Polymerization of propylene was conducted at 0 similar to 150 degrees C with the [ArN(CH2)(3)NAr]TiCl2 (Ar = 2,6-(Pr2C6H3)-Pr-i) complex using a mixture of trialkylaluminium (AlR3, R = methyl, ethyl and isobutyl) and Ph3CB(C6F5)(4) as cocatalyst. When AlMe3 or AlEt3 was employed, atactic polypropylene (PP) was selectively produced, whereas the use of Al(Bu-i)(3) gave a mixture of atactic and isotactic PP. The isotactic index (I.I.; weight fraction of isotactic polymer) depended strongly upon the polymerization temperature, and the highest LI. was obtained at ca. 40 degrees C. The C-13 NMR analysis of the isotactic polymer suggests that the isotactic polymerization proceeds by an enantiomorphic-site mechanism. It was also demonstrated that the present catalyst shows a very high regiospecificity.