Elastomeric polypropylene from unbridged bis(2-arylindenyl) metallocenes: Investigation of the mechanism of stereoblock polypropylene formation

Unbridged group 4 bis(2-arylindenyl)metallocenes are active catalysts for the formation of elastomeric stereoblock polypropylene when activated by a cocatalyst, typically methylaluminoxane (MAO). Careful mechanistic studies of these catalyst systems are compromised by the ill-defined nature of MAO. Well-defined stoichiometric cocatalysts leading to boron-based counteranions have here been used for polymerization and NMR studies.; Propylene polymerizations in liquid propylene by bis(2-phenylindenyl)zirconium dimethyl activated by MAO, trityl tetrakis(pentafluorophenyl)borate, dimethylanilinium tetrakis(pentafluorophenyl)borate, or tris(pentafluorophenyl)borane (B(C ₆F₅)₃) revealed a significant influence of cocatalyst on the polymerization behavior of these catalysts. The productivity and stereoselectivity of the catalyst systems were highest with MAO and lowest with B(C₆F ₅)₃, with B(C₆F₅)₃ yielding amorphous atactic polypropylenes. The presence of aluminum isobutyl groups, either on MAO or from added triisobutylaluminum, increased the stereoselectivity of the polymerizations; the polarity had little influence on the stereoselectivity.; The sterically hindered metallocene bis(2-(3,5-di-tert-butylphenyl)indenyl)zirconium dichloride produced polypropylenes of much higher isotacticity than did bis(2-phenylindenyl)zirconium dichloride. Lower-tacticity polypropylenes generated with the sterically hindered compound contained a high percentage of [mmrm] stereoerrors, indicative of a stereoblock microstructure comprised of isotactic stereosequences of opposite relative configuration. Bis(2-(3,5-di-tert-butylphenyl)indenyl)hafnium dichloride produced polypropylenes of moderate tacticity and did not show a drop in productivity or stereocontrol with increasing temperature.; A series of unbridged bis(2-arylindenyl)zirconocenes varying both in the π indenyl ligand framework and the σ ligand was synthesized. The rotation rates of the dimethyl-substituted metallocenes were too fast to be determined by standard NMR techniques. On-resonance spin lattice relaxation in the rotating frame (T_1ρ) was used to determine the rate of rotation of the aryl ligands in the dimethyl species. The nature of both the σ and the π ligand systems affects the rate with sterically bulky substituents resulting in slower rotation. The effect of the σ ligand is substantially stronger than that of the π ligand.; Variable temperature NMR studies of bis(2-arylindenyl)zirconium dimethyl compounds activated by B(C₆F₅)₃ showed dynamic behavior consistent with fast ligand rotation and slower rearrangement of the ion pairs. Model compounds of the cationic active species were synthesized, and the conformation of the π-ligand framework could be influenced to give a C_s-symmetric syn isomer of the metallocene.