1 to 2 carbon atoms, and groups ##STR8## in which R is a lower alkyl group having 1 to 4 carbon atoms, B. a polymerizable unsaturated monomer having at least one cyano group, which is selected from the group consisting of acrylonitrile and methacrylonitrile, and C. at least one water-insoluble or barely water-soluble polymerizable unsaturated monomer containing a group having a high affinity for s