The RAFT-mediated emulsion polymerization of styrene was carried out in a one-pot, two-step procedure using two poly(methacrylic acid-co-poly(ethylene oxide) methyl ether methacrylate) macroRAFT agents of different compositions carrying a reactive trithiocarbonate end-group. The latter were prepared in situ, directly in aqueous solution at acid pH. In all cases, the synthesis was fast and efficient, leading to very high conversions and very good control over the polymer features. It was moreover particularly reproducible, which is an important outcome for the robustness of the method. Then, styrene was added and directly polymerized in the formed emulsion system until very high conversion in short reaction time. The method led to amphiphilic block copolymers, self-assembled into stable spherical particles. The diameter of the latter was directly governed by the initial concentration of macroRAFT agent, which also controlled the molar mass of the polystyrene block at constant styrene initial concentration. The emulsion polymerization step was studied in detail to provide information on the overall mechanism: nucleation, conversion rate, and chain growth. Because of the reduction of the number of synthesis and purification steps and of the overall reaction time, and due to the use of water as the sole reaction medium, the proposed method is of high interest in terms of both respect of environmental constraints and energy saving.