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

Bis(phenoxyimine)Ti catalysts with crtho-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 origi nates 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 demand ng 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.