The invention concerns a method for preparing polymers by free radical polymerisation comprising (i) preparing a mini-emulsion containing: at least an ethylenically unsaturated monomer, at least a control agent selected among xanthates, dithio-carbamates, thioether-thiones, the xanthates comprising phosphorus and optionally fluorine and dithiophosphoroesters, an aqueous solution, a surfactant, and a co-surfacant, and (ii) reacting said mini-emulsion, in the presence of a free radical source, at a sufficient temperature and/or for a sufficient time interval to form the polymers.