The acid anhydride may also partly be esterified with a compound such as an alcohol.Conversely, the vinyl copolymer thus obtained may be subjected to hydrolysis treatment to cause its acid anhydride group to undergo ring closure so as to be partly made into a dicarboxylic acid.Meanwhile, using a dicarboxylic acid monoester monomer, a vinyl copolymer obtained by suspension polymerization or emulsio