Provided is a polymer having exceptional solubility in hydrophilic polar solvents and stability in solvents. The present invention provides a polymer containing one or two structural units selected from the group consisting of structural units represented by chemical formula (1) and chemical formula (2) (in formula (1) and formula (2), X1 and X2 may be the same or different, and represent H, an optionally substituted C1-12 alkyl group, an optionally substituted C1-12 alkoxy group, an optionally substituted C1-12 alkylene oxide group, an optionally substituted thiocyano group, an optionally substituted amino group, or an optionally substituted thioalkyl group, or an optionally substituted C1-12 alkylene dioxy group or optionally substituted C1-12 alkylene dithio group in which X1 and X2 are linked (excluding combinations in which both X1 and X2 are H or C1-12 alkyl groups and combinations in which one of X1 and X2 is H and the other is a C1-12 alkyl group); R represents a C1-12 alkyl group having an acidic substituent or a salt thereof, a C1-12 alkoxy group, a C1-12 alkylene oxide group having 1-50 repeating units, a phenyl group, a heterocyclic group, or a fused ring group).