Thermoplastic and thermoset polymer compositions

A process for preparing a thermoplastic polymer composition comprising thermoplastic matrix polymer and, distributed throughout the thermoplastic matrix polymer, substantially spherical core-shell polymer particles, said particles having an average diameter of from 2 to 15 .mu.m and a particle size distribution such that at least 90% by weight of the particles fall within .+-.20% of the average particle diameter, a core of rubbery alkyl acrylate polymer and one or more shells, the outer shell of which is compatible with the matrix polymer, wherein the process comprises:

• A) polymerizing a first aqueous emulsion of the one or more monomers which, when polymerized, produce a particulate composition comprising polymer particles;
• B) performing one or more times the steps of
• 1) swelling the particles of the particulate composition produced in A), or if appropriate the particles of B2), with one or more monomers, and
• 2) polymerizing the swelling monomer within the particles until all of the monomers have been polymerized;
• and
• c) polymerizing one or more monomers which are polymerizable to form a polymer compatible with the matrix polymer to produce the core-shell polymer particles,
• and blending the resulting core-shell polymer particles, in an amount of 0.1 to 40% by weight of the total composition, with said thermoplastic matrix polymer.