Triblock copolymers of poly(styrenesulfonate)-b-poly(ethylene glycol)-b-poly(styrenesulfonate) with narrow molecular weight distribution (M-w/M-n = 1.28-1.40) and well-defined structure have been synthesized in aqueous solution at 70 degrees C via reversible addition-fragmentation chain transfer polymerization. Poly(ethylene glycol) (PEG) capped with 4-cyanopentanoic acid dithiobenzoate end groups was used as the macro chain transfer agent (PEG macro-CTA) for sole monomer sodium 4-styrenesulfonate. The reaction was controllable and displayed living polymerization characteristics and the triblock copolymer had designed molecular weight. The reaction rate depended strongly on the CTA and initiator concentration ratio [CTA](0)/[ACPA](0): an increase in [CTA](0)/[ACPA](0) from 1.0 to 5.0 slowed down the polymerization rate and improved the molecular weight distribution with a prolonged induction time. The polymerization proceeded, following first-order kinetics when [CTA](0)/[ACPA](0) = 2.5 and 5.0. (C) 2007 Wiley Periodicals, Inc.