Stereoregular Polymers Ziegler-Natta Polymerization

When a monosubstituted vinyl compound is polymerized, every other carbon atom in the chain becomes a stereogenic center  

The carbons marked with an asterisk have four different groups attached and are therefore stereogenic centers. Three classes of such polymers are recognized (Fig. 14.1)  

An atactic polymer is stereorandom, but an isotactic or syndiotactic polymer is stereoregular. These three classes of polymers, even if derived from the same monomer, will have different physical properties. 

Solution For polypropylene, the group L in eq. 14.19 is —CH3. With the chain extended in zigzag fashion, all methyl substituents occupy identical positions. 

PROBLEM 14.8 Using the definitions just given, draw a chain segment of a. syndiotactic polypropylene b. atactic polypropylene 

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Classification of Polymers Free-Radical Chain-Growth Polymerization Cationic Chain-Growth Polymerization Anionic Chain-Growth Polymerization Stereoregular Polymers Ziegler-Natta Polymerization A WORD ABOUT... Polyacetylene and Conducting Polymers Diene Polymers Natural and Synthetic Rubber Copolymers... 

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). 

Freqnently, the prodnct of Ziegler-Natta polymerization is sterically impure and can be preferentially extracted to give two products a highly crystalline stereoregular fraction and an amorphous atactic one. This may be attributed to the size of the catalyst particles as stereoregularity is enhanced by having large particles, whereas a finely divided catalyst tends to produce an amorphous polymer. 

Before coordination polymerization was discovered by Ziegler and applied to pro-pene by Natta, there was no polypropylene industry. Now, more than 10 pounds of it are 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. WeTl see more about Ziegler-Natta polymerization in Chapter 27 when we examine the properties of synthetic polymers in more detail. 

Venditto, V. Guerra, G. Corradini, R Fusco, R. Possible model for chain end control of stereoregularity in the isospecific homogeneous Ziegler-Natta polymerization. Polymer 1990, 31, 530-537. 

In 1958, Natta et al. published a study on Ziegler-Natta polymerizations of various vinyl monomers [362]. Natta and his co-workers showed that these polymerizations are very sensitive to steric hindrance at the double bound. In addition, Ziegler-Natta catalysts can generate a series of Lewis adds, which lead to complex side reactions. Therefore, only few attempts have been made to polymerize vinyl arenes by Ziegler-Natta catalysts. Heller and Miller obtained stereoregular polymers consisting of 1-vinylnaphthalene, 2-vinylnaphtha-lene and 4-vinylbiphenyl [363]. Polymerization by triethyl aluminum/titanium tetrachloride gave polymers in 75% to 90% conversion, which were characterized by IR and H-NMR spectroscopy and found to be at least 90% isotactic. Only 1-vinylnaphthalene produced a crystallizable polymer [362,363]. 

Ziegler-Natta polymerization of alkenes nCH2=CHR -[CH2-CHR] - Stereoregular polymer Complex of TiClj and AKC2HO,... 

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

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. 

The primary use of TiCl3 is as a catalyst for the polymerization 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 CC-TiCl3 and A1(C2H5)3. The mechanism of this reaction has been described (130). Suppliers of titanium trichloride include Akzo America and Phillips Petroleum in the United States, and Mitsubishi in Japan. 

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