Effects of Internal and External Donors on the Regio- and Stereoselectivity of Active Species in MgCl2-Supported Catalysts for Propene Polymerization

Abstract

Poly(propylene)s prepared using MgCl2-supported catalysts containing different electron donors have been characterized using temperature rising elution fractionation (TREF), gel permeation chromatography (GPC) and 13C NMR analysis. In addition, the regio- and stereochemical composition of oligomeric fractions present in selected polymers has been determined. The results indicate that catalysts in which the internal donor is a diether have a relatively narrow distribution of active species, for which the effects of regioselectivity on chain transfer with hydrogen are particularly prominent. The regio- and stereoselectivity of active species in catalysts in which the internal donor is diisobutyl phthalate is dependent on the nature of the alkoxysilane external donor used in polymerization. The effects of different alkoxysilanes on polymer tacticity and molecular weight distribution are interpreted in terms of lability of donor coordination in the vicinity of active species, a labile as opposed to a stable donor coordination giving rise to a higher proportion of defect-rich sequences in the polymer chain. Such species will also give relatively low molecular weight as a result of an increased probability of chain transfer with decreasing regio- and stereoselectivity. A broad tacticity and molecular weight distribution in poly(propylene) prepared using monoesters as internal and external donors indicates that these systems may contain not only a significant proportion of labile active species, but also species that do not require the presence of an electron donor in their immediate vicinity for high selectivity.