A copolymer or a pharmacologically acceptable salt thereof is provided, which contains, as constitutional units, (a) one or more structural units of formula (I): wherein m is an integer of from 3 to 100, Alk represents alkyline, and R1 and R2 are the same or different and each represents hydrogen or optionally substituted alkyl, and (b) one or more structural units of formula (II): wherein R3 represents hydroxyl, optionally substituted alkoxy, optionally substituted aryloxy, or a group represented by the formula —NR4R5, wherein R4 and R5 are the same or different and each represents hydrogen or optionally substituted alkyl. There is further provided a pharmaceutical composition comprising said copolymer, a protein modifier comprising said copolymer, a complex of said copolymer with a protein, and method of preventing or treating diseases using said complex, use of said complex in the manufacture of a medicament for preventing or treating diseases and methods for the synthesis of said copolymers and said complexes.