Ultra-rigid metallocenes for highly iso- and regiospecific polymerization of propene: The search for the perfect polypropylene helix

Ultra-rigid metallocenes are studied for highly iso- and regiospecific polymerization of propene. A single-crystal X-ray structure is obtained of the rac- and meso- forms of the complex and its rac-Hafnium analogue. For a better understanding typical structural parameters are compared from similar compounds. The observed smaller bite angle is assumed to be a result of this steric interaction between the methoxy substituent and the silicon bridge, a structure element that is indicative for the high steric rigidity of the ansa-metallocene frame. A significant increase of molecular weight, independent of the ligand substitution pattern, is found for iPP produced with hafnium complexes, like the hafnocene. A key step of the chain end epimerization process (CEP) is b-hydride elimination and therefore the amount of stereo-errors and the molecular weight of the produced polypropylenes are influenced directly by the propene concentration.