Insertion mechanisms homogeneous metallocenes

It was found that polypropylene made by such complexes (cocatalyzed by MAO) tended to be isotactic, as opposed to the atactic polypropylene that had been hitherto made by metallocenes such as Cp2ZrCl2. The strong relationship between ligand structure and polymer tacticity is considered to be the most secure proof of the migratory insertion mechanism for polymerization. The lure of homogeneous stereoselective olefin polymerization is responsible for much of the diversity of ligand structures within the bis(Cp) family. 

When propylene is polymerized using homogeneous metallocene/methylaluminoxane (MAO) catalysts, several chain transfer mechanisms occur to release free polymer chains. Their relative frequencies are dependent on the polymerization conditions and the catalyst structure. Three chain transfer mechanisms are identified that form chain-end unsaturated polypropylene. These are (1) P-hydride transfer to metal after a primary (1,2-) propylene insertion, (2) f)-hydride transfer to monomer after a primary propylene insertion, and (3) f)-hydride transfer after a secondary (2,1-) propylene insertion. The formations of these chain ends and associated polymer head groups (chain starts) are shown in Scheme 10.1. Mechanism (2) is commonly referred to as chain transfer to monomer. 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

A detailed molecular mechanics analysis of model catalytic sites has rationalised these observed behaviours for homogeneous propylene isospecific polymerisation catalysts based on bridged rac.-metallocene [357], It is interesting that, in contrast to the discussed case of enantioselectivity of the models for the primary insertion of propylene, the enantioselectivity of the models for the secondary insertion of propylene is due to direct interactions of the... 

The relative configuration of the monomer units can be controlled by the structure of the catalyst or by the configuration of the last inserted unit. These two scenarios are called site control and chain-end control. The isotactic polypropylene generated by the types of stereodefined metallocene catalysts presented in this chapter results from site control. More sophisticated architectures are possible by site-control mechanisms than chain-end control mechanisms, as illustrated by the variety of polyolefins prepared by homogeneous catalysts. 

At the same time, the fact that the homogeneous catalyst precursors are structurally well-defined has provided an extraordinary opportunity to investigate the origin of stereospecificity in olefin polymerization at a level of detail that was difficult if not impossible with the conventional heterogeneous catalysts. For example, NMR analysis of the isotactic polymer produced with HI revealed the stereochemical errors mmmr, mmrr, and mrrm in the ratios of 2 2 1 (Fig.5). This observation is consistent with an enantiomorphic site control mechanism, where the geometry of the catalyst framework controls the stereochemistry of olefin insertion.6 30,31 These results established unambiguously a clear experimental correlation between the chirality of the active site, which could be established by x-ray crystallography of the metallocene catalyst precursor, and the isotacticity of the polymer produced. 

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