It is desirable in many polymerization processes, particularly a gas phase process, to use a supported catalyst. Generally these catalyst systems include a metallocene and alumoxane supported on the same carrier, such as silica, and subsequently dried to a powder. For example, U.S. Pat. No. 4,937,217 generally describes a mixture of trimethylaluminum and triethylaluminum added to an undehydrated silica then adding a metallocene to form a dry catalyst. EP-308177-B 1 generally describes adding a wet monomer to a reactor containing a metallocene, trialkylaluminum and undehydrated silica. U.S. Pat. Nos. 4,912,075, 4,935,937 and 4,937,301 generally relate to adding trimethylaluminum to an undehydrated silica and then adding a metallocene to form a dry supported catalyst. Similarly, U.S. Pat. Nos. 5,008,228, 5,086,025 and 5,147,949 generally describe forming a dry supported catalyst by the addition of trimethylaluminum to a water impregnated silica then adding the metallocene. U.S. Pat. No. 4,914,253 describes adding trimethylaluminum to undehydrated silica, adding a metallocene and then drying the catalyst with an amount of hydrogen to produce a polyethylene wax. U.S. Pat. Nos. 4,808,561, 4,897,455 and 4,701,432 describe techniques to form a supported catalyst where the inert carrier, typically silica, is calcined and contacted with a metallocene(s) and a activator/cocatalyst component. U.S. Pat. No. 5,238,892 describes forming a dry supported catalyst by mixing a metallocene with an alkyl aluminum then adding undehydrated silica. U.S. Pat. No, 5,240,894 generally pertains to forming a supported metallocene/alumoxane catalyst system by forming a metallocene/alumoxane reaction solution, adding a porous carrier, evaporating the resulting slurry to remove residual solvent from the carrier. Polymers produced using these catalyst systems can be difficult to process and more difficult to produce. Particularly in a gas phase polymerization process, using the catalyst systems described above polymers having multiple melting peaks are produced. These multiple melting peaks enhance the potential for sheeting or fouling during polymerization, which can result in reactor shut-down. Furthermore, variable melting peaks reduce polymer product consistency, limit product capability and adversely affect end-use applications properties such as clarity and heat sealability.
Also, during polymerization within a reactor, particularly gas phase polymerization process, there is a tendency for reactor fouling. Typically, in such a process, a continuous cycle is employed where in one part of the cycle, a recycle stream is heated in the reactor by the heat of polymerization. This heat is removed in another part of the cycle by a cooling system external to the reactor. During a typical polymerization process fines within the reactor often accumulate and cling or stick to the walls of a reactor. This phenomenon is often referred to as "sheeting". The accumulation of polymer particles on the reactor walls, the recycling lines and cooling system results in many problems including poor heat transfer in the polymerization process. Polymer particles that adhere to the walls of the reactor continue to polymerize and often fuse together and form chunks, which can be detrimental to a continuous process, particularly a fluidized bed process.
It would be highly desirable to have a polymerization catalyst that in a polymerization process would significantly enhance reactor operability and provide an improved polymer product.
USPTO Backgrounds
USPTO Backgrounds