A well-defined homopolymer of 2-(diethylamino)ethyl methacrylate has been synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization using (4-cyanopentanoic acid)-4-dithiobenzoate as a chain transfer agent. The corresponding protonated homopolymer with a very reactive dithiobenzoate end group has been used as a water-soluble macromolecular chain transfer agent in the batch emulsion polymerization of styrene without any surfactant. The reaction leads to a stable latex, as a result of the in-situ formation of an amphiphilic block copolymer stabilizer, via transfer reaction to the dithioester functions during the nucleation step. The work does not intend to apply controlled free-radical polymerization in an aqueous dispersed system but takes advantage of the RAFT technique to create a well-defined polyelectrolyte, with a high chain-end reactivity.