Cossee mechanism, propylene

The Ziegler-Natta polymerization of ethylene and propylene is among the most significant industrial processes. Current processes use heterogeneous catalysts formed from Ti(IH)Cl3 or MgCl2-supported Ti(IV)Cl4 and some otganoaluminum compounds. The widely accepted Cossee mechanism of ethylene polymerization is illustrated in Scheme 62. 

The basic mechanism of propylene polymerization is the same as that of ethylene polymerization. Here also the cocatalyst initiates polymerization by chain transfer reactions, which are then followed by propagation steps according to the Cossee mechanism. Finally, chain terminations occur either by j3-elimination or by deliberate addition of hydrogen. 

A guide to the manner in which structural theory may be applied to a detailed consideration of the mechanism of a surface-catalyzed reaction is found in papers by Cossee (113), Arlman (114), and Arlman and Cossee (115) concerning the mechanism of the stereoregular heterogeneous catalyzed polymerization of propylene. Particular crystallographic sites are shown to be the active centers at which the reactants combine and ligand field theory is used to demonstrate a plausible relationship between the activation energy for the conversion of adsorbed reactants to the product and the properties of the transition metal complex which constitutes the reaction center. 

The mechanism that is commonly considered to operate in the polymerisation of ethylene and a-olefins in the presence of group 4 metallocene-based catalysts is that devised by Cossee [268, 276, 277] for propylene polymerisation with heterogeneous Ziegler-Natta catalysts, though modifications invoking effects such as a-agostic hydrogen interactions with the metal centre have been proposed [343,344]. 

Most reaction models which describe the mechanism of diene polymerization by Nd catalysts have been adopted from models developed for the polymerization of ethylene and propylene by the use of Ti- and Ni-based catalysts systems. A monometallic insertion mechanism which accounts for many features of the polymerization of a-olefins has been put forward by Cossee and Arlman in 1964 [624-626]. Respective bimetallic mechanisms date back to Patat, Sinn, Natta and Mazzanti [627,628]. The most important and generally accepted mechanisms for the polymerization of dienes by Nd-based catalysts are discussed in the following. 

Natta postulated that for the stereospecific polymerization of propylene with Ziegler-Natta catalysts, chiral active sites are necessary he was not able to verify this hypothesis. However, the metallocene catalysts now provide evidence that chiral centers are the key to isotacticity. On the basis of the Cossee-Arlman mechanism, Pino et al. (164,165) proposed a model to explain the origin of stereoselectivity The metallocene forces the polymer chain into a particular arrangement, which in turn determines the stereochemistry of the approaching monomer. This model is supported by experimental observations of metallocene-catalyzed oligomerization. 

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. 

Fr . 1. Mechanisms of Ziegler polymerization (a) Bimetallic mechanism, (b) Monometallic mechanism [after Cossee]. (c) Monometallic Cossec mechanism (5). Crystal structure of a-TiC. la with active center in the surface. A propylene rnolocule is inserted in the Cl v acancy forming a 7r-bond with Ti ion A. Figures lb and Ic wore reprinted with the permission of the Faraday Society. 

The mechanism of monomer insertion and steric control in polymerizations of a-olefins by the metallocene catalysts received considerable attention [293-297]. There is no consensus on the mechanism of polymerization. Many studies of chain propagation tend to support the Cossee-Arleman mechanism [293-297]. An example is work by Miyake et al. [294] who synthesized unsymmetrical ansa-metallocenes and separated them into threo and erythro isomers. Both isomers coupled with methylaluminoxane polymerize propylene in toluene to highly isotactic polymers of = 105,000. The isotactic placement is greater that 99.6% and the polymer melting point is 161°C. 

In Natta s catalyst, the steric constraints near the active sites due to the presence of the chloride anions make the coordination of propylene stereospecific. In other words, coordination of propylene through one particular enantioface is energetically favored. This mechanism for the stereospecific propylene polymerization is referred to as the Cossee-Arlman mechanism. 

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