Perfectly Isotactic Polypropylene upon In Situ Activation of Ultrarigid meso Hafnocenes

For more than 40 years, the synthesis of C-2-symmetric indenyl-based racemic metallocenes for the isoselective polymerization of propylene relied on a tedious separation of the produced rac and meso isomers. Status quo, latter are considered wasteful as they produce atactic polypropylene (aPP) rather than isotactic polypropylene (iPP) if activated with methylaluminoxane (MAO). Unexpectedly, the in situ activation of meso hafnocene I yielded perfectly isotactic polypropylene. We verified an isomerization of the meso compound to the corresponding racemic one upon triisobutylaluminum (TIBA) addition via nuclear magnetic resonance (NMR) spectroscopy and established an easy and convenient polymerization protocol, enabling productivities comparable to pure rac-I if applied to pure meso-I or a mixture of both isomers. With this established isomerization protocol, the potential yield of iPP was enhanced by more than 400 %. This protocol was also shown to be applicable to other meso hafnocenes and some initial mechanistic insights were received.

Automated summary

For over 40 years, the synthesis of symmetric non-racemic metallocenes for propylene polymerization required tedious separation. However, under specific conditions, isomerization of non-racemic metallocenes resulted in highly isotactic polypropylene, leading to increased yield.