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

Dibenzyl complexes [Ti(OC 6 H 2 -R 1 -6-R 2 -4) 2 {S(CH 2 ) 2 S}(CH 2 Ph) 2 ] ( 2a − c; R 1 = t Bu, R 2 = Me, a; R 1, R 2 = t Bu, b; R 1 = H, R 2 = Me, c ) with a tetradentate dianionic [OSSO]-type ligand were synthesized from the corresponding titanium dichloro complexes [Ti(OC 6 H 2 -R 1 -6-R 2 -4) 2 {S(CH 2 ) 2 S}Cl 2 ] and benzylmagnesium chloride in toluene. NMR spectroscopic monitoring of the reaction of the dibenzyl complex 2b with B(C 6 F 5 ) 3 in bromobenzene indicates the formation of a thermally sensitive benzyl cation [Ti(OC 6 H 2 - t Bu 2 -4,6) 2 {S(CH 2 ) 2 S}(CH 2 Ph)] + . 1 H NMR spectra at −30 °C show two C 1 -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 6 H 2 - t Bu-6-R-4) 2 {S(CH 2 ) 3 S}(CH 2 Ph) 2 ] ( 4a, b; R = Me, a; R = t Bu, 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(C 6 F 5 ) 3 . Using the 1,4-dithiabutanediyl-linked dibenzyl complexes and the activators [PhNMe 2 H][B(C 6 F 5 ) 4 ]/Al n Oct 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.