rising the following three units: (a) a unit having the formula: ##STR21## (b) a unit having the formula: ##STR22## in which R7 is a hydrogen atom or a butyl group, and (c) a unit having the formula: ##STR23## 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 20,00