A well-defined multiarm star copolymer poly(styrene)-b-poly(epsilon-caprolactone) (PSOH-b-PCL) with an average number of PCL arms per molecule of 60 has been prepared. 4-Chloromethyl styrene (4-CMS) was polymerized by means of atom transfer radical polymerization (ATRP) to obtain a hyperbranched poly(styrene) with chlorines as terminal groups. Subsequently, chlorines were substituted by reaction with diisopropanolamine (DIPA) to give the hydroxyl-ended derivative. Finally, the hydroxyl-ended hyperbranched poly(styrene) has been used as a macroinitiator core to polymerize e-caprolactone by means of cationic ring-opening polymerization so as to obtain the star copolymer. In a second step, PSOH-b-PCL was used as re-active modifier of diglycidylether of bisphenol A formulations cured by 1-methyl imidazole (1-MI) obtaining nanostructured thermosets. The curing process was studied by dynamic scanning calorimetry and Fourier transform infrared spectroscopy (FTIR). By rheometry, the effect of this new polymer topology on the complex viscosity (eta*) of the reactive mixture and on the gelation process was also analyzed. The thermomechanical characteristics of the modified materials were determined. (C) 2011 Wiley Periodicals, Inc. J PolymSci Part A: Polym Chem49: 4639-4649, 2011