Unconventional Stereoerror Formation Mechanisms in Nonmetallocene Propene Polymerization Systems Revealed by DFT Calculations

An unconventional
mechanism for the stereoerror formation in propene
polymerization catalyzed by C1-symmetric
salalen-M systems (M = Zr, Hf) is suggested by DFT calculations. While
propagation happens with the ligand in its fac-mer conformation, a change of ligand wrapping mode from fac-mer to fac-fac is the main source of the lower stereoselectivities
obtained with Zr and Hf. This is different for the Ti analogues, where
the ligand fac-mer wrapping mode
does not play a role. Activation strain analysis indicates that the
preference for a chain stationary mechanism of the Zr/Hf species is
due to the energy required to distort the reactants (ΔEStrain) rather than to their mutual interaction
(ΔEInt).