Polymerization-stable anionic ancillary ligand containing alkylimido catalysts and method of using the same

Abstract

Single-site catalyst systems containing alkylimido ligands are disclosed. The catalyst systems comprise a co-catalyst and a Group 3-10 transition metal catalyst. The catalyst includes an alkyl- or arylimido ligand and a polymerization-stable, anionic ligand selected from boraaryl, pyrrolyl, and azaborolinyl. When used to polymerize olefins, the catalyst systems are highly productive, give good comonomer incorporation, and give polymers with narrow molecular weight distributions.

Description

FIELD OF THE INVENTION

This invention relates to a novel metallocene catalyst system containing an alkylimido catalyst. In a preferred embodiment, the catalyst contains at least one polymerization-stable anionic ancillary ligand. The invention further relates to a method of preparation of the catalyst and a method of using the same.

BACKGROUND OF THE INVENTION

Historically, polyolefins have been made with conventional Ziegler catalyst systems. Such catalysts typically consist of transition metal-containing compounds and one or more organometallic compounds. For example, polyethylene has been made using such Ziegler catalysts as titanium trichloride and diethylaluminum chloride, as well as a mixture of titanium tetrachloride, vanadium oxytrichloride, and triethylaluminum.

While these catalysts are inexpensive, they exhibit low activity and therefore must be used at high concentrations. As a result, it is sometimes necessary to remove catalyst residues from the polymer, which adds to production costs. Neutralizing agents and stabilizers must be added to the polymer to overcome the deleterious effects of the catalyst residues. Failure to remove catalyst residues leads to polymers having a yellow or grey color and poor ultraviolet and long term stability. Additionally, for example, chloride-containing residues can cause corrosion in polymer processing equipment.

Furthermore, Ziegler catalysts produce polymers having a broad molecular weight distribution which is undesirable for some applications such as injection molding. They are also poor at incorporating .alpha.-olefin co-monomers. Poor co-monomer incorporation makes it difficult to control the polymer density. Large quantities of excess co-monomer may be required to achieve a certain density and many higher .alpha.-olefins, such as 1-octene, may be incorporated at only very low levels, if at all.

Although significant improvements in Ziegler catalyst systems have occurred since their initial discovery, they lately have been substantially replaced with "single-site," in particular, metallocene, catalyst systems. A traditional metallocene catalyst typically consists of a transition metal compound which has one or more cyclopentadienyl ring ligands bound in an .eta..sup.5 fashion. The cyclopentadienyl ring ligands are polymerization-stable; that is, they remain bound to the metal during the course of the polymerization process. They produce polymers of high molecular weight and display narrow molecular weight distributions, because the cyclopentadienyl ligands deter formation of secondary polymerizing species. These catalysts also incorporate .alpha.-olefin co-monomers well. However, at higher temperatures traditional metallocene catalysts tend to produce lower molecular weight polymers. They are particularly useful for gas phase and slurry polymerizations of ethylene, which are conducted at about 80.degree. C. to about 95.degree. C., but are less useful in solution polymerizations of ethylene, at about 150.degree. C. to about 250.degree. C. Additionally, gas phase and slurry polymerizations using supported metallocene catalysts can suffer from sheeting and equipment fouling problems.

Recently, catalysts have been discovered wherein one or more of the cyclopentadienyl ring ligands associated with the traditional metallocene have been replaced by other polymerization-stable anionic ancillary ligands. These may be ligands which are isolobal to cyclopentadienyl; that is, the frontier molecular orbitals--the highest occupied and lowest unoccupied molecular orbitals--of the ligand and those of the cyclopentadienyl ligand are similar. These isolobal ligands may include tris(pyrazolyl)borates, pentadienyl groups, phospholes, and carbollides.

