Polymerization of Vinyl Aromatic Monomers Using Cyclopentadienyl Zirconium Catalysts

The present invention relates to catalysts. More particularly, the present invention relates to such novel catalysts containing a cyclopentadienyl zirconium complex and an organo aluminum compound. Such catalysts are particularly useful for the preparation of polymers of vinyl aromatic monomers having a high degree of syndiotactic molecular structure.

It is previously known in the art to prepare polystyrene having a high degree of syndiotacticity by polymerizing styrene monomer in the presence of a catalyst containing a titanium compound and an aluminum organo compound. This work has been reported by N. Ishihara, et al., in Macromolecules, 19, Page 2464-2465 (1986).

The same researchers later reported that zirconium based catalysts, specifically ZrCl.sub.4, and biscyclopentadienyl zirconium dichloride in combination with polymethylaluminoxane gave only atactic polystyrene, Polymer Preprints (Japan), vol. 36, No. 2 May 12, 1987.

In EPO No. 210,615 syndiotactic polystyrene polymers were prepared utilizing titaniumtetraethoxide, and oyclopentadienyltitaniumtrichloride complexes in combination with a polymethylaluminoxane cocatalyst.

According to the present invention there is now provided a novel catalyst comprising the reaction product of polymethylaluminoxane and a cyclopentadienyl zirconium (IV) complex corresponding to the formula: CpZrR.sub.3, wherein Cp is a .pi.-bonded cyclopentadienyl or substituted cyclopentadienyl ligand having from 5 to 20 carbons, and R is independently each occurrence selected from the ligand group consisting of halides: alkoxides and aryloxides corresponding to the formula:

OR'; amides corresponding to the formula: NR'.sub.2 ; phosphides corresponding to the formula: PR'.sub.2 ; and .beta.-diketonates corresponding to the formula: R'--C(O)--CH--C(O)--R', and R' is alkyl or aryl of up to 12 carbons. In addition, there is also included within the present invention a process for the use of such novel catalysts in the preparation of polymers of vinyl aromatic monomers having a high degree of syndiotacticity. Such polymers may be usefully employed in the preparation of articles and objects having an extremely high resistance to deformation due to the effects of temperature.

The polymethylaluminoxane may be prepared according to any known technique. One such technique includes the reaction of trimethylaluminum and a hydrated metal salt as disclosed by Kaminsky in U.S. Pat. No. 4,544,762, the teachings of which are herein incorporated in their entirety by reference thereto. For purposes of calculating the molar ratio of aluminum:zirconium in the catalysts of the present invention, the polymethylaluminoxane is assigned a repeating unit structure corresponding to the formula: (Al(CH.sub.3) O).

Following preparation of polymethylaluminoxane a cyclopentadienyl zirconium (IV) complex as previously defined is added to the polymethylaluminoxane along with additional diluent if desired. In addition to unsubstituted cyclopentadienyl derivatives the metal complex employed to prepare the catalysts of the present invention may include alkyl or aryl substituted derivatives and ring fused derivatives thereof such as indenyl zirconium complexes. It is understood that the cyclopentadienyl zirconium complex may be employed in the form of a precursor that is capable of generating the desired cyclopentadienyl zirconium catalyst complex in situ. An example includes such cyclopentadienyl zirconium complexes additionally incorporating one or more molecules of any nature in the crystal structure, such as aliphatic or aromatic alcohol molecules associated with the crystal structure via Van der Waals forces. In the preparation of the catalyst composition, the cyclopentadienyl zirconium (IV) complex and the polymethylaluminoxane can be combined in any order. The catalyst components may be suitably mixed in an inert atmosphere such as nitrogen, argon, xenon, or combinations thereof. The components are mixed at any suitable temperature preferably from about 0.degree. C. to about 50.degree. C., more suitably about 25.degree. C.

The components of the catalyst are employed in quantities which provide an atomic ratio of polymethylaluminoxane to zirconium (Al:Zr) suitably from about 50:1 to about 10,000:1, more suitably from about 100:1 to about 5,000:1, most suitably from about 250:1 to about 1000:1.

