In previous articles, we reported on a novel reactive extrusion process to obtain a compatibilized blend of polymer A and polymer B. It consisted in polymerizing the monomer of polymer A in the presence of polymer B. A fraction of the latter contained initiating sites from which the Polymerization of monomer A took place. As such, both polymer A and a graft copolymer of polymer A and polymer B were formed in the process. That process was called in situ polymerization and in situ compatibilization of polymer blends. Its feasibility was illustrated for in situ polymerized and in situ compatibilized poly(propylene) and polyamide 6 (PP/PA6) blends. The latter were prepared by activated anionic polymerization Of E-caprolactam (CL) in the presence of PP in a batch mixer and a twin-screw extruder, respectively. A fraction of the PP contained isocyanate groups from which PA6 grafts were formed. Sodium caprolactam (NaCL) was used as the catalyst and a diisocyanate compound was used as the activator. In this study, we report on the effects of various parameters on the kinetics of the anionic polymerization of CL in the presence of PP. (C) 2003 Wiley Periodicals, Inc. J Appl Polyrn Sci 91: 1498 -1504, 2004.