Ethylene/1-pentene copolymers were synthesized using CP2ZrCl2(1)/MAO and [(CO)(5)W=C(Me)OZr(CP)(2)Cl](2)/MAO catalyst systems. The copolymers were characterized by SEC, DSC, FTIR and C-13 NMR spectroscopy. The copolymers synthesized with [(CO)(5)W=C(Me) OZr(CP)(2)Cl](2)/MAO had higher average molecular weights and broader polydispersities compared to those produced with Cp2ZrCl2(1)/MAO. The chemical heterogeneity was investigated by SEC-FTIR and fractionation techniques. All copolymers showed a higher incorporation of the I pentene in the low molecular weight fraction as revealed by SEC-FTIR. Crystallization analysis fractionation (CRYSTAF) showed a broad chemical composition distribution (CCD) for all the copolymers synthesized with these two catalyst systems. Selected copolymers were also analyzed using an automated preparative molecular weight fractionation. (c) 2005 Elsevier Ltd. All rights reserved.