Catalyst system to produce highly crystalline polypropylene

Details Catalyst system to produce highly crystalline polypropylene Catalyst system to produce highly crystalline polypropylene

1995-06-07

2003-10-21

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...

C526S107000, C526S125100, C526S128000, C502S103000, C502S104000, C502S115000, C502S116000, C502S118000

Reexamination Certificate

active

06635734

ABSTRACT:

BACKGROUND
1. Technical Field
The present invention provides a catalyst system for the polymerization of propylene to produce highly crystalline polypropylene that includes the combination of a conventional supported Ziegler-Natta catalyst with an electron donor compound having the general formula SiR
m
(OR′)
4-m
where R is selected from the group consisting of an alkyl group, a cycloalkyl group, an aryl group and a vinyl group; R′ is an alkyl group; and m is 0-3, wherein when R is an alkyl group, R may be identical with R′; when m is 0, 1 or 2, the R′ groups may be identical or different; and when m is 1, 2 or 3, the R groups may be identical or different. The electron donor preferably has the formula:
wherein R
1
is an alkyl or cycloalkyl group containing a primary, secondary or tertiary carbon atom attached to the silicon atom; R
2
and R
3
alkyl or aryl groups; and R
4
is an alkyl or cycloalkyl group with a secondary or tertiary carbon atom attached to the silicon atom, R
1
and R
4
can be the same or different and R
2
and R
3
can be the same or different.
2. Description of Prior Art
Catalyst systems for the polymerization of olefins are well known in the art. Typically, these systems include a Ziegler-Natta type polymerization catalyst; a co-catalyst, usually an organoaluminum compound; and an electron donor, usually an organosilicon compound. Examples of such catalyst systems are shown in the following U.S. Pat. Nos. 4,107,413; 4,2394,721; 4,439,540; 4,115,319; 4,220,554; 4,460,701; and 4,562,173; the disclosures of these patents are hereby incorporated by reference. These are just a few of the scores of issued patents relating to catalysts and catalyst systems designed primarily for the polymerization of propylene and ethylene.
A conventional supported Ziegler-Natta polymerization catalyst generally comprises a complex derived from a halide of a transition metal, for example, titanium, chromium or vanadium with a metal hydride and/or a metal alkyl that is typically an organoaluminum compound. The catalyst is usually comprised of a titanium halide supported on a magnesium compound complexed with an alkyaluminum. An external electron donor or selectivity control agent (SCA) can be added to improve the stereoregulation of the catalyst so that a more crystalline polymer is produced.
The development of these polymerization catalysts has improved catalyst and polymer properties. In addition to the improved catalysts, improved activation methods have also lead to increases in the catalyst efficiency. One improved activation method includes a process for pre-polymerizing the catalyst just prior to introducing the catalyst into the reaction zone. One such pre-polymerization process is disclosed in U.S. Pat. No. 4,767,735, the disclosure of which is hereby incorporated by reference.
In addition to the development of new catalysts and new reaction processes, a discovery of an appropriate electron donor to go with a supported Ziegler-Natta catalysts in forming a total catalyst system has been found to be of great benefit to the polymerization art in leading to dramatic improvements in the catalyst and polymer properties. In such a catalyst system, it is believed that a co-catalyst activates the catalyst and provides the initiation of a polymer chain. A co-catalyst that works well with a supported Ziegler-Natta catalyst component is an organoaluminum compound, usually an alkyaluminum and most typically a trialkylaluminum, such as triethylaluminum (TEA) or triisobutylaluminum (TIBAI). Examples of other organoaluminum compounds useful as a co-catalyst for a conventional supported Ziegler-Natta catalyst component include an alkylaluminum dihalide and a dialkylaluminum halide.
An electron donor compound is used in the polymerization reaction to reduce the atactic form of the polymer thereby giving control of and increasing the production of stereoregular or crystalline polymers. Although a broad range of compounds are known generally as electron donors, a particular catalyst may have a specific compound or group of compounds with which it is especially compatible. Discovery of an appropriate type of electron donor which gives good catalyst efficiency as well as improved control of the isotactic index of the desired polymer product and other properties of the product would be highly advantageous. One such group of electron donors are disclosed in U.S. Pat. No. 4,927,797.
The present invention comprises another such discovery. It has been surprisingly discovered that a specific group of silane compounds serving as electron donors in combination with a particular type of catalyst results in significant increases in crystallinity of the polymer produced over the previously known crystallinity for this particular type of catalyst as well as other known catalyst systems.
SUMMARY OF THE INVENTION
The present invention provides a catalyst system for the polymerization of propylene wherein the system includes the combination of a particular type of catalyst with a specific group of electron donor compounds which results in significant increases in the crystallinity of the polymer product. The catalyst comprises a conventional supported Ziegler-Natta catalyst component, an organoaluminum co-catalyst and an organosilicon electron donor. This combination results in a catalyst system that produces a polymer product with significantly higher crystallinity (lower xylene solubles) than provided by previous catalyst systems as illustrated by the comparative examples included below. Further, the catalyst system of the present invention using a particular conventional supported Ziegler-Natta catalyst component provides better control of the xylene solubles of the polymer product than with other types of catalyst components. Further, the catalyst system of the present invention using a particular electron donor provides better control of the xylene solubles of the polymer product than with other types of electron donors. These and other beneficial advantages will become more apparent from the following detailed description of the invention and the accompanying examples.
The above-described catalyst exhibits some increase in crystallinity of the polymer product over previously known catalysts when it is paired with a previously used electron donor, but that increase is significantly greater when the catalyst is paired with electron donors as described by the present invention. The invention provides improved operational control of the properties of the polymer product such as the xylene solubles.
The most significant unexpected result obtained from the combination of the catalyst described in U.S. Pat. Nos. 4,784,983 and 4,861,847 and CMDS, DIDS, DTDS, CPDS and CIDS is the dramatic increase in the crystallinity of the polymer produced by the catalyst system in the polymerization of propylene. The catalyst system has been shown to achieve efficiencies higher than 30 kg/g-cat·h for Si/Ti mole ratios within the range 4-200.
The invention also provides a process for the polymerization of propylene. The process comprises:
forming a catalyst by contacting a supported Ziegler-Natta catalyst component with an organoaluminum compound, preferably with a trialkylaluminum;
contacting the catalyst with an electron donor either simultaneously with or after contact with the organoaluminum, the electron donor being an organosilicon compound as described by the formula above,
optionally, pre-polymerizing the catalyst by contacting a small amount of monomer with the catalyst;
introducing the catalyst into a reaction zone under polymerization conditions containing the monomer and, optionally additional amounts of organoaluminum compound and the electron donor; and
withdrawing a polymer product.
If the monomer is propylene, the xylene solubles are within the range of 0.6-3.00 wt %, while the Si/Ti ratio in the reaction zone is within the range 4-200. If the electron donor is dicyclopentyldimethoxysilane and the monomer is propylene, the crystallinity as defined by the isotacticity (per cent pen

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