Polymerization mechanisms on O-h-propyltitaniumchlorides and T-d-propyltitaniumchlorides, active site models of TiCl3 catalysts and metallocene catalysts, respectively, are studied by using "paired interacting orbitals" (PIO) analysis and "localized frontier orbitals" (LFO) calculation. In the case of TiCl3 catalysts the possible route of an incoming ethylene is constrained by Cl anions located in the ethylene insertion plane. In the case of metallocene catalysts, such a constraint does not appear, and therefore, they should be superior to TiCl3 catalysts in catalytic activity. The low reactivities of TiCl3 catalysts are removed considerably by making Ti-2n clusters on the crystalline surface.