Metallocene polymer tacticity

There has been a great deal of effort reported designed to make sense of how different metallocene complexes regulate polymer tacticity as a function of the symmetry properties of the catalyst. To begin to understand this, we must look at stereochemical possibilities of approach by propene to the metal. Figure 11-4 shows four such options, which may be classified as re-primary,, v/ -primary,. v/ -secondary, and re-secondary. The designations re and si refer to the faces of the propene C=C bond, and Figure 11-4 also details the difference between si... 

It was found that polypropylene made by such complexes (cocatalyzed by MAO) tended to be isotactic, as opposed to the atactic polypropylene that had been hitherto made by metallocenes such as Cp2ZrCl2. The strong relationship between ligand structure and polymer tacticity is considered to be the most secure proof of the migratory insertion mechanism for polymerization. The lure of homogeneous stereoselective olefin polymerization is responsible for much of the diversity of ligand structures within the bis(Cp) family. 

This chapter will discuss all known group 3 and 4 doubly bridged ansa-metallocenes made to date. When polymerization data is unavailable, comments will be made on the perceived viability of the compounds as precatalysts for a-olefin polymerization based on their structure and symmetry. For the a-olefin polymerization precatalysts described herein, the correlation between catalyst structure and polymer tacticity will be discussed. Further, the correlation between catalyst structure and regiocontrol, polymerization activity, and polymer molecular weight will be addressed when there is pertinent data present for a given precatalyst. 

Kelly, W. M. Taylor, N. J. Collins, S. Polymerization of cyclopentene using metallocene catalysts Polymer tacticity and properties. Macromolecules 1994, 27, 4477-4485. 

Stereoisomerism and Connectivity 300 Total Synthesis of an Antibiotic with a Staggering Number of Stereocenters 303 The Descriptors for the Amino Acids Can Lead to Confusion 307 Chiral Shift Reagents 308 C2 Ligands in Asymmetric Synthesis 313 Enzymatic Reactions, Molecular Imprints, and Enantiotopic Discrimination 320 Biological Knots—DNA and Proteins 325 Polypropylene Structure and the Mass of the Universe 331 Controlling Polymer Tacticity—The Metallocenes 332 CD Used to Distinguish a-Helices from [3-Sheets 335 Creating Chiral Phosphates for Use as Mechanistic Probes 335... 

Based on Chien s research results, Collins et al. modified the basic structure of the catalysts and also achieved elastic material [8,18,19]. In both cases the elastic properties of the polymers are justified in a block structure with isotactic and atactic sequences. In 1999 Rieger et al. presented a couple of asymmetric, highly active metallocene catalysts, e.g., the dual-side catalyst rac-[l-(9-r 5-fluorenyl)-2-(5,6-cyclo-penta-2-methyl-l-q5-indenyl)ethane]zirconium dichloride (Fig. 3). These catalysts allowed building of isolated stereoerrors in the polymer chain to control the tacticity and therefore the material properties of the polymers [9],... 

They are based on various metals. Such as zirconium, complexed with cyclopentadienide anions. This type of compound is called a zirconocene and is used with organoalu-minum to make highly regular polymers. The catalyst has the ability to flip back and forth from making atactic to isotactic polypropylene in the same polymerization. The alternating tacticity of the polymer breaks up the crystallinity of the chains and yields an elastomer. Metallocene catalysts are currently very expensive and cannot yet polymerize dienes such as butadiene, so they have only enjoyed limited commercial success in elastomers. However, this is one of the most intense fields of polymer research and many new product breakthroughs are expected in the near future. 

Two examples clearly illustrate the relationship between molecular structures of the metallocene catalysts on the one hand, and the tacticity of the resultant polymers on the other. As shown in Fig. 6.9, complexes 6.32, 6.33, and 6.34 have very similar structures. In 6.33 and 6.34 the cyclopentadiene ring of 6.32 has been substituted with a methyl and a f-butyl group, respectively. The effect of this substitution on the tacticity of the polypropylene is remarkable. As already mentioned, 6.32, which has Cs symmetry, gives a syndiotactic polymer. In 6.33 the symmetry is lost and the chirality of the catalyst is reflected in the hemi-isotacticity of the polymer, where every alternate methyl has a random orientation. In other words, the insertion of every alternate propylene molecule is stereospecific and has an isotactic relationship. In 6.34 the more bulky t-butyl group ensures that every propylene molecule inserts in a stereospecific manner and the resultant polymer is fully isotactic. 

A major breakthrough in polymer production occurred with the discovery of metallocene catalysts [1]. We are now able to make polyolefins with a controlled level of branching (and tacticity). The simplest object is a statistically branched polymer, with a certain overall degree of polymerisation X, and a certain distance (monomer units) between successive branch points, which we shall call b. The basic goal of characterisation is to measure X and b from a minimum number of experiments in dilute solutions. 

The polymers feature two chiral centers per monomer unit and therefore are ditactic. While polymers produced by achiral Pd catalysts seem to be atactic, using chiral metallocene catalysts highly tactic crystalline materials can be produced, featuring extraordinary high melting points (in some cases above the decomposition temperature) and extreme chemical resistance. 

Polymerization of norbornene using chiral metallocenes results in insoluble polymers exhibiting a glass transition temperature of about 210 °C. Although they have been shown by oligomerization to be tactic, no melting up to 500 °C... 

Due to the fact that the polymer chain migrates during insertion, the symmetry of the metallocene is of fundamental importance to the tacticity of the polymer produced. C2-symmetric metallocenes such as the bridged bis(indenyl) compounds mentioned above have homotopic coordination sites and thereby always favor the same orientation of the prochiral monomer during the approach. This leads to the formation of an isotactic polymer (Figure 3). 

The classical heterogeneously catalyzed propene polymerization as discovered hy Natta is a stereospecific reaction forming a polymer with isotactic microstructure. During the development of single-site polymerization catalysts it was found that C2-symmetric chiral metallocene complexes own the same stereospecificity. An analysis of the polymer microstructure hy means of NMR spectroscopy revealed that misinsertions are mostly corrected in the next insertion step, which suggests stereocontrol (Figure 6) hy the coordination site, as opposed to an inversion of stereospecificity hy control from the previous insertion steps (chain-end control). In addition, it was found that Cs-symmetric metallocene catalysts lead to syndio-tactic polymer since the Cosee-Arlmann chain flip mechanism induces an inversion of the stereospecificity at every insertion step. This type of polymer was inaccessible by classical heterogeneous systems. 

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