The crystallization behavior of isotactic propylene-1-hexene (PH) random copolymer having 5.7% mole fraction of hexene content was investigated using simultaneous time-resolved small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. For this copolymer, the hexene component cannot be incorporated into the unit cell structure of isotactic polypropylene (iPP). Only alpha-phase crystal form of iPP was observed when samples were melt crystallized at temperatures of 40 degrees C, 60 degrees C, 80 degrees C, and 100 degrees C. Comprehensive analysis of SAXS and WAXD profiles indicated that the crystalline morphology is correlated with crystallization temperature. At high temperatures (e.g., 100 degrees C) the dominant morphology is the lamellar structure; while at low temperatures (e.g., 40 degrees C) only highly disordered small crystal blocks can be formed. These morphologies are kinetically controlled. Under a small degree of supercooling (the corresponding iPP crystallization rate is slow), a segmental segregation between iPP and hexene components probably takes place, leading to the formation of iPP lamellar crystals with a higher degree of order. In contrast, under a large degree of supercooling (the corresponding iPP crystallization rate is fast), defective small crystal blocks are favored due to the large thermodynamic driving force and low chain mobility. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48:26-32,2010