Polymerization process

A process for the preparation of a sterically stabilised aqueous polymer dispersion, in which at least one ethylenically unsaturated monomer, having a solubility in water in the range 0.08%-8% by weight but giving rise to a polymer which is insoluble in water, is emulsified in water and is subjected in that state to polymerisation in the presence of (a) a non-ionisable free-radical organic azo polymerisation initiator which is water-soluble and is substantially insoluble in the monomer or monomers, having a distribution ratio between water and pure styrene of at least 12 and (b) a precursor for the formation in situ of an amphipathic block or graft copolymer steric stabiliser for the polymer particles produced, the said precursor containing in the molecule a polymer chain derived from a polyethylene glycol, or a monomethyl ether thereof, of molecular weight in the range 2000-4000, and also containing in the molecule an unsaturated grouping with which the monomers can copolymerise, the azo initiator being present in an amount of from 0.1 mol % to 3.0 mol % of the amount of monomer being polymerised and the stabiliser precursor being present in an amount such as to provide, after copolymerization with the monomers, from 0.5% to 20% by weight of stabiliser based on the monomer being polymerised.