The present invention concerns large spherical polymer particles with a narrow size distribution and a procedure for producing such particles. The procedure is characterized in that a seed polymerization is performed in which the start particles have a very high ability to absorb new monomer so that finished particles are obtained after only one stage of polymerization even if the finished particles are to be as large as in the range 10 to 100 .mu.m. The procedure is also characterized in that, when porous particles are produced, a porous structure can be formed in which the fraction of micropores is insignificant. Another characteristic feature of the procedure is that the size distribution of the finished particles is narrow and in some cases narrower than in the start particles.

A PVC mixture which can be converted into plastisol, prepared by a process with comprises mixing a dispersion of spherical and compact PVC particles with a narrow size distribution (CV 0-25%) in the range 10 to 50 micrometers, with a PVC latex produced by standard emulsion-, dispersion-, miniemulsion-, microsuspension-, or seed-polymerisation in such a way that the PVC particles are homegeneously dispersed without agglomeration in the aqueous phase, and processing the resulting dispersion into a free flowing dry powder.

A procedure for preparing compact and spherical self-activated polymer particles of vinyl monomers having a narrow or monodisperse size distribution in accordance with a special precipitation polymerization process in a polar organic medium, possibly in an organic polar medium mixed with water. The medium is a good solvent for the monomer and a poor solvent for the polymer. All ingredients are initially dissolved in the medium and when polymerization has been initiated, formed polymer precipitates from solution and by the aid of the dispersion stabiliser the polymer particles are kept apart as free spherical beads. The polymerization takes place with a temperature profile which produces a scattered chain length distribution, increases the conversion speed and reduces the particle size distribution. The method for producing the particles also achieves an increased degree of freedom for the production of particles with a certain particle size in a given conversion time. The particles, which may be cross-linked, can also be made functional either by an active group through copolymerization or by grafting reactions at a late stage of polymerization or in a subsequent treatment stage.

A procedure for the production of PVC particles with a narrow size distribution in the range 10-50 .mu.m, preferably 10-30 .mu.m, in which, in a first stage, a vinyl monomer or a mixture of monomers is polymerised to form a polymer/oligomer seed particle in the range 1-10 .mu.m. In a second stage, another vinyl monomer or mixture of monomers is swelled into the polymer/oligomer seed particles and polymerisation takes place in such a way that they grow into polymer particles of the desired size. It is preferable to use aromatic vinyl monomers or acrylates as the monomer in the seed particles. The seed particles in the first stage can be produced in a two-stage swelling process or by dispersion polymerisation.