Olefin polymerization process for producing broad MWD polymers

Details Olefin polymerization process for producing broad MWD polymers Olefin polymerization process for producing broad MWD polymers

2000-06-14

2002-09-03

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...

C526S170000, C526S131000, C526S160000, C526S151000, C502S114000, C502S103000

Reexamination Certificate

active

06444765

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a process for preparing polyolefins. The process comprises a first step of premixing a supported boraaryl catalyst with an organoaluminum, followed by olefin polymerization in the presence of the premixed catalyst, an activator, and a second organoaluminum. The organoaluminums may be the same or different. The process is surprisingly useful for the preparation of broad and/or bimodal molecular weight distribution polyolefins.
BACKGROUND OF THE INVENTION
Interest in metallocene and non-metallocene single-site catalysts has continued to grow rapidly in the polyolefin industry. These catalysts are more active than conventional Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of &agr;-olefin comonomers, and lower polymer density. Examples of non-metallocene single-site catalysts include catalysts containing a boraaryl moiety such as borabenzene, boranaphthalene or boraphenanthrene. See U.S. Pat. No. 5,554,775 and PCT Int. Appl. WO 97/23512.
Unfortunately, the uniformity of molecular weight distribution (MWD) reduces the thermal processing ability of polyolefins made with single-site catalysts. These polyolefins also have a higher tendency to melt fracture, especially at higher molecular weights. These disadvantages combine to make it difficult to process polyolefins produced by single-site catalysts under conditions normally used for Ziegler-Natta polymers. Controllable broadening of MWD is therefore a desired advance in single-site catalyst technology.
One method of increasing processability and broadening MWD of polyolefins produced by single-site catalysts is to physically mix two or more different polyolefins to produce a blended polyolefin mixture with a multimodal, broad molecular weight distribution. For example, see U.S. Pat. No. 4,461,873. In addition, olefin polymerization has been performed in a dual reactor system in order to broaden MWD. The olefin is polymerized by a catalyst in one reactor under one set of conditions, and then the polymer is transferred to a second reactor under a different set of conditions. The first reactor typically produces a high-molecular-weight component, and the second reactor produces a low-molecular-weight component. See U.S. Pat. Nos. 4,338,424, 4,414,369, 4,420,592, and 4,703,094. Lastly, a one-reactor, two-catalyst process has also been used to make multimodal, broad-MWD polymers. The olefin is polymerized in one reactor by two catalysts with different reactivity to form a reactor blend having broad and/or multimodal molecular weight distribution. The catalysts may be either two (or more) separate metallocenes or a metallocene and a Ziegler-Natta component. See, for example, U.S. Pat. Nos. 4,937,299 and 4,530,914, in which at least two separate metallocenes are used in one reactor to form multimodal polymers. See U.S. Pat. Nos. 5,032,562 and 5,539,076 for examples of the metallocene/Zeigler-Natta catalyst mixture in one reactor.
A significant disadvantage of each of these methods is the added cost of using two reactors or two catalysts in the polymerization process. A simpler method would use a single catalyst system that produces broad MWD polymers in a one-reactor process. For example, EP 719,797 A2 discloses an olefin polymerization process in which conventional metallocenes and at least two different co-catalysts are used to produce broad/bimodal MWD polyolefins. In addition, copending application Ser. No. 09/439,462 (U.S. Pat. No. 6,294,626) discloses a method for producing broad and/or bimodal polyolefins using a catalyst comprising an activator and an organometallic compound that incorporates a modified boraaryl ligand.
In sum, new processes are needed. Particularly valuable processes are those that would use one catalyst to produce broad MWD polyolefins having greater thermal processing ability.
SUMMARY OF THE INVENTION
The invention is a process for polymerizing olefins. The process comprises preparing a catalyst system by reacting a first organoaluminum with a supported boraaryl catalyst and then polymerizing an olefin in the presence of the premixed catalyst, an activator and a second organoaluminum. The process surprisingly leads to the production of broad MWD polyolefins. The results are particularly surprising since co-pending U.S. application Ser. No. 09/318,009 (U.S. Pat. No. 6,291,386) teaches that olefin polymerization with a boraaryl catalyst produces polyolefins with narrow MWD when organoaluminums are added to the reactor, without a premixing step.
DETAILED DESCRIPTION OF THE INVENTION
The process of the invention comprises: (a) preparing a catalyst system by premixing a first organoaluminum with a supported catalyst comprising a support and an organometallic compound comprising a Group 3-10 transition or lanthanide metal, M, and at least one boraaryl ligand; and (b) polymerizing an olefin in the presence the catalyst system of step (a), an activator, and a second organoaluminum. The second organoaluminum may be the same as or different from the first organoaluminum.
The supported catalyst of the invention comprises a support and an organometallic compound. The organometallic compound useful in the invention contains at least one boraaryl ligand. Suitable boraaryl ligands include substituted or unsubstituted boraaryl groups, such as substituted or unsubstituted borabenzenes, boranaphthalenes or boraphenanthrenes, as described in U.S. Pat. No. 5,554,775, the teaching of which is incorporated herein by reference. The metal, M, may be any Group 3 to 10 transition or lanthanide metal. Preferably, the catalyst contains a Group 4 to 6 transition metal; more preferably, the catalyst contains a Group 4 metal such as titanium or zirconium.
The transition or lanthanide metal may also have other polymerization-stable anionic ligands. Suitable ligands include cyclopentadienyl (substituted or unsubstituted) anions such as those described in U.S. Pat. Nos. 4,791,180 and 4,752,597, the teachings of which are incorporated herein by reference. Suitable ligands also include another boraaryl group or a substituted or unsubstituted azaborolinyl, pyrrolyl, indolyl, quinolinyl, hydroxypyridinyl, or aminopyridinyl group as described in U.S. Pat. Nos. 5,554,775, 5,539,124, 5,637,660, and 5,902,866, the teachings of which are also incorporated herein by reference.
The boraaryl ligand and the other polymerization-stable anionic ligand can be bridged. Groups that can be used to bridge the ligands include, for example, methylene, ethylene, 1,2-phenylene, dialkylsilyls, and diarylsilyls. Normally, only a single bridge is used in the single-site catalyst, but complexes with two bridging groups can be used. Bridging the ligand changes the geometry around the transition metal and can improve catalyst activity and other properties, such as molecular weight, comonomer incorporation, and thermal stability.
Other suitable ligands include halides and C
1
-C
20
alkoxy, siloxy, or dialkylamido ligands. Particularly preferred ligands are halides.
The organometallic compound is immobilized on a support to form the supported catalyst of the invention. The support is preferably a porous material. The support can be inorganic oxides, inorganic chlorides, and organic polymer resins, or mixtures thereof. Preferred inorganic oxides include oxides of Group 2, 3, 4, 5, 13, or 14 elements. Preferred inorganic chlorides include chlorides of the Group 2 elements. Preferred organic polymer resins include polystyrene, styrene-divinylbenzene copolymers, and polybenzimidizole. Particularly preferred supports include silica, alumina, silica-aluminas, magnesias, titania, zirconia, magnesium chloride, and polystyrene.
Preferably, the support has a surface area in the range of about 10 to about 700 m
2
/g, more preferably from about 50 to about 500 m
2
/g, and most preferably from about 100 to about 400 m
2
/g. Preferably, the pore. volume of the support

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