Abstract: A highly heat-resistant and thermally stable polyvinyl chloride resin can be prepared by copolymerizing vinyl chloride monomer and an N-substituted maleimide compound without decreasing the ingerently excellent properties of polyvinyl chloride resins when the monomer mixture is suspension-copolymerized in an aqueous medium containing a suspending agent which is a combination of three different polymers including: (a) a first partially saponified polyvinyl alcohol having an average degree of polymerization in the range from 700 to 3000 and a degree of saponification in the range from 70 to 95%; (b) a second partially saponified polyvinyl alcohol having an average degree of polymerization in the range from 300 to 600 and a degree of saponification in the range from 35 to 55%; and (c) a water-soluble cellulose derivative of which a 0.2% by weight aqueous solution has a surface tension not exceeding 55 dyn/cm at 25.degree. C., in a specified proportion.

Abstract: Foaming above the aqueous polymerization mixture can be efficiently reduced in suspension polymerization of vinyl chloride monomer in an aqueous medium contained in a polymerization reactor equipped with a reflux condenser for removal of the heat of polymerization by admixing the polymerization mixture with additives comprising (a) from 0.002 to 0.007 part by weight of a partially saponified polyvinyl alcohol having a degree of saponification in the range from 20 to 50% by moles and an average degree of polymerization in the range from 200 to 400 and (b) from 0.001 to 0.01 part by weight of a silicone-based defoaming agent, each amount being per 100 parts by weight of the vinyl chloride monomer or the monomer mixture mainly composed of vinyl chloride, at a moment, preferably, before the reflux condenser is brought into operation.

Abstract: A method of charging a suspension stabilizer for suspension polymerization in an aqueous medium of a vinyl monomer comprising vinyl chloride, the suspension stabilizer comprising a partially saponified polyvinyl alcohol having an average polymerization degree of 200 to 600 and a saponification degree of 20 to 60 mole %, wherein the partially saponified polyvinyl alcohol is charged in the form of an aqueous suspension. The method makes it possible to charge a suspension stabilizer which is slightly soluble in water easily and efficiently without using organic solvents, thereby enhancing the quality of the vinyl chloride polymer obtained.