Breaking symmetry-rules for stereoselective polymerization catalysis

Homogeneous catalysts for propylene polymerization are now available for production of isotactic polypropylene (PP) with stereoselectivities approaching that of heterogeneous Ziegler-Natta (ZN) catalysts. Ewen1 and Kaminsky2 demonstrated that the microstructure of PP was predictable when produced with catalysts operating by site control mechanisms and established the relationships, known as “Ewen’s symmetry rules”, which correlate the chirality and symmetry of the catalyst to the microstructures of PP. In this communication we report a theoretical analysis by DFT calculations on the propene polymerization promoted by systems formed by N-heteroaryl-pyridylamido tridentate ligands and the corresponding (N_,N,N_)MIV(Me2)2] pentacoordinate complexes (M=Zr, Hf) (systems 1 and 2 in Chart 1).3 Our results suggest that for these systems: a) the mechanisms of stereoselectivity contradict the “Ewen’s symmetry rules”; b) the tacticity control is much more subtle and difficult to predict than the one performed by tetrahedral ansametallocene4 as well as post-metallocene systems.5 Nevertheless, breaking the generally accepted symmetry-rules open new perspectives to achieve stereoselective catalysis.