Modern Ziegler-Natta Catalyst

As shown by reactions 6.3.1 and 6.3.2, on treatinent with the cocatalyst most of the TiCl is reduced to TiClj. In a polymerization reaction where a hydrocarbon solvent is used, TiClj is produced in a colloidal form. The active catalytic sites are located on the surfaces of these colloidal particles. 

In so far as ethylene polymerization is concerned, the solid-state structure of TiClj is of little consequence. As discussed later, that is not so for propylene polymerization. 

It may be recalled that the cocatalyst can bring about the oligomerization reaction (6.2.1). Instead of an ethyl, the alkyl group in 6.4 could therefore also have been butyl, hexyl, octyl, etc. This is shown by reaction 6.3.3. 

These chain transfer steps produce analogues of 6.2, which are then converted to 6.5 through chain propagation steps. The polymer chain grows by successive insertion of ethylene molecules into the Ti-C bond. Chain propagation by this mechanism is known as the Cossee mechanism. 

Conversion of 6.3 to 6.4 is a simple insertion reaction of ethylene into the Ti-Et bond. This is followed by successive rapid insertions of ethylene molecules into the Ti-carbon bond to give 6.5. Chain termination with release of the polymer can take place by two different pathways either by /3-elimination or by the reaction of 6.5 with hydrogen. 

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A typical Ziegler-Natta catalyst might be made from TiCl or TiCl and Al(C2H )3. Vanadium and cobalt chlorides are also used, as is A1(C2H3)2C1. When these substances are mixed in an inert solvent, a crystalline soHd is obtained. Early catalysts consisted of the finely divided soHd alone, but in modern catalysts, it is often supported on Si02 or MgCl2. 

The type of solvent or diluent should be specified in reporting a Ziegler-Natta catalyst system. Alkene polymerisations are usually carried out in inert solvents, such as aliphatic or aromatic hydrocarbons (e.g. some gasoline fractions or toluene). The use of protic or aprotic polar solvents or diluents instead of the hydrocarbon polymerisation medium can drastically alter the reaction mechanism. This usually results in catalyst deactivation for alkene coordination polymerisation. Modern alkene polymerisation processes are carried out in a gas phase, using fluidised-bed catalysts, and in a liquid monomer as in the case of propylene polymerisation [28,37]. 

Supported catalysts result in dispersed active centers that are highly accessible. Catalyst activity is greatly increased (>5,000 g polyethylene/g catalyst). TEAL is the preferred cocatalyst for supported Ziegler-Natta catalysts. Transition metal residues in polyethylene produced with modern supported catalysts are very low (typically <5 ppm), obviating post-reactor treatment of polymer. 

Modern theoretical concepts of the polymerisation of dienes on Ziegler-Natta catalysts are based on the principle of their inherent polycentrism [49]. Substantial changes in the MW characteristics of polyisoprene and polybutadiene, due to the formation of a microheterogeneous titanium catalyst in the turbulent mode, make it reasonable to analyse the distribution of macromolecular growth centres on their kinetic activity. 

In contrast to early generations of multi-site Ziegler-Natta catalysts, developed on the basis of trial-and-error research, modern metallocene catalysts contain... 

Over the last 60 years, only few discoveries have had such a visible impact on the development of our modern society than Ziegler-Natta olefin polymerization catalysts. They have facilitated large-scale production of synthetic polyolefins and rubbers and subsequently the introduction of cheap commodity materials in our everyday life. 

Catalysts more than a thousand times as active as the original Ziegler-Natta systems have now been developed. In one form magnesium chloride, which has a similar layer lattice to a-TiCl, is milled with ethyl benzoate and then treated with TiCl. The resulting solid retains about 1% titanium and ester it is activated with EtjAl. Polymerization of propene is carried out in the liquid phase under pressure (55°C/20atm) and of ethene, in the gas phase. The reaction is strongly exothermic (p. 124). Such low concentrations of these modern supported catalysts are required that no removal of catalyst residues from the product is necessary. Moreover such highly crystalline polypropene is produced that there is no need to separate unwanted atactic material. 

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