The kinetics of polymerization of PO and ECH using rare earth coordination catalyst composed of Neodynium 2-Ethylhexyl Phosphonatestriisobutyl Aluminum-water in toluene have been studied by a dilatometer method. It has been shown that the rate of polymerization is first order with respect to both monomer and catalyst concentrations. According to Arrhenius equation

it was found that polymerization activstion energies for PO and ECH are 61.3 Kj/mol and 48.9 Kj/mol respectively. Under the same conditions

propylene oxide polymerization proceeded fasfer than ECH polymerization. Al/Nd molar ration and H2O/Al molar ratio also have some effects on the polymerization rate

and the same is catalyst composition. The activity sequence for different rare earth element catalyst systems decresses in the following order:Nd>La>Dy>Yb>Eu;activity se-quence of catalysts containing different ligands decreases in the order: acac>P204>P507>naph; and the activity of Al(i-Bu)3 system is higher than that of catalyst with AlEt3.