Research On Zn-Schiff-base Catalysts And Their Solvent-free Melt Ring-opening Polymerization Of Lactide And Copolymerization With Epoxide

With the characteristic of biodegradable, biocompatible and renewable, polylactide and its copolymers have a very wide range of applications, such as in agriculture, medical field and daily life. But there have the requirements about the molecular weight and the microstructures of polymers to meet specific applications. Hence, it’s continuously a big challenge to exploit the novel and high-efficiency catalysts to realize the controllability of molecular weight and the molecular weight distribution.This thesis successfully synthetized two kinds of symmetrical schiff ligands (H2L1and H2L2) from condensation of aldehydes and diamines, and obtained a series of distinctive catalysts:single-site catalyst (C1), double-sites catalysts (C2) and four-sites catalysts (C3) by the further coordination reaction of ligand with transition metal ions-Zn2+. The structures of all the ligands and catalysts were characterized respectively by elementary analysis (EA), the infrared spectra (IR), the nuclear magnetic resonance spectra (1H NMR) and the X-ray diffraction single-crystal analyses; Then L-lactide was catalyzed by these catalysts from the bulk solvent-free melt ring-opening polymerization. In addition, we attempt to change the nature of the polylactide through the copolymerization modification for the reasons of low molecular weights and simple polymer microstructures:the polymerization behaviors of styrene oxide (SO) catalyzed by the complex C1, C2and C3were verified firstly and the results showed that the polymers were with larger molecular weights (up to Mw=1.5182×104) and narrow molecular weight distributions (PDI=1.23-2.03). Based on the results, the copolymerization behaviors of L-lactide with SO or propylene epoxide (PO) were also verified, respectively. The chracters of all polymers were characterized respectively by IR,1H NMR and gel permeation chromatography (GPC) with polystyrene as the standard. Firstly, three different Zn-schiff-base catalysts were obtained, and their solvent-free melt ring-opening polymerization behaviors of L-lactide were checked in details. The results showed that the structural characters of the catalysts, such as the electronic effect, the steric effect or the number of active centers, were important and influential to the catalytic behaviors.Secondly, the polymerization behaviors of SO catalyzed by the catalysts mentioned above were studied. It suggested that the polymers with higher molecular weight and narrow molecular weight distribution were obtained.Thirdly, the copolymerization behaviors of L-lactide with SO or PO catalyzed by the catalysts mentioned above were also studied. The catalytic results showed that structural characters of the catalysts were the important and influential factors contributing to the catalytic results, including the electronic effect, the steric effect and the sites of active centers.Meanwhile, the procedures of controllable conditions were also disscussed through the studies of the choice of catalysts, reaction temperature, reaction time and molor ratio btween monomers and cantalysts.