Transformation of cationic ring-opening polymerization to atom transfer radical polymerization (ATRP) to form AB and ABA type block copolymers was performed. Syntheses of AB type block copolymers initiated by polyTHF (poly(tetramethylene glycol)) containing one bromopropionyl end group were successful for styrene, methyl methacrylate (MMA), and methyl acrylate (MA). Block copolymers were characterized by GPC, H-1 NMR spectroscopy, and thermal analysis (DSC). An ABA type triblock copolymer formed from a telechelic bifunctional polyTHF backbone that contained bromopropionyl groups at both ends was clearly formed in the case of successive ATRP of MMA. The triblock copolymer was characterized by GPC, H-1 NMR, and DSC. In the case of styrene and MA, the formation of triblock copolymers was significantly slower.