Olefins Cossee-Arlman mechanism

Figure 5.9 The Cossee-Arlman mechanism a) Olefin coordination, b) Olefin concerted insertion, c) Insertion step, d) Chain migration.

The approach and insertion of an olefin molecule may or may not pass through a local minimum or coordination complex (first in brackets in eq. 16) recent theoretical work (128) indicates that the well, if it indeed exists, is very shallow. The insertion of the new molecule into the growing chain is represented in equation 13 as a structure intermediate between reactants and products. The mechanism for this apparently concerted reaction does not involve the participation of metal-based electrons, and can be considered to be a Lewis acid-assisted anionic attack of the zirconium alkyl (ie, the polymer chain) upon one end of a carbon-carbon double bond. The concept of this reaction pre-dates metallocene study, and is merely a variant of the Cossee-Arlman mechanism (129) routinely invoked in Ziegler-Natta polymerization. Computational studies indicate (130) that an a-agostic interaction (131) provides much needed stabilization during the process of insertion. 

Polymer Chain Growth. The essential characteristic of Ziegler-Natta catalysis is the polymerization of an olefin or diene using a combination of a transition-metal compound and a base-metal alkyl cocatalyst, normally an aluminum alkyl. The function of the cocatalyst is to alkylate the transition metal, generating a transition-metal-carbon bond. It is also essential that the active center contains a coordination vacancy. Chain propagation takes place via the Cossee-Arlman mechanism (23), in which coordination of the olefin at the vacant coordination site is followed by chain migratory insertion into the metal-carbon bond, as illustrated in Figure 1. 

The copolymerization of a polar comonomer with nonpolar olefins by coordination polymerization is thought to be possible if the insertion of the polar comonomer takes place on the same active catalyst center as the nonpolar olefin according to the Cossee-Arlman mechanism [131, 132]. The prerequisite for this is that the polar comonomer coordinates to the metal center by its C=C double bond rather than by its polar group [133]. 

The Cossee-Arlman mechanism for the polymerization of olefins is the most widely accepted theory but as yet it is not complete. Cossee developed his early ideas of polyethylene growth at a titanium-carbon bond and supported the theory by molecular orbital calculations. The role of the alkyl aluminium co-catalyst was in the generation of the active species, via the alkylation of the titanium chloride bonds, and to remove impurities in both the gas stream and catalyst preparative procedure. There was also the suggestion that it might be involved in the insertion of each monomer molecule, and also in the regeneration of dormant sites or the formation of new active sites. 

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. 

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