Ethylene polymerization process

Details Ethylene polymerization process Ethylene polymerization process

2000-02-29

2001-07-03

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...

C526S129000, C526S134000, C526S160000, C526S161000, C526S172000, C526S943000, C502S117000

Reexamination Certificate

active

06255415

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to an ethylene polymerization process. More particularly, the invention relates to a gas phase polymerization of ethylene with a single-site catalyst. The process produces polyethylene having a reduced density.
BACKGROUND OF THE INVENTION
Linear low density polyethylene (LLDPE), which has a density from 0.916 to 0.940 g/mL, has penetrated all traditional markets for polyethylene, including film, molding, pipe, and wire and cable. Due to its strength and toughness, LLDPE has been largely used in the film market, such as produce bags, shopping bags, garbage bags, diaper liners, and stretch wrap. LLDPE has been primarily made with conventional Ziegler catalysts. It is typically produced by copolymerization of ethylene with a long chain &agr;-olefin such as 1-butene, 1-hexene, or 1-octene.
In the early 1980's, Kaminsky discovered a new class of olefin polymerization catalysts known as metallocenes (see U.S. Pat. Nos. 4,404,344 and 4,431,788). A metallocene catalyst consists of a transition metal compound that has one or more cyclopentadienyl (Cp) ligands. Unlike Ziegler catalysts, metallocene catalysts are usually soluble in olefins or polymerization solvents and give homogeneous polymerization systems. Since these catalysts have a single reactive site (compared with multiple reactive sites of Ziegler catalysts), they are also called “single-site” catalysts. Metallocene catalysts are more reactive than conventional Ziegler catalysts, and they produce polymers with narrower molecular weight distributions. Because single-site catalysts enhance incorporation of long chain &agr;-olefin comonomers into polyethylene, they are of particular interest in the production of LLDPE.
Over the last decade, non-metallocene single-site catalysts have also been developed rapidly. Non-metallocene single-site catalysts contain non-Cp ligands, which are usually heteroatomic ligands, e.g., boraaryl, azaborolinyl, pyridinyl, pyrrolyl, indolyl, carbazolyl, or quinolinyl groups. The development of non-metallocene single-site catalysts has provided the polyolefin industry with more choices of catalysts and opportunities for optimizing the products or production processes.
Non-metallocene single-site catalysts have most of the characteristics of metallocene catalysts, including high activity. However, they produce polyethylenes that have relatively high density. For example, boraaryl-based single-site catalysts produce polyethylenes that have densities from about 0.93 to about 0.97 g/mL (see U.S. Pat. No. 5,554,775). It is of significant interest to further lower the density of the polyethylenes produced with non-metallocene single-site catalysts.
SUMMARY OF THE INVENTION
The invention is a gas phase polymerization process for making ethylene polymers, particularly polymers that have reduced densities. The process uses a single-site catalyst containing at least one heteroatomic ligand. The process comprises two steps: 1) supporting the single-site catalyst; and 2) polymerizing ethylene in gas phase over the catalyst.
We have surprisingly found that the gas phase process of the invention significantly increases the incorporation of &agr;-olefin into polyethylene and lowers the polyethylene density compared to slurry phase process. Using the gas phase process of the invention, we have successfully prepared ethylene polymers that have densities similar to those prepared with metallocene single-site catalysts.
DETAILED DESCRIPTION OF THE INVENTION
The invention is a gas phase polymerization process for preparing a linear low density polyethylene that has a density within the range from about 0.890 to 0.930 g/mL. The process includes supporting a single-site catalyst. The single-site catalysts suitable for use in the invention are organometallic compounds having a heteroatomic ligand. Suitable metals are Group 3-10 transition or lanthanide metals. Preferably, the metal is titanium, zirconium, or hafnium. Zirconium is particularly preferred. Suitable heteroatomic ligands include substituted or non-substituted boraaryl, azaborolinyl, pyridinyl, pyrrolyl, indolyl, carbazolyl, and quinolinyl, and the like. Preferred heteroatomic ligands are boraaryl and quinolinyl.
In addition to a heteroatomic ligand, other ligands are used. The total number of ligands satisfies the valence of the transition metal. The ligands can be bridged or non-bridged. Other suitable ligands include substituted or non-substituted cyclopentadienyls, indenyls, fluorenyls, halides, C
1
-C
10
alkyls, C
6
-C
15
aryls, C
7
-C
20
aralkyls, dialkylamino, siloxy, alkoxy, and the like, and mixtures thereof. Cyclopentadienyls and indenyls are preferred.
Methods for preparing heteroatomic ligand-containing single-site catalysts are available in the literature. For example, U.S. Pat. Nos. 5,554,775, 5,539,124, 5,756,611, and 5,637,660, the teachings of which are herein incorporated by reference, teach how to make single-site catalysts that contain boraaryl, pyrrolyl, azaborolinyl, or quinolinyl ligands.
The single-site catalyst is immobilized on a support. The support is preferably a porous material such as inorganic oxides and chlorides, and organic polymer resins. Preferred inorganic oxides include oxides of Group 2, 3, 4, 5, 13, or 14 elements. Preferred supports include silica, alumina, silica-aluminas, magnesias, titania, zirconia, magnesium chloride, and crosslinked polystyrene. Preferably, the support has a surface area in the range of about 10 to about 700 m
2
/g, a pore volume in the range of about 0.1 to about 4.0 mL/g, an average particle size in the range of about 10 to about 500 &mgr;m, and an average pore diameter in the range of about 10 to about 1000 Å. They are preferably modified by heat treatment, chemical modification, or both. For heat treatment, the support is preferably heated at a temperature from about100° C. to about 800° C. Suitable chemical modifiers include organoaluminum, organosilicon, organomagnesium, and organoboron compounds.
The single-site catalysts are supported using any known techniques. For example, U.S. Pat. Nos. 5,747,404 and 5,744,417, the teachings of which are incorporated herein by reference, teach how to support single-site catalysts onto a polysiloxane or a silylamine polymer. In one suitable method, the single-site catalyst is dissolved in a solvent and combined with the support. Evaporation of the solvent gives a supported catalyst.
The catalyst is used with an activator. Activators can be either mixed with single-site catalysts and supported together on a support or added separately to the polymerization. Suitable activators include alumoxane compounds, alkyl aluminums, alkyl aluminum halides, anionic compounds of boron or aluminum, trialkylboron and triarylboron compounds, and the like. Examples are methyl alumoxane, ethyl alumoxane, triethylaluminum, trimethylaluminum, diethylaluminum chloride, lithium tetrakis(pentafluorophenyl) borate, triphenylcarbenium tetrakis(pentafluorophenyl) borate, lithium tetrakis(pentafluorophenyl) aluminate, tris(pentafluorophenyl) boron, tris(pentabromophenyl) boron, and the like. Other suitable activators are known, for example, in U.S. Pat. Nos. 5,756,611, 5,064,802, and 5,599,761, and their teachings are incorporated herein by reference.
Activators are generally used in an amount within the range of about 0.01 to about 100,000, preferably from about 0.1 to about 1,000, and most preferably from about 0.5 to about 300, moles per mole of the single-site catalyst.
The process of the invention includes polymerizing ethylene in the gas phase over the supported catalyst. Methods and apparatus for gas phase polymerization of ethylene with Ziegler catalysts are well known, and they are suitable for use in the process of the invention. For example, U.S. Pat. No. 5,859,157, the teachings of which are herein incorporated by reference, teaches in detail a gas phase polymerization of ethylene with a Ziegler catalyst.
In one suitable method, the polymerization is conducted batchwise where ethylene is gradually fed into a react

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