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

摘要

Dibenzyl complexes [Ti(OC6H2-R-1-6-R-2-4)(2){S(CH2)(2)S}(CH2Ph)(2)] (2a-c; R-1 = Bu-t, R-2) Me, a; R-1, R-2 = Bu-t, b; R-1) H, R-2 = Me, c) with a tetradentate dianionic [OSSO]-type ligand were synthesized from the corresponding titanium dichloro complexes [Ti(OC6H2-R-1-6-R-2-4)(2){S(CH2)(2)S}Cl-2] and benzyl-magnesium 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-Bu-t(2)-4,6)(2){S(CH2)(2)S}(CH2Ph)](+). H-1 NMR spectra at -30 degrees 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((OC6H2Bu)-Bu-t-6-R-4)(2){S(CH2)(3)S}(CH2Ph)(2)] (4a,b; R = Me, a; R = Bu-t, 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)]/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 < similar to 1.2). A linear relation between the number-averaged molecular weights M-n and the monomer conversion was observed.