Among the above-mentioned polymers (1), copolymers of N[2-methacryloyloxy)ethyl]-2,3-dimethylmaleimide and methacrylic acid or acrylic acid, i.e. a monomer having an alkali-soluble group, as disclosed on pages 71 to 91 of Die Angewandte Makromoleculare Chemie Vol 128 (1984) are preferred.