The olefin polymerization reaction by silylene-bridged zirconocene catalysts has been investigated theoretically.With the ab initio molecular orbital (MO) method the structure and energetics of the reactants, the ir-complex, the transition state, and the product for insertion of ethylene into (SiH2Cp2)ZrCH3+ have been determined. The nearly C,-symmetric transition state has a low activation barrier (about 6.0 kcal/mol from the x-complex). The product shows a strong C6-H agostic interaction.Based on the above transition state structure, molecular mechanics (MM) calculations have been carried out to investigate the effects of alkyl substituents on olefins and methyl groups and other substituents on the Cp rings as well as the effects of the alkyl group on Zr, a model polymer chain end. The regioselectivity in propylene polymerization, that the primary insertion is preferred to the secondary insertion, is reproduced well by the energy difference at the transition state but not by that of the 7r-complex. As to the stereoselectivity in isotactic polymerization of propylene by SiH2(CpMe")2ZrR+, the substituents on the Cp rings have been found to determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion at the transition state. The direct steric effect of the Cp methyl groups on the olefin insertion stereochemistry is negligibly small. The same direct control mechanism is operational also in syndiotactic polymerization of propylene and 4-methyl-l-pentene.(34) Sherrod, M.

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A series of ansa-cyclopentadienyl-phenoxy titanium complexes (PHENICS: phenoxy-induced complex of Sumitomo) (1a-e), [{Me 2 CCp(OC 6 H 2 -3-R-5-R 0 )}TiCl 2 ] (1a, R = R 0 = H; 1b, R = t Bu, R 0 = Me) and [{Me 2 Si(C 5 Me 4 )(OC 6 H 2 -3-R 00 -5-Me)}TiCl 2 ] (1c, R 00 = i Pr; 1d, R 00 = t Bu; 1e, R 00 = Adm), have been synthesized and characterized. It is noteworthy that the dihedral angles of the cyclopentadienyl and phenoxy moieties of the complexes 1b and 1d, revealed by X-ray crystal structure analysis, are 63.0°(1b) and 68.2°(1d), indicating a distortion from a mutual perpendicular arrangement, whereas the symmetrical patterns of the 1 H NMR spectra indicate a dynamic behavior in solution. The PHENICS-type complexes exhibit good to excellent catalytic activities for copolymerization of ethylene and 1-hexene upon activation with Al i Bu 3 /[Ph 3 C][B(C 6 F 5 ) 4 ] cocatalyst. Particularly, the complexes with a bulky substituent at the ortho position of the phenoxy moiety showed excellent catalytic features (1d, 27 200 kg mol -1 h -1 at 80 °C, 6000 kg mol -1 h -1 at 180 °C). These data show that the catalytic activity of the PHENICS-type catalysts is higher than that reported for the so-called constrained geometry catalyst (CGC) precursors. This becomes even more evident in light of the results for polymerization at 180 °C. Whereas the PHENICS catalyst system acts as a single-site catalyst at 180 °C, the CGC-type catalysts proved to be inactive under analogous conditions. Moreover, the copolymers obtained with PHENICS incorporate much higher content of 1-hexene than those obtained with CGC.

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