Preferred cyclopentadienyl zirconium (IV) complexes are cyclopentadienyl zirconium trichloride and cyclopentadienyl zirconium triphenoxide.

Polymerizations utilizing the catalyst of the present invention may be conducted at temperatures from about 25.degree. C. to about 100.degree. C. Preferred temperatures are from about 30.degree. C. to about 60.degree. C.

Preferred polymerizations are those involving vinyl aromatic monomers due to the discovery that such polymers have a high degree of syndiotacticity. By the term "high degree" is meant at least about 50% syndiotacticity on a molar basis as determined by C.sup.13 nuclear magnetic resonance spectroscopy. Suitable vinyl aromatic monomers include those monomers corresponding to the formula: ##STR1## wherein each R is independently hydrogen, an aliphatic, cycloaliphatic or aromatic hydrocarbon group having up to about 10 carbon atoms, or a halogen atom. A preferred vinyl aromatic monomer is styrene. Initially it is desirable to provide catalyst concentrations so as to provide a molar ratio of vinyl aromatic monomer to zirconium of from about 100:1 to about 30,000:1, most preferably 1,000:1 to about 10,000:1. Additionally, preferred polymers according to the invention have a syndiotacticity of greater than about 65%.

The polymerization is normally oonducted in the presence of a solvent for the monomer. Examples include aliphatic and aromatic hydrocarbons, as well as mixtures thereof. Preferred solvents comprise the C.sub.4-20 alkanes, especially branched chain alkanes and mixtures thereof. A particularly desirable solvent for the polymerization is isooctane, or blends thereof such as Isopar E.RTM., available from Exxon. Suitable amounts of solvent are employed to provide a monomer concentration from about 5% to about 100% by weight. During the polymerization, polymer may precipitate from the reaction mixture as it is formed.

As in other similar polymerizations it is highly desirable that the monomers and solvents employed be of sufficiently high purity that catalyst deactivation not occur. Any suitable technique for monomer purification such as devolatilization at reduced pressures, or contacting with molecular sieves or high surface area alumina may be employed.

Purification of the resulting polymer to remove entrained catalyst may also be desired by the practitioner. Entrained catalyst may generally be identified by residues of ash on pyrolysis of the polymer that are attributable to aluminum and zirconium values. A suitable technique for removing such compounds is by solvent extraction, eg. extraction utilizing hot, high boiling chlorinated solvents, followed by filtration.

Having described the invention, the following example is provided as further illustrative and is not to construed as limiting.

In a nitrogen atmosphere glove box, a glass reactor was charged with Isopar E.RTM. (32 milliliters) along with a suspension of polymethylaluminoxane (PMA) (1.3 grams, 22.4 mmole, in 29 milliliters of Isopar E.RTM.). A 10 percent toluene solution of cyclopentadienyl zirconium trichloride (480 microliters, 22.4 mmole slurried in 10 ml of Isopar-E, ) was added with stirring at room temperature. After approximately 30 seconds, highly pure deariated styrene monomer (15.4 ml, 163 mmoles) was added. The reactor was sealed and placed in a shaker in a hot water bath and maintained at 50.degree. C. for 20 hours. The mixture was then hydrolyzed by the addition of 20 milliliters of methanol. The reactor contents were added to 50 milliliters of aqueous 1N. hydrochloric acid for five minutes. The resulting syndiotactic polystyrene was collected by filtration, washed with methanol and vacuum dried at 75.degree. C. for 36 hours. Yield was 1.1 grams (7.9 percent). Melting point as determined by differential scanning calorimetry was 212.degree. C. Syndiotacticity as determined by C.sup.13 NMR was greater than 65%.

The procedure of Example 1 was substantially repeated employing 22.4 mmoles of cyclopentadienyl zirconium triphenoxide catalyst. After working up according to the procedure previously described, the yield of polystyrene was 0.4 grams (3.0 percent). The melting point was 210.degree. C. Syndiotacticity was greater than 90%.