HIGHLY CROSSLINKED POLYMER PARTICLES AND PROCESS FOR PRODUCING THE SAME

Highly crosslinked polymer particles, which may be porous, consisting essentially of a nonhydrophilic polymer of a polymerizable monomer mixture comprising at least 20% by weight of at least one non-hydrophilic, crosslinking polyvinyl monomer and satisfying the following conditions (a) to (e): (a) the average particle diameter (rm) is 0.1-1.0 .mu.m and the proportion of particles having particle diameters falling within the range of 0.9rm 1.1rm to the whole of particles is 80% by weight or more, (b) the polymer particles are substantially insoluble in toluene and substantially nonswellable with toluene, (c) when the polymer particles are heated on a thermobalance in a nitrogen atmosphere while elevating the temperature at a rate of 10.degree.C/min, the temperature at which the weight of the particles reduces by 10% is at least 380.degree.C, (d) when the polymer particles are heated at 300.degree.C for 5 hours in a nitrogen atmosphere, the weight reduction is at most 30%, and (e) the polymer particles are not melt-adhered to one another at 200.degree.C in a nitrogen atmosphere. Said highly crosslinked polymer particles can be produced by adding a polymerizable monomer mixture comprising at least 20% by weight of at least one nonhydrophilic, crosslinking polyvinyl monomer to an aqueous dispersion containing a non-crosslinked, low molecular weight polymer particles having a weight average molecular weight of 500-10,000 as a dispersoid, with a non-reactive solvent in a proportion of not more than 1 part by weight per part by weight of the polymerizable monomer mixture, the proportion of the polymerizable monomer mixture plus the non-reactive solvent to the non-crosslinked, low molecular weight polymer particles being 4 to 20 parts by weight per part by weight of the non-crosslinked, low molecular weight polymer particles, and then subjecting the polymerizable monomer mixture to radical emulsion polymerization with stirring.