valent copolymer residue comprising the following three units: (a) a unit having the formula: ##STR18## (b) a unit having the formula: ##STR19## in which R7 is a hydrogen atom or a butyl group, and (c) a unit having the formula: ##STR20## 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 mol