The first example for the transformation of living cationic polymerization into a living anionic one was described. The cationic propagation ends of living poly(tetrahydrofuran) (poly(THF)) were quantitatively reduced into anionic ones by 2 equiv of samarium(II) iodide in the presence of hexamethylphosphoramide (HMPA). The transformed poly(THF) macroanion initiated the polymerization of epsilon-caprolactone (CL) quantitatively, resulting in the formation of the block copolymer of THF with CL with narrow molecular weight distributions (<1.21). The formation of a homopolymer of THF or CL could not be observed. The polymerization of CL with the poly(THF) macroanion proceeded in living fashion without termination and backbiting reactions. The unit ratio of THF and CL could be facilely controlled by both the polymerization time of THF and the amount of CL.