Well-defined multiarm star copolymers, with hyperbranched poly(ethyleneimine) (PEI) as the core and poly(e-caprolactone) (PCL) arms with different degree of polymerization were synthesized by cationic ring-opening polymerization of e-caprolactone from a hyperbranched poly(ethyleneimine) core and used to modify diglycidylether of bisphenol A formulations cured with 1-methylimidazole as anionic initiator. The curing process was studied by dynamic scanning calorimetry (DSC) and FTIR. By rheometry the complex viscosity of the multiarm stars synthesized and the influence of their addition to the reactive mixture was analyzed in detail. The resulting materials were characterized by thermal and mechanical tests. The addition of the multiarm star to the formulation led to homogeneous materials with a slightly toughened fracture in comparison to neat DGEBA thermosets without compromising thermal characteristics.