritic blocks consisting of difunctional dendritic block and monofunctional dendritic end-group was accomplished by an aromatic nucleophilic substitution reaction of hexakis(4-(4-fluorophenylsulfonyl)phenyl)benzene with varying amounts of 1-(4-hydroxyphenyl)-2,3,4,5,6-pentaphenylbenzene, which provides a number of pendant phenyl rings as postsulfonation sites. Polycondensation of a controlled molar ratio between 4,4'-dihydroxybenzophenone and bis(4-fluorophenyl)sulfone monomers was carried out in the presence of the difunctional and monofunctional dendritic blocks in sulfolane at 215 C for 3.5 h. Essentially six sulfonic acid groups were introduced into each hexaphenylbenzene moiety on the dendritic blocks by reaction with a large excess of chlorosulfonic acid, followed by hydrolysis with KOH aqueous solution. The introduction of longer average blocks (n = 60-80) into the dendritic PESs improved the mechanical properties of the resulting sulfonated dendritic PES membranes. At a level of IEC (0.92-1.26 meq/g) similar to Nafion (0.91 meq/g), PIES membranes showed proton conductivities (58-67 mS/cm) comparable with that of Nafion (99 mS/cm). (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5461-5473, 2009