Characteristics of Ziegler-Natta Catalysts

Ziegler-Natta catalysts polymerize ethylene under very mild conditions relative to those needed for free radical polymerization. For example, conditions for free radical processes are often 200 °C and pressures of 140 MPa (-20,000 psig). In contrast, Ziegler s group showed that Ziegler-Natta catalysts are capable of polymerizing ethylene at atmospheric pressure and ambient temperature 

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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. 

Olig omerization and Polymerization. Siace an aHyl radical is stable, linear a-olefins are not readily polymerized by free-radical processes such as those employed ia the polymerization of styrene. However, ia the presence of Ziegler-Natta catalysts, these a-olefins can be smoothly converted to copolymers of various descriptions. Addition of higher olefins during polymerization of ethylene is commonly practiced to yield finished polymers with improved physical characteristics. 

Thus, modification using diene additives exerts a substantial influence on the dispersion characteristics of Ziegler-Natta microheterogeneous catalytic systems and therefore, on their activity. The effect is intensified by the preliminary hydrodynamic impact on an in situ prepared catalyst in a turbulent mode. 

The use of Ziegler-Natta catalysts in the polymerization of conjugated dienes has been widely investigated. It is characteristic of these catalysts that the resulting polymers often contain a very high proportion of one type of structural unit. By appropriate choice of catalyst, polydienes comprised almost exclusively of c -l,4-, /ra s-l,4-, 1,2-, and 3,4-units have been obtained. The mechanisms of such reactions are, at present, somewhat obscure but presumably the diene molecule co-ordinates with the metal-carbon bond of the catalyst in a manner similar to that involving the lithium-carbon bond mentioned above. 

Since the beginning of the use of Ziegler-Natta catalysts for the production of isotactic polypropylene, numerous improvements of both production processes and product properties were promoted by the dramatic progress of the catalyst performance. In the industrial manufacture of polypropylene, four major performance characteristics of the catalytic system have to be balanced and maximized the catalyst productivity, the polymer stereoregularity and molecular weight control, and the powder morphology. 

Heterogeneous Ziegler-Natta catalysts used to polymerize olefins exhibit phenomena characteristic of active site heterogeneity (1- 5). Complex kinetic models which account for this likelihood have been developed and used only in simulation studies (6-7). 

A characteristic of polyolefins synthesized with metallocene catalysts is their significantly lower polydispersity compared to one obtained by using heterogeneous Ziegler-Natta catalysts. Such narrower molecular mass distributions can lead to different mechanical properties of the resulting material. 

Soluble Ziegler-Natta catalysts can exhibit unique stereochemical properties. Group IV metallocenes in combination with methylaluminoxanes produce isotactic polypropylene with two different isotactic microstructures. The usual enantio-morphic site control is characteristic of enantiomeric racemic titano- and zirco-nocene complexes (e.g., ethylene-bridged indenyl derivatives279,349). In contrast, achiral titanocenes (e.g., [Cp2TiPh2]) yield isotactic polypropylene with microstructure 49, which is consistent with a chain end control mechanism 279,349-351... 

The first example of Iiving polyolefin with a uniform chain length was found in the low-temperature polymerization of propylene with the soluble catalyst composed of V(acac)3 and Al(C1Hi)2Cl. The mechanism of the living coordination polymerization is discussed on the basis of the kinetic and stereochemical data. Subsequently, some applications of living polypropylene are introduced to prepare tailor-made polymers such as terminally functionalized polymers and block copolymers which exhibit new characteristic properties. Finally, new types of soluble Ziegler-Natta catalysts are briefly surveyed in connection with the synthesis of living polyolefins. 

The activation energies available for binary and ternary catalyst systems are in a range from 20 to 70 kj mol x, which according to Odian is characteristic for polymerizations mediated by Ziegler/Natta-catalysts [456]. It is interesting to note that the polymerization of dienes catalyzed by NdCl3-based catalyst systems show lower activation energies than Nd-carboxylate-based catalyst systems. The influence of halide donors on the temperature depen-... 

The following are some of the characteristics of the Phillips and Ziegler-Natta catalysts that indicate possible improvements ... 

Table 6.1 Contrasting characteristics of single-site and Ziegler-Natta catalysts.

In terms of catalyst development, the next sequence of reactions shows the evolution of one series of Ziegler-Natta materials. The initial reaction exhibits the simple characteristics of a chlorination reaction. The addition of the ethylorthosilicate reactant increases the rate of reaction and yields a purer solid phase. By optimizing the reactivity of the chlorination promoter, the conversion of the Grignard reagent to magnesium chloride can be forced to occur even faster. The magnesium chloride product from this reaction is composed of fine particles that lack appreciable crystallinity and exhibit high BET surface... 

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