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Tim Lenz, Speaker at Catalysis Conference
Tim Lenz
Technical University of Munich, Germany
Title : Perfectly isotactic polypropylene upon In situ activation of ultrarigid meso metallocenes

Abstract:

In 1984, Ewen disclosed the mechanism of stereocontrol exhibited by group IV metallocene catalysts in propylene polymerization.[1] While the racemic (rac) isomers of such complexes yield isotactic polypropylene (iPP), the meso isomers were shown to yield atactic polypropylene (aPP). Generally, both metallocene isomers accumulate upon catalyst syntheses and since aPP finds far less applications, the rac species need to be separated from the meso ones in a rather tedious manner.[2] While there exist isomerization methods to convert meso metallocene complexes into their corresponding rac analogues,[3] such procedures add one additional step to the catalyst syntheses and do not lead to complete isomerization in every case.[4]

Recently, we observed an unprecedented isomerization of the hafnocene complex meso-I upon reaction with triisobutylaluminum (TIBA) at elevated temperatures (Scheme 1).[5] Since TIBA is needed to convert hafnocene complexes into their catalytically active forms, anyway,[6] this process can be considered an in situ isomerization. Such was verified using 1H and 29Si NMR spectroscopy, as well as UV-Vis spectroscopy and we were able to increase the yield of perfectly isotactic PP per employed amount of ligand by up to 400%.

In addition to mentioned published results, we were able to expand the catalyst scope onto far less bulky systems lacking the tBu groups in position 3’ and 5’ of the aryl groups and onto the corresponding zirconocene complexes, as well. Recent experiments indicate that the methoxy groups at position 7 of the indenyl moieties are necessary for a TIBA-promoted isomerization to occur and studies regarding the mechanism of the isomerization are ongoing.

Audience Take Away 

  • A way to gain a new perspective on a process (homogenous olefin polymerization) that has been established for more than 40 years
  • Interpreting unexpected experimental results and elucidating as well as utilizing the underlying molecular mechanisms
  • The strive to push chemical properties (isotacticity and molecular weight of polypropylene) to their limits to obtain unprecedented material properties
  • Catalyst design on a molecular level to tune material characteristics

Biography:

Tim Lenz studied Chemistry at the Technical University of Munich, where he graduated with a B. Sc. degree in 2020 and an M. Sc. degree in 2022. He started his PhD studies under the supervision of Prof. Bernhard Rieger in the same year. His research focuses on designing new catalysts for the homogeneous polymerization of propylene and elucidating organometallic reaction mechanisms of such.

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