Cationic d.sup.o metal complexes of the formula L.sub.2 MCH.sub.3.sup.+, where L is a cyclopentadienyl-type ligand and M is a Group IVB metal (titanium, zirconium, or hafnium) have been implicated as the active species in metallocene-based Ziegler-Natta olefin polymerization catalyst systems of the type L.sub.2 MCl.sub.2 /Al.sub.n Cl.sub.3-n.
Interest in this type of catalyst resumed in the early 1980's and a variety of zirconocene complexes were found to form extremely active olefin polymerization catalysts when they were combined with methylaluminoxane (MAO). The activity increases with an increase of the Al/Zr ratio, while MAO itself cannot polymerize ethylene. The neutral compounds L.sub.2 MR (where L is as defined above, M is lutetium or scandium) and (L.sub.2 MH).sub.2 (where M is lanthanum, neodymium, samarium, or lutetium), which are isoelectronic with L.sub.2 MCH.sub.3.sup.+ (where M is titanium, zirconium, or hafnium), were also found to be highly active ethylene polymerization catalysts. On the other hand, the cationic titanocene complex could be trapped by the insertion of TMSC=CPh into the titanium-methyl bond. Unfortunately, such a cationic complex is catalytically inactive, possibly due to the bulkiness of the highly substituted vinyl ligand. Direct isolation of the cationic titanium methyl complex stabilized by conventional anions such as [BF.sub.4 ] or [PF.sub.6 ] can only be achieved from acetonitrile solutions resulting in the adducts of the formulae (L.sub.2 MCH.sub.3 (CH.sub.3 CN)).sup.+ (BF.sub.4) or (L.sub.2 MCH.sub.3 (CH.sub.3 CN)).sup.+ (PF.sub.6).sup.-, where M is zirconium or titanium and L is a cyclopentadienyl-type ligand. These are deactivated due to the ligand coordination of acetonitrile. Attempts to exchange acetonitrile with a weaker base, e.g., tetrahydrofuran ("THF"), produces L.sub.2 MFCH.sub.3 due to facile fluorine abstraction.
By using [BPh.sub.4 ].sup.- as the counterion, however, active cations can be obtained. The THF-coordinated cationic zirconocene complex of the formula (L.sub.2 ZrCH.sub.3 (THF)).sup.+ (B(C.sub.6 H.sub.5).sub.4).sup.- was isolated by the reaction of dicyclopentadienyl zirconium dimethyl with silver tetraphenylborate in acetonitrile followed by recrystallization from tetrahydrofuran. The NMR spectrum and the X-ray structure clearly show the coordination of the tetrahydrofuran. Tetrahydrofuran-coordinated cationic titanocene complexes of the formula (L.sub.2 TiCH.sub.3 (THF)).sup.+ (B(C.sub.6 H.sub.5).sub.4).sup.- were synthesized later. All of these complexes are weakly active ethylene polymerization catalysts (N.sub.t =7.2 min.sup.-1 and 1.9 min.sup.-1, respectively, at room temperature). It is worth noting that a similar reaction does not take place for analogous thorium complexes under mild conditions.
While the concept of an active cationic center was established, research interest was gradually passing from confirming the cationic nature of the catalysts to searching for more active catalysts. The effect of base-coordination on the activity of the cationic catalysts has been studied systematically, and it is commonly believed that the coordination of the Lewis base decreases the catalytic activity of the cationic species. Extensive efforts to isolate base-free catalysts were undertaken. The pivotal concept of such research is to develop an anion which is bulky, highly stable, and noncoordinating to the cationic catalytic center, which exhibits very strong Lewis acidity and very high reactivity. Attempts at the isolation of the base-free cationic zirconocene complex, (L.sub.2 ZrCH.sub.3).sup.+ (B(C.sub.6 H.sub.5).sub.4).sup.-, failed and produced instead the neutral compound, L.sub.2 ZrCH.sub.3 (C.sub.6 H.sub.5), by the ligand redistribution reaction between the anion and the cation. The Lewis base-free cationic thorium complex (L.sub.2 ThCH.sub.3).sup.+ (B(C.sub.6 H.sub.5).sub.4).sup.- was synthesized as the first example of the base free catalysts by quantitative protonolysis of the neutral thorium alkyl complex L.sub.2 Th(CH.sub.3).sub.2 with ((C.sub.2 H.sub.5).sub.3 NH).sup.+ B(C.sub.6 H.sub.5).sub.4).sup.- in noncoordinating solvents. However, the low temperature solution NMR data and room temperature solid state NMR data show that the cation and the anion interact strongly with each other as a tight ionic pair. This could be the reason for the modest activity of the catalyst in ethylene polymerization (N.sub.t =1 min.sup.-1 at room temperature). Protonolysis of dicyclopentadienyl zirconium dimethyl with ((C.sub.4 H.sub.9).sub.3 NH).sup.+ (B(C.sub.6 H.sub.4 R).sub. 4).sup.-, where R can be hydrogen, methyl, or ethyl, gives a zwitterion of the formula L.sub.2 Zr.sup.+ (m-C.sub.6 H.sub.3 R)B(C.sub.6 H.sub.4 R).sub.3.sup.-. The activity of this catalyst for ethylene polymerization is 3.75.times.10.sup.5 gPe/mol.atm.h. (N.sub.t =3.75s.sup.-1) at 80.degree. C. The X-ray structure shows an agostic interaction between the cationic zirconium center and the C-H bond ortho to the boron.
