Propene/Ethene-[1-¹³C] Copolymerization as a Tool for Investigating Catalyst Regioselectivity. MgCl₂/Internal Donor/TiCl₄−External Donor/AlR₃Systems

Abstract

The propene/ethene-[1-13C] copolymerization method has been applied to measure precisely the regioselectivity of two modern MgCl2-supported Ziegler−Natta catalyst systems for the industrial production of isotactic polypropylene, namely MgCl2/di(isobutyl)-o-phthalate/TiCl4−cyclohexylmethyldimethoxysilane/AlR3 (1) and MgCl2/2,2-di(isobutyl)-1,3-dimethoxypropane/TiCl4−AlR3 (2). System 2 turned out to be slightly less regioselective than system 1 (on average, 0.26% 2,1 misinsertions instead of 0.18%), but the distribution of the regiodefects throughout the polymer was found to be more uniform, most likely as a result of a lower differentiation of the active species. This provides a simple explanation for the different response of the two catalysts to molecular hydrogen as a chain transfer agent. In system 1, indeed, part of the active species are almost completely regioselective, which means that H2 finds very few preferential “cutting points” in the form of growing chains with a sterically hindered 2,1 last-inserted unit; therefore, a higher H2 partial pressure is neededcompared with system 2in order to achieve the same decrease of average polymer molecular mass.