1. Field of the Invention
This invention relates to catalyst system for the polymerization of olefins, particularly, to a catalyst system comprising a conventional supported Ziegler-Natta catalyst and a lithium cyclopentadienide or lithium indene compound used to polymerize .alpha.-olefins, such as propylene.
2. Description of the Prior Art
Polyolefin manufacturing processes typically involve the polymerization of olefin monomer with an organometallic catalyst of the Ziegler-Natta type. Catalyst systems for the polymerization of olefins are well known in the art. Typically, these systems include a Ziegler-Natta type polymerization catalyst component; a co-catalyst, usually an organoaluminum compound; and an external electron donor compound or selectivity control agent, usually an organosilicon compound. Examples of such catalyst systems are shown in the following U.S. Pat. Nos.: 4,107,413; 4,294,721; 4,439,540; 4,115,319; 4,220,554; 4,460,701; and 4,562,173; the disclosures of these patents are hereby incorporated by reference.
A Ziegler-Natta type polymerization catalyst is basically a complex derived from a halide of a transition metal, for example, titanium, chromium or vanadium with a metal hydride and/or a metal alkyl, typically an organoaluminum compound, as a co-catalyst. The catalyst is usually comprised of a titanium halide supported on a magnesium compound complexed with an alkylaluminum co-catalyst.
Lithium cyclopentadienide and lithium indene compounds are known in the synthesis of metallocene compounds useful as catalysts. U.S. Pat. No. 5,168,111; 5,096,867; 5,057,475 and 5,055,438 disclosed using a cyclopentadienyl lithium compound in a synthesis of a bridged metallocene-alumoxane catalyst system for the production of polyolefins and ethylene-.alpha.-olefin copolymers. U.S. Pat. No. 5,120,867 disclosed use of lithium indene in the synthesis of a silicon-bridged metallocene.
Lithium compounds have been used in hydrogenation of conjugated diolefin polymers. U.S. Pat. No. 4,673,714 disclosed the use of hydrocarbon lithium compounds, specifically alkyl lithium, with bis(cyclopentadienyl)titanium compound to hydrogenate the unsaturated double bonds of conjugated diolefins and an improved process which eliminated the need for these alkyl lithium compounds.
Molecular weight can be increased in polymerization processes using conventional supported Ziegler-Natta catalysts in several known ways.
It would be advantageous to increase molecular weight in polymerization processes using conventional supported Ziegler-Natta catalyst by the addition of a compound which interacts or modifies the catalyst without changing process condition or other catalyst components.