| 摘要 |
Polyvinyl alcohol of high viscosity e.g. 100 to 300 centipoises, is prepared by polymerizing vinyl acetate at the reflux temperature of the vinyl acetate reaction mixture in the presence of a low concentration, preferably between 0,0035 and 0,0055 weight per cent based on the weight of vinyl acetate, of an alkyl peroxydicarbonate ester catalyst to convert not more than 77% of the vinyl acetate to polyvinyl acetate, and hydrolysing the polyvinyl acetate until at least 75% of its acetyl groups has been converted to hydroxyl groups. Lower viscosity polyvinyl alcohol, e.g. of 25 to 75 centipoises, is produced by polymerizing the vinyl acetate in the presence of a small amount of acetaldehyde, preferably between 8 to 30 c.c. thereof per 20 pounds of vinyl acetate. The peroxydicarbonate ester catalysts have the formula: <FORM:0870356/IV (a)/1> where R1 and R2 are alkyl radicals such as those each containing up to 18 carbon atoms, e.g. methyl, ethyl, isopropyl, n-propyl, isobutyl and n-butyl. Polymerization is by suspension or bead techniques in which vinyl acetate is reacted in aqueous medium in the presence of protective colloids, e.g. small quantities of polyvinyl alcohol. The catalyst is added to a reaction mixture refluxing at between 65 DEG C. and 75 DEG C. where a vinyl acetate conversion of 50% to 77% is required. Acetaldehyde, if employed, is added to the reaction mixture prior to, or together with, the catalyst. Hydrolysis is defined as including alcoholysis, e.g. with an alkanol of 1 to 4 carbon atoms such as methanol or ethanol, as well as acid hydrolysis using sulphuric or phosphoric acid as a strong acid catalyst. In alcoholysis alkali metal alkoxides may be used as catalysts, employing between 0,5% and 5% by weight thereof based on the polyvinyl acetate. Hydrolysis may be continued until 95% conversion of acetyl groups is achieved. Specification 596,779 is referred to. |
