(14) a copolymer or a pharmacologically acceptable salt thereof according to any one of (1) to (13), wherein R3 is a hydroxyl group, an alkoxy group having from 1 to 6 carbon atoms or a group represented by the formula ???NR4R5, wherein R4 and R5 are the same or different and each represents a hydrogen atom or an alkyl group having from 1 to 6 carbon atoms;