We report an ab initio emulsion reversible addition-fragmentation chain transfer (RAFT) polymerization of styrene using poly(acrylic acid-b-styrene) trithiocarbonate as surfactant and RAFT agent. The system is characteristic or high polymerization rate, controlled polymer molecular weight, and free of coagulum. Design of the amphiphilic RAFT agent is found to be critical for the success. Poly(acrylic acid) neutralization increased the colloidal stability but led to uncontrolled molecular weight development and broad molecular weight distribution, attributed to small size and thus large number of micelles resulting in continuous particle nucleation. Polystyrene with molecular weight LIP to 120 kg/mol and block styrene-butylacrylate copolymer with well-controlled molecular weight were readily synthesized with this method.