Ziegler-Natta catalysts active centre models

Kinetic models referred to as adsorption models have been proposed, especially for olefin polymerisation with highly active supported Ziegler-Natta catalysts, e.g. MgCl2/ethyl benzoate/TiCU AIR3. These models include reversible processes of adsorption of the monomer (olefin coordination at the transition metal) and adsorption of the activator (complexation via briding bonds formation). There are a variety of kinetic models of this type, most of them considering the actual monomer and activator concentrations at the catalyst surface, m and a respectively, described by Langmuir-Hinshelwood isotherms. It is to be emphasised that M and a must not be the same as the respective bulk concentrations [M] and [A] in solution. Therefore, fractions of surface centres complexed by the monomer and the activator, but not bulk concentrations in solution, are assumed to represent the actual monomer and activator concentrations respectively. This means that the polymerisation rate equation based on the simple polymerisation model should take into account the... 

A number of models of the active centres in Ziegler Natta catalysts have been postulated. The diversity of these models arises from the multitude of products found to be formed or believed to be formed in the reaction of the catalyst precursor with the activator [e.g. schemes (4) to (8) and (12)]. The proposed active centres fall into either of two general categories those containing monometallic species with the central transition metal atom (e.g. Ti), and those containing bimetallic species with the central transition metal atom linked via bridges with the metal atom originating from the activator (e.g. Al). 

Figure 9 Simplified model for the formation of active centres of Ziegler-Natta catalysts by adsorption ofTiCl4 on solid MgCl2, reduction ofTi(IV) to Ti(III) chloride-alkyl exchange with aluminium alkyls and polyethylene (PE) formation by coordination and insertion of ethylene molecules.

It is the opinion of the author that it is unwise to expect any one model to fit all cases because of the very wide range of Ziegler-Natta and related catalysts diich have been developed. Nevertheless it is essential in order to interpret the accumulated active centre concentration data to have fairly well defined kinetic models. This requirement will become evident in the next section of this paper and it is for this reason that these kinetic models have been presented in some detail. 

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