A new tetrafunctional initiator, di(hydroxyethyl)-2,9-dibromosebacate (DHEDBS) [HOCH2CH2OOCCHBr(CH2)(6)CHBrCOOCH2CH2OH], was synthesized and used in preparation of A(2)B(2) miktoarm star copolymers, (polystyrene)(2)/ [poly(1,3-dioxepane)](2) [S-(PSt)(2)(PDOP)(2)], by transformation of atom transfer radical polymerization (ATRP) to cationic ring-opening polymerization (CROP). First, two-armed PSt with two primary hydroxyl groups sited at the center of macromolecule [(PStBr)(2)(OH)(2)] was obtained by ATRP of St with the initiation system of DHEDBS/CuBr/bpy, and used as a chain-transfer agent in the CROP of DOP with triflic acid as the initiator. Therefore, A(2)B(2) miktoarm star copolymer S-(PSt)(2)(PDOP)(2) was formed. Its structure was confirmed by the H-1 NMR spectrum. Gel permeation chromatography (GPC) curves show that the polymers obtained have a relatively narrow molecular weight distribution. The hydrolysis product of S-(PSt)(2)(PDOP)(2) was also characterized by H-1 NMR and GPC. (C) 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 437-445, 2001.