Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The present invention relates to the polymeriza-
tion of ethylene in the presence of supported catalysts
known in the art as Ziegler catalysts.
Ziegler catalysts are commonly formed by reducing
a transition metal compound with an organometallic compound.
The reduced transition metal compound is then used, in
conjunction with an activator, which may be the same or a
; different organometallic compound, to polymerize olefins,
especially ethylene, in the presence of an inert solvent
or in the gas phase. A molecular weight regulator, such as
hydrogen, may be used with these catalyst systems, as
taught by Vandenberg in U.S. 3,051,690.
Such catalysts are often rather unefficient
because the catalyst particles tend to agglomerate. To
obviate this problem many systems of supporting the catalyst
on solid carriers have been proposed.
Kashiwa et al, in U.S. 3,642,746, describe
~; cataly~ts utilizing a magnesium chloride support pretreatedwith an electron donor, such as methanol, and then treated
with a titanium compound. The electron donor must be
coordinated with the magnesium chloride when the titanium
compound is added to the support.
Diedrich et al, in U.S. 3,644,318, describe
catalysts supported on magnesium alcoholates.
Stevens et al, in U.S. 3,718,636 describes
catalysts obtained by reacting a magnesium oxide support
with an organometallic compound, separating the resulting
solid product and reacting this product with a titanium
compound. The polyethylene produced with this catalyst
had low melt index and broad molecular weight dlstribution.
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BRIEF SUMMARY OF THE IN~rENTION
It has now been found that a catalyst having
high efficiency for polymerizing ethylene is obtained by
treating magnesium oxide with methanol, drylng the oxide
free of alcohol, impregnating the oxide with a mixture
of titanium tetrachloride and tetrabutyltitanate (1:1 mole
ratio) and adding an alkylaluminum compound to reduce the
titanium mixture. The polyethylene made with this
catalyst has melt indices in the injection molding range,
i.e. between 3.1 and 12.5 g./10 min., relatively narrow
molecular weight distribution, and high bulk density
which prevents reactor fouling and facilitates physical
handling of the polymer.
DETAILED DESCRIPTION OF THE IN~rENTION
The catalyst of the invention comprises a solid
complex component obtained by heating a support of
magnesium oxide with methanol, removing all the methanol
by drying the support under vacuum, refluxing the treated
magnesium oxide with a 1:1 molar mixture of titanium
tetrachloride and tetrabutyltitanate, removing any excess
titanium compound by washing the support with an inert
hydrocarbon solvent, reacting said support containing
titanium compound with an organoaluminum compound of
formula RnAlX3_n, wherein R is a hydrocarbon radical
selected from branched or linear alkyl, alkenyl, cycloalkyl,
aryl, alkylaryl or arylalkyl radicals having 1 to 20 carbon
atoms, X is hydrogen or halogen, and n is 1, 2 or 3, and
aging the resulting supported complex.
rrhe polymerization of ethylene involves subject-
ing ethylene in an inert solvent, or in the gas phase, to
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low pressure polymerization conditions in the presence of a
catalytic amount of the above-described catalyst and
sufficient organoaluminum compound to activate the catalyst
and scavenge any undesirable impurities in the system.
The catalyst is prepared by first thoroughly
drying the magnesium oxide support by heating under vacuum
at temperatures of from 200C. to 6000C. for times up to
24 hours. The dried oxide is then suspended in inert
hydrocarbon and stirred for 2 to 4 hours at 600C. with
about 20 percent by mole fraction, based on the oxide, of
methanol. The support material is then carefully separated
from the liquid medium and dried under vacuum. Analysis by
infrared shows no alcohol or alcoholate groups remaining
on the magnesium oxide at this point. The support is
then refluxed with a solution of dibutoxytitanium dichloride
in an inert hydrocarbon for 15 to 24 hours. The dibutoxy-
titanium dichloride is made by mixing equimolar amounts of
titanium tetrachloride and tetrabutyltitanate. me excess
titanium compound is removed from the support by repeated
washing with the inert hydrocarbon solvent. The resulting
magnesium oxide support containing the titanium salt is then
dispersed in the inert hydrocarbon, and sufficient organo-
aluminum compound added to produce an aluminum to titanium
ratio of between 0.05 and 0.5. The organoaluminum compound
useful for the catalyst preparation has been described earlier
herein and may be, preferably, triethylaluminum, tri-
hexylaluminium, triisobutylaluminum, diisobutylaluminum
hydride, and the like. The resulting catalyst may be
used immediately to polymerize ethylene. It is preferred,
however, to age the catalyst for times of 12
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hours to 1 day prior to the polymerization.
