Process for preparing a seed latex of a methyl methacrylate polymer (PMMA) by radical polymerization in aqueous emulsion of methyl methacrylate and optionally at least one comonomer.Process for preparing a chlorinated vinyl polymer latex by radical polymerization in aqueous emulsion of at least one chlorinated vinyl monomer, according to which the polymerization takes place in the presence of at most 3% by weight, expressed relative to the total weight of the monomers, of the PMMA seed latex.