Fractionated crystallization of Ziegler-Natta PP (ZN-PP) droplets produced from ZN-PP/polystyrene (PS) multilayered films resulted in four crystallization exotherms at 40, 64, 90, and 103 degrees C. The origins of these crystallization peaks were investigated in this article. The nucleation of ZN-PP droplets at various crystallization exotherms was attributed to (1) various additives in ZN-PP and PS, (2) catalyst residues present in ZN-PP, or (3) substrate nucleation by confining matrix. Effect of various additives on the fractionated crystallization an additive-free grade of ZN-PP (cPP) was investigated. Crystallization of cPP droplets from 200-nm layers showed that Irganox was responsible for crystallization of droplets at 103 degrees C. Irgafos and ZnS did not affect the crystallization of cPP droplets. We also investigated three grades of PP, which were synthesized using different catalysts. The PP grades were ZN-PP, developmental PP (D-PP) and metallocene PP (M-PP). D-PP droplets from 200-nm layers showed crystallization at 100 degrees C, while ZN-PP resulted in crystallization at 90 and 103 degrees C. It was suggested that the 90 degrees C peak for ZN-PP droplets was most likely due to the presence of Ziegler-Natta catalyst residues. Crystallization of M-PP droplets produced from 200 nm showed multiple crystallization exotherms (60 to 90 degrees C). The effect of confining substrates on the fractionated crystallization of ZN-PP droplets revealed heterogeneous nucleation by polycarbonate (PC) substrate. When the confining matrix was changed from polystyrene (PS) to polymethyl methacrylate (PMMA), the crystallization thermograms of ZN-PP droplets were unaffected. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012