Homogeneous and silica-supported CP2ZrCl2/methylaluminoxane (MAO) catalyst systems have been used for the copolymerization of ethylene with 1-butene, 1-hexene, 4-methylpentene-1 (4-MP-1), and 1-octene in order to compare the "comonomer effect" obtained with a homogeneous metallocene-based catalyst system with that obtained using a heterogenized form of the same metallocene-based catalyst system. The results obtained indicated that at 70 degrees C there was general rate depression with the homogeneous catalyst system whereas rate enhancement occurred in all copolymerizations carried out with the silica-supported catalyst system. Rate enhancement was observed for both the homogeneous and the silica-supported catalyst systems when ethylene/4-MP-1 copolymerization was carried out at 50 degrees C. Active center studies during ethylene/4-MP-1 copolymerization indicated that the rate depression during copolymerization using the homogeneous catalyst system at 70 degrees C was due to a reduction in the active center concentration. However, the increase in polymerization rate when the silica-supported catalyst system was used at the same temperature resulted from an increase in the propagation rate coefficient. (C) 2007 Wiley Periodicals, Inc.