Mechanism of Isotactic Styrene Polymerization with a C6F5‑Substituted Bis(phenoxyimine) Titanium System

We report a combined, experimental and theoretical, study of styrene polymerization to clarify the regio- and stereocontrol mechanism operating with a C6F5-substituted bis(phenoxyimine) titanium dichloride complex. Styrene homopolymerization, styrene–propene and styrene–ethene–propene copolymerizations have been carried out to this aim. A combination of 13C NMR analysis of the chain-end groups and of the microstructure of the homopolymers and copolymers reveals that styrene polymerization is highly regioselective and occurs prevalently through 2,1-monomer insertion. DFT calculations evidenced that steric interaction between the growing chain and the monomer in the transition state insertion stage is at the origin of this selectivity. The formation of isotactic polystyrene with a chain-end like microstructure is rationalized on the base of a mechanism similar to that proposed for the syndiospecific propene polymerization catalyzed by bis(phenoxyimine) titanium dichloride complexes. Finally, a general stereocontrol mechanism operative in olefin polymerization with this class of complexes is proposed.