In particular, U.S. Pat. No. 5,554,775, incorporated herein by reference, discloses catalysts wherein one or both cyclopentadienyl moieties are replaced by a borabenzene moiety including boranaphthalene and boraphenanthrene. Further, U.S. Pat. No. 5,539,124, incorporated herein by reference, discloses catalysts in which one or both cyclopentadienyl moieties have been replaced by a nitrogen-containing heteroaromatic compound containing a pyrrolyl ring, i.e., an azametallocene, variously substituted. The heteroaromatics disclosed in the latter patent include, e.g., indolyl, isoindolyl, and carbazolyl, and other homologous heteroaromatic moieties. The foregoing heteroaromatic catalysts may be referred to generally as heterometallocenes. In addition, PCT International Application WO 96/34021 discloses azaborolinyl heterometallocenes wherein at least one aromatic ring is complexed with a transition metal. Such rings include both a boron atom and a nitrogen atom. These specifically will be referred to as, e.g., azaborolines and the catalysts derived therefrom as azaborolinyl catalysts. The latter catalysts also fall into the general group referred to as heterometallocenes. The foregoing metallocene and heterometallocene catalysts have been developed to include bulky ligands attached to the aromatic moieties. Increased control of the polymerization process may therefore be provided.

Because supported catalysts are more stable, may produce higher molecular weight polymers, and may produce useful changes in the morphology of the polymer, metallocene catalysts are often used in conjunction with a support, such as silica gel.

For the purposes of the present disclosure, it is to be understood that when the term "metallocene" is used, both traditional metallocenes and heterometallocenes such as those disclosed in the above referenced U.S. patents and applications, including those containing bulky ligands, are contemplated to fall within the scope of the term. Thus, "metallocene" is considered to be a generic term for all such transition metal-bonded aromatic organic polymerization catalysts. Likewise, it is to be understood that when the term "single-site" catalyst is used, both metallocenes as well as other metal complexes containing polymerization-stable ancillary ligands are contemplated to fall within the scope of the term.

SUMMARY OF THE INVENTION

The invention relates to a novel catalyst system comprising a catalyst containing an alkylimido group and a Group 3-10 metal. The alkylimido group is attached to the metal, M. This group presumably stabilizes the active metal center to achieve high productivity, good comonomer incorporation, and narrow molecular weight distribution characteristic of the single site catalyst.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A novel metallocene catalyst for the polymerization of olefin homopolymers and co-polymers is of the general formula:

Ak--N=MX.sub.m Q.sub.n Ak is a C.sub.1 -C.sub.12 alkyl or a C.sub.7 -C.sub.18 aralkyl group optionally substituted with a C.sub.1 -C.sub.12 haloalkyl, halogen, or a C.sub.1 -C.sub.12 alkoxy group; M is a transition metal of Groups 3-10, preferably Groups 3-7, more preferably Groups 4-6, and most preferably Groups 4-5, of the Periodic Table; Q is a polymerization-stable anionic ancillary ligand; X is a halogen (preferably --Cl or --Br), alkoxy from C.sub.1 to C.sub.20, siloxy from C.sub.1 to C.sub.20, --N(R.sub.1).sub.2 , a hydrocarbyl group containing up to about 12 carbon atoms, hydrogen or another univalent anionic ligand, or mixtures thereof; preferably X is chloride, methyl, benzyl, methoxy, ethoxy, dialkyl amido, N(R.sub.1).sub.2 or siloxy, (R.sub.1).sub.3 SiO--, where R.sub.1 is alkyl from C.sub.1 to C.sub.12, preferably C.sub.1 -C.sub.6 alkyl group; and m+n equals the valency of the M minus two, provided m+n.gtoreq.1, and wherein Q and Ak of the alkylimido group may be bridged.

While the metal, M may be any Group 3 to 10 metal or a metal from the lanthanide or actinide series, in a preferred embodiment, the catalyst contains a Group 4, 5, or 6 transition metal. In a particularly preferred embodiment, the catalyst contains a Group 4 metal, particularly zirconium, titanium or hafnium.