The inert hydrocarbon diluent used for preparing
the catalyst solutions is that to be used as a reaction
medium for the olefin polymerization process. Suitable
inert hydrocarbons are the paraffinic and cycloparaffinic
hydrocarbons having from 4 to 10 carbon atoms, such as
butane, isobutane, pentane, isopentane, hexane, heptane,
octane, decane, cyclopenbane, cyclohexane, methylcyclohexane
and aromatic hydrocarbons, such as benzene, xylene, toluene
and the like. The choice of hydrocarbon may vary with the
olefin to be polymerized. The use of hydrocarbons of 6 to
10 carbon atoms will reduce the pressure required for the
reaction and may be preferred for safety and equipment cost
considerations.
The activator-scavenger used in the polymerization
process may be any of the organoaluminum compound known
to be useful in Ziegler polymerization systems. The
activator may be the same as or different from the organo-
aluminum compound used to form the supported catalyst.
The polymerization of ethylene is conveniently
carried out in an autoclave or other suitable pressure
apparatus. The apparatus is charged with solvent, if used
and an activator-scavenger and allowed to equilibrate. The
supported catalyst is then added and the reactor pressured
with ethylene and a molecular weight regulator such as
hydrogen, if used. Polymerization pressures depend mainly
on the limitations of the equipment used, but a normal
range of pressures would be from 1 to 50 atmospheres with
a preferred range of from 6 to 40 atmospheres. Temperatures
of polymerization usually are from 400C. to 200C.,
preferably between 70 and 100C. The catalyst concentration
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suitable for the invention are between 0.001 and 10
millimoles of transition metal per liter of solvent,
preferably between 0.005 and 0.25 millimoles per liter.
The polyethylenes produced with the catalyst
of this invention has melt indices in the injection molding
range, i.e. between 3.1 and 12.5 grams per 10 minutes,
as measured by ASTM-1238 at 190C. using a 2160 g. applied
weight. The molecular weight distirbutions are relatively
narrow i.e. 7.6-7.8 as measured by the ratio of melt index
at lOK~g weight to the melt index at 2 Kg weight as compared
to 8.0-10. for polyethylenes prepared by other supported
catalysts. The catalyst of the invention gives a poly-
ethylene in the slurry process which has exceptionally high
bulk density, i.e. greater than 20 pounds per cubic foot
(pcf.), which makes physical handling of the polymer simpler
and allows greater amounts of polymer to be produced per
unit weight of solvent without the concommitant fouling of
the reactor
The following examples illustrate, but are not
meant to limit the invention.
Example I
a. Catalyst Preparation
Anhydrous magnesium oxide was thermally activated
by heating under vacuum at 210 to 300C. for 18 hours.
The oxide was then blanketed under purified nitrogen. In
a 100 ml Schlenk type flask, 3.8 g. of this activated
magnesium oxide was suspended in 60 ml. of n-hexane. To
the suspension was added o.76 ml. of methanol and the
slurry stirred at 600C. for 2 hours. The solvent was
carefully drained off and the solid residue was dried
under vacuum. After the solid support was completely dried,
purified nitrogen was introduced to blanket the support
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along with 60 ml. of n-hexane to cover the support.
