Polyamide-6 (PA6)/polybutadiene (PB) block copolymers were synthesized with macroactivators (MAs) based on hydroxyl-terminated polybutadiene functionalized with diisocyanates and having three N-acyllactam chain-growing centers per molecule. Two different diisocyanates, hexamethylene diisocyanate and isophorone diisocyanate, were applied as precursors for the MAs. The sodium salt of e-caprolactam was chosen as an initiator. The influence of the MA type and concentration on the anionic ring-opening polymerization of epsilon-caprolactam at 180degreesC was studied. A large percentage of the gel fraction in the copolymers was estimated, indicating crosslinked macromolecules. The structure and phase behavior of the copolymers were investigated with differential scanning calorimetry, wide-angle X-ray scattering, thermogravimetric analysis, and dynamic mechanical thermal analysis. In the copolymers, only the PA6 chains crystallized, and the crystallinity depended on the PB content. Different glass-transition temperatures for the PB blocks and PA6 blocks were observed, indicating microphase separation in the copolymers. The mechanical properties of the copolymers were studied by notched impact testing and hardness measurements. The impact strength increased linearly with the soft component concentration up to 10 wt % and reached values six times higher than those of the PA6 homopolymer. (C) 2003 Wiley Periodicals, Inc.