Stereoregularity, olefin polymerization

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. 

Polymerization. Supported catalysts are used extensively in olefin polymerization, primarily to manufacture polyethylene and polypropylene. Because propylene can polymerize in a stereoregular manner to produce an isotactic, or crystalline, polymer as well as an atactic, or amorphous, polymer and ethylene caimot, there are large differences in the catalysts used to manufacture polyethylene and polypropylene (see Olefin 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. 

Figure 9 Non-metallocene complexes for stereoregular o-olefin polymerization.

As in the case of a-olefins polymerization, the differences in stereoregularity observed in the polymers of racemic and optically active monomers can be hardly interpreted (65). 

Olefin polymerization using heterogeneous catalysts is a very important reaction and stereochemical aspects have been studied extensively. For a review on this topic see Pino et al. [9], Briefly, the origin of stereoregularity in polyolefins (47) is explained by the chiral nature of the acdve site during polymerization. If the absolute configuration of the first intermediate can be controlled by chiral premodification then we should obtain a non-racemic mixture of R - and "S"-chains. This has indeed been observed e.g. with catalyst M4 for the polymerization (partial kinetic resolution) of racemic 3,7-dimethyl-l-octene (ee 37%) and also for the racemic monomer 46 using Cd-tartate M5. 

Ziegler-Natta catalysts can polymerize a variety of structurally different monomers. Examples of stereoregular homopolymers (Table IV), elastomeric or crystalline copolymers, as well as block copolymers may be found in the patent and open literature (, 49-51). Ethylene polymerizes easily with many soluble and heterogeneous Ziegler catalysts. Some ethylene-active catalysts, for example, Cp2TiCl2 + aluminum alkyl (52). are not active for a-olefin polymerizations. However, all known Ziegler catalysts that polymerize propylene are also active in ethylene polymerization. 

The polymerization of 1,3-dienes (e.g., 1,3-butadiene and isoprene) with Ziegler-Natta catalysts began in 1954, soon after the first results obtained in a-olefin polymerization since then many transition metal and lanthanide catalysts have been examined and several stereoregular diene polymers have been obtained [30, 31], 1,3-Dienes can generate several types of polymers having different stmctures trans-1,4 cis-1,4 1,2 and, in the case of asymmetric monomers (e.g., isoprene), 3,4. Stereoregular 1,2- or 3,4-polydienes may also exhibit iso- or syndiotacticity. (Figure 11.1). 

Keywords metallocene catalyst, Ziegler-Natta catalyst, olefin polymerization, polyolefins, homogeneous catalysts, supported catalysts, stereoregularity, molecular weight distribution (MWD), chemical composition distribution, Unipol , Novolen , stereoselectivity, single site catalyst, multiple site catalyst, gas phase process, slurry process, homopolymerization, copolymerization. 

Stereoregular a-olefin polymerization by early transition metal Ziegler-type compounds was first reported in 1955 by Natta et al. Since then, major industrial and academic research efforts have focused mainly on the development of highly efficient, stereoselective catalyst systems for propylene polymerization. Concurrently, the polymerization of higher a-olefins was also investigated, with the hope of revealing new opportunities in the field of polymeric materials. 

Abstract The use of methylaluminoxane (MAO) as cocatalyst for the polymerization of olefins and some other vinyl compounds has widely increased the possibilities for more precisely controlling the polymer composition, polymer structure, tacticity, and special properties. Highly active catalysts are obtained by different transition metal complexes such as metallocenes, half-sandwich complexes, and bisimino complexes combined with MAO. These catalysts allow the synthesis of polyolefins with different tacticities and stereoregularities, new cycloolefins and other copolymers, and polyolefin composite materials of a purity that cannot be obtained by Ziegler-Natta catalysts. The single-site character of metaUocene/MAO or other transition metal/ MAO catalysts leads to a better understanding of the mechanism of olefin polymerization. 

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