. . . . "As the segment R2, a segment obtained by the radical polymerization of a radically polymerizable monomer, such as methyl methacrylate, ethyl methacrylate, vinyl acrylate, butyl acrylate, styrene and acrylonitrile; and a segment obtained by the ring-opening polymerization of a cyclic monomer, such as lactone, lactam, 2-oxazoline and cyclic ethers, are preferred." . . .