The present invention concerns a method for preparing a polymer comprising a step (EI) for dispersed phase polymerization in the presence of a reactive stabiliser, wherein the following are brought into contact in an aqueous phase:—at least one ethylenically unsaturated monomer； —at least one source of free radicals； and—a reactive stabiliser comprising a polymer chain including monomer units (N-vinyl lactam) and a thiocarbonylthio-S group (C═S)—.
