Living Isospecific Styrene Polymerization by Chiral Benzyl Titanium Complexes That Contain a Tetradentate [OSSO]-Type Bis(phenolato) Ligand

Dibenzyl complexes [Ti(OC6H2-R1-6-R2-4)2{S(CH2)2S}(CH2Ph)2] (2a−c; R1 = tBu, R2 = Me, a; R1,
R2 = tBu, b; R1 = 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(OC6H2-tBu-6-R-4)2{S(CH2)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
[PhNMe2H][B(C6F5)4]/AlnOct3, the living isospecific polymerization of styrene could be achieved for
the first time. The resulting isotactic polystyrenes show narrow molecular weight distributions (Mw/Mn
< ∼1.2). A linear relation between the number-averaged molecular weights Mn and the monomer
conversion was observed.