(Butadiene)metallocene/B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> Pathway to Catalyst Systems for Stereoselective Methyl Methacrylate Polymerization:  Evidence for an Anion Dependent Metallocene Catalyzed Polymerization Process

The <i>ansa</i>-zirconocene dichlorides [Me<sub>2</sub>Si(C<sub>5</sub>H<sub>4</sub>)(3-R-C<sub>5</sub>H<sub>3</sub>)]ZrCl<sub>2</sub> <b>7a</b><b>−</b><b>e</b> (R = H, CH<sub>3</sub>, cyclohexyl, −CHMe<sub>2</sub>, −CMe<sub>3</sub>) were reacted with butadiene−magnesium to yield the respective (η<sup>4</sup>-butadiene)metallocenes <b>17a</b><b>−</b><b>e</b>. The chiral examples give a mixture of two s-cis and two s-trans diastereomers. The strong Lewis acid B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> adds selectively to a terminal butadiene carbon atom to yield the (butadiene)metallocene/B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> betaine complexes <b>18a</b><b>−</b><b>e</b>. Initially, the formation of the <i>Z</i>-<b>18</b> isomers is preferred. These consecutively rearrange to the thermodynamically favored isomers <i>E</i>-<b>18</b>. The dipolar systems <b>18</b> 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<sub>2</sub>Si(C<sub>5</sub>H<sub>4</sub>)(3-R-C<sub>5</sub>H<sub>3</sub>)]ZrCH<sub>3</sub><sup>+</sup> catalysts (<b>9a</b><b>−</b><b>e</b>) that were conventionally prepared by methyl abstraction from the corresponding <i>ansa</i>-zirconocene dimethyl complexes by treatment with B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>. 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 <i>E</i>-<b>18a</b> was experimentally observed and characterized by NMR spectroscopy at 253 K. The subsequently formed series of [PMMA−C<sub>4</sub>H<sub>6</sub><sub>-</sub>B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]<sup>-</sup> anion oligomers (at the catalyst <b>18c</b>) was monitored (after quenching) and characterized by electrospray mass spectrometry.