Supported metallocene-alumoxane catalysts for the preparation of polyethylene having a broad monomodal molecular weight distribution

The present invention relates to the production of high density polyethylene homopolymers or copolymers having a broad and monomodal molecular weight distribution wherein the polymerization process is conducted in the presence of supported metallocene-alumoxane catalysts wherein the metallocene is bridged, comprises at least a hydrogenated indenyl or fluorenyl and a metal M which may be Ti, Zr or Hf, wherein a plurality of conformers of the metallocene are formed and isolated on the support by reaction of the metallocene with the alumoxane and depositing the product formed on the support at a temperature in the range 85.degree. C. to 110.degree. C.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to new supported metallocene-alumoxane 
catalysts. More particularly, the present invention relates to the 
production of polyolefins, particularly of high density polyethylene 
homopolymers or copolymers, having a broad monomodal molecular weight 
distribution wherein the polymerization process is conducted in the 
presence of the new supported metallocene-alumoxane catalysts. 
2. Description of the Prior Art 
For polyolefins in general and high density polyethylene in particular, 
hereinafter referred to as polyethylene, the molecular weight distribution 
(MWD) is one of the basic properties that determines the properties of the 
polymer, and thus its end-uses. 
Although it may be difficult to evaluate the influence of each property 
taken independently, it is generally accepted that the molecular weight 
mostly determines the mechanical properties while the molecular weight 
dispersion mostly determines the rheological properties. 
There is a demand for high molecular weight polyethylene, because an 
increase of the molecular weight normally improves the physical properties 
of the resins However, high molecular weights tend to make polymers harder 
to process. On the other hand, an increase in the MWD tends to improve the 
flowability at high shear rate during the processing. Thus, broadening the 
MWD is one way to improve the processing of high molecular weight (=low 
melt flow index) polyethylene, in applications requiring fast processing 
at fairly high die swell, such as in blowing and extrusion techniques. 
It is generally believed that, in polyethylene having a high molecular 
weight combined with a broad MWD, the lower molecular weight portion aids 
in processing while the higher molecular weight portion contributes to the 
good impact resistance of the film, such polyethylene being processed at 
higher throughput rates with lower energy requirements. 
The MWD may be described completely by the curve obtained by gel permeation 
chromatography. The MWD is generally described by a figure which is a good 
evaluation, also called the polydispersity index, representing the ratio 
of the weight average to the number average molecular weight. 
There are several known methods of producing polyethylene having a broad 
and multimodal MWD; however, each method has its own disadvantages. 
Polyethylene having a multimodal MWD can be made by employing two distinct 
and separate catalysts in the same reactor each producing a polyethylene 
having a different MWD; however, catalyst feed rate is difficult to 
control and the polymer particles produced are not uniform in size and 
density, thus, segregation of the polymer during storage and transfer can 
produce non-homogeneous products. A polyethylene having a bimodal MWD can 
also be made by sequential polymerization in two separate reactors or 
blending polymers of different MWD during processing; however, both of 
these methods increase capital cost. 
European Patent No 0128045 discloses a method of producing polyethylene 
having a broad molecular weight distribution and/or a multimodal MWD. The 
polyethylenes are obtained directly from a single polymerization process 
in the presence of a catalyst system comprising two or more metallocenes 
each having different propagation and termination rate constants, and 
aluminoxane. 
It is interesting to note that the known methods of preparing broad 
molecular weight distribution polyolefins show a bimodal or multimodal 
MWD. Indeed, the gel permeation chromatograph curves show a more or less 
marked bimodal or multimodal MWD of the polyolefin. The MWD and shear rate 
ratios of the polymer and the catalyst activity disclosed in the known 
methods are rather low. Further the known metallocene catalyst systems for 
producing broad MWD use aluminoxane as cocatalyst during the 
polymerization which is not suitable for the slurry, bulk and gas phase 
processes and which causes severe fouling inside the reactor and renders 
the use of such a type of catalyst in continuous processes almost 
impossible. 
SUMMARY OF THE INVENTION 
The Applicants have unexpectedly found that it was possible to solve all 
these prior art problems. It is indeed an object of the present invention 
to provide a process for the polymerization of olefins, preferably for the 
homopolymerization or copolymerization of ethylene to form ethylene 
homopolymers or copolymers, having a broad molecular weight distribution 
with good processability, good physical properties and diverse 
applicability. 
In accordance with the present invention, there is provided a supported 
metallocene-alumoxane catalyst for use in the preparation of polyolefins, 
preferably ethylene homopolymers and copolymers, having at the same time a 
broad and monomodal molecular weight distribution wherein the metallocene 
consists of a particular bridged meso or racemic stereoisomers, preferably 
the racemic stereoisomers. 
