The thermodynamics of the reaction of an ethylene molecule with the Cp2TiCH3Cl/ Al(CH3)(2)Cl system (Cp = eta(5)-C5H5), as a model for olefin polymerization with homogeneous Ziegler-Natta catalysts, was investigated via quantum mechanical DFT calculations. The comparison of the calculated energies for three possible titanium-olefin coordinated intermediates, the ionic complex Cp2TiCH3(C2H4)(+)//Al(CH3)(2)Cl-2(-) the bimetallic complex Cp2TiCH3(C2H4)(delta+). Al(CH3)(2)Cl-2(delta-) and the olefin-separated ion pair Cp2TiCH3+/ C2H4/Al(CH3)(2)Cl-2(-), shows that the most feasible polymerization mechanism occurs via olefin-separated ion pair.