Polystyrene-based polyampholyte having upper critical solution temperature, and application for same

A polyampholyte whose main chain is a repeating phenylethylene structure having upper critical solution temperature (UCST) thermoresponsiveness, consisting of the following structural unit (A) and the following structural unit (B), a content of the structural unit (A) being 36 to 64 mol % with respect to the total of the structural units (A) and (B):

• Structural unit (A):
• at least one selected from the group consisting of:
• a vinylpyridine structural unit represented by General formula (2)
• [Formula 2]
• wherein, in the formula (2), Q represents a halogen ion, OH−, HSO4−, NO3−, RaSO3−, or RaCO2−, and Ra represents an alkyl group having 1 to 3 carbon atoms;
• and
• a quaternized vinylpyridine structural unit represented by General formula (3)
• [Formula 3]
• wherein, in the formula (3), Xa represents a halogen ion or HSO4−, and Rb represents an alkyl group having 1 to 3 carbon atoms;
• and
• Structural unit (B):
• General formula (4)
• [Formula 4]
• wherein, in the formula (4), M represents a hydrogen atom, an alkali metal, or an alkali earth metal,
• wherein a number average molecular weight measured by gel permeation chromatography of the polyampholyte is 500 to 100,000 daltons (Da), and
• wherein the polyampholyte is a copolymer of an anionic monomer and a cationic monomer.