Polymerization process

Claim 1

A process for the production of a stable dispersion in a non-aqueous liquid medium of microparticles of diameter from 0.01 to 10 microns composed of a crosslinked polymer, the process comprising the steps of (1) polymerising one or more ethylenically unsaturated monomers, including at least one crosslinking monomer, from which the crosslinked polymer is to be derived, in an aqueous medium comprising (a) at least 30% by weight of water and (b) not more than 70% by weight of a second constituent which is miscible with water, the nature and proportion of which are such that the mixture as a whole is capable of dissolving the monomer or monomers being polymerised to the extent of at least 3% by weight but is a non-solvent for the polymer formed, the polymerisation being carried out at a temperature at least 10° higher than the glass transition temperature of the polymer in the presence in the aqueous medium as steric stabiliser of a block or graft copolymer which contains in the molecule (i) a polymeric component which is solvatable by the aqueous medium and which consists of at least one polymer chain derived from a polyethylene glycol of mol. wt. at least 1000, and (ii) another polymeric component which is not solvatable by the aqueous medium, the concentration of free monomer in the polymerisation mixture being maintained throughout this process step at a level such that at no time does the free monomer form a separate phase and the total amount of monomer polymerised being such that the resulting dispersion contains at least 20% by weight of microparticles, and (2) transferring the polymer microparticles from the resulting dispersion into a non-aqueous liquid medium, the said liquid medium being one which is capable of dissolving, to the extent of at least 10% by weight at room temperature, the polyethylene glycol from which the above-mentioned solvatable component of the block or graft copolymer is derived when the said polyethylene glycol is in a non-hydrated state.