Poly(epsilon-caprolactam)-poly(butadiene-co-acrylonitrile) block copolymers have been synthesized that were suitable for the pultrusion process. The effects of amine-terminated butadiene acrylonitrile copolymer (ATBN) content on copolymerization rate, conversion, crystallization behaviors, and mechanical properties and morphology of NY 6-ATBN copolymers were investigated. The conversion was over 90% and the copolymerization was completed within 3 min (up to 20 wt % ATBN) at 145-165-degrees-C. The higher the [MgBr - CPL] (epsilon-caprolactam magnesium bromide) initiator concentration, the lower the molecular weight [ or intrinsic viscosity (IV) ] of the copolymers and the less the copolymerization time. Relationships among mechanical properties, morphology, and structure of copolymers have been studied. The notched Izod impact strength of the NY 6-ATBN copolymers (> 10.5 wt % ATBN) increased more than 10-fold, while the flexural modulus only reduced 10%. Transmission electron micrographs showed that ATBN rubber particle size changed slightly with ATBN content, but interparticle distance (T) decreased with increasing ATBN content. As T decreased, the ductility of the copolymers increased. When ATBN content reached 10.5-13 wt % (T = 0.32-0.45 mum), the copolymers showed a very high ductile behavior.