Polymerization of vinyl monomer, both mono-olefins and conjugated dienes, has been focused on transition metal catalysts since the work of Ziegler and Natta. These catalysts are based on a central transition metal ion or atom surrounded by a set of coordinating ligands and modified by various co-catalysts. These polymerization systems when brought in contact with addition polymerizable monomers polymerize the monomers into polymers.
By controlling the nature of the ligand system, the central transition metal ion or atom and the co-catalyst, highly active catalytic agents can be made. In addition, catalysts can be made that yield polymers with high degrees of additions regularity and in the case of non-ethylene type monomers, stereoregularity or tactiospecificity.
U.S. Pat. No. 3,051,690 discloses a process of polymerizing olefins to controlled, high molecular weight polymers by the controlled addition of hydrogen to a polymerization system that includes a hydrocarbon insoluble reaction product of a Group IVB, VB, VIB and VIII compound and an alkali metal, alkaline earth metal, zinc, earth metal or rare earth metal organometallic compound. It is further known that certain metallocenes such as bis(cyclopentadienyl) titanium or zirconium dialkyls in combination with aluminum alkyl/water cocatalyst form homogeneous catalyst systems for the polymerization of ethylene.
German Patent Application 2,608,863 discloses the use of a catalyst system for the polymerization of ethylene consisting of bis(cyclopentadienyl) titanium dialkyl, aluminum trialkyl and water. (Cyclopentadienyl is sometimes abbreviated as Cp.) While German Patent Application 2,608,933 discloses an ethylene polymerization catalyst system of the general formula (Cp).sub.n ZrR.sub.4-n where n is a number from 1 to 4 and R is a hydrocarbyl group or a metalloalkyl in combination with an aluminum trialkyl cocatalyst and water.
European Patent Appln. No. 0035242 discloses a process for preparing ethylene and atactic propylene polymers in the presence of a halogen-free Ziegler catalyst system of the general formula (Cp).sub.n MR.sub.4-n where n is an integer from 1 to 4, M is a transition metal, especially zirconium, and R is either hydrogen, a C.sub.1 -C.sub.5 alkyl, metalloalkyl group or a other radical in combination with an alumoxane. While U.S. Pat. No. 5,324,800 discloses a catalyst system for polymerizing olefins including a metallocene catalyst represented by the general formula (C.sub.5 R'.sub.m).sub.p R".sub.s (C.sub.5 R'.sub.m)MQ.sub.3-p and R".sub.s (C.sub.5 R'.sub.m).sub.2 MQ' where (C.sub.5 R'.sub.m) is a substituted Cp group and an alumoxane.
Polyolefins can be prepared in a variety of configurations that correspond to the manner in which each new monomer unit is added to a growing polyolefin chain. Four basic configurations are commonly recognized for polyolefins, atactic, hemi-isotactic, isotactic and syndiotactic. Of course, a given polymer may incorporate regions of each configurational type, yet not exhibit the pure or nearly pure configuration and polyethylene can have no tacticity.
Atactic polymers exhibit no regular order of repeat unit orientation in the polymer chain, i.e., the substituents are not regularly ordered relative to a hypothetical plane containing the polymer backbone (the plane is oriented such that the substituents on the pseudo-asymmetric carbon atoms are either above or below the plane). Instead, atactic polymers exhibit a random distribution of substituent orientations.
Besides metallocene catalysts that produce polyethylene and atactic polyolefins, certain metallocenes are also known to produce polymers with varying degrees of stereoregularity or tactiospecificity, such as isotactic, syndiotactic, and hemi-isotactic polymers which have unique and regular repeating stereochemistries or substituent orientations relative to the plane containing the polymer backbone.
Isotactic polymers are typically described as having the substituents attached to the pseudo-asymmetric carbon atoms oriented on the same side relative to the polymer backbone, i.e., the substituents are all either configured above or below a plane containing the polymer backbone. Isotacticity can be determined through the use of NMR. In Bovey's NMR nomenclature, an isotactic pentad is represented by . . . mmmm . . . with each "m" representing a "meso" dyad or successive monomer units oriented with the substituents oriented on the same side relative to the polymer backbone. As is well known in the art, any deviation, disruption, or inversion about a pseudo asymmetric carbon in the chain will lower the degree of isotacticity and crystallinity of the polymer.
In contrast, the syndiotactic structure is typically described as having the substituents, that are attached to the pseudo-asymmetric carbon atoms, pseudo-enantiomorphically disposed, i.e., the substituents are oriented alternately and regularly above and below the plane containing the polymer chain. Syndiotacticity can also be determined through the use of NMR. In NMR nomenclature, a syndiotactic pentad is represented by . . . rrrr . . . in which each "r" represents a "racemic" dyad, i.e., successive substituents on alternate sides of the plane. The percentage of "r" dyads in the chain determines the degree of syndiotacticity of the polymer.
There are other variations in polymer structures as well. One such variant is the so-called hemi-isotactic polymers. Hemi-isotactic polymers are ones in which every other pseudo-asymmetric carbon atom has its substituent oriented on the same side relative to the plane containing the polymer backbone. While, the other pseudo-asymmetric carbon atoms can have their substituents oriented randomly either above or below the plane. Since only every other pseudo-asymmetric carbon is in an isotactic configuration, the term hemi is applied.
Isotactic and syndiotactic polymers are crystalline polymers and are insoluble in cold xylene. Crystallinity distinguishes both syndiotactic and isotactic polymers from hemi-isotactic or atactic polymers that are soluble in cold xylene and are non-crystalline. While it is possible for a catalyst to produce all four types of polymers (atactic, hemi-isotactic, isotactic and syndiotactic), it is desirable for a catalyst to produce predominantly or essentially polymer type with little or no other polymer type and few stereochemical defects.
Several catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403, as well as European Pat. Appln. 0,537,130. Several catalysts that produce syndiotactic polyolefins are disclosed in U.S. Pat. Nos. 3,258,455, 3,305,538, 3,364,190, 4,852,851, 5,155,080, and 5,225,500.
Besides neutral metallocenes, cationic metallocenes are known to result in polymers with varying degrees of tactiospecificity. Cationic metallocene catalysts are disclosed in European Patent Applications 277,003 and 277,004. Catalysts that produce hemi-isotactic polyolefins are disclosed in U.S. Pat. No. 5,036,034.
In addition to monoolefins homopolymers, polymerization catalysts for preparing copolymers of monoolefins or polymers of di-functional olefins or copolymers of di-functional olefins and monoolefins can be prepared using coordinated metal catalysts including metallocene catalysts.
Although many metallocene catalysts are now available, the need for new ligand systems to make metallocene catalysts for the polymerization of olefins is still important and represents a significant advancement in the art. Such new ligand systems and the catalyst derived therefrom can offer new design approaches for making highly stereoregular or tactiospecific polymers essentially free of defects, polymers with controlled defect statistics, copolymers with controlled properties, new approaches for molecular weight control and the control of other polymer properties.
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