"Living" propene polymerization with bis(phenoxyimine) group 4 metal catalysts: New strategies and old concepts

Bis(phenoxyimine)Ti catalysts with ortho-F-substituted phenyl rings on the N can be "living" propene polymerization catalysts. On the basis of DFT calculations, it has been proposed that the "living" behavior originates from an unprecedented attractive interaction between the said ortho-F atoms and a β-H of the growing polymer chain, which would render the latter less prone to be transferred to the metal (or to the monomer). In this paper, we report on a thorough full-QM and combined QM/MM investigation of representative model catalysts, demonstrating that the key factor is instead the repulsive nonbonded contact of the F-substituted rings with the growing polymer chain and an incoming propene molecule, which destabilizes the sterically demanding six-center transition structure for chain transfer to the monomer. A conceptually similar substituent effect has been reported before for several metallocene and non-metallocene catalysts; in the present case, though, this is partly alleviated by a weak attractive interaction between the ortho-F and a close-in-space α-H of the growing chain.