The kinetics of ab initio reversible addition:fragmentation chain transfer (RAFT) emulsion polymerization of styrene using oligo(acrylic acid-b-styrene) trithiocarbonate as both polymerization mediator and surfactant were systematically investigated. The initiator concentration was set much lower than that in the conventional emulsion polymerization to significantly suppress the irreversible termination reaction. It was found that n decreased rapidly but the nucleation efficiency of micelles increased with the decrease of the initiator concentrations due to the significant radical exit. The particle number (iVp) did not follow the classic Smith Sward equation but was proportional to [1]-0.4 [st. It was suggested that RAFT emulsion polymerization could be fast enough for commercial use even at extremely low initiator concentrations and low macro-RAFT agent concentrations due to the higher particle nucleation efficiency at lower initiator concentration. (C) 2016 American Institute of Chemical Engineers