Carboxyl group-containing polymer and composition containing the same

A carboxyl group-containing polymer comprising:

• a structure unit (a) derived from an ether bond-containing monomer (A);
• a structure unit (b) derived from a sulfonic acid group-containing monomer (B);
• and
• a structure unit (c) derived from an acrylic acid-based monomer (C),
• the structure unit (a) represented by the formula (1):
• wherein R0 represents a hydrogen atom or a methyl group;
• R1 represents a CH2 group, a CH2CH2 group, or a direct bond;
• X represents a hydroxyl group or a group represented by the formula (2) or (3):
• wherein R2s, which may be the same or different, represent a C2-C4 alkylene group;
• n represents an average addition number of moles of an oxyalkylene group (—O—R2—) and is 0 to 2;
• and R3, R4 and R5 independently represent a C1-C4 alkyl group;
• Y represents a hydroxyl group or a group represented by the formula (2) or (3);
• and one of X and Y is a hydroxyl group and the other is a group represented by the formula (2) or (3),
• wherein the structure unit (a) is present at a level of from 0.5 to 15% by mass based on 100% by mass of all structure units derived from all monomers in the carboxyl group-containing polymer,
• the structure unit (b) is present at a level of from 0.5 to 30% by mass based on 100% by mass of all the structure units derived from all the monomers in the carboxyl group-containing polymer,
• the structure unit (c) is present at a level of from 55 to 99% by mass based on 100% by mass of all the structure units derived from all the monomers in the carboxyl group-containing polymer, and
• the carboxyl group-containing polymer has a weight average molecular weight of 20,000 to 60,000.