Suitable polymerization stable ancillary ligands for Q are the cyclopentadienyl, boraaryl, pyrrolyl, or azaborolinyl or homologous ligands. Such polymerization stable ancillary ligands include the mono-, bi-, or tri- cyclopentadienyl or substituted cyclopentadienyl radicals, especially those of the formulae:

(C.sub.5 R.sup.1.sub.w).sub.f R.sup.2.sub.s (C.sub.5 R.sup.1.sub.w)(I) and R.sup.2.sub.s (C.sub.5 R.sup.1.sub.w).sub.2 (II) wherein (C.sub.5 R.sup.1.sub.w) is a cyclopentadienyl or substituted cyclopentadienyl; each R.sup.1 is the same or different and is hydrogen or a hydrocarbyl radical such as alkyl, alkenyl, aryl, alkaryl or aralkyl radical containing from 1 to 20 carbon atoms of which two carbon atoms may be joined together to form a C.sub.4 -C.sub.6 ring; R.sup.2 is a C.sub.1 -C.sub.20 alkylene radical, a dialkyl germanium or silicon [such as silyl or a radical of the formula --SiR.sub.5 wherein each R.sub.5 is H, a C.sub.1 -C.sub.10 (preferably a C.sub.1 -C.sub.4) alkyl group, an aryl such as benzyl or phenyl or a benzyl or phenyl group substituted with one or more C.sub.1 -C.sub.4 alkyl groups] or an alkyl phosphine or amine radical bridging two (C.sub.5 R.sup.1.sub.w) rings; s is 0 or 1; f is 0, 1 or 2 provided that when f is 0, s is 0; w is 4 when s is 1; and w is 5 when s is 0.

Particularly good results are obtained where the cylcopentadienyl ring is of the structure: ##STR1## where each substituent group, R.sub.2, is independently selected from a C.sub.1 to C.sub.20 hydrocarbyl group and r is a number from 0 to 5. In the case in which two R.sub.2 groups are adjacent, they can be joined to produce a ring which is fused to the cyclopentadienyl ring. Examples of alkyl substituted cyclopentadienyl rings include n-butylcyclopentadienyl, methylcyclopentadienyl and pentamethylcyclopentadienyl. Examples of fused cyclopentadienyl ring ligands include indenyl, tetrahydroindenyl, fluorenyl and 2-methylindenyl.

The ligand Q for use in the olefin polymerization catalyst for the invention may further contain 4 to 30 carbon atoms and may contain a fused ring, one of which is a pyrrolyl ring. Included within this group are heterocyclic radicals of the formula: ##STR2## wherein, R' is independently hydrogen or R.sup.80 or with F forms a C.sub.4 to C.sub.10 fused ring;

each R.sup.80 is independently selected from hydrogen, a C.sub.1 to C.sub.20, preferably a C.sub.1 to C.sub.6, aliphatic or cycloaliphatic radical; a C.sub.6 -C.sub.30, preferably a C.sub.6 -C.sub.15, aryl radical, or a C.sub.7 -C.sub.30, preferably a C.sub.7 -C.sub.15, aralkyl or alkaryl radical; E independently represents a trivalent atom selected from nitrogen, phosphorus, arsenic, antimony and bismuth; and F is independently selected from carbon and E.

Exemplary compounds include those wherein R' is --H or a C.sub.1 to C.sub.6 alkyl group or C.sub.6 to C.sub.10 aryl group. Preferred compounds include 2-methylpyrrolyl, 3-methylpyrrolyl, 2,5-dimethylpyrrolyl, 2,5-di-tert-butylpyrrolyl, aryl substituted pyrrolyl rings such as 2-phenylpyrrolyl, 2,5-diphenylpyrrolyl, indolyl and alkyl substituted indolyis of the formula: ##STR3## such as 2-methylindolyl, 2-tert-butylindolyl, 3-n-butylindolyl, 7-methylindolyl, and 4, 7-dimethylindolyl and carbazolyl and alkyl substituted carbazolyls of the formula: ##STR4## where m is 0 to 6 in formula V and 0 to 8 in formula VI and R' is defined as above. Note that the alkyl and aryl substituents on the pyrrolyl ring-containing ligand are not on the nitrogen atom in the ring but are on the carbon atoms of the ring.

