Benzoxazole phenoxide ligand supported group IV catalysts and their application for the ring‐opening polymerization of rac ‐lactide and ε ‐caprolactone

Monomeric titanium [C 48 H 62 N 2 O 6 Ti] ( 1 ) and zirconium [C 48 H 62 N 2 O 6 Zr] ( 2 ) complexes were synthesized by combining the monoanionic salicylbenzoxazole pro‐ligand 2‐(5‐X‐benzoxazol‐2‐yl)‐6‐R 1 ‐4‐R 2 ‐phenol, LH (R 1 , R 2 , X =  t Bu, t Bu, H, respectively) with Ti(O i Pr) 4 or Zr(O i Pr) 4 ∙( i PrOH) in a 2:1 stoichiometric ratio. Controlled hydrolysis of the monomeric Ti complex with deionized water yielded a new unusual hexanuclear titanium complex [C 126 H 144 N 6 O 21 Ti 6 ] ( 3 ). In addition, the cage formed in this complex 3 was surrounded by all‐oxygen and penta‐coordinated Ti(IV) atoms, which leads to an hexanuclear drum‐shaped Ti complex 3 with a trigonal‐bipyramidal geometry at the metal center. The catalytic activity of these complexes toward ring‐opening polymerization (ROP) of ε ‐caprolactone ( ε‐ CL) and rac ‐lactide ( rac ‐LA) was studied. All these complexes showed moderate to good catalytic activity without using any co‐catalyst additives. Single‐site zirconium alkoxide 2 showed the best catalytic activity and was able to convert 400 units of ε ‐CL or rac ‐LA into the polymer product in high yield (≥90%) within 1 hour under solvent‐free conditions. The obtained polymers showed good control in molecular weight ( M n  = 46.2–6.3 kg/mol) and molecular weight distributions (MWD's = 1.37–1.15). The polymerization of rac ‐LA by 2 yielded iso‐ enriched polylactide ( P m  = 0.60).