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This invention relates to monodisperse cross-linked polymer particles, comprising particles with a substantially smooth outer surface and an average diameter of less than 1 μm, wherein the particles are solid or porous, and wherein the coefficient of variation (CV) % of the particles, when measured

This invention relates to monodisperse cross-linked polymer particles, comprising particles with a substantially smooth outer surface and an average diameter of less than 1 μm, wherein the particles are solid or porous, and wherein the coefficient of variation (CV) % of the particles, when measured by CPS disk centrifugation analysis, is less than 15%. These monodisperse cross-linked polymer particles may comprise magnetic material and are useful in various application. This invention also relates to monodisperse polymer particles for use as seed particles in the Ugelstad process.

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1. A process for the preparation of monodisperse particles for use as Ugelstad seed particles, the process comprising an emulsion polymerization process comprising: forming an aqueous dispersion comprising a vinylic monomer and a water soluble polymerization initiator; andagitating until the dispers

1. A process for the preparation of monodisperse particles for use as Ugelstad seed particles, the process comprising an emulsion polymerization process comprising: forming an aqueous dispersion comprising a vinylic monomer and a water soluble polymerization initiator; andagitating until the dispersion becomes visibly cloudy,wherein the aqueous dispersion comprises a surfactant and a chain transfer agent is added after the dispersion becomes visibly cloudy, such that the polymerization forms monodisperse seed particles having a z-average diameter of from 50 nm to 200 nm and, when measured by gel permeation chromatography, the polymer has a mean weight average molecular weight of more than 1,000 and less than 70,000. 2. The process of claim 1, wherein the surfactant is present below its critical micelle concentration. 3. The process of claim 1, wherein the surfactant is present in a concentration of not more than 2.5 g/L. 4. The process of claim 1, wherein the surfactant is sodium dodecyl sulfate. 5. The process of claim 1, wherein the vinylic monomer is an acrylic monomer, a styrene monomer or a methacrylate monomer. 6. The process of claim 1, wherein the chain transfer agent comprises a haloalkane. 7. The process of claim 6, wherein the haloalkane is bromotrichloromethane. 8. The process of claim 1, wherein the chain transfer agent comprises an alkylmercaptan. 9. The process of claim 8, wherein the alkylmercaptan is butyl mercaptan, isooctyl 3-mercaptopropionate or octylmercaptan. 10. The process of claim 1, wherein the addition of chain transfer agent starts about 5 to 15 minutes after the dispersion becomes visibly cloudy. 11. The process of claim 1, further comprising subjecting the monodisperse seed particles to an Ugelstad process wherein the percentage by weight of cross-linker monomer included in the total monomer used in the suspension polymerization stage of the Ugelstad process (the final polymerization stage where there are plural polymerization stages) is >25% wt cross-linker, to form submicron monodisperse particles having a z-average diameter of less than 1 μm. 12. The process of claim 11, wherein the submicron monodisperse particles are porous particles having a smooth outer surface. 13. The process of claim 11, wherein the submicron monodisperse particles have a specific surface area of from 300 m2/g to 700 m2/g when measured by gas adsorption analysis. 14. The process of claim 11, wherein the Ugelstad process comprises: (i) forming an aqueous dispersion comprising the monodisperse seed particles,finely divided droplets comprising an organic compound of molecular weight below 5,000 and water solubility at 25° C. of less than 10-2 g/L, andan organic solvent in which the organic compound is soluble, the organic solvent being optional when the polymer forming the seed particles has an average molecular weight which corresponds to up to 50 monomer units;(ii) allowing the organic compound to diffuse into the monodisperse seed particles, causing the seed particles to become activated;(iii) removing the organic solvent, where present from inside the seed particles, and contacting the activated seed particle with an aqueous vehicle containing a monomer that is at least 10 times more soluble in water than the organic compound, and a crosslinker;(iv) allowing the monomer to diffuse into the activated seed particles to form an aqueous dispersion of swollen seed particles; and(v) initiating polymerization of the monomer in the swollen seed particles. 15. The process of claim 1, wherein the surfactant is present in a concentration of not more than 2 g/L. 16. The process of claim 1, wherein the surfactant is present in a concentration of not more than 1.7 g/L. 17. The process of claim 1, wherein the surfactant is present in a concentration of not more than 1.5 g/L.