Additional examples of ring structures include:

1-Phospha-2,3,4,5-tetramethylcyclopentadienyl, 1-Phospha-3,4-diphenylcyclopentadienyl, 1-Phospha-3,4-dimethylcyclopentadienyl, 1-Phosphaindenyl, 1-Phospha-3-methoxycyclopentadienyl, 1,3-Diphospha-4,5-diphenylcyclopentadienyl, 1,2,4-Triphospha-3,5-diphenylcyclopentadienyl, 1,2,3,4-Tetraphospha-5-phenylcyclopentadienyl, Pentaphosphacyclopentadienyl, 1-Phospha-3-benzoyloxycyclopentadienyl, Imidazolyl, Pyrazolyl, 1,2,3-triazolyl, 1,2,4-triazolyl, Tetrazolyl, and Pentazolyl.

Still further, the ligand Q may be of the formula: ##STR5## wherein R.sub.10 is independently selected from R.sub.25 alkaryl from C.sub.6 to C.sub.12 aralkyl from C.sub.6 to C.sub.12 hydrogen, or Si(R.sub.25).sub.3, R.sub.25 is alkyl from C.sub.1 to C.sub.12, or aryl from C.sub.6 to C.sub.12, R.sub.20 is R.sub.10 or COR.sub.25, R.sub.30 is R.sub.20, OR.sub.25, N(R.sub.25).sub.2, SR.sub.25, or a fused ring system and n is 0 to 3.

The R.sub.25 group is preferably alkyl from C.sub.1 to C.sub.4, the R.sub.10 group is preferably C.sub.1 to C.sub.6 alkyl or Si(R.sub.25).sub.3 and the R.sub.30 group is preferably hydrogen or methyl. Examples of fused ring structures that can be used include: ##STR6##

The Q ligand may further be a boratabenzene ligand. A boratabenzene ring has the structure: ##STR7## where R.sub.40 can be hydrogen, N(R.sub.50).sub.2, OR.sub.50, or R.sub.50, where each R.sub.50 is independently selected from alkyl from C.sub.1 to C.sub.10, aryl from C.sub.6 to C.sub.15, alkaryl from C.sub.7 to C.sub.15, and aralkyl from C.sub.7 to C.sub.5. The R.sub.40 group is preferably --N(R.sub.50).sub.2, methyl or phenyl and, if R.sub.40 is --N(R.sub.50).sub.2, then the R.sub.50 in --N(R.sub.50).sub.2 is preferably methyl.

Exemplary of the boratabenzene ligands include: ##STR8## where "p" is 0 to the maximum number of substitutable positions, and is preferably 0 or 1. Each R.sub.60 is independently selected from halogen, alkoxy from C.sub.1 to C.sub.10, silyl (--Si(R.sub.50).sub.3 and R.sub.50. Particularly preferred boratabenzene ligands are 1-methyl boratabenzene, 2-phenyl-2-boratanaphthalene, 9-mesityl-9-boraanthracene, and 1-methyl-2-trimethyl silyl-boratabenzene.

The catalysts of this invention can be prepared by a variety of different methods. It is possible to react an alkylamine directly with certain transition metal complexes of Q. For example, contact of a certain cyclopentadienyl metal halide or borabenzene metal halide complex with the alkyl amine will liberate HCl and form the alkylimido complex directly. This is the case with niobium.

In the case of other transition metal-Q complexes, a metal-nitrogen single bond is easily prepared, but a good leaving group is needed in order to produce the metal-nitrogen double bond of the alkylimido group. So, for example, Cp*TiCl.sub.3 reacts with .sup.t Bu--N--Sn(CH.sub.3).sub.3 to produce: ##STR9## which contains a Ti--N single bond. The double bond can be formed by treating this with a strong base to remove the hydrogen and chlorine as HCl, which separates from the reaction mixture as a salt.

These reactions can occur at room temperature, but low temperatures (-100.degree. C. to 0.degree. C.) are preferred in order to reduce unwanted by-products. Stoichiometric quantities are typically used. The by-products are generally removed by filtration, the solvent evaporated, and the metal ligand catalyst collected.

