Block copolymer

A method for producing a water-soluble block copolymer of poly(N-formylethyleneimine) or poly(N-acetylethyleneimine) with poly(N-acylethyleneimine) or poly(N-acylpropyleneimine) having a molecular weight of 500 to 50,000 represented by the formula ##STR18## and R.sub.1 represents a hydrogen atom or an alkyl or aralkyl group, R.sub.2 represents a hydrogen atom or a methyl group, R.sub.3 represents a C.sub.6 -C.sub.18 alkyl, aralkyl or aryl group, R.sub.4 represents a C.sub.6 -C.sub.18 alkyl, aralkyl or acyl group, m and n represent the polymerization degrees of the respective blocks determined in connection with the molecular weight, the ratio of m:n being 0.1 to 10, and X represents a terminal group.

• which comprises conducting, at a temperature of 40.degree. to 150.degree. C. and in the presence of a polymerization initiator selected from the group consisting of sulfuric acid esters, sulfonic acid esters, alkyl halides, Lewis acids and protonic acids, the substantially complete first-step cationic ring-opening polymerization of a 2-oxazoline compound of the formula ##STR19## wherein R.sub.2 represents a hydrogen atom or a methyl group, or a 2-oxazoline compound of the formula ##STR20## wherein R.sub.3 represents a C.sub.6 -C.sub.18 alkyl, aralkyl or aryl group, or a 2-substituted-5,6-dihydro-4H-1,3-oxazine compound of the formula ##STR21## wherein R.sub.4 represents a C.sub.6 -C.sub.18 alkyl, aralkyl or aryl group, the amount of the initiator being 0.1 to 25 mole% based on the compound (2), (3) or (4), and then when said first-step cationic ring-opening polymerization is the polymerization of the compound (2), conducting the second-step cationic ring-opening polymerization of the compound (3) or (4) at a temperature of 40.degree. to 150.degree. C.;
• or when said first-step cationic ring-opening polymerization is the polymerization of the compound (3) or (4), conducting the second-step cationic ring-opening polymerization of the compound (2) at a temperature of 40.degree. to 150.degree. C.