Activity and mechanism of propylene polymerization by bis(2-arylindenyl)zirconocenes

Metallocene catalysts derived from bis(2-arylindenyl)zirconium dichloride catalysts yield elastomeric polypropylene. The mechanism by which these metallocenes produce elastomeric polypropylene is proposed to be through creation of a stereoblock polymer via interconversion between rac-like and meso-like catalyst geometries. A study of the steric and electronic effects of varying the 4-substituent of bis(2-(4-R-C₆ H₄)indenyl)ZrCl₂ [R = H, Me, Et, nBu, tBu, SiMe₃, CF₃, Cl] on propylene polymerization at 25, 50, 75 psig and bulk propylene revealed that the polymerization behavior of these catalysts is not strongly influenced by the nature of the substituent in the 4-position of the 2-aryl substituent.; Catalysts of the type bis(2-(3,5-R₂-C₆H₃)indenyl)ZrCl ₂/MAO (R = H, R = CF₃) were investigated for regioregularity of monomer insertion in bulk propylene polymerization and compared to the isospecific catalyst rac-(ethylene)bis(indenyl)ZrCl ₂/MAO. Regioirregular insertions were found to be correlated to catalyst activity. The amount of regioirregular propylene insertions were in the range of 0.1–0.5 mol %. Comparison of polymerizations in the presence and absence of hydrogen revealed a 4–10 fold increase in productivity for catalysts (2PhInd)₂ZrCl₂/MAO and (2-(3,5-(CF ₃)₂-C₆H₃)Ind)₂ZrCl ₂ in the presence of hydrogen but only a slight increase in productivity with rac-(ethylene)bis(indenyl)ZrCl₂/MAO.; The kinetics of propylene polymerization in toluene solution by (2-phenylindenyl) ₂ZrCl₂/MAO and the substituted catalyst (2-(3,5-(tBU) ₂-C₆H₃)Ind)₂ZrCl₂/MAO were investigated. The rates of propylene polymerization reach a maximum after 10–20 minutes and then decrease. The increase of the isotactic dyads and pentads ([m] and [mmmm]) with increasing monomer concentration reveals an additional kinetic event which competes with the stereodifferentiating olefin insertion step. For both catalysts, the rate of polymerization increased as temperature increased from 20°C to 40°C to 60°C while isotacticity and molecular weights of the polymers decreased.