关键词: 聚酯, 有机钛化合物, 缩聚反应, 催化活性, 配位基团

Abstract: Polyethylene terephthalate (PET) has excellent physical and chemical properties and is the most commonly used polyester material. The polycondensation stage of polyester synthesis requires the participation of catalysts. At present, antimony catalyst is widely used in industrial production. Antimony catalyst is a kind of heavy metal compound, which has biological toxicity and will cause harm to human health and ecological environment. With the increasing awareness of healthy, the design and development of non-toxic and non-hazardous polyester catalysts has become an important hotspot in the field of polyester research. The titanium-based catalysts are expected to replace the antimony-based catalysts due to the advantages of environmental protection, strong catalytic activity and abundant resources. The high activity of titanium catalysts increases the reaction rate of the PET polycondensation process, but also promotes side reactions such as thermal degradation and thermo-oxidative degradation, which is one of the most prominent problems limiting the application of titanium catalysts in polyester synthesis. The regulation of the catalytic activity of titanium catalysts is therefore crucial. In order to investigate the catalytic performance of organotitanium compounds in the condensation synthesis of polyesters under different coordination environments, firstly, density functional theory (DFT) quantum chemistry method was used to study tetramethylphenol titanium(Ti·[C7H7O]4，Cat1), acetyl acetone titanium oxide(TiO·[C5H7O2]2，Cat2), isopropyl tri (dioctylpyrophosphoxy) titanate(C51H112O22P6Ti，Cat3), and the coupling mechanism of electronic effects and steric hindrance of coordination groups on catalytic activity of organotitanium was analyzed theoretically. The Gaussian software was used to carry out structural optimization and energy calculations for the three organotitanium compounds, and the wave functions were extracted from the calculation results. The wave function analysis was carried out using the Multiwfn software package to obtain the Hirshfeld charge values of the titanium metal centres and the surface distance projection maps used to assess the effect of steric hindrance. Theoretical calculations show that Cat1 is more capable of forming titanium alcohol salt active species than Cat2 and Cat3; the Hirshfeld charge values for the central titanium atoms of the three active species are: 0.661, 0.524 and 0.600, respectively, and the Hirshfeld charge is strongly correlated with the electrophilic ability of the atoms, the higher the charge value, the stronger the electrophilic ability.By analyzed the distance between ligands in the surface distance projection，, respectively, and the size of the coordination space is Cat2、Cat1、Cat3. The catalytic reaction kinetics experiments results show that the reaction activation energies of Cat1, Cat2 and Cat3 catalyzed the synthesis of polyesters from BHET(Dihydroxyethyl terephthalate) were 72.41, 80.16 and 102.47 kJ/mol, respectively, and the highest catalytic activity was obtained for tetramethylphenolato-titanium (Ti·[C7H7O]4，Cat1), indicating that the electronic effect of the ligand group and the spatial site resistance jointly affect the catalytic activity of the titanium catalysts, with the electronic effect being more influential than the spatial site resistance. This work is a guide to the screening of ligands when adjusting the activity of titanium catalysts in the future.

Key words: polyester, organic titanium compounds, polycondensation, catalysis, coordination group