Chromium based catalysts are used in the commercial polymerization of small alpha-olefins such as ethylene and propylene. One such catalyst is prepared by depositing chromocene (bis(cyclopentadienyl) chromium (II)) on an inorganic metal oxide support, as disclosed in British Patent No. 1,253,063 to Karapinka. U.S. Pat. No. 4,015,059, issued Mar. 29, 1977 to Karol, describes the use of bis(indenyl)--and bis(fluorenyl)-chromium (II) compounds supported on activated inorganic oxide supports as catalysts for the polymerization of ethylene.
Recently, new synthetic methods have been described for preparing Cr.sup.+3 organometallic compounds. Theopold, J, Am. Chem Soc. (1988), 110, 5902 entitled "Cationic Chromium (III) Alkyls as Olefin Polymerization Catalysts," Theopold, Acc. Chem. Res. (1990), 23, 263 entitled "Organochromium (III) Chemistry: A Neglected Oxidation State" and Thomas et at., J. Am. Chem. Soc., (1991), 113, 893 et seq. disclose that certain pentamethylcyclopentadienyl chromium (III) alkyls can be prepared, and that they can be used for making polyethylene homogeneously in CH.sub.2 Cl.sub.2. However, these homogeneous Cr (III) polymerization catalysts have several deficiencies. These include low polymer productivity, rapid deactivation, and the need to use polar, non-coordinating solvents. Additionally, since they are homogeneous catalysts, they are unsuitable for gas phase olefin polymerizations.
U.S. Pat. No. 4,530,914, issued Jul. 23, 1985 to Ewen et at., discloses a catalyst system for the polymerization of alpha-olefins which comprises two or more metallocenes, each having different propagation and termination rate constants, and aluminoxane. The metallocenes are cyclopentadienyl derivatives of a transition metal of Group 4b, 5b, and 6b metals of the Periodic Table. They are described by the formulas (C.sub.5 R'.sub.m).sub.p R".sub.s (C.sub.5 R'.sub.m)MeQ.sub.3-p and R".sub.s (C.sub.5 R'.sub.m).sub.2 MeQ' where (C.sub.5 R'.sub.m) is a cyclopentadienyl or substituted cyclopentadienyl, each R' is hydrogen or a hydrocarbyl radical, R" is an alkylene radical, a dialkyl germanium or silicon or an alkyl phosphine or amine radical bridging two (C.sub.5 R'.sub.m) rings, Q is a hydrocarbon radical, Me is a Group 4b, 5b, or 6b metal, s is 0 or 1, p is 0, 1, or 2; when p=0, s=0; m is 4 when s is 1 and m is 5 when s is 0.
U.S. Pat. No. 4,939,217, issued Jul. 3, 1990 to Stricklen, also discloses a process for polymerizing olefins where the polymerization is conducted in the presence of hydrogen, and a catalyst system is used which contains aluminoxane and at least two metallocenes, each having different olefin polymerization termination rate constants. The metallocenes disclosed are similar to those described in aforementioned U.S. Pat. No. 4,530,914.
U.S. Pat. No. 4,975,403, issued Dec. 4, 1990 to Ewen, discloses a catalyst system for use in the polymerization of olefins. The catalyst system includes at least two different chiral, stereo-rigid metallocene catalysts of the formula R"(C.sub.5 (R').sub.4).sub.2 MeQ.sub.p (where Me is a Group 4b, 5b, or 6b metal and (C.sub.5 (R').sub.4) is a cyclopentadienyl or substituted cyclopentadienyl ring) and an aluminum compound.
Canadian Patent Application No. 2,000,567, published Apr. 13, 1990, discloses a process for producing polyethylene using a composite catalyst made up of a solid catalyst component typified by a selected chromium compound, a modified aluminum compound typified by a trialkylaluminum, and an alkylaluminum alkoxide compound. The chromium compound may be chromium oxide, and the modified aluminum compound may be the reaction product of an organoaluminum compound and water.
