Patent ID: 7851568

Claim:
A method for production of a vinylpyrrolidone-based copolymer by using a metallic reaction vessel, wherein the vinylpyrrolidone-based copolymer comprises: a constitutional unit (A) derived from N-vinylpyrrolidone and a constitutional unit (B) derived from hydroxyl group-containing (meth)acrylate at a ratio such that the constitutional unit (A) is 50 to 95% by mass and the constitutional unit (B) is 5 to 50% by mass with respect to the total of both, wherein: K value according to Fikentscher is 12 or more; water insoluble matter is 0.5% by mass (on the basis of solid content) or less; and each content of iron, nickel, chromium, sodium and calcium is 500 ppb (on the basis of mass; the same hereinafter or less, and wherein: polymerization is performed by using a nitrogen-containing basic compound in an amount of 0.0001 to 0.5% by mass with respect to a monomer component, a nonionic azo initiator and a solvent containing an alcohol having 1 to 5 carbon atoms by 10% by mass (ratio with respect to the total solvent) or more; and polymerization reaction is performed so that 0 to 100% by mass in 100% by mass of N-vinylpyrrolidone and 0 to 10% by mass in 100% by mass of hydroxyl group-containing (meth)acrylate are initially charged into said reaction vessel and the residuals of N-vinylpyrrolidone and hydroxyl group-containing (meth)acrylate are successively added into the reaction vessel.