Metallocene polymerisation catalysis

Whether or not such electrophilic organometallic species can be identified, or indeed isolated, depends primarily on the stability of the counteranion. The per-fluorophenyl boron compounds B(C,sF5)3 and [B(C6F5)4] , first prepared by Stone and co-workers in 1963 [33], proved particularly useful in this respect. Their use in metallocene polymerisation catalysis [34, 35] led to significantly more active catalysts and well-defined catalyst systems that proved mechanistically informative. These results have then enabled similar species to be detected in the more complex MAO-activated catalyst systems (vide infra). 

I 8 The Use of Spectroscopy in Metallocene-based Polymerisation Catalysis Chain-end control ... 

Over the last 20 years, there has been an enormous increase in the use of NMR spectroscopy in metallocene-based polymerisation catalysis and these studies have provided an unprecedented increase in our understanding this has allowed the properties of homo- and co-polymers to be tailored and transition state energies to be lowered by 1-2 kcal mol , which makes all the difference between a poor and a highly successful catalyst. 

Cyclopentadienyl ligands have become extremely important in catalysis for metal such as Ti, Zr, and Hf (Chapter 10) and in academic studies of related elements such as Ta. Ethylene polymerisation with the use of CpiTiCE (alkylated with aluminium alkyl compounds) has been known for many decades, but the intensive interest in derivatives of these compounds started in the early 1980 s following the discovery of MAO (methaluminoxane - see chapter 10) which boosted metallocene catalyst activities by several orders of magnitude. Commercial interest focussed on ethylene copolymers (LLDPE where more homogeneous comonomer incorporation resulted in greatly improved copolymer properties) and in enantiospecific polymerisations for propene, styrene, etc. 

A foamable isotactic polypropylene homopolymer is obtained by metallocene catalysis and has a molecular weight distribution and density, which fall within broad ranges. It may be prepared in a multiple stage polymerisation process nsing the same metallocene component in at least two stages. 

One of the earliest examples of the use of zirconium in catalysis is the area of Ziegler-Natta polymerisation, the catalysts employed are typically based on a family of Cp2ZrCl2 metallocene complexes." With judicious choice of the cyclopentadienyl ligand either atactic (bridged Cp-derived systems),... 

Weiss, K., V teh-Pfeifer, C., Hofmann, M., Botzenhardt, S., Lang, H., Brtining, K., and Meichel, E. 2002. Polymerisation of ethylene or propylene with heterogeneous metallocene catalysts on clay minerals. Journal of Molecular Catalysis A Chemical 182-183 143-149. 

Metallocenes, in combination with the conventional aluminum alkyl cocatalysts used in Ziegler systems, are indeed capable of polymerising ethene, but only at a very low activity. Only with the discovery and application of methylaluminoxane (MAO) it was possible to enhance the activity, surprisingly, by a factor of 10000 [113]. Therefore, MAO plays a crucial part in the catalysis with metallocenes. 

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