A possible unified mechanism of like and unlike chain-end stereocontrol for primary propene-coordinated polymerizations

Theoretical studies relative to propene primary insertion for polymerization catalysts based on Brookhart-type Ni(II) and biscylopentadienyl Ti(IV) complexes are reported. On the basis of geometrical considerations relative to calculated transition states, a unified-mechanism for like (isospecific) and unlike (syndiospecific) chain-end stereo-control for primary propene polymerizations, which can be considered as a sub-case of the more general mechanism of the chiral orientation of the growing chain, is suggested. As usual, the selection between monomer enantiofaces would be determined by the conformation relative to the Mt-C bond (negative or positive values of θ₁), which for these catalysts would be determined by the configuration of the tertiary carbon bond of the growing chain being closest to the metal, but would also be critically depending on the conformation of the first C-C bond of the chain. In particular, chain-end stereo-controlled polymerization catalysts imposing that the first tertiary H atom would point toward the metal θ_2H≈0°) or toward the monomer (θ_2H≈100°) would lead to preference for isospecific and syndiospecific propagation, respectively.