In accordance with the present invention, polyethylene having a broad 
monomodal molecular weight distribution is prepared by contacting in a 
reaction mixture under polymerization conditions ethylene and a catalyst 
system comprising a supported metallocene-alumoxane catalyst characterized 
in that the metallocene consists of a particular bridged meso or racemic 
stereoisomer, preferably the racemic stereoisomers.

DETAILED DESCRIPTION OF THE INVENTION 
The metallocenes used in the process of the present invention can be any of 
those known in the art as suitable for the (co)polymerization of olefins 
with the proviso that the metallocene is bridged, that it comprises at 
least a hydrogenated indenyl or fluorenyl and that it is isolated on its 
support under the form of all its conformers. 
The preferred bridged metallocenes of the present invention can be selected 
from hydrogenated bisindenyl compounds having the following formula: 
EQU (IndH.sub.4).sub.2 R"MQ.sub.2 
wherein Ind is an indenyl or a substituted indenyl, R" is a C.sub.1 
-C.sub.4 alkylene radical, a dialkyl germanium or silicon or siloxane, or 
an alkyl phosphinidine or imido group bridging the indenyls, Q is a 
hydrocarbyl radical such as aryl, alkyl, alkenyl, alkylaryl, or aryl alkyl 
radical having from 1-20 carbon atoms, hydrocarboxy radical having 1-20 
carbon atoms or halogen and can be the same or different from each other, 
and M is Ti, Zr or Hf. Among these, 
ethylenebis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride is the most 
preferred. 
According to the present invention, the metallocene used in the catalyst 
system can be prepared by any known method. A preferred preparation method 
is described in an article of Hans H. Brintzinger published in the 
"Journal of Organometallic Chemistry", 288 (1985) p.63-67, which is 
incorporated herein by reference. 
Any alumoxane known in the art can be used in the present invention. The 
preferred alumoxanes comprise oligomeric linear and/or cyclic alkyl 
alumoxanes represented by the formulae: 
##STR1## 
for oligomeric, linear alumoxanes and 
##STR2## 
for oligomeric, cyclic alumoxanes, wherein n is 1-40, preferably 10-20, m 
is 3-40, preferably 3-20 and R is a C.sub.1 -C.sub.8 alkyl group and 
preferably methyl. Generally, in the preparation of alumoxanes from, for 
example, trimethyl aluminum and water, a mixture of line ar and cyclic 
compounds is obtained. Methylalumoxane is preferably used. 
The alumoxane is usually delivered as a concentrated solution of alumoxane 
in toluene. 
The support used in the process of the present invention can be any organic 
or inorganic solids, particularly porous supports such as talc, inorganic 
oxides, and resinous support materials such as polyolefin. Preferably, the 
support material is an inorganic oxide in its finely divided form. 
Suitable inorganic oxide materials which are desirably employed in 
accordance with this invention include Group 2a, 3a, 4a or 4b metal oxides 
such as silica, alumina, and silica-alumina and mixtures thereof, silica 
being the most preferred one. Other inorganic oxides that may be employed 
either alone or in combination with the silica, alumina or silica-alumina 
are magnesia, titania, zirconia, and the like. Other suitable support 
materials, however, can be employed, for example, finely divided 
functionalized polyolefins such as finely divided polyethylene. 
Preferably, the support is a silica having a surface area comprised between 
200 and 600 m.sup.2 /g and a pore volume comprised between 0.5 and 3 ml/g. 
According to the present invention, the catalyst system used in the process 
for producing polyethylene having a broad and monomodal molecular weight 
distribution can be made by any known method as long as the metallocene of 
the resulting supported metallocene-alumoxane catalyst is bridged, that it 
comprises at least a hydrogenated indenyl or fluorenyl and that it is 
isolated on its support under the form of all its conformers. 
According to a preferred embodiment of the present invention, the supported 
metallocene-alumoxane catalyst is prepared as follows: 
a) reacting a bridged metallocene stereoisomer comprising at least a 
hydrogenated indenyl or fluorenyl with an alumoxane at a temperature 
comprised between 15 and 50.degree. C. 
b) recovering from step a) a mixture of an alkylmetallocenium cation and an 
anionic alumoxane oligomer 
c) reacting the mixture from step b) with a support at a temperature 
comprised between 85 and 110.degree. C. 
d) recovering a supported metallocene-alumoxane catalyst as a free flowing 
catalyst wherein the metallocene stereoisomer is isolated on its support 
under the form of all its conformers. 
