The use of chromium compounds in the polymerization of olefins is well-known. U.S. Pat. Nos. 2,825,721 and 2,951,816 teach the use of CrO.sub.3 supported on an inorganic material such as silica, alumina or combinations of silica and alumina and activated by heating at elevated temperatures to polymerize olefins. When these catalyst systems are used in various polymerization processes such as the well-known particle-form process, the resins produced, while useful in many applications, are unsatisfactory for others because of a deficiency in certain properties such as melt index.
Attempts to improve deficient properties of polyolefins produced using supported, heat-activated chromium oxide catalysts have been made by adding various metal and non-metal compounds to the supported chromium oxide prior to activation by heating. For example, in U.S. Pat. No. 3,622,522 it is suggested that an alkoxide of gallium or tin be added to supported chromium oxide prior to heat activation, and the performance of aluminum isopropoxide compared. U.S. Pat. No. 3,715,321 suggests adding a compound of a Group II-A or Group III-B metal to supported chromium oxide prior to heat treatment.
U.S. Pat. No. 3,484,428 discloses an olefin polymerization catalyst composite prepared by impregnating a support such as silica, alumina, zirconia, and thoria with a chromium compound, convertible to chromium oxide upon calcination, and a borane or an alkyl-substituted borane, and then heating and activating both the boron and chromium compounds of the resulting composite on the support in an oxygen-containing atmosphere at a temperature of about 750.degree. to 1800.degree. F. U.S. Pat. No. 3,780,011 discloses an olefin polymerization catalyst prepared by forming an intimate mixture of a silica, alumina, zirconia or thoria support and chromium oxide. The mixture is dried by fluidizing with dry air. An oxygenated titanium, boron or vanadium compound, e.g., alkyl esters of such metals is then added to the fluidized bed, and the resulting fluidized mixture is activated with an oxygen-containing, dry gas at elevated temperatures.
U.S. Pat. No. 3,202,645 defines catalysts, including the combination of a halide of titanium, zirconium, chromium, etc. with the reaction product of e.g., tri-n-butyl borane with a particulate inorganic solid such as silica. U.S. Pat. No. 3,625,864 relates to a supported catalyst including a titaniaboron complex. U.S. Pat. No. 2,898,326 describes a catalyst comprising an activated Group VI-A oxide support impregnated with boranes.
It is also known to utilize other chromium compounds as catalysts for the polymerization of olefins. Such compounds include various silyl chromate and polyalicyclic chromate esters as described, for example, in U.S. Pat. Nos. 3,324,095; 3,324,101; 3,642,749; and 3,704,287. The use of phosphorus-containing chromate esters in olefin polymerization catalysts has also been disclosed in the aforesaid U.S. Pat. No. 3,704,287 and in U.S. Pat. No. 3,474,080.
Use of the above chromium compound catalysts in Ziegler-type coordination catalyst systems has also been proposed. As is well-known in the art, such catalysts frequently additionally comprise organometallic reducing agents such as, for example, trialkyl aluminum compounds. Ziegler-type catalyst systems incorporating supported chromium compound catalysts and organometallic reducing agents, particularly organoaluminum compounds, are disclosed, for example, in U.S. Pat. Nos. 3,324,101; 3,642,749; 3,704,287; 3,806,500; and in the copending application Ser. No. 532,131, filed Dec. 16, 1974.
The preparation and use of improved high pore volume silica xerogel materials suitable as catalyst supports is described in Belgium Pat. No. 741,437 and U.S. Pat. Nos. 3,652,214; 3,652,215 and 3,652,216; whereas moderate pore volume silica xerogel supports are disclosed in U.S. Pat. No. 3,453,070.