Shin-etsu Chemical Co., Ltd.

Sadaichi Arima

Shigehiro Hoshida

Kouzou Kuwabara

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.

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Ibaraki, Japan

Sadaichi Arima