The Applicants have unexpectedly found that the metallocenes of the present 
invention, which comprise bulky substituents (hydrogenated indenyl or 
fluorenyl), are present on the support in the form of all their conformers 
which exhibit considerable differences of energy barrier. Said conformers 
can be trapped in the alumoxane anionic cages and the steric restriction 
of said metallocenes prevents their interconversion. The presence of said 
isolated conformers on the support explains the production of a 
polyethylene having at the same time a broad and monomodal MWD when 
prepared with the catalyst system of the present invention. 
According to the preferred catalyst preparation method, the reaction 
between the metallocene and the alumoxane is performed at a temperature 
comprised between 15 and 50.degree. C., preferably about 25.degree. C. 
This reaction is usually conducted in the presence of a solvent, 
preferably toluene. 
The amount of alumoxane and metallocene usefully employed in the 
preparation of the solid support catalyst can vary over a wide range. 
Preferably, the aluminum to transition metal mole ratio is comprised 
between 1:1 and 100:1, preferably between 5:1 and 50:1. 
The order of addition of the support to the mixture comprising the 
metallocene-alumoxane can be reversed. In accordance with a preferred 
embodiment of the present invention, the mixture metallocene-alumoxane is 
added to the support material slurried in a suitable hydrocarbon solvent. 
Preferred solvents include mineral oils and the various hydrocarbons which 
are liquid at temperature and pressure conditions and which do not react 
with the individual ingredients. Illustrative examples of the useful 
solvents include the alkanes such as pentane, iso-pentane, hexane, 
heptane, octane and nonane; cycloalkanes such as cyclopentane and 
cyclohexane, and aromatics such as benzene, toluene, ethylbenzene, xylene 
and diethylbenzene, the preferred being toluene. 
The reaction between the support and the mixture alumoxane-metallocene is 
conducted at a temperature comprised between 85 and 110.degree. C., more 
preferably around 110.degree. C. 
An advantage of the preferred catalyst preparation method is the facility 
and rapidity with which the catalyst is prepared Indeed said preparation 
process does not require the time-consuming washing steps of the prior 
art; the final catalyst system is prepared within 1-2 hours. Further the 
present preparation method does not require the consumption of large 
amounts of solvent which is needed in prior art methods. 
According to the present invention, there is also provided an improved 
process for the (co)polymerization of ethylene to produce a broad 
monomodal molecular weight distribution polyethylene characterized in that 
the polymerization is conducted in the presence of a supported 
metallocene-alumoxane catalyst according to the present invention. 
The Applicants have unexpectedly found that the (co)polymerization of 
ethylene in the presence of a supported metallocene-alumoxane catalyst 
according to the present invention gives a polyethylene showing a broad 
monomodal molecular weight distribution. 
The catalyst of the present invention can be used in gas, solution or 
slurry polymerizations. Preferably, according to the present invention, 
the polymerization process is conducted under slurry phase polymerization 
conditions. It is preferred that the slurry phase polymerization 
conditions comprise a temperature of about 20 to 125.degree. C. and a 
pressure of about 0.1 to 5.6 MPa for a time between 10 minutes and 4 
hours. 
It is preferred that the polymerization reaction be run in a diluent at a 
temperature at which the polymer remains as a suspended solid in the 
diluent. Diluents include, for examples, isobutane, n-hexane, n-heptane, 
methylcyclohexane, n-pentane, n-butane, n-decane, cyclohexane and the 
like. The preferred diluent is isobutane. 
According to a preferred embodiment of the present invention, a continuous 
reactor is used for conducting the polymerization. This continuous reactor 
is preferably a loop reactor. During the polymerization process, at least 
one monomer, the catalytic system and a diluent are flowed in admixture 
through the reactor. 
While alumoxane can be used as cocatalyst, it is not necessary to use 
alumoxane as cocatalyst during the polymerization procedure for preparing 
polyolefins according to the process of the present invention. Further, 
the use of alumoxane as a cocatalyst during the polymerization may lead to 
the fouling of the reactor. 
According to a preferred embodiment of the present invention, one or more 
aluminum alkyl represented by the formula AlR.sub.X are used wherein each 
R is the same or different and is selected from halides or from alkoxy or 
alkyl groups having from 1 to 12 carbon atoms and x is 3. Especially 
suitable aluminum alkyls are trialkylaluminums selected from 
trimethylaluminum, triethylaluminum, triisobutylaluminum, 
tri-n-octylaluminum or tri-n-hexylaluminum, the most preferred being 
triisobutylaluminum. 
In accordance with the present invention the broadness of the molecular 
weight distribution and the average molecular weights can be controlled by 
the introduction of some amount of hydrogen during polymerization. Another 
preferred embodiment of the present invention implies the use of a 
comonomer for this control; examples of comonomer which can be used 
include 1-olefins butene, hexene, octene, 4-methyl-pentene, and the like, 
the most preferred being hexene. 
