PURPOSE: The titled polymer useful as a material for artificial organs of excellent biocompatibility, prepared by surface-modifying the polymer by covalently bonding a specified molecular chain to its surface.

CONSTITUTION: A surface-modified polymer prepared by covalently bonding a molecular chain of a large mobility [e.g., a molecular chain of the formula (wherein B is the residue derived from a compound having 0W10 active hydrogen atoms, AO is a 2W4C oxyalkylene or dimethylsiloxane, X and Y are each a residue derived from an active hydrogen-containing compound, H, a 1W22C alkyl or perfluoroalkyl, or the like, i is 0 or 1, a and c are each 0W50, b and d are each 1W300, m and n are each 0W24, and m+n=1W24)] to the surface of a (co) polymer having an amino, hydroxyl, carboxyl, isocyanato or urethane group or a (co)polymer to which these functional groups can be introduced, e.g., PVA, ethylene/vinyl alcohol copolymer, polyester, polybutadiene, or PVAc. The selection of the molecular chain of a large mobility makes it possible to obtain a surface-modified polymer having a structure similar to that of a biomembrane and having hydrophilicitry, hydrophobicity or hydrophilicity/hydrophobicity combination of any desired ratio.