A mathematical model was developed to describe ethylene-1-hexene copolymerization with a tandem catalysis system. A series of semibatch polymerization runs catalyzed by a trimerization catalyst bis(2-dodecylsulfanyi-ethyl)amine-CrCl3 and a copolymerization catalyst Et(Ind)(2)ZrCl2 in toluene at 74 degrees C were carried out to verify the model. Both experimentation and modeling showed that adjusting the Cr/Zr ratio yielded various branching densities and thus melting temperatures, as well as molecular weights and polydispersities. Broad composition distributions and thus broad DSC curves were observed at high Cr/Zr ratios. Modeling results elucidated that this is due to an accumulation of I-hexene component and to composition drifting during the copolymerization. It was also found that applying a short time period of pre-trimerization improved homogeneity in chain microstructure and minimized broadening in DSC curves. (C) 2008 Elsevier Ltd. All rights reserved.