Synthesis of Rare Earth Metal Complexes Stabilized by Amine Bridged Bis(phenolato) Ligands and Their Performance in the Polymerization of <i>rac</i>‐β‐Butyrolactone

A series of rare earth alkoxides bearing amine-bridged bis(phenolato) ligands were synthesized through sequential reactions of RE(C₅H₅)₃(THF) (RE = Y, Lu) or Nd[N(SiMe₃)₂]₃ with bis(phenols) LH₂ and CF₃CH₂OH. Complexes REL(OCH₂CF₃)(THF)_n (1-6) bearing different aryl-substituents were obtained in good yields of 59-70 %. They were applied in the ring-opening polymerization (ROP) of rac-β-butyrolactone (rac-BBL), which showed good activity (TOF up to 27,300 h^-1), resulting in syndiotactically enriched poly(3-hydroxybutyrate) (PHB) (P_r up to 0.86) with narrow polydispersities (PDI ≤ 1.27). The yttrium complex 3 bearing bulky o-1,1-diphenylethyl substituents outperformed other complexes, suggesting that the smaller ionic radii of metal centers and bulky ortho substituents of ancillary ligands play crucial roles in controlling the activity and stereoselectivity in ROP of rac-BBL. Kinetics of the polymerization of rac-BBL initiated by complex 3 was investigated, which revealed first order dependences on the monomer and initiator concentrations, respectively.