Organo-modified montmorillonites and poly(p-dioxanone) (PPDO) diol prepolymers were used to prepare Poly(p-dioxanone)-PU/organic montmorillonite (PPDO-PU/OMMT) nanocomposites by chain-extending reaction. The crystallization behavior and spherulitic morphology of PPDO-PU/OMMT nanocomposites were investigated by WXRD, differential scanning calorimetry, and polarized optical microscopy. The results show that the regularity of the chain structure plays a dominant role during the crystallization process rather than that of OMMT content and its dispersion status in PPDO matrix. With similar molecular weight and same OMMT content, PPDO-PU/OMMT nanocomposite, which derived from lower molecular weight PPDO prepolymer, exhibits lower crystallization rate, melting point, and crystallinity. The influence of the clay content on the crystallization behavior highly depends on its dispersing state. The nucleating effect of OMMT can be only observed at high loading percentage. For the nanocomposites with low clay loading percentage, the retarding effect of exfoliated platelets on the chain-ordering into crystal lamellae became the key factor. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013