Production of broad molecular weight polyethylene

#The present invention relates to the use of chromium-based catalysts with alkyl silanols such as triphenyl silanol with or without aluminum alkyl activators. The use of aluminum alkyls and alkyl silanols allows for the control of polymer molecular weight and molecular weight distribution and results in a catalyst with behavior similar to silylchromate on silica catalyst. Bimodal polyethylene may be produced with the present invention.

#Ethylene polymers have been used generally and widely as resin materials for various molded articles and are required to have different properties depending on the molding method and purpose. For example, polymers having relatively low molecular weights and narrow molecular weight distributions are suitable for articles molded by an injection molding method. On the other hand, polymers having relatively high molecular weights and broad molecular weight distributions are suitable for articles molded by blow molding or inflation molding. In many applications, medium-to-high molecular weight polyethylenes are desirable. Such polyethylenes have sufficient strength for applications which call for such strength (e.g., pipe applications), and simultaneously possess good processability characteristics.

#Ethylene polymers having broad molecular weight distributions can be obtained by use of a chromium catalyst obtained by calcining a chromium compound carried on an inorganic oxide carrier in a non-reducing atmosphere to activate it such that at least a portion of the carried chromium atoms is converted to hexavalent chromium atoms (Cr+6). This is commonly referred to in the art as the Phillips catalyst. The respective material is impregnated onto silica, fluidized and heated in the presence of oxygen to about 400° C.-860° C., converting chromium from the +3 oxidation state to the +6 oxidation state. A second chromium catalyst used for high density polyethylene applications consists of silylchromate (bis-triphenylsilyl chromate) absorbed on dehydrated silica and subsequently reduced with diethylaluminum ethoxide (DEALE). The resulting polyethylenes produced by each of these catalysts are different in some important properties. Chromium oxide-on-silica catalysts have good productivity (g PE/g catalyst), also measured by activity (g PE/g catalyst-hr) but produce polyethylenes with molecular weight distributions lower than that desired for certain applications. Silylchromate-based catalysts produce polyethylenes with desirable molecular weight distribution characteristics (broader molecular weight distribution with a high molecular weight shoulder on molecular weight distribution curve, indicative of two distinct molecular weight populations).

#For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:

#FIG. 1. Possible structure of chromium oxide-on-silica (“Phillips”) catalyst.

#FIG. 2. Possible structure of silylchromate-on-silica catalyst.

#As used herein, “a” or “an” is defined herein as one or more.

#As used herein, “in situ,” in reference to the mode of addition of a component to the catalyst, is defined herein as addition to the catalyst in the reactor. Therefore, when a catalyst component is added in situ, it is added to the remaining catalyst components in the reactor and is not combined with the other catalyst components prior to their transport to the reactor. “In reactor” is synonymous with and used interchangeably herein with “in situ.”

#As used herein, “in catalyst” or “on catalyst,” in reference to the mode of addition of a component to the catalyst, is defined herein as addition directly to the catalyst prior to introduction of the catalyst to the reactor. Therefore, when a component is added to the catalyst “in catalyst” or “on catalyst,” it is added to the other catalyst components prior to the transport of the aggregate to the reactor.