Air‐stable copper derivatives as efficient catalysts for controlled lactide polymerization: Facile synthesis and characterization of well‐defined benzotriazole phenoxide copper complexes

Air‐stable copper catalysts supported by bis‐ BTP ligands ( BTP = N,O ‐bidentate benzotriazole phenoxide) were synthesized and structurally characterized. The reactions of Cu(OAc) 2 ·H 2 O with 2.0 molar equivalents of sterically bulky 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐bis(1‐methyl‐1‐phenylethyl)phenol ( CMe2Ph BTP ‐H) and 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐di‐ tert ‐butylphenol ( t‐Bu BTP ‐H) in refluxing ethanol solution afforded monomeric copper complexes [( CMe2Ph BTP) 2 Cu] ( 1 ) and [( t‐Bu BTP) 2 Cu] ( 2 ), respectively. The four‐coordinated copper analogue [( TMCl BTP) 2 Cu] (3 ) resulted from treatment of 2‐ tert ‐butyl‐6‐(5‐chloro‐2H‐benzotriazol‐2‐yl)‐4‐methylphenol ( TMCl BTP ‐H) as the ligand under the same synthetic method with ligand to metal precursor ratio of 2:1, but treatment of complex 3 in acetone gave five‐coordinated monomeric complex [( TMCl BTP) 2 Cu(Me 2 CO)] (4 ). X‐ray diffraction of single crystals indicates that Cu complex 4 assumes a distorted square pyramidal geometry, penta‐coordinated by two BTP ligands, and one Me 2 CO molecule. Catalysis for lactide (LA) polymerization of BTP ‐containing Cu complexes in the presence of various alcohol initiators was investigated. Complex 3 initiated by 9‐anthracenemethanol catalyzes the ring‐opening polymerization effectively not only in a “living” fashion but also in an “immortal” manner, yielding polymers with the predictable molecular weights and narrow molecular weight distributions. Initiations from multifunctional alcohols were able to produce PLLAs with two‐arm linear and three‐arm star‐shaped molecular architectures. The controlled character of Cu complex 3 also enabled us to synthesize the PEG‐ b ‐PLLA copolymer.