PURPOSE: To obtain a new monomer which is suitable for a starting material of a polymer which is soluble in water, excellent in the force to disperse in organic particles, high in capability of capturing heavy metal ions and large in the capture of metallic ions per unit weight because the monomer has many carboxyl groups in the positions apart from the double bond.

CONSTITUTION: This monomer is represented by formula I {R1 is H, OH, group of the formula: -COOR5 (R5 is Na, K, NH4), group (A1) of formula II [R3 is H, OH, the formula: -CH2COOR4 (R4 is H, R5); R2 is H, methyl, A1}, for example, sodium (1,2,3-tricarboxyl)propan-2-yl]oxycarbonylvinylenecarboxylate. The compound of formula I is obtained by reaction of a corresponding ethylenically unsaturated carboxylic acid and a hydroxyl group-bearing polycarboxylic acid.