Polystyrene (PSt) with end-terminal bromine (Br-PSt-Br) was synthesized by the atom transfer radical polymerization of styrene with the difunctional initiator 1,2-bis(2'-bromobutyryloxy)ethane in combination with CuBr and bipyridine. The Br-PSt-Br reacted with silver perchlorate at -78 degrees C, and the resulting macromolecular initiator was used to initiate the polymerization of tetrahydrofuran. Triblock poly(tetrahydrofuran)-polystyrene-poly(tetrahydrofuran) (PTHF-PSt-PTHF) diol was obtained after propagation at -15 degrees C. The conversion of the polymerization was measured by gas chromatography. The structures of the triblock copolymer PTHF-PSt-PTHF diol were characterized by H-1 NMR and gel permeation chromatography. The mechanism of cationic ring-opening polymerization is discussed.