One-step synthesis of star copolymers by reversible addition-fragmentation chain transfer (RAFT) and ring-opening polymerization (ROP) by using a novel dual initiator is reported. Triarm block copolymers comprising one polystyrene (or polyacrylamide) arm and two poly(beta-butyrolactone) arms were synthesized in one-step by simultaneous RAFT polymerization of styrene (St) (or acrylamide, designated as AAm) and ROP of beta-butyrolactone (BL) in the presence of a novel trifunctional initiator, 1,2-propanediol ethyl xanthogenate (RAFT-ROP agent). This dual initiator was obtained through the reaction of 3-chloro-1,2-propanediol with the potassium salt of ethyl xanthogenate. The principal parameters such as monomer concentration, initiator concentration, and polymerization time that affect the one-step polymerization reaction were evaluated. The characterization of the products was achieved using Fourier-transform infrared spectroscopy (FTIR), H-1-nuclear magnetic resonance (H-1-NMR), C-13-nuclear magnetic resonance (C-13-NMR), Gas chromatography-mass spectrometry (GC-MS), gel-permeation chromatography (GPC), thermogravimetric analysis (TGA), and fractional precipitation (7) techniques. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 1638-1645, 2010