The hafnocene-based catalyst ethylenebis(1-indenyl)hafnium dichloride/methylalumoxane, as well as its zirconium analogue, is able to oligomerize styrene, p-methylstyrene, and p-tert-butylstyrene in the presence of hydrogen to produce hydrooligomers. The composition of the product mixture compared to that obtained using zirconium-based catalysts indicates that the primary insertion occurs with greater frequency. This difference in regioselectivity is likely to be related to the electronic differences between the two metal centers. Previous experimental and theoretical evidence suggests that increasing the electron density at the incoming metal-carbon bond decreases the preference for the secondary insertion; so by exploiting the contribution from the electron-releasing substituent of the monomer and changing the metal center, from zirconium to hafnium, the regiochemistry of insertion can be inverted from secondary to prevailingly primary. The styrene regiochemistry of insertion is engraved into the structure of ethylene-styrene copolymers. Some relevant differences can be observed by comparing the 13C NMR spectra of copolymer obtained with zirconium- and hafnium-based catalysts. In fact the structure of the chains obtained with hafnium, compared to zirconium, shows higher styrene uptake and the presence of styrene homosequences and of regioirregularly arranged units. © 2010 American Chemical Society.
|Data di pubblicazione:||2010|
|Titolo:||Comparison of the regiochemical behavior of zirconium and hafnium in the polyinsertion of styrenes|
|Digital Object Identifier (DOI):||10.1021/om1001342|
|Codice identificativo ISI:||WOS:000282930900007|
|Codice identificativo Scopus:||2-s2.0-77958088139|
|Appare nelle tipologie:||Articolo su Rivista|