The shift of the polymers’ molecular weight to the ultrahigh molecular weight regime is linked with outstanding properties, such as enhanced mechanical and thermal characteristics. So far, we isolated ultrahigh molecular weight isotactic polypropylene (UHMW-iPP) with a molecular weight of 5800 kg/mol, a perfect isotacticity and the highest melting transition ex reactor of 171 °C. With the expected outperformance of iPP by syndiotactic PP (sPP) in processing and application of these polymers, we investigated the isolation of UHMW-sPP with highly active Cs-symmetric Hafnocenes. However, the activation of these Hafnocene dichlorides with the commonly used methylaluminoxane (MAO) proved problematic, often accompanied by a a reduced activity and a lower molecular weight of the polymers. Therefore, we adapted for sPP the in situ activation using TIBA (triisobutylaluminum) and tritylborate (TrBCF) forming the catalytic active cationic species. The stereoregularity, productivity and molecular weight of various C and Si bridged Cs-symmetric ansa-Hafnocenes was compared for the in situ and the commonly used MAO activation. All four Hafnocenes produced UHMW-sPP with up to 2500 kg/mol and a high syndiotacticity (up to 73%) using TIBA/TrBCF. The different bridging motifs of the Hafnocenes (C and Si) and the thereby induced change in geometry clarified the big impact of the ligand structure onto the performance of the catalyst. Si bridged Hafnocenes resulted in a loss of activity (4-fold to 20-fold, respectively) and stereoregularity, whereas the molecular weight was enhanced. Contrary to the in situ activation, MAO led to a by far lower productivity and molecular weight (max. 530 kg/mol), whereas the degree of syndiotacticity was slightly increased (up to 80%). With this series of polymerizations, we highlighted the outstanding benefits of the in situ activation in terms of productivity and molecular weight compared to MAO but also disclosed possible limitations of this activation method.

In Situ Activation: Chances and limitations to form ultrahigh molecular weight syndiotactic polypropylene

ACS Spring 2022

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In Situ</i> Activation: Chances and limitations to form ultrahigh molecular weight syndiotactic polypropylene