According to the present invention when hydrogen is used it is preferred 
that the relative amounts of hydrogen and olefin introduced into the 
polymerization reactor be within the range of about 0.001 to 15 mole 
percent hydrogen and 99.999 to 85 mole percent olefin based on total 
hydrogen and olefin present, preferably about 0.2 to 3 mole percent 
hydrogen and 99.8 to 97 mole percent olefin. 
The invention will now be further described by the following examples. 
EXAMPLES 
1. Catalyst Preparation 
The support used is a silica having a total pore volume of 4.217 ml/g and a 
surface area of 322 m.sup.2 /g. This silica is further prepared by drying 
in high vacuum on a Schlenk line for three hours to remove the physically 
absorbed water. 5 g of this silica are suspended in 50 ml of toluene and 
placed in a round bottom flask equipped with magnetic stirrer, nitrogen 
inlet and dropping funnel. 
An amount of 0.31 g of racemic metallocene is reacted with 25 ml of 
methylalumoxane (MAO 30 wt % in toluene) at a temperature of 25.degree. C. 
during 10 minutes to give a solution mixture of the corresponding 
metallocenium cation and the anionic methylalumoxane oligomer. 
Then the resulting solution comprising the metallocenium cation and the 
anionic methylalumoxane oligomer is added to the support under a nitrogen 
atmosphere via the dropping funnel which is replaced immediately after 
with a reflux condenser. The mixture is heated to 110.degree. C. for 90 
minutes. Then the reaction mixture is cooled down to room temperature, 
filtered under nitrogen and washed with toluene. 
The catalyst obtained is then washed with pentane and dried under a mild 
vacuum. 
The type of metallocene and the amount of catalyst obtained are given in 
Table 1 hereafter. 
2. Polymerization Procedure 
Three minutes before the introduction of the catalyst into the reaction 
zone 1 ml of 25 wt % of triisobutylaluminum (TIBAL) in toluene is added to 
the catalyst. 
All polymerizations were performed in a four liters bench reactor. The 
reactor contained two liters of isobutane as diluent. 
The catalyst type, the polymerization conditions and the polymer properties 
are given in Table 2 hereafter. 
The polymers were analyzed by Gel Permeation Chromatography (GPC-WATERS 
MILLIPORE) and Differential Scanning Calorimetry (DSC). The graphs are 
given in FIGS. 1 and 2 (FIGS. 1 and 2 respectively correspond to examples 
1 and 2 of table 2). "D" represents the ratio Mw/Mn (MWD), "D'" the ratio 
Mz/Mw and "A" the area under the curve. 
TABLE 1 
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Silica 
Type Metallocene 
MAO 
T Time 
Catalyst 
Example 
(g) 
(racemic) 
(g) (ml) 
(.degree.C.) 
(min) 
(g) 
__________________________________________________________________________ 
A1 5 .sub.-- (IndH.sub.4).sub.2 EtZrCl.sub.2 
0.31 25 110 
90 8.2 
A2 5 .sub. (Ind).sub.2 EtZrCl.sub.2 
0.31 25 110 
90 10 
A2 comparative 
__________________________________________________________________________ 
(IndH.sub.4).sub.2 EtZrCl.sub.2 
ethylenebis(4,5,6,7tetrahydro-1-indenyl)zirconiumdichloride. 
(Ind).sub.2 EtZrCl.sub.2 ethylenebis(indenyl)zirconiumdichloride. 
In the two examples, the mixture alkylmetallocenium cationanionic 
alumoxane oligomer has been added to the support material. 
TABLE 2 
__________________________________________________________________________ 
Catalyst Pressure 
Polymerization 
Monomer Hydrogen 
Hexene 
Activity 
Example 
(mg) 
type 
(MPa) 
T (.degree.C.) 
Time (min) 
Type 
(wt %) 
(NI) (wt %) 
(g/g .multidot. h) 
Bulk (1) 
MI (2) 
HLMI 
MWD 
__________________________________________________________________________ 
1 50 
A.1 
2.2 70 60 C.sub.2 
6 0.25 2.44 
17280 
0.25 
0.08 
7.77 7.4 
2 (comp) 
100 
A.2 
2.2 80 60 C.sub.2 
6 0.25 2.44 
10280 
0.37 
0.03 
4.49 6.3 
__________________________________________________________________________ 
C.sub.2 ethylene 
(1) Bulk density (ASTMD-1895) 
(2) Melt Index (ASTMD-1238-89A) 
(3) High Load Melt Index (ASTMD-1238-89A)