关键词: 苯并噁唑取代氨基酚配体, 锌氯化物, 外消旋丙交酯, 开环聚合, 环状聚丙交酯

Four zinc chloride complexes supported by aminophenolate ligands bearing a benzoxazolyl group were synthesized via the reactions of the corresponding sodium salts of the proligands HL^1-4 with one equiv. of zinc dichloride at room temperature respectively, and their structures were well-characterized by ¹H NMR, ¹³C NMR and elemental analysis (EA) methods. The molecular structure of complex 1 was further determined by X-ray single crystal diffraction analysis, where the phenoxy oxygen atom, the skeleton N atom, and the benzoxazolyl N atom of the ligand all coordinate with the metal center, forming a disordered tetrahedral coordination geometry around the metal center together with the chloride ligand. This series of zinc chloride complexes showed high stability, and could be applied to catalyze the ring-opening polymerization (ROP) of industrial grade racemic lactide (rac-LA) with good activities. The substituent on the skeleton N atom exhibited a significant impact on the catalytic performance of the complex. Complex 1 with a linear n-butyl group on the skeleton N atom displayed the highest catalytic activity. At 80 ℃, 50000 equiv. of industrial grade rac-LA could be polymerized by complex 1 using cyclohexane oxide (CHO) as a solvent, and a high turnover frequency (TOF) value of 2716 h^-1 was reached, resulting in a cyclic polymer with a molecular weight of M_n=2.03×10⁴ g/mol. Complex 3 with a cyclohexyl group on the skeleton N atom showed the highest isotactic selectivity among these zinc chloride complexes toward the ROP of rac-LA, at 80 ℃, P_m=0.67, which could be increased to P_m=0.70 when the polymerization was carried out at 25 ℃. By adopting a high monomer concentration of 14 mol/L in the polymerization, cyclic polylactide with a molecular weight of M_n=3.10×10⁴ g/mol was achieved using complex 1. Based on the matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopic analysis of oligomers, it was found that in the absence of benzyl alcohol, the polymer formed was mainly cyclic, containing small amount of linear chains end-capped with -OH/-COOH, -OH/-OC₆H₁₀Cl groups etc.; while in the presence of benzyl alcohol, the resultant polymer was found to be a mixture of linear ones end-capped with -OH/-OBn and cyclic ones, both in a reasonable amount. It is thus suggested that, in the absence of external alcohol a coordination-insertion polymerization mechanism is involved, which integrates with end-to-end and non-selective cyclization to produce mainly cyclic polymers, wherein CHO acts as activator to zinc chloride complexes by forming zinc alkoxide species via the nucleophilic attack of chlorine to the coordinated CHO; when benzyl alcohol was used, it acts as a chain transfer reagent with the active propaga- tion chain end-capped with -OH/-OC₆H₁₀Cl to generate zinc benzyloxide species, which initiates the ROP of rac-LA to form linear polymers end-capped with -OH/-OBn groups, meanwhile the cyclization of the active propagation chain end-capped with -OH/-OC₆H₁₀Cl still takes place in this system.

Key words: benzoxazolyl-based aminophenol proligands, zinc chloride complex, rac-lactide, ring-opening polymerization, cyclic polylactide