In this study, we developed a mathematical model for olefin copolymerization using soluble Ziegler-Natta catalysts in a semibatch reactor to predict the reaction rate and polymer characteristics (i.e., molecular weight, polydispersity, and ethylene content) as functions of the reaction parameters (i.e., time, temperature, pressure, concentrations, and so on) accurately. The proposed model differs from others because it considers the olefin copolymerization as a dynamic process and applies double moments for two reactants (ethylene and propylene) in the presence of hydrogen. To establish the model validity, the copolymerization was performed with VOCl3-Al2Et3Cl3 systems with hydrogen as a molecular weight controlling agent. The dynamic model was able to reproduce the experiment at data within experimental accuracy and accurately demonstrated the fundamental importance of the polymerization variables on the final properties of the polymer material in the copolymerization of ethylene and propylene with AI/V ratios of up to 28 before synthesis. (c) 2006 Wiley Periodicals, Inc.