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·H₂O with 2.0 molar equivalents of sterically bulky 2-(2H-benzotriazol-2-yl)-4,6-bis(1- methyl-1-phenylethyl)phenol (^CMe2PhBTP-H) and 2-(2H-benzotriazol-2- yl)-4,6-di-tert-butylphenol (^t-BuBTP-H) in refluxing ethanol solution afforded monomeric copper complexes [(^CMe2PhBTP)₂Cu] (1) and [(^t-BuBTP)₂Cu] (2), respectively. The four-coordinated copper analogue [(^TMClBTP)₂Cu] (3) resulted from treatment of 2-tert-butyl-6-(5-chloro-2H-benzotriazol-2-yl)-4-methylphenol (^TMClBTP-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 [(^TMClBTP)₂Cu(Me ₂CO)] (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 Me₂CO 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.