Groups that can be used to bridge two ligands include methylene, ethylene, 1,2-phenylene, dimethyl silyl, diphenyl silyl, diethyl silyl, and methyl phenyl silyl. Normally, only a single bridge is used in a catalyst. It is believed that bridging the ligand changes the geometry around the catalytically active transition metal and improves catalyst activity and other properties, such as comonomer incorporation and thermal stability.

The resins produced with the catalysts of the invention have a narrow molecular weight distribution and typically exhibit a ratio of MI.sub.20 vs. MI.sub.2 between about 10 to about 25. The catalysts of the invention are characterized as being single site catalysts.

Since the catalyst of the invention is normally used in conjunction with a co-catalyst, it is preferable to dissolve the organometallic compound in a solvent in which the co-catalyst is also soluble. For example, if methylalumoxane (MAO) is the co-catalyst, then toluene, xylene, benzene, or ethyl benzene could be used as the solvent.

Representative co-catalysts for use in the invention include alumoxanes and aluminum alkyls of the formula Al(R.sup.7).sub.3 where R.sup.7 independently denotes a C.sub.1 -C.sub.8 alkyl group, hydrogen or halogen. Exemplary of the latter of such co-catalysts arc triethylaluminum, trimethylaluminum and triisobutylaluminum. The alumoxanes are polymeric aluminum compounds typically represented by the cyclic formulae (R.sup.8 --Al--O).sub.s and the linear formula R.sup.8 (R.sup.8 --Al--O).sub.s AlR.sup.8 wherein R.sup.8 is a C.sub.1 -C.sub.5 alkyl group such as methyl, ethyl, propyl, butyl and pentyl and s is an integer from 1 to about 20. Preferably, R.sup.8 is methyl and s is about 4 to about 10. Representative but non-exhaustive examples of alumoxane co-catalysts are (poly)methylalumoxane (MAO), ethylalumoxane and diisobutylalumoxane.

The preferred co-catalyst is MAO as it results in high catalyst activity, good comonomer incorporation, and a polymer having a narrower molecular weight distribution. It is preferable not to premix the catalyst and the co-catalyst as this may result in lower catalyst activity. Rather, the catalyst and co-catalyst are preferably injected separately into a reactor containing the monomer to be polymerized. And, preferably, the co-catalyst is injected first. The amount of cocatalyst used with the transition metal compound can be in a molar ratio ranging from about 1:1 to about 15,000:1.

The co-catalyst can further be a substituted or unsubstituted trialkyl or triaryl boron derivative, such as tris(perfluorophenyl)boron as well as ionic compounds such as tri(n-butyl)ammonium tetrakis (pentafluorophenyl) boron or trityl tetrakis(perfluorophenyl)boron which ionize the neutral metallocene compound. Such ionizing compounds may contain either an active proton, or a cation which is associated with, but not coordinated or only loosely coordinated to, the remaining ion of the ionizing compound. See, for instance, U.S. Pat. Nos. 5,153,157; 5,198,401; and 5,241,025, all of which are herein incorporated by reference.

The catalyst and co-catalyst can also be used on a support such as silica gel, alumina, magnesia, or titania. Supports are not generally preferred as they leave additional contaminants in the polymer. However, a support may be required depending upon the process being utilized. For example, a support is generally needed in gas phase polymerization processes and slurry polymerization processes in order to control the particle size of the polymer being produced and in order to prevent fouling of the reactor walls. In order to use a support, the catalyst is dissolved in a solvent and is deposited onto the support material by evaporating the solvent. The cocatalyst can also be deposited on the support or it can be introduced into the reactor separately from the supported catalyst.

Once the catalyst has been prepared it should be used as promptly as possible as it may lose some activity during storage. Storage of the catalyst should be at a low temperature, such as -100.degree. C. to 20.degree. C. The catalyst is used in a conventional manner in the polymerization of unsaturated olefinic monomers.

The catalyst is also useful for copolymerizing mixtures of ethylene with unsaturated monomers such as 1-butene, 1-hexene, 1-octene, and the like; mixtures of ethylene and di-olefins such as 1,3-butadiene, 1,4-hexadiene, 1,5-hexadiene, and the like; and mixtures of ethylene and unsaturated comonomers such as norbornadiene, ethylidene norbornene, vinyl norbornene, and the like.

