Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2003-01-01
2004-11-16
Choi, Ling-Siu (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S124300, C526S124100, C526S348000, C502S103000, C502S126000
Reexamination Certificate
active
06818711
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to using a catalyst system for the polymerization of olefins. More specifically, the invention relates to the polymerization of olefins using a catalyst system comprising a Ziegler-Natta catalyst and a diether-based external donor.
BACKGROUND OF THE INVENTION
Olefins, also called alkenes, are unsaturated hydrocarbons whose molecules contain one or more pairs of carbon atoms linked together by a double bond. When subjected to a polymerization process, olefins are converted to polyolefins, such as polyethylene and polypropylene. One commonly used polymerization process involves contacting the olefin monomer with a Ziegler-Natta catalyst system that includes a conventional Ziegler-Natta catalyst, a co-catalyst, and one or more electron donors. Examples of such catalyst systems are provided in U.S. Pat. Nos. 4,107,413; 4,294,721; 4,439,540; 4,114,319; 4,220,554; 4,460,701; 4,562,173; and 5,066,738, which are incorporated by reference herein.
Conventional Ziegler-Natta catalysts comprise a transition metal compound, e.g., titanium tetrachloride (TiCl
4
), supported on an inert solid, such as a magnesium compound. An internal electron donor is typically added to the Ziegler-Natta catalyst during its preparation and can be combined with the support or otherwise complexed with the transition metal compound. Examples of internal electron donors include amines, amides, esters, ketones, nitriles, and phosphines. The internal electron donor is used to reduce the atactic form of the resulting polymer, thus decreasing the amount of xylene solubles. A polymer is “atactic” when its pendant groups are arranged in a random fashion on both sides of the chain of the polymer. In contrast, a polymer is “isotactic” when all of its pendant groups are arranged on the same side of the chain and “syndiotactic” when its pendant groups alternate on opposite sides of the chain.
During the polymerization process, an external electron donor may be added to the catalyst to further control the amount of atactic polymer produced. Examples of commonly used external electron donors include organosilicon compounds, such as diphenyldimethoxysilane (DPMS), cyclohexylmethyl dimethoxysilane (CMDS), and dicyclopentyl dimethoxysilane (CPDS). A co-catalyst, such as an organoaluminum compound, also may be used in conjunction with the Ziegler-Natta catalyst to activate the catalyst by initiating the polymer chain.
In the polymerization process, hydrogen can be fed to the catalyst system to terminate the chain formation of the polymer, thereby altering the molecular weight and the melt flow rate of the polymer. The hydrogen response of the Ziegler-Natta catalyst affects the molecular weight of the polymer produced. In particular, an increase in hydrogen response produces a lower molecular weight polymer (i.e., shorter chain length), and a decrease in hydrogen response produces a higher molecular weight polymer (i.e., longer chain length). As molecular weight decreases, the melt flow rate (MFR) of the polymer increases. Polyolefins having relatively high MFR values are desirable because they offer numerous processing advantages. For example, lower temperatures and pressures are required for the extrusion of such polyolefins. Further, the use of such polyolefins reduces the wear on the extrusion equipment. Also, high MFR values are useful in high throughput (i.e., fast rates) molding applications as the resin flows more easily.
The present invention provides an improved catalyst system having an external electron donor that is particularly compatible with conventional Ziegler-Natta catalysts. The addition of this external electron donor forms a catalyst system that is more responsive to hydrogen. As such, lower molecular weight polyolefins with higher MFR values may be formed from the catalyst system.
SUMMARY OF THE INVENTION
The present invention includes a catalyst system for the polymerization of olefins comprising a conventional Ziegler-Natta catalyst and an external election donor selected from the group consisting of diethers and combinations thereof. The catalyst system comprises a Ziegler-Natta catalyst having a transition metal compound generally represented by the formula:
MR′
x
where M is a transition metal, R′ is a halogen or a hydrocarboxyl, and x is the valence of the transition metal. The transition metal compound can be TiCl
4
. The Ziegler-Natta catalyst may also comprise an internal electron donor, such as phthalate. The catalyst system further includes an external electron donor selected from the group consisting of diethers and a co-catalyst selected from the group of organoaluminum compounds. In one embodiment, the external electron donor is 2,2-diisobutyl-1,3-dimethoxypropane, and the co-catalyst is triethylaluminum.
The present invention further includes a process for polymerizing olefins that includes introducing a catalyst system comprising a Ziegler-Natta catalyst and an external election donor selected from the group consisting of diethers and combinations thereof into a reaction zone, and feeding one or more olefin monomers to the reaction zone under conditions suitable for polymerization. The Ziegler-Natta catalyst can comprise a transition metal compound, e.g., TiCl
4
, and an internal electron donor, e.g., phthalate. The external election donor can comprise 2,2-diisobutyl-1,3-dimethoxypropane. The catalyst system may further comprise an organoaluminum compound acting as a co-catalyst. The polymerization process can also include extracting polyolefin polymers. The polymerization process can produce polyolefins having a relatively high hydrogen response and containing a relatively low level of xylene solubles, such as for example less than 3 wt %.
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Choi Ling-Siu
Fina Technology, Inc.
Madan Mossman & Sriram P.C.
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