The other type of noncoordinating anions that have been used are carborane-derived. Dicyclopentadienyl zirconium dimethyl reacts with the diprotic carborane complex nido-C.sub.6 B.sub.9 H.sub.13, giving off methane and resulting in (L.sub.2 ZrCH.sub.3).sup.+ (C.sub.2 B.sub.9 H.sub.12).sup.- as an active ethylene polymerization catalyst (N.sub.t =2.7s.sup.-1 at 40.degree. C.). The X-ray structure shows that the cationic zirconium center is bound to the anion through a terminal hydride on the nido face of the anion. Metallocarboranes of the formula (M(C.sub.2 B.sub.9 H.sub.11).sub.2).sup.-,where M is iron, cobalt, or nickel are also stable anions for the zirconocene and thorocene cations. For example, (L.sub.2 ZrCH.sub.3).sup.+ (M(C.sub.2 B.sub.9 H.sub.11).sub.2).sup.-, where M can be one of the aforementioned metals, has an ethylene polymerization activity similar to that of (L.sub.2 ZrCH.sub.3).sup.+ (C.sub.2 B.sub.9 H.sub.11).sup.-, whereas similar thorium complexes, e.g., (L.sub.2 ThCH.sub.3).sup.+ (Fe(C.sub.2 B.sub.9 H.sub.11)).sub.2.sup.-, are completely inactive for catalyzing ethylene polymerization due to strong Th-H-B interactions.
In general, both the [B(C.sub.6 H.sub.4 R).sub.4 ].sup.- type anions and the carborane anions exhibit a significant degree of coordination to the cations, and this suppresses the catalytic activity.
The advantage for the use of perfluorinated tetraphenyl borate as the counteranion is that it can made the catalysts more stable, and meanwhile largely overcome strong anioncation interactions. This makes the olefin polymerization activity of the homogeneous characterizable metallocene catalysts comparable to the IVB transition metal complex/MAO catalyst systems and the heterogeneous catalyst systems. The difference in activity between the cations with the fluorinated anions and cations with the nonfluorinated anions is dramatic. For example, (Me.sub.5 C.sub.5).sub.2 ThCH.sub.3.sup.+ B(C.sub.6 F.sub.5).sub.4.sup.- has an ethylene polymerization activity 3300 times higher than of (Me.sub.5 C.sub.5).sub.2 ThCH.sub.3.sup.+ B(C.sub.6 H.sub.5).sub.4.sup.-. An .sup.19 F NMR spectroscopic study shows that the four phenyl groups on the borane are equivalent at temperatures as low as -70.degree. C. in toluene-d.sub.8 while the X-ray structure shows a weak coordinative/interaction between the cation and m- and o-fluorine atoms from one of the four phenyl groups. In the case of zirconium, the difference in activity is around several hundred times.
A wholly different and convenient way to generate the cationic species has also been reported. The neutral tris(pentafluorophenyl)borane can directly abstract a methide or hydride ligand from the neutral zirconocene complexes and thereby generate a cation and an anion. The NMR spectrum of (L.sub.2 ZrCH.sub.3).sup.+ (CH.sub.3 B(C.sub.6 H.sub.5).sub.3.sup.-, where L in this case is 1,3-dimethylcyclopentadienyl shows two signals for the methyl groups on the cyclopentadienyl rings in toluene-d.sub.8, indicating that the methyl groups are diastereotopic and the compound is a tight ion pair. The X-ray structure shows a weak linkage between the anion and cation through the abstracted methyl. This reaction also serves as a model for the generation of the cationic active centers in the heterogeneous system. The ethylene polymerization activity is 4.5.times.10.sup.6 gPE/mol.atm.h. (Nt=45s.sup.-1) at 25.degree. C.
To summarize, base-free group IVB or thorium cationic metallocene species are the active centers for olefin polymerization. The more open, both sterically and coordinatively, the more active the cation is. And the stability and noncoordinativity of anions are the key to keeping the cations stable and open.