Catalysis, Ziegler-Natta

In order to obtain good mixing of ethene with the catalyst, the original Ziegler-Natta processes used hexane as a solvent. Although the solvent is almost completely recovered, the use of a hazardous material such as hexane detracts from the greenness of the process. Since the catalyst is highly moisture sensitive it needs to be deactivated at the end of the process by addition of water or alcohol, and this produces a small waste 

Monomers that contain electron-releasing groups (such as isobutylene) can be polymerized by cationic polymerization. 

Cationic polymerization can lead to the formation of linear polymers particularly if carried out at low temperatures. 

Chain-transfer reactions are slow at lower temperatures. 

We have seen that free-radical polymerization of ethylene leads to the formation of low-density polyethylene (LDPE). The reason for the low density of LDPE is the lack of crystallinity owing to branching in the polymer. Straight-chain polyethylene can be obtained from Ziegler-Natta polymerization, which involves the use of an organometallic catalytic system [1-5, 18-20]. 

Ziegler, a German chemist, observed an Aufbaureaktion (growth reaction) when ethylene is polymerized in the presence of triethylaluminum [1,18] (see Eq. 2.24). 

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Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Ziegler-Natta catalysis, 431, 449 Zinc diacetate catalysts, 71 Zirconium alkoxides, 68... 

Several articles on these topics have appeared in earlier volumes. For the convenience of readers they are listed here. In addition, articles on Ziegler-Natta catalysis and on organolithium compounds in diene polymerization are planned for the next volume of this serial publication. 

Key words Olefin polymerization, Ziegler-Natta catalysis... 

Metal Complex Initiation (Ziegler-Natta Catalysis)... 

Figure 14.6 Ziegler-Natta catalysis in the polymerization of propylene.

The versatility of Ziegler-Natta catalysis is shown in the polymerization of butadiene. Polybutadiene may have either a 1,2 or 1,4 configuration. The 1,4 polymer has a double bond as part of the main chain and this can be atactic, isotactic, or syndiotactic. Thus many different polybutadienes can be made and all of them have been made with the aid of Ziegler-Natta catalysts. 

Finally, one last type of natural polymer is natural rubber, obtained from the rubber tree and having the all cw-l,4-polyisoprene structure. This structure has been duplicated in the laboratory and is called synthetic rubber, made with the use of Ziegler-Natta catalysis. 

Solution and emulsion polymerization Ziegler-Natta catalysis Mostly cis configuration... 

Under the reaction conditions employed, the oligomerization is living the aluminium is removed from the chain in a separate process step [2]. Further research on the Aufbau reaction led to the discovery of transition metal catalyzed olefin polymerization (Ziegler-Natta catalysis) [3]. 

Garlin, R. T. and Wilkes, J. S., Complexation of CP2MCI2 in a chloroaluminate molten-salt. Relevance to homogeneous Ziegler-Natta catalysis, /. Mol. Catal., 63,125,1990. 

Infrared spectroscopic studies show that hdpe produced by Ziegler-Natta catalysis has about 10 to 14 ethyl groups per 1000 repeating units. However, hope produced by the use of the Phillips catalyst has less branching and fewer ethyl groups. Most of the commercial hope used in the United States is produced by use of the Phillips catalyst. 

J. Boor, Jr., Ziegler-Natta Catalysis and Polymerization, Academic Press, New York, 1979. 

One of the great discoveries of organomctallic chemistry was the catalyzed polymerization of alkenes at atmospheric pressure and ambient temperature. Vast quantities of polyethylene and polypropylene (over 15 million tons annually) are made by Ziegler-Natta catalysis. Ziegler and Natta received the Nobel prize in chemistry in 1963, and the importance of their work w stimulating interest in organometallic chemistry should not be underestimated. 

Ziegler. K.. 718 Ziegler-Natta catalysis. 718 Zinc. 582-587 Zinc blende, 96-97 ZinH salts. 527. 817 ZSM-5. 747-748... 

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