The effects of catalyst support pore size and reaction conditions (T = 40- 100 degreesC; ethylene pressure = 1.4 MPa; 1-hexene concentration = 0-47 mol/m(3)) on gas-phase polymerization rates and product properties were studied. Catalysts were prepared by impregnation of mesoporous molecular sieves (pore sizes of 2.5-20 nm) with methylaluminoxane and (n-BuCp)(2)ZrCl2. Temperature rising elution fractionation, differential scanning calorimetry and size exclusion chromatography were used to characterize the products. The results showed that these catalysts contained multiple types of catalytic sites and that the types of sites were a strong function of the support pore size. Ethylene polymerization and 1-hexene incorporation rates were strong functions of support pore size, 1-hexene concentration, and temperature. Large-pored catalysts had higher 1-hexene incorporation rates and the rate of 1-hexene incorporation was a function of polymerization time. Highest polymerization rates were obtained at 80 degreesC and 1-hexene concentration of 4-12 mol/m(3); high 1-hexene concentrations resulted in large decreases in polymerization rates. (C) 2003 Elsevier Science Ltd. All rights reserved.