We have prepared in situ molded products of morphologically different nylon 6/polyethylene glycol (PEG) copolymers and their blends via anionic polymerization of -caprolactam in the presence of several kinds of PEG derivatives using sodium caprolactamate as a catalyst and carbamoyl caprolactam derivative as an initiator. Three carbamoyl caprolactams, such as tolylene dicarbamoyl dicaprolactam (TDC), hexamethylene dicarbamoyl dicaprolactam (HDC), and cyclohexyl carbamoyl caprolactam (CCC), with different functionalities and activities were used. Phase separation behavior was investigated by dynamic mechanical thermal analysis (DMTA) and DSC during in situ polymerization and melt crystallization. The mechanical properties of these molded products were evaluated. PEG segments in the block copolymers showed amorphous characteristics, whereas a large fraction of unreacted PEG segments was crystallized in as-polymerized samples, except for the products obtained using the CCC activator. The presence of PEG derivatives retarded the crystallization of nylon 6 part during in situ polymerization as well as melt crystallization. However, PEG segments did not alter the crystalline structure of nylon 6, showing ␣-crystalline modification. The nylon 6 -PEG-nylon 6 triblock copolymers showed the highest impact strength, whereas the nylon 6 -PEG diblock copolymers and in situ nylon 6 -PEG blends showed no improved toughness.