The ansa-zirconocene dichlorides [Me(2)Si(C(5)H(4))(3-R-C(5)H(3))]ZrCl(2) 7a-e (R = H, CH(3), cyclohexyl, -CHMe(2), -CMe(3)) were reacted with butadiene-magnesium to yield the respective (eta(4)-butadiene)metallocenes 17a-e. The chiral examples give a mixture of two s-cis and two s-trans diastereomers. The strong Lewis acid B(C(6)F(5))(3) adds selectively to a terminal butadiene carbon atom to yield the (butadiene)metallocene/B(C(6)F(5))(3) betaine complexes 18a-e. Initially, the formation of the Z-18 isomers is preferred. These consecutively rearrange to the thermodynamically favored isomers E-18. The dipolar systems 18 are active single component metallocene catalysts for the stereospecific polymerization of methyl methacrylate. With increasing steric bulk of the attached single alkyl substituent an increasingly isotactic poly(methyl methacrylate) is obtained. A similar trend is observed in the methyl methacrylate polymerization at the [Me(2)Si(C(5)H(4))(3-R-C(5)H(3))]ZrCH(3)(+) catalysts (9a-e) that were conventionally prepared by methyl abstraction from the corresponding ansa-zirconocene dimethyl complexes by treatment with B(C(6)F(5))(3). A comparison of the poly(methyl methacrylates) obtained at these two series of catalysts has revealed substantial differences in stereoselectivity that probably originate from an influence of the respective counteranions. An initial reactive intermediate of methyl methacrylate addition to the dipolar single component metallocene catalyst E-18a was experimentally observed and characterized by NMR spectroscopy at 253 K. The subsequently formed series of [PMMA-C(4)H(6)(-)B(C(6)F(5))(3)](-) anion oligomers (at the catalyst 18c) was monitored (after quenching) and characterized by electrospray mass spectrometry.
Addition von B(C6F5)3 an den Butadienliganden in 1 liefert mit dem zwitterionischen Komplex 2 (charakterisiert durch Einkristall‐Röntgenbeugung; Ar=2,6‐Diisopropylphenyl) einen aktiven Katalysator für die Ethenpolymerisation. Die Bildung von 2 könnte Modellcharakter für einen Aktivierungsweg bei homogenen Ziegler‐Natta‐Katalysatoren später Übergangsmetalle ohne σ‐Alkylliganden haben.
Vorwort und Dank der Verfasser Due Diligence-Analysen sind seit einigen Jahren Gegenstand wissenschaftlicher Auseinandersetzung in der Literatur und praktischer Anwendung bei Mergers & Acquisitions (M&A). Ursprünglich als spezielles Instrument der angloamerikanischen Transaktionspraxis charakterisiert, hat sich gezeigt, daß die Notwendigkeit zur Durchführung einer Due Diligence im Vorfeld von M&A auch in Deutschland besteht. Bei den im Rahmen der vorliegenden Untersuchung berichteten Akquisitionen im Zeitraum von 1997 bis 2000 wurde zu 74% eine Due Diligence durchgeführt.
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