Star-block copolymers comprised of poly (styrene) (S) core and four poly(epsilon-caprolacton) (epsilon-CL) arms were synthesized by the combination of free radical polymerization (FRP) of S and ring opening polymerization (IMP) of epsilon-CL in one-step in the presence of tetrafunctional ineferter. The block copolymers were characterized by H-1-NMR and FTIR spectroscopy, gel permeation chromatography (GPC), and fractional precipitation method. H-1-NMR and FTIR spectroscopy and GPC studies of the obtained polymers indicate that star-block copolymers easily formed as result of combination FRP and ROP in one-step. The gamma values (solvent/precipitant volume ratio) were observed between 1.04-2.72 (mL/mL) from fractional measurements. The results show that when the initial S feed increased, the molecular weights of the star-block copolymers also increased and the polydispersities of the polymers decreased. M-w/M-n values of the products were measured between 1.4 and 2.86 from GPC. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 3277-3281, 2010