Further work using propene and other alkenes

7 Further work using propene and other alkenes 

One could also imagine a complex that at one site will insert only one certain type of alkene due to high steric constraints (e.g. ethene) and a second alkene at the other site thus giving an alternating copolymer that may even contain stereoregularity, thus obtaining for instance an alternating norbomene/ethene copolymer [34], 

Here we have discussed only very few of the many ligands that have been studied. Many variations of substituents at the cyclopentadienyl ligand have been studied, and there are more to come as well as variation in the structure of the bridge, the anions, and the central metal. In summary, the toolbox is quite extensive prediction of properties of new polymers to be made can guide the catalysis research in the design of new catalysts. 

Many of the older bis-indenyl catalysts are less selective at higher temperatures, which was ascribed initially to a lower selectivity of the insertion reaction itself. More recent work by Busico, based on deuteration studies and again based on very detailed and elegant analysis of 13C NMR spectra of the polymers, has shown that in fact epimerisation of the growing alkyl chain occurs via a series of (3-hydride eliminations and re-insertion reactions [36] involving even tertiary alkyl zirconium species. 

If this is the case and if the rate of polymerisation increases with the propene concentration, the latter influences the tacticity. 

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