European Patent Application 0,509,294 A2 published Oct. 21, 1992 Bulletin 92.backslash.43 discloses a catalyst system for the homopolymerization and copolymerization of alpha-olefins having 2-8 carbon atoms. The catalyst system comprises a cyclopentadienyl Group 6b metal hydrocarbyl compound in which the metal has an oxidation state of +3, the Group 6b metal compound being supported on an inorganic support. Page four of the European Patent Application discloses dimer compounds having the formula: EQU [C.sub.5 (R').sub.5).sub.a MX.sub.b ].sub.c
wherein M is a Group 6b metal such as chromium, molybdenum and tungsten;
(C.sub.5 (R').sub.5) is a cyclopentadienyl or substituted cyclopentadienyl ring;
R' is at each independent occurrence hydrogen, a hydrocarbyl radical having 1-20 carbon atoms, or adjacent R' groups may together form one or more rings;
X is a hydrocarbyl radical having 1-20 carbon atoms (for example, a monovalent saturated aliphatic or alicyclic radical or a monovalent aromatic radical, or combinations thereof);
a=1 or 2, b=1 or 2 where a+b=3;
c=1 or 2 with the proviso that when c=2 then x is alkyl.
When c is 1, the catalyst is a monomer and X is defined as a hydrocarbyl radical having 1-20 carbon atoms.
When c is 2, the catalyst is a dimer and X is alkyl. On page 4, lines 21-22, the term "hydrocarbyl" refers to "alkyl, alkenyl, aryl, aralkyl and alkaryl radicals and the like." On page 5, lines 3 and 4 indicate [Cp*Cr(CH.sub.3).sub.2 ].sub.2 is the preferred dimeric compound. In all cases, the Group 6b metal atom in the compound has an oxidation state of +3.
U.S. Pat. No. 5,240,895 issued Aug. 31, 1993 to Michael J. Carney and David L. Beach discloses a catalyst system for the homopolymerization and co-polymerization of alpha-olefins having 2-8 carbon atoms. The catalyst system comprises a dimeric or tetrameric cyclopentadienyl Group 6b metal compound in which the metal has an oxidation state of +2, the Group 6b metal compound being supported on an inorganic support. In column 5, lines 10 et seq, examples of the Group 6b metal compounds are disclosed having the formula: EQU [(C.sub.5 (R').sub.5)MX].sub.a
wherein M is a Group 6b metal such as chromium, molybdenum and tungsten;
(C.sub.5 (R').sub.5) is a substituted cyclopentadienyl ring;
R' is at each independent occurrence hydrogen, a hydrocarbyl radical having 1-20 carbon atoms, or adjacent R' groups may together form one or more hydrocarbyl rings, with the proviso that at least one R' is alkyl;
a=2 or 4;
X is at each independent occurrence a hydrocarbyl radical having 1-20 carbon atoms (for example a monovalent saturated aliphatic or alicyclic radical or a monovalent aryl, alkaryl radical, or combinations thereof), or an organosilyl group, such as trimethylsilylmethyl, when a=2 or hydrogen when a=4.
Examples of compounds having formula (I) above include, but are not limited to, [Cp*Cr(CH.sub.3)].sub.2, [Cp*Cr(Bzyl)].sub.2, [Cp*Cr(Ph)].sub.2, [Cp*Cr(TMSM)].sub.2, where Bzyl is benzyl, Ph is phenyl, and TMSM is trimethylsilylmethyl.
It is also taught in the '895 patent, Column 5, line 1 et seq, that the strong chromium-chromium multiple bond present in [Cp*Cr(CH.sub.3)].sub.2 makes it virtually unreactive towards ethylene (reference is made to an article by Heintz, R. A. et al; Angew. Chem. (1992), 104, 1100). The invention in the '895 patent was the discovery that depositing [Cp*Cr(CH.sub.3)].sub.2 on a solid support generates a highly active, ethylene polymerization catalyst (See column 5, lines 5-7). In all cases the Group 6b metal compounds in the '895 reference have the metal in the +2 oxidation state.
Thus the prior an discloses the preparation of various Group 6b cyclopentadienyl type catalysts for the polymerizations of alpha-olefins and especially ethylene. The Group 6b metal, exemplified by chromium, is in all cases in the oxidation state of +2 or +3. It is not readily or reliably predictable whether a given chromium-cyclopentadienyl based catalyst system will be catalytically active for the polymerization of ethylene either in homogeneous type reactions or heterogeneous type reactions where the catalyst is deposited on an inorganic support. Further, it is also not readily or reliably predictable as to the nature of the polymer, if any, which will be produced, i.e., whether it will be of a low or high molecular weight or have a narrow or broad molecular weight distribution.
The type of ligands sigma bonded to the cyclopentadienyl-Cr are also important. The European Patent Application 0,509,294 A2, referred to above, teaches that the ligand ("X") is alkyl, i.e., CH.sub.3 when a dimeric compound is employed with the chromium in the +3 oxidation state while the teachings of U.S. Pat. No. 5,240,895 above indicate the ligand "X") is hydrocarbyl when the chromium is in the +2 oxidation state.