Multi-stage process for the polymerization of olefins

Details Multi-stage process for the polymerization of olefins Multi-stage process for the polymerization of olefins

2001-06-18

2002-11-12

Wu, David W. (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S160000, C526S943000, C526S113000, C526S082000, C526S065000, C525S053000, C525S242000, C525S247000, C525S245000, C525S248000

Reexamination Certificate

active

06479609

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a multi-stage process for preparing ethylene homo and copolymers having broad molecular weight distributions and low content of soluble fractions, said process comprising a first polymerization stage (A) in the presence of Ti or V catalysts, a treatment stage (B) wherein the Ti or V catalyst is deactivated and a specific metallocene is supported on the polymer produced in stage (A), optionally in the presence of a suitable cocatalyst, and a final polymerization stage (C).
The invention also relates to the broad molecular weight distribution polyethylene obtainable by the above process.
PRIOR ART DISCLOSURE
Multistage processes for the polymerization of olefins, carried out in two or more reactors, are well known in the state of the art and are particularly interesting in industrial practice. Said processes are generally carried out using the same catalyst in the various stages/reactors, by utilizing tandem reactors operated in series: the product obtained in one reactor is usually discharged and sent directly to the following stage/reactor without altering the nature of the catalyst.
Broad or multi-modal molecular weight distribution (MWD) polyethylenes are commonly prepared by employing a multi-reactor process, wherein ethylene is polymerized in various reactors containing the same catalyst but in the presence of different concentrations of hydrogen, as molecular weight regulator. The thus obtained polymers contain a high molecular weight (HMW) fraction and a low molecular weight (LMW) fraction, therefore showing a broad total molecular weight distribution (MWD=Mw/Mn).
Nevertheless, said polymers present various drawbacks due to the LMW fraction obtained in the reactor with the higher amount of hydrogen; said fraction, having itself a broad MWD, contains undesired soluble products of very low molecular weight, which impair the mechanical properties of the final polymer and do not allow its use in medical and alimentary fields.
Polyolefins having broader MWD are products of notable commercial value, since they show high workability due to the HMW fractions and, at the same time, they provide excellent mechanical properties, due to the LMW fractions. Said polyolefins have been produced even in a single polymerization process, by employing two distinct and separate catalysts in the same reactor each producing a polyolefin having a different MWD. For instance, the European patent application EP 0 128 045 describe the use of a bimetallic catalyst system, i.e. a catalysts system comprising two or more metallocenes, each one having different propagation and termination rate constants, and alumoxanes; the polymers obtained by using said system in a single polymerization process have a broader multimodal molecular weight distribution than using a single metallocene, that usually produces polyethylenes with Mw/Mn of from 2 to 4; nevertheless, the obtained polymers have Mw/Mn lower than 8.
Also the European patent applications EP 0 619 325 and EP 0 705 851 describe the production of multimodal MWD polyolefin from a single polymerization process in the presence of a catalytic system comprising two different metallocenes, wherein at least one of the metallocenes is bridged, generally in the presence of hydrogen as MW regulator. The amount and productivity of the two metallocenes used is varied to control the relative amounts of the HMW and LMW fractions in the final polymer; nevertheless, these parameters are difficult to control and lead to non-homogeneous final products, not uniform in size, having unpredictable properties.
The European patent application EP 0 673 950 describes the preparation of polyethylene having a broad MWD in a gas phase reactor, in the presence of a prepolymer containing a Ti/Zr bimetallic catalyst and of hydrogen. The thus obtained polymers show very low Mn values and are obtained with low activities.
The European patent application EP 0 514 594 describes the production of polymers having multimodal molecular weight distribution in a single polymerization reactor wherein the catalytic system comprises a Mg support, a Ti or V based component, a zirconocene and suitable cocatalysts. Said catalytic system allows bimodal MWD polymers to be obtained; more specifically, the LMW polymer fraction is produced by the metallocene component, while the HMW fraction is due to the Ti or V component.
The several known methods employing mixtures of catalytic systems in one stage polymerization, as described above, have many drawbacks; in particular, the catalyst feed rate is difficult to control and the polymer particles produced are not uniform in size; segregation of the polymer during storage and transfer usually produce non-homogeneous products.
In order to overcome the above difficulties, various solutions have been proposed. The International patent application WO 96/07478 describe a single polymerization process carried out in the presence of a catalyst comprising two different transition metal compounds, each having different hydrogen response characteristics. The proportion of different weight fractions in broad or bimodal molecular weight distribution is controlled by adding a further amount of one of the two components of the bimetallic catalyst. Another solution is that disclosed in the International patent application WO 96/09328, wherein water and/or carbon dioxide are co-fed to the polymerization reactor, containing a bimetallic catalyst, at levels necessary to modify the weight fractions of the HMW and LMW components, thus achieving a target molecular weight distribution. However, the above solutions are still not satisfactory.
The International patent application WO 96/11218, in the name of the same Applicant, describes a multi-stage process for olefin polymerization, in particular for the preparation of heterophase copolymers of propylene, comprising a first stage wherein a first olefin polymer is prepared in the presence of titanium or vanadium catalysts; a second stage of deactivation of the catalyst used in the first stage; and a third stage of polymerization of one or more olefins in the presence of the polymer of the first stage and of a complex of a transition metal M (M being Ti, Zr, V or Hf) containing at least one M-&pgr; bond and/or of their reaction products. Said process proved to be particularly useful in the production of polymers with broad MWD, even if the metallocenes used did not allow a sufficiently low Mw to be obtained.
Therefore, it is desirable to provide a novel, high activity process for the production of broad MWD polyethylenes having low xylene soluble fractions, without incurring in the disadvantages of the above discussed prior art.
SUMMARY OF THE INVENTION
It has now been found that broad MWD ethylene homo and copolymers can be obtained through a multi-stage process, by using a new class of zirconocenes which are unexpectedly able to produce, in high yields, ethylene polymers having fairly low molecular weights and narrow MWD, without necessitating the use of molecular weight regulators, such as hydrogen.
More specifically, the present invention provides a multi-stage process for the polymerization of ethylene, optionally in the presence of one or more (x-olefins, comprising from 3 to 10 carbon atoms, to produce a polymer having a broad MWD; said process comprises the following stages:
(A) polymerizing ethylene, and optionally said &agr;-olefin, in one or more reactors, in the presence of a catalyst comprising the reaction product between:
(i) a solid component comprising a compound of a transition metal M
I
selected from Ti and V, not containing M
I
-&pgr; bonds, and a halide of Mg in active form, optionally comprising an electron-donor compound (internal donor);
(ii) an alkyl-Al compound and optionally an electron-donor compound (external donor);
in order to produce an ethylene homo or copolymer;
(B) contacting the product obtained in stage (A), in any order whatever, with:
(a) a compound capable of deactivating the catalyst of stage (A);
(b) a zirconocene compound o

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