Patent ID: 7001960

Claim:
A process for producing a vinyl chloride-based polymer, comprising a suspension polymerization of either vinyl chloride, or a monomer mixture of vinyl chloride and another copolymerizable monomer, in an aqueous medium, and in presence of a dispersion stabilizer, wherein said dispersion stabilizer comprises (A) hydroxypropylmethylcellulose with a degree of methoxy group substitution of 26 to 30% by mass, a degree of hydroxypropoxy group substitution of 4 to 15% by mass, and a viscosity at 20° C. for a 2% by mass aqueous solution thereof of 5 to 4000 mPa·s, (B) a partially saponified polyvinyl alcohol with a saponification degree of 75 to 85 mol % and an average polymerization degree of 1000 to 3000, and (C) a partially saponified polyvinyl alcohol with a saponification degree of 20 to 57 mol % and an average polymerization degree of 150 to 600, said suspension polymerization is initiated in presence of said (A), (B), and (C), and respective quantities of said (A), (B), and (C) at initiation satisfy conditions prescribed by a formula (1) and a formula (2) shown below: mass of ( A )/mass of ( B )=2 or greater (1) and when the polymerization conversion is within a range from 15 to 50%, an additional quantity of said (C) is added to said polymerization reaction system.