Nanostructured polymer composites (NPC) based on polyamide 6 (PA6) are prepared by activated anionic ring-opening polymerization (AAROP) of mixtures of e-caprolactam (ECL) and organically treated montmorillonite (o-MMT). The polymerization is performed in bulk, at 165 degrees C, i.e., below the melting point of the resulting APA6, the reaction time being in the range of 10-15 min. The o-MMT content is varied in the 0.5-10% range. X-ray diffraction (XRD) and transmission electron microscopy (TEM) show that exfoliated NPC can be produced with clay loads of 0.5-1.0%. Larger clay amounts lead to various degrees of intercalation of the MMT layers. FT-IR imaging proves that all NPCs contain MMT aggregates with sizes in the 10-20 mm range. The formation of the matrix crystalline structure is followed directly by performing AAROP of an activated ECL/o-MMT blend in a synchrotron beamline. Irrespective of the o-MMT type and concentration, it is the alpha-PA6 that forms first and in larger amounts. The gamma-PA6 polymorph can be found in predominating amounts only after melting and recrystallization of the already produced polymer matrix. (C) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1228-1238, 2013