The kinetics of gas phase propylene polymerization was studied at elevated pressures by using a spherical TiCl4/MgCl2 catalyst. The experiments were carried out in a semi-batch olefin polymerization reactor equipped with an on-line computer control system. Important kinetic data such as instantaneous reaction rate of monomer were obtained. The effects of polymerization conditions on reaction kinetics were systematically studied. The Flory-Huggins equation was used to estimate the monomer concentration in the polymer amorphous region. It was found that reaction rate had a linear relationship with monomer concentration. The time evolution of polymerization rate under all conditions followed the same pattern

with a fast decay in the early stage of polymerization and a relatively slow decay in the later stage. An nth decay model was developed to describe the dynamic changes of polymerization rate. Experimental data were used to estimate the model parameters. The decay order n was found to be 2.5 and the lumped propagation activation energy was estimated to be 77.1 kJ8226;mol-1

which was similar to the value in the liquid phase reported by other researchers.The model described the experiments well.