A series of new mono beta-diiminato titanium complexes [(N(Ar)C(CH3))(2) CH]TiCl3 (3a: Ar = 2.6-F2C6H3; 3b: Ar = C6F5; 3c: Ar = 2.6-Me2C6H3) have been synthesized and characterized. The crystal structure of 3a revealed that the fl-diiminato ligand in our complex is more close to the eta(2)-coordination mode with little delocalization of the double bonds, which is different from the strong delocalization in the ligands of eta(5)-coordinated (Tolnacnac)TiCl3 and eta(2-)coordinated (Dipnacnac)ZrCl3. The significant electronic effects of fluoro-substituents on the olefin polymerization activity of mono beta-diiminato titanium complexes were found. Titanium complexes with fluorine-containing beta-diiminato ligands, on activation with MMAO, are extremely active catalysts for polymerization of ethylene. The activity of copolymerization of ethylene and 1-hexene is higher than homopolymerization of ethylene and increases with the increase of 1-hexene concentrations, which show the positive "comonomer effect." The molar percentage of 1-hexene incorporation and polymer microstructures can also be modulated by the initial comonomer concentrations. (c) 2007 Wiley Periodicals, Inc.