and (c) a unit having the formula: ##STR11## in which the carbonyl carbon atom of unit (c) is bound to (SOD) via a nitrogen atom of an amino group of (SOD), the copolymer residue having a weight-average molecular weight of 800 to 20,000, and x is an integer of 1 to 6 which corresponds to the number of the groups of (SOD) each derived from an amino group by removal of one hydrogen atom. 2.