Modified vinyl alcohol polymer, method for producing same, dispersion stabilizer for suspension polymerization, and method for producing vinyl polymer

The present invention provides a modified vinyl alcohol polymer having a side chain with an ethylenic double bond, and having reduced a water-insoluble content and superior storage stability in the form of an aqueous solution, methods of making thereof, a modified vinyl alcohol polymer (A) having a degree of saponification of 68 mol % or more and less than 99.9 mol %, and a viscosity-average degree of polymerization of 200 or more and less than 3,000, and comprising a side chain with 0.01 mol % or more and less than 0.50 mol % of a double bond derived from an esterification agent, the esterification agent being one or more selected from the group consisting of (i) itaconic acid and/or derivatives thereof, and (ii) methacrylic acid and/or derivatives thereof, and the modified vinyl alcohol polymer (A) having a peak width W0.05h of at least 2.85 minutes and less than 3.70 minutes at a height of 5% from the baseline in a peak measured by HPLC.