The process comprises the following phases: formation of a pre-emulsion of monomers in water; transfer into the polymerisation reactor with stirring, of (1) reaction stock consisting of a water solution of a surfactant, in an amount by weight relative to the weight of final polymer dispersion of 1 15%; (2) a portion of the pre-emulsion prepared in the previous phase, in an amount of 1% 10% by weight of the initial pre-emulsion; phase C.1: initiation of the polymerisation ; phase C.2: polymerisation of the monomer mixture at 30 90.degree. C., by gradually adding into the reactor, separately, the initiator solution and the remaining portion of the pre-emulsion with an added salt, which is soluble in water at 25.degree. C. to at least 1% by weight, in an amount so as to have a salt concentration in the added pre-emulsion of 0.02 0.25% by weight; phase C.3: scrubbing of the residual monomers.