Three-site mechanism and molecular weight: Time dependency in liquid propylene batch polymerization using a MgCl2-supported Ziegler-Natta catalyst

This article demonstrates that the molecular weight of propylene homopolyme r decreases with time, and that the molecular weight distribution (MWD) nar rows when a highly active MgCl2-supported catalyst is used in a liquid pool polymerization at constant H, concentration and temperature. To track the change in molecular weight and its distribution during polymerization, smal l portions of home polymer samples were taken during the reaction. These sa mples were analyzed by Cross Fractionation Chromatograph (CFC), and the res ulting data were treated with a three-site model. These analyses clearly sh owed that the high molecular weight fraction of the distribution decreases as a function of time. At the same time, the MWD narrows because the weight -average molecular weight decreases faster than the number-average molecula r weight. A probable mechanism based on the reaction of an external donor w ith AIEt(3) is proposed to explain these phenomena. (C) 2001 John Wiley & S ons, Inc.