| Aliphatic polyesters, as the most typical representative of the variety of biodegradable polymers known, had become increasingly important in the development of biomedical polymer materials and drug delivery systems due to their characteristic properties of high mechanical strength, biocompatibility and easily processed ability. As the key to synthesis, the development of catalysts was one of the important breakthroughs in aliphatic polyester industry.In this thesis, one Zn(Ⅱ) unilateral Schiff-base complex and seven Zn(Ⅱ) asymmetric bis-Schiff-base complexes were obtained, respectively. They were characterized by element analysis (EA), X-ray single-crystal diffraction (XRD), powder X-ray diffraction (PXRD), nuclear magnetic resonance spectrum (NMR), ultraviolet-visible absorption spectrum (UV-Vis), infrared spectrum (FT-IR) and thermo-gravimetric analysis (TGA). Moreover, all the eight complexes were used as catalysts for the ring-opening polymerization of lactide as well as the copolymerization of L-lactide (LLA) and maleic anhydride (MA), LLA and cyclohexene oxide (CHO) or MA and CHO. The molecular weight size (Mn or Mw) and the molecular weight distribution (PDI=Mw/Mn) of the obtained polymers were determined by gel permeation chromatography (GPC). In addition, the microstructures and properties of polymers were characterized by the FT-IR,1H NMR and TGA. During the process of polymerization, the study on the corresponding relationship between catalysts structure and the polymerization behavior was focused. The catalysts with excellent performance were selected to investigate the effects of the molar ratio of monomer and catalyst, co-catalyst, polymerization time and polymerization temperature on detailed polymerization.Firstly, based on the unilateral Schiff-base ligand L synthesized from the selective condensation reaction of1-Phenyl-3-methyl-4-benzoyl-2-pyrazolin-5-one (PMBP) and one-NH2of1,2-diaminobenzene, the zinc(Ⅱ) unilateral Schiff-base complex LZn was obtained by the coordination of Zn ion with two ligands L. Furthermore, utilizing the Zn-templating effect, seven asymmetric zinc(Ⅱ) bis-Schiff-based complexes (LZn-1～LZn-7) were obtained from the condensation reaction of L and one of aldehyde derivatives (salicylaldehyde,5-bromo-2-hydroxy-benzaldehyde,3,5-dibromo-2-hydroxy-benzal-dehyde, o-vanillin,5-bro-mo-3-methoxy-2-hydroxy-benzaldehyde and3,5-ditert-butyl-2-hydroxybenzaldehyde). The X-ray single crystal diffraction determinations showed that five (LZn-1～LZn-5) of the complexes were monomers and one (LZn-7) of complexes had homo-trinuclear framework.Secondly, the bulk polymerization behaviors of LA (LLA and L,D-LA) were studied in detail by using complexes LZn and LZn-1～LZn-6as the catalysts. The results showed that all the zinc(II) catalysts had a relatively higher catalytic activity (0.650-4.749×103g·mol-1h-1) for the polymerization of LLA in contrast to that of L,D-LA. On the condition of better catalytic behaviors (Catalytic activity of4.160×103g·mol-h-1, Mn of4.298×103g·mol-1and PDI of1.19) from the selected catalyst LZn-3, the controllable polymerization process was based on the molar ratio of monomer and catalyst of1000:1, the polymerization time of24h and the polymerization temperature of160℃.Thirdly, the bulk copolymerization behaviors of LLA and MA were studied in detail by using the eight complexes LZn and LZn-1-LZn-7as the catalysts in presence of4-dimethylaminopyridine (DMAP). The results showed that the effective copolymerization of LLA and MA was realized with the moderate catalytic activity (0.429～1.641×103g·mol-1h-1). Using the selected LZn-3as the suitable catalyst from the copolymerization condition of the molar ratio of LLA, MA, catalyst and DMAP of250:250:1:1, the reaction time of24h and the polymerization temperature of130℃, the relatively better catalytic behaviors were obtained, where the catalytic activity was up to2.880×103g·mol-1h-1, the Mn of the P(LLA-MA) was1.023×104g·mol-1and PDI was1.50.The bulk copolymerization behaviors of LLA and CHO were also studied in detail by using the eight complexes LZn and LZn-l-LZn-7as the catalysts in presence of DMAP. The results showed that the effective copolymerization of LLA and CHO was realized with the moderate catalytic activity (0.265×103～0.919×103g·mol-1h-1). Using the selected LZn-3as the suitable catalyst from the copolymerization condition of the molar ratio of LLA, CHO, catalyst and DMAP of250:250:1:1, the reaction time of24h and the polymerization temperature of115℃, the relatively better catalytic behaviors were obtained, where the catalytic activity was0.913×103g·mol-1h-1, the Mn of the P(LLA-CHO) was2.404×103g·mol-1and PDI was1.07.Fourthly, the copolymerization behaviors of MA and CHO in both bulk and in solvent were studied in detail by using the eight complexes LZn and LZn-1-LZn-7as the catalysts in presence of DMAP. The results showed that the effective copolymerization of MA and CHO was realized with the excellent catalytic activity (0.52×103～6.57×103g·mol-1h-1). Using the selected LZn-3as the suitable catalyst from the copolymerization condition of the molar ratio of MA, CHO, catalyst and DMAP of250:250:1:1, the reaction time of150min and the polymerization temperature of110℃in bulk, the relatively better catalytic behaviors were obtained, where the catalytic activity was6.10×103g·mol-1h-1, the Mn of the P(MA-CHO) was1.656×104g·mol-1and the PDI was1.66. Using the selected LZn-7as the suitable catalyst from the copolymerization condition of the molar ratio of MA, CHO, catalyst and DMAP of150:150:1:1, the reaction time of150min and the polymerization temperature of110℃in bulk, the relatively better catalytic behaviors were obtained, where the catalytic activity was3.22×103g·mol-1, the Mn of the P(MA-CHO) was1.403×104g·mol-1and the PDI was1.17.Finally, all the performances of the series zinc(Ⅱ) catalysts demonstrated that it could achieve the polymerization of LLA, the co-polymerization of LLA and MA, of LLA and CHO and of MA and CHO. The effect of polymerization was clearly influenced by the structure of the catalyst, especially the steric effects and the electronic effects of the catalysts. The asymmetric zinc(Ⅱ) bis-schiff-based catalysts had better catalytic effect than the zinc(Ⅱ) unilateral schiff based ones. The zinc(Ⅱ) catalyst with trinuclear structure had better catalytic effect than the zinc(Ⅱ) with mononuclear ones. The catalyst with the withdrawing substituent groups or having a large steric hindrance groups showed superior catalytic effect. |
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