PURPOSE: To produce a particulate polymer having a broad-range particle size distribution and controlled internal and external surface areas at a high reaction rate in high space-time yields by applying high shear force during the polymerization of acrylonitrile and/or methacrylonitrile.

CONSTITUTION: In the polymerization of acrylonitrile and/or methacylonitrile, the monomers are dissolved, suspended, or emulsified in a liquid inert under the reaction conditions; high shear force is applied before or during the polymerization in the state in which the total monomer concentration is below 10 wt.% at the start of or during the polymerization so that the formed homopolymer or copolymer particles may have a D50% value of below 10 μm; a polymerization initiator is added to the reaction mixture; and the entire mixture is polymerized to produce a homo- or copolymer containing at least 50 wt.%, desirably, 90 wt.% acrylonitrile repeating units and methacrylonitrile repeating units. The reaction temperature is 30-75°C, and the reaction time is 30 min to 5 hr. A cationic surfactant, or the like, are also added.