9  5,925,690  10  wherein  R$_{1 }$ is hydrogen atom or a methyl group,  R$_{3 }$ is an  organic group having a valence of 2 to 6 and 1 to 30 carbon  atoms, which may have an ether linkage and/or an ester  linkage,  R$_{6 }$ is hydrogen atom or a carboxyl group, n$_{1 }$ is an  integer of 1 to 5, and n$_{3 }$ is 1 or 2.  Preferred examples of the carboxylic acid groups- containing monomer represented by the general formula (4)  are as follows.  The carboxylic acid groups-containing monomer  (B)  of  the present invention is a monomer having in one molecule  a plurality of carboxyl groups or groups which readily react  with water to produce a carboxyl group, such as acid  anhydrides or acid halides, and at least one polymerizable  unsaturated group.  Illustrative examples of the compound include trimellitic  acid derivatives represented by the following general for› mula (4) and acid anhydrides and acid halides thereof;  pyromellitic acid derivatives represented by the following  general formula (4) and (5) and acid anhydrides and acid  halides thereof; malonic acid derivatives represented by the  following general formula (6) and acid anhydrides and acid  halides thereof; 6-(meth)acryloxyethylnaphthalene-1,2,6- tricarboxylic acid; N-(meth)acryloylaspartic acid; and the  like.  -continued  (5)  (6) 