The liquid rubbers Hycar ATBN and HTBN were used in the preparation of poly(ε‐caprolactam)—poly(butadiene‐co‐acrylonitrile) block copolymers intended for reaction injection molding by the anionic polymerization of ε‐caprolactam. The conversion of Hycar end groups to polymerization growth centers and the conditions of polymerization influence the crystallization, morphology, and mechanical properties of the product through its molecular structure. The contribution of individual reactions to this molecular structure is discussed.