Process for making polyolefins

Details Process for making polyolefins Process for making polyolefins

1999-09-22

2001-02-27

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...

C526S089000, C526S160000, C526S161000, C526S943000, C526S133000, C526S134000, C526S348600, C526S352000, C526S904000, C502S152000, C502S202000

Reexamination Certificate

active

06194527

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a process for making polyolefins. In particular, the invention provides an efficient way to make polyolefins with high molecular weights and low residual aluminum contents using single-site catalysts.
BACKGROUND OF THE INVENTION
Interest in single-site (metallocene and non-metallocene) catalysts continues to grow rapidly in the polyolefin industry. These catalysts are more reactive than 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, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Traditional metallocenes include cyclopentadienyl, indenyl, or fluorenyl groups, which may contain other substituents or bridging groups. More recently, single-site catalysts in which a heteroatomic ring ligand replaces a cyclopentadienyl group have appeared, including boraaryl (see, e.g., U.S. Pat. No. 5,554,775), pyrrolyl (U.S. Pat. No. 5,539,124), and azaborolinyl groups (U.S. Pat. No. 5,902,866).
Many single-site catalysts require high levels of an alumoxane activator (e.g., polymethalumoxane). When used at such high concentrations, alumoxanes cause chain-transfer reactions that undesirably limit polyolefin molecular weight. In addition, high residual aluminum in the polymer adversely impacts mechanical properties, so the polyolefin product is normally treated after manufacture to remove it.
Boron compounds such as triphenylcarbenium tetrakis-(pentafluorophenyl)borate or tris(pentafluorophenyl)borane can be used instead of alumoxanes to activate some single-site catalysts. Unfortunately, however, these catalyst systems are usually less active and less stable than alumoxane-activated catalysts. Because boron-containing activators eliminate the need for high levels of alumoxanes, it would be valuable to develop a process that retains this advantage, yet overcomes the activity and stability issues.
Single-site catalysts also generally lack thermal stability compared with Ziegler-Natta catalysts. Olefin polymerizations catalyzed by single-site catalysts are normally performed at relatively low reaction temperatures (less than about 100° C.) to prolong catalyst lifetime. Higher reaction temperatures would ordinarily be desirable, however, because polymerization rates generally escalate with increasing temperature. Thus, a process that enhances the thermal stability of single-site catalysts and allows higher reaction temperatures to be used would be valuable.
Prepolymerization of a small proportion of olefin with single-site catalysts and alumoxane activators to make prepolymer complexes is known. For example, U.S. Pat. No. 4,923,833 teaches to prepolymerize a portion of ethylene with bis(cyclopentadienyl)zirconium dichloride and an activating amount of methalumoxane, followed by addition of the prepolymer complex to a second reactor that is used for the main polymerization. U.S. Pat. No. 5,308,811 similarly teaches to use a prepolymerization technique with an alumoxane-activated single-site catalyst (see column 11 and Examples 23-26). Neither reference explains why prepolymerization is used, and neither suggests making the prepolymer complex substantially in the absence of an alumoxane or in the presence of a boron-containing compound.
An improved process for making polyolefins with single-site catalysts is needed. In particular, a process that gives products with low residual aluminum is required. A preferred process would avoid chain-transfer reactions and allow the production of high-molecular-weight polymers. A valuable process would use a boron-activated catalyst with both high activity and good stability. Ideally, the polymerization process could be performed at relatively high reaction temperatures without significantly deactivating the catalyst.
SUMMARY OF THE INVENTION
The invention is a two-step process for making a polyolefin. First, a single-site catalyst precursor reacts with a boron-containing activator and a first olefin substantially in the absence of an alumoxane to produce a stable, prepolymer complex. The prepolymer complex is then used as a catalyst to polymerize a second olefin in the presence of a scavenging amount of an alumoxane to produce a polyolefin.
The process of the invention is easy to practice and affords high-molecular-weight polyolefins with low residual aluminum contents. Surprisingly, boron-containing activators can be used—and high levels of alumoxanes can be avoided—while retaining high catalyst activity. Moreover, the process can be performed at temperatures greater than 100° C. while maintaining good catalyst activity.
DETAILED DESCRIPTION OF THE INVENTION
In the first step of the process of the invention, a single-site catalyst precursor reacts with a boron-containing activator and a first olefin to give a stable, prepolymer complex.
Single-site catalyst precursors are organometallic complexes that can be converted to active olefin polymerization catalysts, usually by contacting them with a suitable activator. “Single-site” catalysts are distinct chemical species rather than mixtures of different species. They typically give polyolefins with characteristically narrow molecular weight distributions (Mw/Mn<3) and good, uniform comonomer incorporation.
Single-site catalyst precursors useful in the invention include a transition or lanthanide metal, M, preferably from Group 3 to Group 10 of the Periodic Table. More preferred catalyst precursors include a Group 4 to 6 transition metal; most preferably, the precursor contains a Group 4 metal such as titanium or zirconium.
The single-site catalyst precursor preferably includes one or more polymerization-stable, anionic ligands. Examples include substituted and unsubstituted cyclopentadienyl, fluorenyl, and indenyl, or the like, 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. A preferred group of polymerization-stable ligands are heteroatomic ligands such as boraaryl, pyrrolyl, indolyl, quinolinyl, pyridinyl, and azaborolinyl 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 incorporated herein by reference. The catalyst precursor also usually includes one or more labile ligands such as halides, alkyls, alkaryls, aryls, dialkylaminos, or the like. Particularly preferred are halides, alkyls, and alkaryls (e.g., chloride, methyl, benzyl).
The polymerization-stable ligands can be bridged. Groups that can be used to bridge the ligands include, for example, methylene, ethylene, 1,2-phenylene, and dialkyl silyls. Normally, only a single bridge is included. Bridging changes the geometry around the transition or lanthanide metal and can improve catalyst activity and other properties such as comonomer incorporation.
Exemplary catalyst precursors: bis(cyclopentadienyl)zirconium dimethyl, bis(cyclopentadienyl)zirconium dichloride, bis(indenyl)titanium dibenzyl, bis(fluorenyl)zirconium dimethyl, 8-quinolinoxy(cyclopentadienyl)titanium dimethyl, bis(2-pyridinoxy)titanium diethyl, (1-dimethylaminoborabenzene)cyclopentadienylzirconium dichloride, bis(1-methylborabenzene)zirconium dimethyl, bis(indolyl)zirconium dimethyl, and the like.
A boron-containing activator ionizes the catalyst precursor to produce an active polymerization catalyst. Preferred activators are organoboranes, which include boron and one or more alkyl, aryl, or aralkyl groups. Particularly preferred activators incorporate perfluoroaryl groups. Suitable activators include substituted and unsubstituted trialkyl and triarylboranes such as tris(pentafluorophenyl)borane, triphenylborane, tri-n-octylborane, and the like. Tris(pentafluorophenyl)borane is especially preferred. Suitable activators also include ionic borates in which a boron is bonded to four alkyl, aryl, or aralkyl groups, most pref

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