Strategies for Synthesis of Elastomeric Polypropylene: Fluxional Metallocenes with C1-Symmetry

The effects of replacing one of the 2-phenylindenyl-type ligands in bis(2-phenylindenyl)zirconium dichloride, (2-PhInd) 2 ZrCl 2 ( 7 ), a catalyst precursor for the production of elastomeric polypropylene (ELPP-7), with a cyclopentadienyl-type ligand have been studied. The mixed-ring compounds: (pentamethylcyclopentadienyl)(2-phenylindenyl)zirconium dichloride, Cp*(2-PhInd)ZrCl 2, ( 8 ), (pentamethylcyclopentadienyl)(1-methyl-2-phenylindenyl)zirconium dichloride, Cp*(1-Me-2-PhInd)ZrCl 2 ( 9 ), and (cyclopentadienyl)(2-phenylindenyl)zirconium dichloride, Cp(2-PhInd)ZrCl 2 ( 10 ), have been synthesized through the reaction of the corresponding lithium indenide with C 5 R 5 ZrCl 3 ( 8 and 9: R = CH 3; 10: R = H). Crystal structures have been determined for complexes 8 and 9 . The behavior of the catalysts derived from complexes 8, 9, and 10 upon their activation by methylaluminoxane (MAO) for ethylene and propylene polymerization has been studied. Of all mixed-ring catalysts only 8 /MAO produces elastomeric polypropylene (ELPP-8) with properties consistent with a stereoblock microstructure. The stereosequence distributions in ELPP-8 determined from 13 C NMR spectra are very similar to those in elastomeric polypropylene generated with 7 /MAO under comparable conditions. However, the melting points and IR indices of 8 /MAO-derived samples are significantly lower than for samples of ELPP-7 with similar isotacticity which may be attributable to shorter isotactic block lengths in ELPP-8. At the same time, comparison of 8 /MAO to another metallocene system capable of producing elastomeric polypropylene rac -MeHC(Me 4 Cp)(Ind)TiCl 2 /MAO, 1 /MAO, described by Chien reveals significant similarities both in the catalyst symmetry and properties of generated polypropylene. The increase of isotacticity of ELPP-8 with propylene pressure similar to the trend observed for ELPP-7 favors the two-state mechanism of stereoblock formation with 8 /MAO.