The effects of molecular characteristics and processing conditions on melt-drawing behavior of ultrahigh molecular weight polyethylene (UHMW-PE) are discussed, based on a combination of in situ X-ray measurement and stress-strain behavior. The sample films of metallocene- and Ziegler-catalyzed UHMW-PEs with a similar viscosity average MW of similar to 10(7) were prepared by compression molding at 180 degrees C. Stress profiles recorded at 160 degrees C above the melting temperature of 135 degrees C exhibited a plateau stress region for both films. The relative change in the intensities of the amorphous scattering recorded on the equator and on the meridian indicated the orientation of amorphous chains along the draw axis with increasing strain. However, there was a substantial difference in the subsequent crystallization into the hexagonal phase, reflecting the molecular characteristics, that is, MW distribution of each sample film. Rapid crystallization into the hexagonal phase occurred at the beginning point of the plateau stress region in melt-drawing for metallocene-catalyzed UHMW-PE film. In contrast, gradual crystallization into the hexagonal phase occurred at the middle point of the plateau stress region for the Ziegler-catalyzed film, suggesting an ease of chain slippage during drawing. These results demonstrate that the difference in the MW distribution due to the polymerization catalyst system dominates the phase development mechanism during melt-drawing. The effect of the processing conditions, that is, the including strain rate and drawing temperature, on the melt-drawing behavior is also discussed. The obtained results indicate that the traditional temperature-strain rate relationship is effective for transient crystallization in to the hexagonal phase during melt-drawing, as well as for typically oriented crystallization during ultradrawing in the solid state. (c) 2006 Wiley Periodicals, Inc.