The crystallization behavior and crystalline structure of a series of chlorine-containing polyethylenes (PEs) with either a random or a precise placement of the Cl atom on each and every 21st, 19th, 15th, and 9th backbone carbon have been studied by DSC, NMR, Raman spectroscopy, WAXD, SAXS and AFM in samples cooled from the melt at 1 degrees C/min. All precision Cl-substituted PEs crystallize as homopolymers based on their DD/MAS solid-state C-13 NMR spectra that reveal a uniform distribution of the content of halogen between crystalline and noncrystalline regions. In addition, unique WAXD crystallographic patterns that differ from those of the linear PE and random polymers, relatively large crystal thicknesses, and sharp crystallization and melting peaks also support this homopolymer pattern in precision systems. In contrast, a high concentration of Cl in the noncrystalline regions in the case of the random systems suggest that the crystallization process for randomly Substituted samples is being led by the selection of the most crystalline sequences, resulting in broad thermal transitions and lower crystallities. Increasing chlorine content in both precision and randorn systems decreases crystallinity, melting temperature, heat of fusion, and crystallite thicknesses suggesting that Cl atoms are defects that hinder the crystallization of these polyolefins. Both systems are found to exhibit a lamellar crystalline morphology. Precision substituted polyethylenes form long, straight, and stacked lamellae with axialitic (PE2 I Cl, PE19Cl, and PE15Cl) or spherulitic (PE9Cl) organization as typical of homopolymers of similar molar mass. In contrast, the lamellae of random analogs are curved and more segmented as found in other PE random copolymers. The distribution of Cl within the crystalline regions of systems with precision substitution is strongly affected by the sequence length of the methylene groups. Packing around the substitution point of PE21Cl, PE19Cl, and PE15Cl render nearly all-trans conformations and asymmetric Cl distribution, while close staggering of Cl atoms in the crystals of PE9Cl exert conformational distortions as observed by a strong increase of TG conformers in the Raman spectra.