Slurry polymerization of propene using MgCl2-supported TiCl4/dioctylphthalate catalysts were carried out in a semibatch reactor at a constant pressure to examine the effects of polymerization conditions on catalyst activity and polymer isotacticity. The catalysts were prepared at 80, 90, and 105 degrees C, which gave different compositions of chemical complexes associated with the diester. Five alkyl aluminums (triethyl, triisobutyl, tri-n-hexyl, tri-n-octyl, and isoprenyl) were studied as cocatalysts. Among these, triethyl aluminum was found to be most effective for the catalysts prepared at 80 and 95 degrees C, and tri-n-hexyl aluminum for the catalyst prepared at 105 degrees C. Dimethoxydiphenyl silane and 2,2,6,6-tetramethyl piperidine were employed to study their effects as an external Lewis base for the catalyst prepared at 105 degrees C. In both cases, a small amount of either base resulted in significant increase in activity and isotacticity, which can be attributed to the high level of phthaloyl chloride complex in the catalyst.