The synthesis of chain-end sulfonated polystyrene [PS (omega-sulfonated PS)] by reversible addition fragmentation chain transfer (RAFT) polymerization followed by postpolymerization modification was investigated by two methods. In the first method, the polymer was converted to a thiol-terminated polymer by aminolysis. This polymer was then sulfonated by oxidation of the thiol end-group with m-chloroperoxybenzoic acid (m-CPBA) to produce a sulfonic acid end-group. In the second method, the RAFT-polymerized polymer was directly sulfonated by oxidation with m-CPBA. After purification by column chromatography, omega-sulfonated PS was obtained by both methods with greater than 95% end-group functionality as measured by titration. The sulfonic acid end-group could be neutralized with various ammonium or imidazolium counter ions through acid-base or ionic metathesis reactions. The effect of the ionic end-groups on the glass transition temperature of the PS was found to be consistent with what is known for PS ionomers. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 5100-5108, 2011