While unsaturated monomers such as styrene can be polymerized using the catalysts of this invention, it is particularly useful for polymerizing .alpha.-olefins such as propylene, 1-butene, 1-hexene, 1-octene, and especially ethylene.

The catalysts of this invention can be utilized in a variety of different polymerization processes. They can be utilized in a liquid phase polymerization process (slurry, solution, suspension, bulk phase, or a combination of these), in a high pressure fluid phase, or in a gas phase polymerization process. The processes can be used in series or as individual single processes. The pressure in the polymerization reaction zones can range from about 15 psia to about 50,000 psia and the temperature can range from about -100.degree. C. to about 300.degree. C.

EXAMPLES

Example 1

This example describes the synthesis of (pentamethylcyclopentadienyl) t-butyl imidotitanium chloride.

A solution of pentamethylcyclopentadienyl titanium trichloride (1.14 g. 3.94 mmol) in 22 ml dry tetrahydrofuran (THF) in a 100 ml Schlenk flask was cooled to 0.degree. C. and treated dropwise with a solution of 0.92 g (3.94 mmol) of (N-tert-butylamido)trimethylstannane (prepared independently by reacting trimethyltin chloride with lithium tert-butylamide in hexane) in 7 ml THF. The ice bath was then removed and the solution was stirred for 24 hours at room temperature. The solvent was removed by pumping under vacuum. Trimethyltin chloride by-product was removed under vacuum by heating the reaction mixture in a 40.degree. C. water bath for 1 hour leaving the desired precursor, (N-tert-butylamido)pentamethylcyclopentadienyltitanium dichloride in the Schlenk flask as a red solid.

An oven-dried 5 ml Schlenk flask was evacuated, purged with argon, and charged with 0.9 mol (0.689 g, 4.27 mmol) of bis(trimethylsilyl)amine followed by 5 ml of dry ether. The solution was cooled to 0.degree. C. with an ice bath. To this solution was added n-butyllithium in hexane (2.4 ml of 1.6M, 3.84 mmol) dropwise. A white suspension was obtained. The ice bath was removed and the mixture was refluxed for 0.5 hours. After cooling to room temperature, the solvents were pumped off under vacuum to give a white crystalline solid of lithium hexamethyldisilazide (LiHMDS). To the solid was added 40 ml of dry toluene to obtain a clear colorless solution.

To the red solid precursor obtained above was added a solution of 5.2 ml dry pyridine dissolved in 40 ml of toluene. A clear red solution resulted. To this solution with magnetic stirring was added the toluene solution of LiHMDS obtained above. The reaction mixture turned dark red with formation of solids. After being stirred overnight (15 hours), the solution was filtered and the solvent was evaporated. To the residue, 7 ml hexane was added and the dark supernatant solution was filtered off. The solids on the frit were rinsed with 5 ml hexane and vacuum-dried to give 0.42 g (29%) of the desired final product. The purity of the material was verified by its .sup.1 H NMR spectrum in C.sub.6 D.sub.6.

Example 2

This example describes the synthesis of (cyclopentadienyl) t-butyl imidoniobium dichloride.

A solution of 0.7 g (9.5 mmol) of tert-butylamine dissolved in 20 ml dry dichloromethane was added slowly over a period of 1.5 hours to a cooled (-40.degree. C. bath) suspension of cyclopentadienylniobium tetrachloride (0.93 g, 3.1 mmol) in 20 ml dichloromethane. A clear red solution resulted. After allowing the bath to warm by itself to room temperature, the reaction mixture was filtered over Celite (pre-dried at 200.degree. C. and stored under argon). The filtrate was concentrated to afford a yellow powder (0.5 g, 54%) whose .sup.1 H NMR spectrum in C.sub.6 D.sub.6 showed it to be pure.

Examples 3-10

These examples demonstrate the polymerization of an olefin in the presence of either of the catalysts of Example 1 or Example 2.

