Gas-phase polymerization of C2-C8-alk-1-enes

Details Gas-phase polymerization of C2-C8-alk-1-enes Gas-phase polymerization of C2-C8-alk-1-enes

2000-03-14

2003-02-18

Teskin, Fred (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S065000, C526S072000, C526S901000, C526S905000

Reexamination Certificate

active

06521722

ABSTRACT:

The present invention relates to a process for the polymerization of C
2
-C
8
-alk-1-enes by means of a Ziegler-Natta catalyst system, in which polymerization is carried out from the gas phase in at least one reaction zone at from 40 to 120° C. and pressures of from 1 to 100 bar.
Polymers of C
2
-C
8
-alk-1-enes can be prepared both by liquid-phase polymerization and by polymerization in a slurry or by gas-phase polymerization. Since the solid polymer formed can easily be separated from the gaseous reaction mixture, the polymerization is increasingly carried out from the gas phase. The polymerization is here carried out with the aid of a Ziegler-Natta catalyst system which usually comprises a titanium-containing solid component, an organic aluminum compound and an organic silane compound (EP-B 45 977, EP-A 171 200, U.S. Pat. Nos. 4,857,613, 5,288,824).
Polymers of C
2
-C
8
-alk-1-enes include the corresponding homopolymers, copolymers and also block or impact copolymers. The latter are usually mixtures of various homopolymers or copolymers of C
2
-C
8
-alk-1-enes which display, in particular, a good impact catalyst. They are usually prepared in reactor cascades comprising at least two reactors connected in series and often in an at least two-stage process in which the polymer obtained in a first reactor is transferred in the presence of still active Ziegler-Natta catalyst constituents to a second reactor where further monomers are polymerized onto it.
In the exothermic polymerization of C
2
-C
8
-alk-1-enes, it is necessary to remove the heat of polymerization continuously and in a controlled manner. This is usually carried out by removing unreacted monomer mixtures from the reaction zone, then cooling them and subsequently reintroducing them into the reaction zone as cooled gaseous monomer mixtures. EP-B 89 691 discloses cooling the unreacted monomers removed from a fluidized-bed reactor to such an extent that they partly condense. The resulting mixture of gaseous and liquid monomers is then fed back into the lower part of the fluidized bed reactor.
To achieve a stable reaction and a good productivity of the Ziegler-Natta catalyst system used, the mixing behavior of the polymer bed present in the reactor is of critical importance. If mixing is insufficient, locally overheated zones frequently occur in the polymer bed in the reactor and these result in inhomogeneity of the reaction temperatures. Such inhomogeneous reaction temperatures lead, inter alia, to reduced productivity of the catalyst used and to a poorer morphology of the polymer obtained.
It is an object of the present invention to remedy the disadvantages indicated and to develop a process for the polymerization of C
2
-C
8
-alk-1-enes which is improved compared to the previously known processes and, inter alia, has an improved productivity of the catalyst used and leads to polymers having an improved morphology.
We have found that this object is achieved by a novel process for the polymerization of C
2
-C
8
-alk-1-enes by means of a Ziegler-Natta catalyst system, in which polymerization is carried out from the gas phase in at least one reaction zone at from 40 to 120° C. and pressures of from 1 to 100 bar, wherein the pressure and temperature in the reaction zone are set so that an operating point formed by these parameters in a pressure-temperature diagram is located at from 0.2 to 5.0 bar below the dew line of the respective reaction mixture above which condensation of the C
2
-C
8
-alk-1-enes occurs.
C
2
-C
8
-alk-1-enes which can be used in the process of the present invention are, in particular, ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene and 1-octene, with preference being given to using ethylene, propylene or 1-butene. The process is suitable for preparing homopolymers of the C
2
-C
8
-alk-1-enes or copolymers of the C
2
-C
8
-alk-1-enes, preferably containing up to 30% by weight of copolymerized, other alk-1-enes having up to 8 carbon atoms. For the purposes of the present invention, copolymers are both random copolymers and also block or impact copolymers.
In general, the process of the present invention is carried out in at least one reaction zone, frequently in two or more reaction zones, i.e. the polymerization conditions differ in the reaction zones so that polymers having different properties are produced. In the case of the homopolymers or random copolymers, this can be, for example, the molar mass, i.e. the molar mass distribution is broadened by preparing polymers having different molar masses in the reaction zones. Preference is given to polymerizing different monomers or monomer compositions in the reaction zones. This usually then leads to block or impact copolymers.
The process of the present invention is particularly suitable for preparing homopolymers of propylene or copolymers of propylene with up to 30% by weight of copolymerized, other alk-1-enes having up to 8 carbon atoms. The copolymers of propylene are here random copolymers or block or impact copolymers. If the copolymers of propylene have a random structure, they generally contain up to 15% by weight, preferably up to 6% by weight, of other alk-1-enes having up to 8 carbon atoms, in particular ethylene, 1-butene or a mixture of ethylene and 1-butene.
The block or impact copolymers of propylene are polymers in the case of which a propylene homopolymer or a random copolymer of propylene with up to 15% by weight, preferably up to 6% by weight, of other alk-1-enes having up to 8 carbon atoms is prepared in the first stage and then, in the second stage, a propylene-ethylene copolymer having an ethylene content of from 15 to 80% by weight, where the propylene-ethylene copolymer can additionally comprise further C
4
-C
8
-alk-1-enes, is polymerized on. In general, the propylene-ethylene copolymer is polymerized on in such an amount that the copolymer produced in the second stage makes up from 3 to 60% by weight of the end product.
According to the present invention, the polymerization is carried out by means of a Ziegler-Natta catalyst system. Here, use is made, in particular, of catalyst systems comprising a titanium-containing solid component a) plus cocatalysts in the form of organic aluminum compounds b) and electron donor compounds c).
However, Ziegler-Natta catalyst systems based on metallocene compounds or based on polymerization-active metal complexes can also be used in the process of the invention.
To prepare the titanium-containing solid component a), the halides or alkoxides of trivalent or tetravalent titanium are generally used as titanium compounds. Here, it is also possible to use titanium alkoxyhalide compounds or mixtures of various titanium compounds. Preference is given to using titanium compounds which contain chlorine as halogen. Preference is likewise given to titanium halides which, apart from titanium, contain only halogen, and among these especially the titanium chlorides and in particular titanium tetrachloride.
The titanium-containing solid component a) preferably comprises at least one halogen-containing magnesium compound. In this context, halogen is chlorine, bromine, iodine or fluorine, with preference being given to bromine and in particular chlorine. The halogen-containing magnesium compounds are either used directly in the preparation of the titanium-containing solid component a) or are formed during its preparation. Magnesium compounds which are suitable for preparing the titanium-containing solid component a) are especially the magnesium halides, in particular magnesium dichloride or magnesium dibromide, or magnesium compounds from which the halides can be obtained in a customary fashion, e.g. by reaction with halogenating agents, for example magnesium alkyls, magnesium aryls, magnesium alkoxy or magnesium aryloxy compounds or Grignard compounds. Preferred examples of halogen-free compounds of magnesium which are suitable for preparing the titanium-containing solid component a) are n-butylethylmagnesium or n-butyloctylmagnesium. Preferred halogenating agents are chlorine or hydrogen ch

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