Ziegler-Natta polymerization stereoregulation

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. 

Resurgent interest in Ziegler-Natta polymerization was triggered by Kaminsky s report of the high catalytic activity of Cp2ZrX2 and, to a lesser extent, the Hf counterpart coupled with methylaluminoxane (MAO) in a-olefin polymerizations [4]. There is evidence that the cationic complexes play an active role stereoregulation, isotactic or syndiotactic, became possible with the advent of structurally tailored metallocenes (Fig. 12) [27]. 

RECENT DEVELOPMENTS IN STEREOREGULATION BY ZIEGLER-NATTA POLYMERIZATION OF a-OLEFINS... 

The mechanisms and kinetics of Ziegler-Natta polymerization are known to be complex and are not yet fully understood. It is clear that the reaction takes place at the surface of the solid catalyst and that there is some type of complex formed at the surface. It is thought that the stereoregulation occurs from the way in which the monomer molecules are oriented during addition to the growing chain at the catalyst surface. 

The copolymerization theory presented in Chapter is of limited applicability to processes involving heterogeneous Ziegler-Natta catalysis. The simple copolymer model assumes the existence of only one active site for propagation, whereas the supported catalysts described above have reaction sites that vary in activity and stereoregulating ability. In addition, the catalytic properties of the active sites may vary with polymerization time. The simple copolymer model can be used with caution, however, by employing average or overall reactivity ratios to compare different catalysts and monomers. 

The structure of the polymer obtained in the polymerization of butadiene and isoprene with heterogeneous Ziegler-Natta catalysts depends on the nature of the monomer, catalyst system, and reaction conditions. Previously reported results are reviewed and a mechanism is proposed for the stereoregulated polymerization of conjugated dienes. The polymerization of cyclopentadiene with LiAlH -TiCl4 or LiAlR4-TiCl4 catalyst system yields a readily oxidized polymer for which a 1,2-structure is proposed. 

The most important characteristic of Ziegler-Natta catalysts is their ability to produce stereoregular polymers. On the basis of the monometallic mechanism of Cossee and Arlman, described above, stereoregulation of propylene polymerization can be explained as follows. 

Montagnoli, G Pini, D. Lucherini, A. Ciardelli, R Pino, P. Quantitative aspects of the stereoselectivity in the stereoregulated polymerization of racemic a-olefins by Ziegler-Natta catalysts. Macromolecules 1969, 2, 684-686. 

Natta, a consultant for the Montecatini company of Milan, Italy, applied the Zeigler catalysts to other vinyl monomers such as propylene and found that the polymers were of higher density, higher melting, and more linear than those produced by the then classical techniques such as free-radical-initiated polymerization. Ziegler and Natta shared the Nobel Prize in 1963 for their efforts in the production of vinyl polymers using what we know today as solid state stereoregulating catalysts. 

In 1953, Ziegler s work then inspired Giulio Natta of Milan to apply these insights to the stereoregulated polymerization of propylene. This work led to discovery of isotactic, syndiotactic, and atactic PP as well as to research into their structural property differences. The CH3 groups can be arranged in various different orders along the carbon chain, a property termed tacticity [3]. 

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