Ziegler stereoregularity

Among other possible reactions, these free radicals can initiate ordinary free-radical polymerization. The Ziegler-Natta systems are thus seen to encompass several mechanisms for the initiation of polymerization. Neither ionic nor free-radical mechanisms account for stereoregularity, however, so we must look further for the mechanism whereby the Ziegler-Natta systems produce this interesting effect. 

The primary use of TiCl is as a catalyst for the polymerisa tion of hydrocarbons (125—129). In particular, the Ziegler-Natta catalysts used to produce stereoregular polymers of several olefins and dienes, eg, polypropylene, are based on a-TiCl and A1(C2H3)3. The mechanism of this reaction has been described (130). SuppHers of titanium trichloride iaclude Akso America and Phillips Petroleum ia the United States, and Mitsubishi ia Japan. 

As a result of the work of Ziegler in Germany, Natta in Italy and Pease and Roedel in the United States, the process of co-ordination polymerisation, a process related to ionic polymerisation, became of significance in the late 1950s. This process is today used in the commercial manufacture of polypropylene and polyethylene and has also been used in the laboratory for the manufacture of many novel polymers. In principle the catalyst system used governs the way in which a monomer and a growing chain approach each other and because of this it is possible to produce stereoregular polymers. 

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

Before coordination polymerization was discovered by Ziegler and applied to propene by Natta, there was no polypropylene industry. Now, more than 10 ° pounds of it aie prepared each year in the United States. Ziegler and Natta shared the 1963 Nobel Prize in chemistry Ziegler for discovering novel catalytic systems for alkene polymerization and Natta for stereoregular- polymerization. 

Even more important is the stereoregular catalytic polymerization of ethene and other alkenes to give high-density polyethene ( polythene ) and other plastics. A typical Ziegler-Natta catalyst can be made by mixing TiCU and Al2Eti in heptane partial reduction to Ti " and alkyl transfer occur, and a brown suspension forms which rapidly absorbs and polymerizes ethene even at room temperature and atmospheric pressure. Typical industrial conditions are 50- 150°C and 10 atm. Polyethene... 

Stereoregular polymerization of ethene and propene by catalysts developed by K. Ziegler and by G. Natta (shared Nobel Prize 1963). 

Polymerizations catalyzed with coordination compounds are becoming more important for obtaining polymers with special properties (linear and stereospecific). The first linear polyethylene polymer was prepared from a mixture of triethylaluminum and titanium tetrachloride (Ziegler catalyst) in the early 1950s. Later, Natta synthesized a stereoregular polypropylene with a Ziegler-type catalyst. These catalyst combinations are now called Zieglar-Natta catalysts. 

Ziegler-Natta catalysts currently produce linear polyethylene (non-branched), stereoregular polypropylene, cis-polybutadiene, and other stereoregular polymers. 

Polypropylene (PP) is a major thermoplastic polymer. Although polypropylene did not take its position among the large volume polymers until fairly recently, it is currently the third largest thermoplastic after PVC. The delay in polypropylene development may be attributed to technical reasons related to its polymerization. Polypropylene produced by free radical initiation is mainly the atactic form. Due to its low crystallinity, it is not suitable for thermoplastic or fiber use. The turning point in polypropylene production was the development of a Ziegler-type catalyst by Natta to produce the stereoregular form (isotactic). 

Alkene polymerization can be carried out in a controlled manner using a Ziegler-Natta catalyst. Ziegler-Natta polymerization minimizes the amount of chain branching in the polymer and leads to stereoregular chains—either isotactic (substituents on the same side of the chain) or syndiotactic (substituents on alternate sides of the chain), rather than atactic (substituents randomly disposed). 

Alkenes undergo addition polymerization. When a Ziegler-Natta catalyst is used, the polymer is stereoregular and has a high density. 

We can employ coordination polymerization to produce stereoregular polystyrene. By performing this type of reaction at low temperatures, using Ziegler-Natta or single-site catalysts, we can prepare isotactic and syndiotactic versions of polystyrene. 

Ziegler-Natta catalysts are used commercially for the production of stereoregular polymers, especially isotactic polypropylene and high-density linear polyethylene. The resultant polymers have number and weight average molecular weights (Mn and Mw, respectively) that are defined as... 

Catalysts which bring about a spatially regular (stereoregular) configuration in high polymers. The main types are Ziegler, Phillips, Standard Oil and Metallocene Stereospecific Polymers... 

Natta A process for polymerizing propylene and other higher olefins, catalyzed by crystalline titanium trichloride and an alkyl aluminum compound such as triethyl aluminum. The polymer can exhibit various types of stereoregularity, depending on the catalyst and the conditions. Invented in 1954 by G. Natta at the Istituto de Chimica Industrial del Politecnico di Milano, Italy, and commercialized in 1957. Now used widely, worldwide. See also Ziegler, Ziegler-Natta. 

Chapter 1, by Guerra, Cavallo, and Corradini, discusses in detail both the historical and modem aspects of stereoregular olefin polymerization, a field that arose with the first uses of the catalyst that came to be known as Ziegler-Natta, and a field that these authors greatly contributed to and therefore write about with an intimate knowledge. 

Ziegler-Natta catalyst. Gi ulio Natta developed a catalyst based on his work with Karl Ziegler for polymerizing vinyl monomers to give stereoregular, tailored, three-dimensional chains. The catalyst is based on aluminum alkyls and TiCU or other transition metal halides. 

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