Nylon 12 was successfully synthesized in a twin-screw extruder via the anionic ring-opening polymerization of lauryllactam (LL). Maleated low-density polyethylene (LDPE-MAH) was added to improve the mechanical properties of nylon 12. The in situ blends of nylon 12 and LDPE-MAH were characterized by mechanical testing and scanning electron microscopy. With increasing LDPE-MAH content, the tensile strength and flexural strength decreased, whereas the blend had improved impact strength and achieved supertoughness when the content of LDPE-MAH was 30 wt%. In the in situ formed low-density polyethylene-g-PA12 copolymer, the domain of the LDPE-MAH phase was finely dispersed in the nylon 12 matrix. The good interface between the two phases demonstrated that LDPE-MAH could be used as a macromolecular activator to induce the polymerization of LL. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2662-2672, 2009