Ethylene was polymerized using the catalysts of Examples 1 and 2. Examples 3 through 9 used the catalyst prepared in Example 1. Example 10 used the catalyst prepared in Example 2. The polymerizations were conducted in a stirred 1.7 liter stainless steel autoclave at 80.degree. C. and 110.degree. C. Dry oxygen-free toluene (840 ml) was charged to the dry oxygen-free reactor. 10% methyl alumoxane in toluene (from Ethyl Corporation) is typically added by syringe without further purification. A solution of catalyst was prepared by dissolving 0.100 grams of product in 100 ml of toluene. The reactor was then heated to the desired temperature and sufficient ethylene was added to bring the reactor pressure to 150 psig. The reactor was allowed to equilibrate at the desired temperature and pressure. The desired amount of catalyst solution was then added.

At the end of one hour the ethylene flow was stopped and the reactor was rapidly cooled to room temperature. The polymer was filtered from the toluene, dried in a vacuum oven, and weighed. In the following, Table 1 lists polymerization conditions and Table 2 lists the results of the polymerizations.

The melt index of the polymer was measured according to ASTM D-1238, Condition E and F. MI is the melt index measured with a 2.16 Kg weight (Condition E). HLMI is the melt index measured with a 21.6 kg weight (Condition F). The melt flow ratio (MFR) is defined as the ratio of HLMI (or MI.sub.20) of MI (or MI.sub.2) and is a measure of molecular weight distribution. A MFR below 25 indicates narrow molecular weight distribution and is likely to demonstrate improved properties characteristics of a single site catalyst (or an metallocene). Typically a Ziegler-Natta catalyst yields polymer with a MFR of around 30 or greater.

TABLE 1__________________________________________________________________________Polymerization Conditions Reactor Hydrogen Temp. Time, delta P, Catalyst Co- Al/M Example (.degree. C.) Min psi Comonomer mmoles catalyst (molar)__________________________________________________________________________3 80 60 0 None 0.0084 MAO 1607 4 80 60 20 None 0.0084 MAO 1607 5 80 60 0 Butene, 20 0.0084 MAO 1607 ml 6 110 60 0 None 0.0084 MAO 1607 7 110 60 0 None 0.0084 MAO 1607 8 110 60 10 None 0.0084 MAO 1607 9 110 60 0 Butene, 20 0.0084 MAO 1607 ml 10 80 60 10 None 0.005 MAO 2000__________________________________________________________________________ TABLE 2______________________________________Polymerization Results Catalyst Wt. PE Activity MI.sub.2 MI.sub.20 Example (g) kg/g M/hr dg/min dg/min MFR______________________________________3 65.9 164 0.11 1.47 13.4 4 52.8 144. 0.083 1.67 20.1 5 44.0 109 0.154 2.11 13.7 6 10.9 27 1.48 10.76 7.27 7 48.4 120 0.83 13.02 15.7 8 37.7 94 1.89 36.52 19.3 9 36.0 89 1.61 23.63 14.7 10 3.3 7.1 -- -- --______________________________________

Claims

1. A catalyst system which comprises:

(a) a co-catalyst; and

(b) a catalyst which comprises:

(i) a Group 3 to 10 transition metal;

(ii) an alkyl- or arylimido ligand;

(iii) a polymerization-stable, anionic ligand selected from the group consisting of boraaryl, pyrrolyl, and azaborolinyl; and

(iv) optionally, a ligand selected from the group consisting of halide, C.sub.1 -C.sub.20 alkoxy, C.sub.1 -C.sub.20 siloxy, dialkylamido, C.sub.1 -C.sub.12 hydrocarbyl, and hydride.

2. The catalyst system of claim 1 wherein the catalyst includes a Group 4 to Group 6 transition metal.

3. The catalyst system of claim 1 wherein the co-catalyst is an alumoxane.

4. The catalyst system of claim 1 wherein the co-catalyst is a triaryl or trialkylboron compound or an ionic borate.

5. A supported catalyst system of claim 1.

6. A method which comprises polymerizing ethylene or a mixture of ethylene and an .alpha.-olefin in the presence of the catalyst system of claim 1.