Catalyst for the polymerization of olefins

Details Catalyst for the polymerization of olefins Catalyst for the polymerization of olefins

2002-02-28

2004-04-06

Choi, Ling-Siu (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S151000, C526S153000, C526S158000, C526S348000, C526S352000, C502S103000, C502S132000

Reexamination Certificate

active

06716940

ABSTRACT:

The present invention relates to catalysts for the polymerization of olefins CH
2
═CHR, wherein R is hydrogen or a hydrocarbon radical having 1-12 carbon atoms. In particular, the present invention relates to a catalyst obtained by reacting a solid catalyst component, based on Mg, Ti and halogen, with a particular pair of alkyl-Al compounds. This kind of catalyst is particularly suitable for the preparation of copolymers of ethylene with &agr;-olefins due to its high capacity for incorporating the comonomer while at the same time maintaining high yields.
Accordingly, another object of the present invention is the use of said catalysts in a process for the copolymerization of olefins in order to produce ethylene/&agr;-olefin copolymers.
Linear low-density polyethylene (LLDPE) is one of the most important products in the polyolefin field. Due to its characteristics, it finds application in many sectors and in particular in the field of wrapping and packaging of goods where, for example, the use of stretchable films based on LLDPE constitutes an application of significant commercial importance. LLDPE is commercially produced with liquid phase processes (solution or slurry) or via the more economical gas-phase process. Both processes involve the widespread use of Ziegler Natta MgCl
2
-supported catalysts that are generally formed by the reaction of a solid catalyst component, in which a titanium compound is supported on a magnesium halide, with an alkylaluminium compound.
In order to be advantageously usable in the preparation of LLDPE, said catalysts are required to show high comonomer incorporation properties and good comonomer distribution suitably coupled with high yields.
The requirement of high comonomer incorporation is particularly important in the case of gas-phase production processes because the use of excessively large amounts of &agr;-olefin in the feed mixture can cause condensation phenomena in the gas-phase reactor. Therefore, the use of a catalyst having a high capacity for incorporating the comonomer would make it possible to lower the amount &agr;-olefin monomer in the feed.
It is known in the art that the use of different co-catalysts can modulate certain;properties of the solid catalyst component like, for example, polymerization activity, ability to produce higher or lower molecular weights polymers, comonomer distribution, etc. In particular, it is reported in the art that the use of dimethylaluminium chloride with respect to a trialkylaluminium, would give catalysts capable of producing ethylene polymers with a broader Molecular Weight Distribution (MWD) and also capable of incorporating a higher amount of comonomer. However, all the above improvements are made redundant by the fact that the yields are dramatically decreased.
International patent application WO 95/17434 discloses a catalyst system aimed at solving this problem. It is characterized by the use of DMAC/trialkylaluminium (TAA) co-catalyst mixtures in molar ratios from 30 to 300. Table 1 of said application shows that when the DMAC/TAA molar ratio is higher than 30, a high Melt Flow Ratio (indicating a broad MWD) and a melt index in the range 10-20 are obtained. The incorporation of a comonomer in this range of DMAC/TAA molar ratio appears to increase slightly as a function of the TAA content (it passes from 2.1% with the use of pure DMAC to 2.3% with the use of a DMAC/TAA molar ratio of 30). The yields however are very low in this range if compared with the TAA alone. In particular, the activity of the best invention example of Table 1 (Example 4) is about 160 times lower than the activity obtained with triethylaluminium (TEAL) alone. On the other hand, said application shows that when DMAC/TAA molar ratios lower than 30 are used, the Molecular Weight of the polymer decreases (the melt index in the range 20-60), the MWD becomes narrower (Melt Flow Ratios lower than 30 are obtained) and, most importantly, at the same time the incorporation of comonomer does not increase (the value of 2.3% remains constant). All the above drawbacks are not offset by the slight increase in activity which, for a DMAC/TEAL molar ratio of 20, remains about 85 times lower than that for TEAL alone.
Contrary to the strong suggestion of using a large excess of DMAC with respect to the alkylaluminium, we have surprisingly discovered that the use of DMAC/alkylaluminum compound co-catalyst mixtures having lower molar ratios gives catalysts with completely unexpected properties. Said catalysts in fact have a very high capacity for incorporating the co-monomer while at the same time displaying activity which is higher than that obtainable by the use of the aluminium alkyl alone.
Accordingly, an object of the present invention is a catalyst system comprising the product of the reaction between (a) a solid catalyst component comprising Mg, Ti, halogen and optionally an electron donor compound, (b) dimethylaluminium chloride (DMAC) and (c) an compound in which the molar ratio between (b) and (c) is lower than 10.
In the reaction with component (a), the DMAC and the alkyaluminium compound are preferably used in molar ratios from 0.01 to 5 and more preferably between 0.3 and 3.
The alkylaluminium compound can be selected from the compounds of formula AlR
1
3−y
H
y
where y is from 0 to 2 and R
1
is a hydrocarbon group having from 1 to 15 carbon atoms. Preferably, the alkylaluminium compound (c) is selected from those of the above formula in which y is 0 and R
1
is a C2-C10 alkyl radical. Examples of suitable aluminium alkyl compounds are di-(2,4,4-trimethylpentyl)aluminium hydride, triethylaluminum, triisopropylaluminum, triisobutylaluminum, tri-n-hexylaluminum and tri-(2,4,4-trimethylpentyl)aluminium. The use of triethyl- or triisobutylaluminium is especially preferred.
As explained above, the component (a) of the invention is a solid catalyst component comprising Ti, Mg and halogen. In particular, the said catalyst component comprises a titanium compound supported on a magnesium halide. The magnesium halide is preferably MgCl
2
in active form, which is widely known from the patent literature as a support for Ziegler-Natta catalysts. Patents U.S. Pat. Nos. 4,298,718 and 4,495,338 were the first to describe the use of these compounds in Ziegler-Natta catalysis. It is known from these patents that the magnesium dihalides in active form used as support or co-support in components of catalysts for the polymerization of olefins are characterized by X-ray spectra in which the most intense diffraction line that appears in the spectrum of the non-active halide is diminished in intensity and is replaced by a halo whose maximum intensity is displaced towards lower angles relative to that of the most intense line.
The preferred titanium compounds are those of formula Ti(OR
2
)
n−y
X
y
, where X is halogen, preferably chlorine, n is the valence of titanium, y is a number between 0 and n, and the R
2
groups, which may be identical or different, are hydrocarbon radicals having from 1 to 10 carbon atoms. Particularly preferred titanium compounds are TiCl
4
, TiCl
3
, titanium (IV) butoxide and titanium (IV) isopropoxide, trichlorobutoxy titanium (IV) and dichlorobutoxytitanium (III).
The preparation of the solid catalyst component can be carried out according to several methods. According to one of these methods, the product obtained by co-milling the magnesium chloride in an anhydrous state and the titanium compound is treated with halogenated hydrocarbons such as 1,2-dichloroethane, chlorobenzene, dichloromethane, etc. The treatment is carried out for a time between 1 and 4 hours and at a temperature ranging from 40° C. to the boiling point of the halogenated hydrocarbon. The product obtained is then generally washed with inert hydrocarbon solvents such as hexane.
According to another method, magnesium dichloride is pre-activated according to well-known methods and then treated with an excess of Ti compound at a temperature of about 80 to 135° C. The treatment with the Ti compound is repeated and the solid is washed with

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