Stereochemistry of Polymerization Ziegler-Natta Catalysts

Metallocenes are far more versatile in controlling polymer stereochemistry compared to Ziegler-Natta catalysts, as extensively demonstrated in the case of PP. Also in 1-butene polymerization, all kinds of chain microstructures can be obtained with different metallocenes. The 13C NMR pentad analysis of polybutene is somewhat less immediate than that of PP, and has been reported for both ZN 886,887 and metallocenes.180 The 13C NMR spectrum of atactic polybutene, with pentad assignments of the C(3) methylene region, is shown in Figure 37. 

One of the special features of the Ziegler-Natta catalyst is the stereochemistry associated with polymerization. Radical- and cationic-chain polymerization of... 

In the polymerization of dienes with Ziegler/Natta catalyst systems it is a well-established fact that the presence of halide donors is essential in order to achieve high catalytic activities and high cis-1,4-contents [360,361]. The halide free catalyst system NdO/TIBA is a good example for a catalyst with a poor performance and a high trans- 1,4-specificity [362,363]. For various binary and ternary catalyst systems the qualitative impact of chlorides on the stereochemistry of BR is demonstrated in a series of fundamental experiments the results of which are summarized in (Table 5) [364],... 

Natta postulated that for the stereospecific polymerization of propylene with Ziegler-Natta catalysts, chiral active sites are necessary he was not able to verify this hypothesis. However, the metallocene catalysts now provide evidence that chiral centers are the key to isotacticity. On the basis of the Cossee-Arlman mechanism, Pino et al. (164,165) proposed a model to explain the origin of stereoselectivity The metallocene forces the polymer chain into a particular arrangement, which in turn determines the stereochemistry of the approaching monomer. This model is supported by experimental observations of metallocene-catalyzed oligomerization. 

The 2000 Nobel Prize in Chemistry was awarded for work on poly acetylenes. Acetylene can be polymerized using a Ziegler-Natta catalyst. The cis or trans stereochemistry of the products can be controlled by careful selection and preparation of the catalyst. The resulting polyacetylene is an electrical semiconductor with a metallic appearance. cw-Polyacetylene has a copper color, and frawi-polyacetylene is silver. 

Although radical polymerization can t control stereochemistry, Ziegler-Natta catalysts can yield polymers of desired stereochemical orientation. 

Zam belli and Tosi have extensively studied the stereochemistry of the propagation step in propylene polymerization on Ziegler-Natta catalysts. Specific features of this process are shown in Table 4. Cis-addition of the olefin to the active metal-carbon bond has been observed both in isospecific and syndiospecific polymerization. The olefin addition to the active bond proceeds with the participation of the primary (L,(Mt—CH2—CHR—P) and secondary (L,Mt—CHR—CH2—P) carbon atoms of the growing polymer chain using isospecific and syndiospecific catalysts, respectively. 

The mechanism for polymerization of propylene using Ziegler-Natta catalysts is analogous to that discussed in section 3.7 with ethylene. However, unlike ethylene, propylene can be said to have "head" and "tail" portions and regiochem-istry can vary. More importantly, the orientation (stereochemistry) of the methyl group in the polymer has a dramatic effect on polymer properties. These factors make polymerization of propylene (and other a-olefins) more complex (17). 

The isoselective polymerization of styrene can be achieved by different mechanisms depending on the catalyst used. With a Ziegler-Natta catalyst, such as HCU/TIBA/MgCh, the insertion of the monomer into the metal-carbon bond of the active site is primary (1,2-), and the stereochemistry of the insertion is controlled by the chirality of the active sites (enantiomorphic-site control). ... 

Addition polymerization can be accomplished not only through a free radical initiator as mentioned above, but also by some other means. The most important polymerization catalyst is of the type known as Ziegler-Natta catalyst. These two chemists discovered that a combination of chemicals titanium tetrachloride and triethyl aluminum is an excellent catalyst for polymerizing a number of olefins. They were awarded Nobel Prize in 1963 for this discovery. Subsequent research by others found that similar combinations of chemicals a transition element compound and triethyl aluminum or similar alkylating agent do catalyze polymerization of olefins. Specific combination of such chemicals allow formation of polymers of specific stereochemistry. 

Propylene is the only ex -olefin which can be polymerized to either isotactic or syndiotactic polymers by means of Ziegler - Natta catalyst. The stereochemistry of the two types of stereospecific polymerization has been elucidated by structural analysis of polymerization products (mainly propylene homopolymers and ethylene-propylene copolymers) obtained in the presence of different catalytic systems (7). 

Butadiene can also be stereospecifically polymerized to give either of two isomeric polymers. The stereochemistry depends on the conditions of the reaction and the Ziegler-Natta catalyst used. 

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