The nascent ultra-high molecular weight polyethylene (UHMWPE) reactor powders are prepared by polymerizing ethylene in the presence of soluble magnesium complexes under various reaction conditions. It is found that the polymerization process applied in the study provides a high yield of UHMWPE even at low polymerization temperature, and the reactor powders exhibit high melting temperatures (137-143 degrees C) and heat of melting (180-210 J/g). X-ray diffraction (XRD) patterns and solid state C-13 magic angle spinning (MAS) NMR spectra of the powders synthesized under various reaction conditions are examined and compared with those of a commercial UHMWPE powder. Our results reveal that a significant portion of the monoclinic phase is identified when the resin is polymerized at relatively low temperatures. Temperature dependent XRD and NMR experiments show that UHMWPE reactor powders synthesized in the study exhibit more drastic thermal behaviors than the commercial UHMWPE, and this difference becomes more pronounced with the reactor powders polymerized at low temperatures. The increased crystallinity and crystal size in newly synthesized UHMWPE powders at elevated temperatures (100-125 degrees C) may significantly improve the processability of UHMWPE reactor powders into high strength tapes below their melting temperature.