Ziegler-Natta catalysts specificity

Zam belli and Tosi have extensively studied the stereochemistry of the propagation step in propylene polymerization on Ziegler-Natta catalysts. Specific features of this process are shown in Table 4. Cis-addition of the olefin to the active metal-carbon bond has been observed both in isospecific and syndiospecific polymerization. The olefin addition to the active bond proceeds with the participation of the primary (L,(Mt—CH2—CHR—P) and secondary (L,Mt—CHR—CH2—P) carbon atoms of the growing polymer chain using isospecific and syndiospecific catalysts, respectively. 

At present it is not possible to determine which of these mechanisms or their variations most accurately represents the behavior of Ziegler-Natta catalysts. In view of the number of variables in these catalyzed polymerizations, both mechanisms may be valid, each for different specific systems. In the following example the termination step of coordination polymerizations is considered. 

Curves presented in Figure 3.13 testify to the large specificity of supported Ziegler-Natta catalysts regarding the kind of monomer some centres that polymerise ethylene do not polymerise propylene (or higher a-olefins, which may also be differentiated by particular catalyst centres, depending on the structure of the oc-olefin, e.g. branched as in 3-methyl-1-butene or not branched). Therefore, no hints about the monomer reactivity can be obtained by simple comparison of polymerisation rates without simultaneous estimation of the concentration of active sites [241]. 

Many attempts have been made to synthesise ethylene/propylene block copolymers, referred to as polyallomers, with isospecific Ziegler-Natta catalysts. However, true block copolymers can hardly be synthesised. This is due to the short life of the growing polymer chains [68,241]. Therefore, only in a few cases, when the copolymers are synthesised by adding two comonomers sequentially and under very specific conditions in order to reduce chain transfer reactions, does unambiguous evidence for true block copolymer formation with isospecific catalysts exist [457]. 

In the polymerization of dienes with Ziegler/Natta catalyst systems it is a well-established fact that the presence of halide donors is essential in order to achieve high catalytic activities and high cis-1,4-contents [360,361]. The halide free catalyst system NdO/TIBA is a good example for a catalyst with a poor performance and a high trans- 1,4-specificity [362,363]. For various binary and ternary catalyst systems the qualitative impact of chlorides on the stereochemistry of BR is demonstrated in a series of fundamental experiments the results of which are summarized in (Table 5) [364],... 

There are relatively many different catalysts that are usually used for specific chemical reactions. Types include Ziegler-Natta Catalyst (Z-N), metallocene, and others including their combinations. These systems are available and used worldwide from different companies.73... 

It is in the stereospecific polymerization of propylene that metallocene complexes display their astonishing versatility. Commercial Ziegler-Natta catalysts for isotactic polypropylene - based on combinations of TiCU, MgCl2, Lewis bases and aluminum alkyls - depend on a metal-centered chirality which exists at specific edge and defect sites on the crystal lattice to direct the incoming monomer in a particular orientation. These catalysts produce small amounts of undesirable atactic material due to the presence of achiral active sites. 

The earliest Ziegler-Natta catalysts were insoluble bimetallic complexes of titanium and aluminum. Other combinations of transition and Group I-III metals have been used. Most of the current processes for production of high-density polyethene in the United States employ chromium complexes bound to silica supports. Soluble Ziegler-Natta catalysts have been prepared, but have so far not found their way into industrial processes. With respect to stereo-specificity they cannot match their solid counterparts. 

Novel data on the composition of active centers of Ziegler-Natta catalysts and on the mechanism of propagation and chain transfer reactions are reviewed. These data are derived from the following trends in the study of the mechanism of catalytic polymerization a) determination of the number of active centers (mainly with the use of radioactive CO as a tag) b) analysis of the microstructure of polymers with the use of C-NMR c) analysis of specific features of highly active supported catalysts d) quantum-chemical calculation of the electronic structure of active centers and their reactions. 

For the higher activity Ziegler-Natta catalysts (Table II) based on reaction products of specific magnesium, titanium, and aluminum compounds, the similarity in size, coordination preference, electronic structure, and electronegativity of Ti(IV), Mg(II), and Al(III) ions is reflected in structural parameters and chemical properties (38) (Table III). The similarity in size between Mg(II) and Ti(IV) probably permits an easy substitution between ions in a catalyst framework. 

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