Infrared analysis showed this support to have no methoxide
or free methanol retained at this point.
In a 50 ml. Schlenk type flask, 31.8 millimoles
(mm.) of titanium tetrachloride was added to 10 ml. of
n-hexane and 29.2 mm. of tetrabutyltitanate was mixed
with this solution at room temperature for 5 minutes.
This titanium solution was then added to the support
slurry above and refluxed for 21 hours. The liquid was
drained out and the solid was washed with 60 ml. portions
of hexane five times. A stock solution was made by
adding 80 ml. of hexane to the thus formed catalyst slurry.
A preactivated catalyst slurry solution was made
by diluting 20 ml. of the stock solution with 20 ml. of
hexane and adding 0.73 mm. of triethylaluminum. The pre-
activated catalyst slurry solution was then aged for 1
day before use. The catalyst contained 0.34 mm. of titanium
per gram of catalyst.
b. Polymerization of Eth~lene
Under purified nitrogen atmosphere, 1.5 1. of
dry hexane was placed in a 1 gallon autoclave and 6.8 mm.
of triethylaluminum was added as activator-scavenger. The
autoclave was heated to 400C. and 1 ml. of the aged, pre-
activated catalyst slurry solution was added. The tempera-
ture was then raised to 90C. and the reactor pressured to
~5 psig. with hydrogen. The reactor pressure was raised
to 150 psig and maintained at that pressure by adding
ethylene as needed during the polymerization. The ethylene
uptake rate was measured with a stainlesssteelball flowmeter
manuf`actured by Matheson. At the end of 2 hours, the
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polymerization was stopped by venting the autoclave, open~
ing the reactor, and filtering the polyethylene from the
liquid medium. The ethylene up-take was still 267 units
per gram of catalyst at the 2 hour point. The polymer
was dried under vacuum at 40O C. overnight. The yield
was 207 g. polyethylene, melt index 4.2 g./10 minutes
under a load of 2160 g. at 190C., and bulk density of
24.1 pounds per cubic foot (pcf.). The catalyst efficiency
was s8,ooo g. PE/g.Ti. The polyethylene had a molecular
weight distribution as measured by the ratio of melt index
at 10 Kg. weight to the melt index at 2 Kg.weight (MIlo/MI2)
of 7.6.
Comparative Example
For comparison purposes, the methanol treated
magnesium chloride supported catalyst of U.S. 3,642,746
was prepared and utilized as follows:
a. Catalyst Preparation
A support of anhydrous magnesium chloride was
treated with methanol by the procedure of Example Ia.
Infrared analysis showed the methanol remàined attached
to the magnesium chloride after vacuum drying. The support
was then refluxed with equimolar amounts of titanium tetra-
chloride and tetrabutyltitanate for 21 hours, separated
and washed as in Ia. Preactivation with triethylaluminum
(aluminum/Titanium = 0.25) gave a catalyst having o.o3
millimoles of titanium per gram of catalyst.
b. Polymerization of Ethylene
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The procedure of Example Ib. was followed using
the above-prepared magnesium chloride supported catalyst
and triethylaluminum as activator-scavenger. After two
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hours, the ethylene uptake was only 123 units per gram of
catalyst (measured by the Matheson flowmeter). The
catalyst efficiency was 425,000 g. PE/g. Ti, but the
polyethylene formed had a lower melt index of 2.3 g./10 min.,
a broader distribution of 8.2 (MIlo/MI2), and a low bulk
density of 16.9 pcf.
It can thus be seen that the methanol treated
magnesium oxide supported catalyst of the present invention,
although having lower catalyst efficiency for the first
two hours of polymerization than the known methanol-treated
magnesium chloride supported catalyst, retains a higher
ethylene uptake and hence may have a longer catalyst life.
The polymer prepared by the oxide supported catalyst
has better injection molding properties, i.e. higher melt
index, higher bulk density, and narrower molecular weight
distribution.
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