The cationic propagation end of poly(tetrahydrofuran) (polyTHF) was quantitatively reduced into an anionic one by using samarium(II) iodide in the presence of hexamethylphosphoramide (HMPA). Any side reactions such as dimerization or termination did not occur during the reduction. Transformed poly(THF) macroanion initiated the polymerization of tert-butyl methacrylate (TBMA), affording the block copolymer of THF with TBMA with narrow molecular weight distributions (1.07-1.22) without the formation of the respective homopolymer of THF or TBMA. The amount of HMPA was found to affect the molecular weight distribution of the copolymer. The unimodal copolymer was obtained by using 4.5 equiv of HMPA to SmI2. The unit ratio of TBF and TBMA and molecular weight of the copolymer could be facilely controlled by both the polymerization time of THF and the amount of TBMA.