Ziegler-Natta Catalysts and Polymer Stereochemistry

Ziegler-Natta catalysts (Section 30.4) Polymerization catalysts prepared from an organoaluminum compound and a Lewis acid such as TiCl4, which afford polymer chains without significant branching and with controlled stereochemistry. 

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). 

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. 

The regular stereoisomers are possible, and the Ziegler-Natta catalysts promote formation of stereoregular polymers. Two types of regular stereochemistry are shown below. Isotactic polymers have the same stereochemistry at each asymmetric carbon, whereas in syndiotactic polymers the configuration alternates regularly down the chain ... 

The strict connection between chiral structure of the active site and polymer stereochemistry, pointed out by the investigations on the heterogeneous Ziegler-Natta catalytic polymerization, has been fully confirmed and extended by the studies concerning polymerization of prochiral a-olefins in the presence of metallocene catalysts. 

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. 

Ethylene is the simplest olefin. Propylene (propene) is the next simplest. The polypropylene produced by a Ziegler-Natta catalyst turned out to be of the structure of a special stereochemistry, and to form a fairly crystalhne plastic. It was too hard for most of applications and people who developed this polymer had to invent some use for it. One of the uses they came up with was hoola hoop. This is an example in which a product had been produced before they knew what they could use it for. Is this a rare exception Would not people know the uses for a product before they decide to produce it It is usually the case. However, there have been a number of cases in which a product had been produced first for no good reason or other purposes and only afterward they discovered its good application. 

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. 

Extensive efforts have been made to develop catalyst systems to control the stereochemistry, addition site, and other properties of the final polymers. Among the most prominant ones are transition metal-based catalysts including Ziegler or Ziegler-Natta type catalysts. The metals most frequentiy studied are Ti (203,204), Mo (205), Co (206-208), Cr (206-208), Ni (209,210), V (205), Nd (211-215), and other lanthanides (216). Of these, Ti, Co, and Ni complexes have been used commercially. It has long been recognized that by varying the catalyst compositions, the trans/cis ratio for 1,4-additions can be controlled quite selectively (204). Catalysts have also been developed to control the ratio of 1,4- to 1,2-additions within the polymers (203). 

ZIEGLER-NATTA POLYMERIZATION. Polymerization of vinyl monomers under mild conditions using aluminum alkyls and TiCL lor other transition element halide) catalyst to give a stereoregulated, or tactic, polymer. These polymers, in which the stereochemistry of the chain is not random have very useful physical properties. 

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