Living isospecific styrene polymerization by chiral benzyl titanium complexes that contain a tetradentate [OSSO]-type bis(phenolato) ligand

Article abstract of DOI:10.1021/om060047e

Dibenzyl complexes [Ti(OC₆H₂-R¹-6-R ²-4)₂{S(CH₂)₂S}(CH ₂Ph)₂] (2a-c; R¹ = ^tBu, R ² = Me, a; R¹, R² = ^tBu, b; R ¹ = H, R² = Me, c) with a tetradentate dianionic [OSSO]-type ligand were synthesized from the corresponding titanium dichloro complexes [Ti(OC₆H₂-R¹-6-R²-4) ₂{S(CH₂)₂S}Cl₂] and benzylmagnesium chloride in toluene. NMR spectroscopic monitoring of the reaction of the dibenzyl complex 2b with B(C₆F₅)₃ in bromobenzene indicates the formation of a thermally sensitive benzyl cation [Ti(OC₆H₂-^tBu₂-4,6) ₂{S(CH₂)₂S}(CH₂Ph)]⁺. ¹H NMR spectra at -30°C show two C₁-symmetric diastereomers in a 55:45 ratio, suggesting slow epimerization on the NMR time scale at this temperature. The benzyl complex 2c, which does not bear bulky ortho-substituents, was found to be fluxional and unstable in solution. Related benzyl complexes of the 1,5-dithiapentanediyl-linked ligand [Ti(OC ₆H₂-^tBu-6-R-4)₂{S(CH ₂)₃S}(CH₂Ph)₂](4a,b; R = Me, a; R = ^tBu, b) were synthesized in an analogous manner ' and were also shown to be highly fluxional and unstable in solution, decomposing immediately upon addition of B(C₆F₅)₃. Using the 1,4-dithiabutanediyl-linked dibenzyl complexes and the activators [PhNMe ₂H][B(C₆F₅)₄]/AlⁿOct ₃, the living isospecific polymerization of styrene could be achieved for the first time. The resulting isotactic polystyrenes show narrow molecular weight distributions (M_w/M_n < ～1.2). A linear relation between the number-averaged molecular weights M_n and the monomer conversion was observed.