hosen from a bond, ???(CH2)4???CO??? groups, ???(CH2)7???CO??? group groups, and X is a chlorine atom, or, alternatively, D may be chosen from block copolymers formed from units corresponding to the polymers when D is chosen from a bond and ???(CH2)4???CO??? groups; and copolymers of acrylic acid and of dimethyldiallylammonium chloride. 40.