Ethylene polymerization process

Details Ethylene polymerization process Ethylene polymerization process

2001-05-23

2003-06-24

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...

C526S131000, C526S151000, C526S134000, C526S148000, C526S160000, C526S170000, C526S348000, C526S348200, C526S348500, C526S348600, C526S351000, C526S352000

Reexamination Certificate

active

06583240

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to ethylene polymerization using single-site catalysts. More particularly, the invention relates to a process that produces polyethylene having an increased long-chain-branch index.
BACKGROUND OF THE INVENTION
Single-site catalysts for olefin polymerization are known. They can be divided into metallocenes and non-metallocenes. Metallocene single-site catalysts are transition metal compounds that contain cyclopentadienyl (Cp) or Cp derivative ligands. Non-metallocene single-site catalysts contain ligands other than Cp but have similar catalytic characteristics to the metallocenes. The non-metallocene single-site catalysts often contain heteroatomic ligands, e.g., boraaryl (see U.S. Pat. No. 6,034,027), pyrrolyl (U.S. Pat. No. 5,539,124), azaborolinyl (U.S. Pat. No. 5,756,611) and quinolinyl (U.S. Pat. No. 5,637,660). Single-site catalysts produce polyethylenes having many properties that are not available to those made with Ziegler catalysts, for example, narrow molecular weight distribution and low density.
Successful production of polyethylene with the newly developed single-site catalysts offers many challenges. First, the catalysts often need to be modified for desirable activity and stability. For example, copending Appl. Ser. No. 09/318,009 teaches in-situ alkylation of a transition metal complex that has at least one labile ligand with an alkyl aluminum compound in the polymerization system.
Polyethylene and other olefin polymers made with single-site catalysts are highly desirable if they can be produced in existing equipment. Successful commercial production often requires the polyethylene to have a relatively high bulk density (usually greater than about 0.30 g/cm
3
). Low bulk density gives a low production rate, causes difficulty in operation, and often results in inferior product quality. Copending Appl. Ser. No. 09/593,878 (Docket No. 88-1023A) teaches how to increase bulk density of polyethylene by premixing supported boraaryl single-site catalysts with an alkyl aluminum.
Moreover, single-site catalysts often produce olefin polymers of narrow molecular weight distributions. The uniformity of molecular weight distribution, although improving tensile strength and other physical properties of polymer products, makes the thermal processing more difficult. U.S. Pat. No. 6,127,484, for example, teaches a multiple-zone, multiple-catalyst process for making polyethylene. The polymer produced has a broad molecular weight distribution and improved processability.
It is also known that increasing long-chain branching can improve processability of polyethylene made with single-site catalysts (see WO 93/08221). The existence of long-chain branching in polyethylene is particularly important for blown film extrusion and blow molding processes. However, achieving long-chain branching often requires the use of specific catalysts. For example, WO 93/08221 teaches how to increase the concentration of long-chain branches in polyethylene by using constrained-geometry single-site catalysts.
New methods for increasing long-chain branching in polyethylene are needed. Ideally, the method would use a readily available single-site catalyst and would be easy to practice.
SUMMARY OF THE INVENTION
The invention is a process for producing polyethylene. The process uses a Group 4 metal single-site catalyst that contains at least one boraaryl ligand. The catalyst is alkylated with triisobutyl aluminum (TIBAL). The process gives a polyethylene having an increased long-chain-branch index (LCBI).
We have surprisingly found that when the catalyst is alkylated with TIBAL, the polyethylene produced has LCBI greater than about 1. When the catalyst is not alkylated or is alkylated with triethyl aluminum, the polyethylene has LCBI less than 1.
DETAILED DESCRIPTION OF THE INVENTION
The invention is a process for making polyethylene having an increased long-chain-branch index (LCBI). LCBI is a rheological index used to characterize low levels of long-chain branching in essentially linear polyethylenes. LCBI is defined as:
LCBI
=
η
0
0.179
4.8
·
[
η
]
-
1
where &eegr;
0
is the limiting, zero-shear viscosity (Poise) at 190° C. and [&eegr;] is the intrinsic viscosity in trichlorobenzene at 135° C.(dL/g). LCBI is based on observations that low levels of long-chain branching, in an otherwise linear polymer, result in a large increase in melt viscosity, &eegr;
0
, with no change in intrinsic viscosity, [&eegr;]. See R. N. Shroff and H. Mavridis, “Long-Chain-Branching Index for Essentially Linear Polyethylenes,” Macromolecules, Vol. 32 (25), pp. 8454-8464 (1999). Higher LCBI means a greater number of long-chain branches per polymer chain. When LCBI equals zero, there is no long-chain branching.
A Group 4 metal single-site catalyst is used. Preferably, the metal is zirconium. The catalyst contains at least one boraaryl ligand. Suitable boraaryl ligands include substituted or unsubstituted borabenzenes, boranaphthalenes, boraanthracenes, and boraphenanthrenes. Preferably, the boraaryl ligand is borabenzene or a substituted borabenzene, e.g., 1-methylborabenzene. U.S. Pat. Nos. 5,554,775, 5,637,659, and 6,034,027, the teachings of which are herein incorporated by reference, teach how to prepare catalysts that contain a boraaryl ligand.
In addition to a boraaryl ligand, the catalyst contains other ligands. The total number of ligands satisfies the valence of the transition metal. The ligands can be bridged or non-bridged. Examples include substituted or unsubstituted cyclopentadienyls, indenyls, fluorenyls, halides, C
1
-C
10
alkyls, C
6
-C
15
aryls, C
7
-C
20
aralkyls, dialkylamino, thioether, siloxy, alkoxy, and the like, and mixtures thereof. Halides, cyclopentadienyls, and indenyls are preferred.
Examples of boraaryl-based single-site catalysts are (borabenzene)(cyclopentadienyl)zirconium dichloride, (1 -methylborabenzene)(cyclopentadienyl)zirconium dichloride, (borabenzene)(indenyl)-zirconium dichloride, (1-methylborabenzene)(indenyl)zirconium dichloride, (boranaphthalene)(cyclopentadienyl)zirconium dichloride, and (boraanthracenes)(cyclopentadienyl) zirconium dichloride.
Optionally, the 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, titanias, zirconias, magnesium chloride, and crosslinked polystyrene. Silica is most preferred.
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 about 50° C. to about 800° C. More preferably, the temperature is from about 50° C. to about 300° C.
Suitable chemical modifiers include organoaluminum, organosilicon, organomagnesium, and organoboron compounds. Organosilicon and organoboron compounds, such as hexamethyldisilazane and triethylborane, are preferred. Suitable techniques to support a single-site catalyst are taught, for example, in U.S. Pat. No. 6,211,311, the teachings of which are incorporated herein by reference.
The catalyst is used with an activator. Activators can be combined with the Group 4 metal catalyst and the optional support or they can be added separately to the polymerization reactor. Suitable activators include anionic compounds of boron and aluminum, trialkylborane and triarylborane compounds, and the like. Examples are lithium tetrakis(pentafluorophenyl) borate, triphenylcarbenium tetrakis(pentafluorophenyl) borate, tris(pentafluorophenyl) borane, methyl alumoxane (MAO), the like and mixtures thereof. Activators are generall

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