Polyvinyl chloride is generally produced by subjecting a vinyl chloride monomer to suspension polymerization. The suspension polymerization is performed dispersing the vinyl chloride monomer in water in the presence of a dispersant such as polyvinyl alcohol.
Various types of dispersants are studied from the viewpoint of suppressing foaming during polymerization, the viewpoint of producing polyvinyl chloride having a desired particle diameter and particle diameter distribution, and other viewpoints.
JP 2004-250695 A (hereinafter, referred to as “PTL1”) describes a polyvinyl alcohol-based dispersant having a carbonyl group in a molecule thereof, having a block character in a particular range, and exhibiting a particular ultraviolet absorption spectrum. International Publication No. WO 2016/141256 (hereinafter, referred to as “PTL2”) describes a dispersant containing polyvinyl alcohol having a degree of saponification, an absorbance at 320 nm, a block character, a clouding point, and the like each adjusted in a particular range.
In the case where the dispersant described in PTL1 is used, there is a tendency that the particle diameter of polyvinyl chloride which is produced is relatively large, and it is considered that the dispersing ability needs to be further improved. In addition, in the case where the dispersant described in PTL2 is used, the dispersant has a high dispersing ability, and therefore the particle size of polyvinyl chloride which is produced becomes too fine, so that handling becomes difficult. Moreover, the porosity of polyvinyl chloride which is obtained becomes low, so that the processability is lost. When the amount of the dispersant which is added is made small to adjust the particle diameter appropriately, the porosity becomes lower and foaming during polymerization of vinyl chloride is observed frequently, and therefore there is room for improvements in terms of productivity.