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Sterically Encumbered (Perfluoroaryl) Borane and Aluminate Cocatalysts for Tuning Cation−Anion Ion Pair Structure and Reactivity in Metallocene Polymerization Processes. A Synthetic, Structural, and Polymerization Study

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journal contribution
posted on 11.06.1998, 00:00 by You-Xian (Eugene) Chen, Matthew V. Metz, Liting Li, Charlotte L. Stern, Tobin J. Marks
The synthesis and dialkyl abstraction chemistry as well as the unusual cocatalytic characteristics in metallocene-mediated polymerization of two distinctive borane and aluminate cocatalysts tris(2,2‘,2‘‘-nonafluorobiphenyl)borane (PBB) and triphenyl carbenium tris(2,2‘,2‘‘-nonafluorobiphenyl)fluoroaluminate, (Ph3C+PBA-) are reported. Reaction of PBB with Cp‘2ThMe2 (Cp‘ = η5-C5Me5), CGCZrMe2 (CGC = Me2Si(η5-Me4C5)(tBuN)), and Cp‘MMe3 (M = Zr, Hf) cleanly affords base-free cationic complexes Cp‘2ThMe+MePBB- (1), CGCZrMe+MePBB- (5), and Cp‘MMe2+MePBB- (M = Zr, 7; Hf, 8). In case of CGCTiMe2 and dimethyl zirconocenes, μ-methyl dinuclear cationic complexes [(CGCTiMe)2(μ-Me)]+MePBB- (6) and [(L2ZrMe)2(μ-Me)]+MePBB- (L = η5-C5H5 (Cp), 2; η5-1,2-Me2C5H3 (Cp‘‘), 3; Cp‘, 4; L2 = Me2Si(Ind)2, Ind = η5-C9H6, 9; L2 = Me2C(Flu)(Cp), Flu = η5-C13H8, 10) are formed. A similar reaction with Ph3C+PBA- results in the corresponding complexes CGCZrCH3+PBA- (M = Zr, 19; Ti, 20) and L2ZrCH3+PBA- (L = Cp, 15b; Cp‘‘, 16; η5-1,3-(SiMe2)2C5H3, 17; Cp‘, 18; L2 = Me2Si(Ind)2, 21; L2 = Me2C(Flu)(Cp), 22). Two dinuclear complexes 3 and 13 ([Me2C(Flu)(Cp)Zr(C6F5)]2(μ-F)+MeB(C6F5)3-) derived from borane PBB and B(C6F5)3, respectively, and three other PBA--based monomeric complexes 14 (Ph3C+PBA-), 19, and 21 have been characterized by X-ray diffraction, and these determinations allow detailed analysis of the ion pairing in the solid state. In combination with solution dynamic NMR, all data indicate MePBB-−cation interactions to be considerably weaker than those involving MeB(C6F5)3-, while the strongly ion-paired chiral PBA- converts previously enantiomeric cations into pairs of diastereomers. As revealed by dynamic 1H NMR studies, ion pair reorganization/symmetrization in 5 is significantly more rapid than in the MeB(C6F5)3- analogue, suggesting much looser ion pairing in 5. On the other hand, PBA- racemization is a rapid process (e.g., ΔG(58 °C) = 16.9(2) kcal/mol for 16), while cation−PBA- ion pairs have higher barriers for ion pair symmetrization than in analogous fluoroaryl borates. Dinuclear complexes 2 and 3 initiate efficient polymerization of methyl methacrylate (MMA) to produce syndiotactic poly(methyl methacrylate) (PMMA), while 9 produces highly isotactic PMMA, and sterically more accessible complexes 6 and 10 exhibit no activity. For olefin polymerization and copolymerization, PBB-derived cationic complexes, both monomeric and dinuclear, generally exhibit higher catalytic activity and comonomer incorporation levels than the MeB(C6F5)3- analogues, with CGC catalysts exhibiting the greatest activity contrasts. On the other hand, PBA--derived complexes exhibit a remarkable sensitivity of olefin polymerization characteristics and ion pairing to ancillary ligand bulk, with activity differences of up to 106-fold observed. In regard to stereospecific polymerization, PBA--derived chiral complex 21 produces highly isotactic polypropylene while B(C6F5)4--derived analogue produces isotactic polypropylene with lower isotacticity under similar conditions. Microstructure analyses of poly(ethylene-co-1-hexene) samples indicate that PBB enhances comonomer incorporation randomness.