Ring-Opening Polymerization of rac-β-Butyrolactone Catalyzed by Zinc Complexes Bearing Multidentate Aminophenolate Ligands

The stereotacticity of poly(3-hydroxybutyrate) (P3HB) has a great impact on its physical and mechanical properties, thus the design and synthesis of high-performance catalysts capable of catalyzing the stereoselective ring-opening polymerization of racemic β-butyrolactone (rac-β-BL) are challenging and becoming a hot issue. In this work, a series of aminophenolate zinc complexes with pendant benzoimidazolyl groups Zn1—Zn7 were synthesized via the reaction of the corresponding proligands L¹H—L⁷H with the same amount of ZnN(SiMe₃)₂₂, respectively. Among them, the proligands L⁴H, L⁷H and zinc complexes Zn4, Zn7 were newly reported, which were further characterized by elemental analysis and NMR spectroscopy. The molecular structure of complex Zn4 was further determined by X-ray diffractional analysis, where the metal center is four-coordinated by the tridentate aminophenolate ligand and one silylamido group. Complexes Zn1—Zn7 could catalyze the ring-opening polymerization of rac-β-BL with moderate activities, producing P3HBs with syndiotacticities up to P_r = 0.66. The substituents on the ligand frameworks showed noticeable influences on the activities of these complexes, with the influence of ortho-substituent of the phenolate ring most remarkable. Complex Zn1 with an ortho-tert-butyl group of less steric bulkiness meanwhile electron-donating exhibited the highest activity, TOF = 116 h⁻¹; the introduction of electron-withdrawing chlorine brought unfavorable effect on the polymerization of rac-β-BL, leading to complex Zn4 exhibiting a decreased activity. The molecular weights of resultant P3HBs are in good consistency with the theoretical values, and the molecular weight distributions are generally narrow (M_w/M_n ≈ 1.15—1.38). On the basis of ¹H-NMR tracking reactions and the result of ¹H-NMR spectrum of typical oligomer, it is proposed that the polymerization of rac-β-BL is initiated by the in-situ formed unique zinc isopropoxide species and the P3HB chain is propagated via coordination-insertion process to give linear polymers end-capped with one isopropoxyl group and one hydroxy group. Moreover, an easy way of evaluating the stereoregularity of P3HB using inexpensive and convenient ¹H-NMR spectrometry was developed.