Elaboration of rac-Like Active Species from Lewis Base Functionalized Unbridged Zirconocenes for Isospecific Propylene Polymerization

• Published in: Chemistry, an Asian journal an Asian journal, 2008-11, Vol.3 (11), p.1912-1921
• Main Authors: Kim, Hwa Kyu, Kim, Seong Kyun, Park, Jun Ha, Yoon, Seung Woong, Lee, Min Hyung, Do, Youngkyu
• Format: Article
• Language: English
• Subjects: Lewis bases; metallocenes; polymerization; polymers; zirconium
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.200800227

A series of ortho or meta Lewis base functionalized unbridged zirconocenes, [{1‐(En‐Ph)‐3,4‐Me2C5H2}2ZrCl2] (E=NMe2, OMe; n=1, 2), and a half‐functionalized zirconocene, [{1‐(p‐Me2NC6H4)‐3,4‐Me2C5H2}{1‐(p‐tolyl)‐3,4‐Me2C5H2}ZrCl2], were prepared. The crystal structures of these compounds determined by X‐ray diffraction revealed the presence of only C2‐symmetric rac‐like isomers in the asymmetric units. In combination with methylaluminoxane (MAO) cocatalyst, the meta‐functionalized complexes afforded mixtures of polymers that exhibit multimelting transition temperatures and broad molecular‐weight distributions (MWDs) in propylene polymerization at atmospheric monomer pressure, whereas the ortho‐functionalized complexes did not give rise to polymerization. Stepwise solvent extraction of the polymer mixtures showed that the polymers consist of amorphous, moderately isotactic, and highly isotactic portions, the weight ratio of which is dependent on the reaction temperature. 13C NMR spectral analysis indicated that the [mmmm] methyl pentad value of the isotactic portion reached around 90 %. Among the meta‐functionalized zirconocenes, the di‐OMe‐substituted one afforded the largest amount of the isotactic portion at all temperatures, and the portion comprised 82 wt % of the crude polymer obtained at 25 °C. In contrast, propylene polymerization with the half‐functionalized unbridged zirconocene resulted in the formation of nearly atactic polypropylene with a narrow MWD of around 2. These results corroborate the proposition that the rigid rac‐like cation–anion ion pair of type [rac‐L2ZrP]+[Me‐MAO]− generated in situ, through Lewis acid–base interactions between the functional groups and [Me‐MAO]−, is responsible for the isospecific propylene polymerization with the given class of functionalized unbridged zirconocenes and further indicate that the formation of such ion pairs can be favored by difunctionalization at the meta position of the phenyl ring with OMe groups. A meta‐phor for uniformity: meta Lewis base functionalized unbridged zirconocenes lead to the generation in situ of rigid rac‐like cation–anion ion pairs of active species that are capable of inducing isospecific polymerization of propylene upon activation with methylaluminoxane (MAO).