Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. While these catalysts are inexpensive, they exhibit low activity and are generally poor at incorporating α-olefin comonomers. To improve polymer properties, single-site catalysts, in particular metallocenes, are beginning to replace Ziegler-Natta catalysts.
Catalyst precursors that incorporate a transition metal and an indenoindolyl ligand are known. U.S. Pat. Nos. 6,232,260 and 6,451,724 disclose the use of transition metal catalysts based upon indenoindolyl ligands.
U.S. Pat. No. 6,559,251 discloses a process for polymerizing olefins with a silica-supported, indenoindolyl Group 4-6 transition metal complex having “open architecture.” WO 01/53360 discloses similar open architecture indenoindolyl catalysts that may be supported on an inert support. U.S. Pat. No. 6,211,311 teaches that many heterometallocenes are inherently unstable and this causes difficulties in supporting these catalysts and poor catalyst activity. This problem is avoided by using chemically treated supports.
U.S. Pat. No. 6,908,972 discloses a process for polymerizing ethylene in the presence of a silica-supported Group 3-10 transition metal catalyst that has two bridged indenoindolyl ligands. The catalyst is effective for copolymerizing ethylene with α-olefins such as 1-butene or 1-hexene to make low density polyolefins.
When ethylene is polymerized in a slurry process, the polyethylene forms as a powder slurry. This isolated powder has a certain bulk density. The bulk density should not be confused with the more commonly reported polyethylene density, which is the density of the polyethylene prepared from the melt and depends upon branching. Bulk density depends upon the characteristics of the powder. A higher bulk density is desirable because less volume is required to store and ship the powder. Despite the considerable work done with catalysts based upon indenoindolyl ligands, there is a need for improvement, especially with regard to improving bulk density.