DISPERSING AGENT COMPRISING VINYL ALCOHOL POLYMER HAVING CONJUGATED DOUBLE BOND IN ITS MOLECULE AND PROCESS FOR PREPARING THEREOF

摘要

PURPOSE: Provided are a polyvinyl alcohol-based dispersing agent capable of suppressing foaming in suspension polymerization of vinyl compounds and providing vinyl polymers having excellent properties and a uniform particle size distribution, and a process for economically preparing the dispersing agent for suspension polymerization of vinyl compounds. CONSTITUTION: The dispersing agent comprises a polyvinyl alcohol resin containing carbonyl groups in its molecule and having a block character of remaining fatty acid ester groups of at least 0.5 and an absorbance of at least 0.1 measured at each of wavelengths of 215nm, 280nm and 320nm with respect to a 0.1% by weight aqueous solution of the polyvinyl alcohol resin, wherein the ratio of absorbance at 320nm to absorbance at 280nm is at least 0.3. The process comprises steps of (a) feeding a polyvinyl alcohol resin containing carbonyl groups and having a degree of hydrolysis of 65 to 98% by mole to an extruder; and (b) melt-kneading therein the resin at a resin temperature of 175 to 250 deg.C.