Novel high and ultrahigh molecular weight poly(propylene) plastomers by asymmetric hafnocene catalysts

The novel asymmetric ansa-complexes [1-(9-η⁵-fluorenyl)-2- (2,5,7-trimethyl-indenyl)ethane]hafnium dichloride (7a) and [1-(9- η⁵-fluorenyl)-2-(2,4,6-trimethyl-indenyl)ethane]hafnium dichloride (7b) were prepared and used as catalysts for propylene homopolymerization reactions after in situ activation. The synthetic route allows to separate the 4,6- and 5,7-substituted ligand isomers before the complexation step. The orientation of the methyl groups to the "front" (4,6) or to the "back" (5,7) of the tetrahedral hafnocene dichloride species influences their performances in polymerization reactions. Whereas hafnocene (7b) which bears trimethyl substitution at 2,4,6-positions of the indenyl moiety exhibits only moderate activity, the 2,5,7-trimethyl substituted structure (7a) produces isotactic poly(propylene)s with high molecular weights (up to M̄_w = 9.0 × 10⁵ g · mol ^-1) and high activities [up to 3.2 × 10⁵ kg of PP (mol Hf × h)^-1]. A comparative analysis of polymerization data and mechanical behavior of 7a, and previously reported 6,7-indenyl substituted complex 6b are reported.