Synthesis Of Thermostable Iron Complexes And Application In The Ethylene Polymerization

A series of tridentate iron complexes with asymmetric diiminopyridine structure and fused ring diiminopyridine structure were synthesized and applied in the catalysis of ethylene polymerization in this thesis.The effects of ligand framework and imine aromatic substituents on the thermal stability of the catalysts were investigated systematically.The polymerization conditions（i.e.molecular weight,molecular microstructure,and thermal properties of the polyethylene）were studied.The tridentate iron complex was also supported in boron nitride to further improve the thermal stability.The specific work is divided into the following three parts:1.Based on previous studies,a series of asymmetrical tridentate iron complexes were prepared creatively by replacing the classical tridentate imine phenyl with imine naphthalyl.These tridentate iron complexes exhibited good catalytic activity(16.7×10⁶ g（PE）mol^-1（Fe）h^-1,70℃,Fe4)and good thermal stability(9.74×10⁶g（PE）mol^-1（Fe）h^-1,80℃,Fe2)for ethylene polymerization.DFT simulation shows that the introduction of large steric groups hinders the coordination and insertion of ethylene,so the catalytic activity of the complex is reduced.At the same time,the introduction of large steric hindrance groups also makes chain transfer more difficult to occur,resulting in high molecular weight of polyethylene.This part of work shows that the introduction of naphthalene ring as substituent and phenylethyl as ortho substituent of imine aromatic group in the ligands of classic Brookhart type iron complexes is beneficial to improve the catalytic activity and thermal stability of the catalysts2.A series of tridentate iron complexes containing fused rings of pyridine ring frame structure was synthesized and applied in ethylene polymerization.Both activity(15.7×10⁶ g（PE）（Fe）mol^-1h^-1,Fe8,70℃)and thermal stability(9.26×10⁶g（PE）（Fe）mol^-1h^-1,Fe8,90℃)achieved by electronic-donating groups,which is higher than the previous report.These results indicate that the introduction of pyridine framework with fused rings structure is beneficial to improve the catalytic activity and thermal stability of the complex.Besides,the molecular weight of polyethylene obtained by MAO as co-catalyst is closely related to the structure of the complex and the polymerization conditions.The molecular weight of polyethylene prepared by Fe8 with a large steric hindrance group is 280 kg mol^-1 while 1.842kg mol^-1 is obtained by Fe6.The molecular weight of polyethylene prepared at 20℃is 692kg mol^-1,while the molecular weight of polyethylene prepared at 90℃is only 10.8 kg mol^-1.Different from MAO,when MMAO is selected as the co-catalyst,the polyethylene obtained under the optimal polymerization condition is linear polyethylene wax with narrow molecular weight distribution.3.Boron nitride（BN）was used as a carrier to load Fe8.By SEM characterization,it was proved that the tridentate iron complex was successfully loaded on BN,and the Fe content on each catalyst was characterized by inductively coupled plasma mass spectrometry（ICP-MS）.By catalyzing ethylene polymerization experiments,the loaded catalyst showed higher activity(26.9×10⁶ g（PE）（Fe）mol^-1h^-1,0.1%Fe8@BN,70℃)to ethylene polymerization than Fe8(11.1×10⁶ g（PE）（Fe）mol^-1h^-1,70℃),and the molecular weight of the obtained polyethylene(467 kg mol^-1)is significantly higher than the Fe8(57.23 kg mol^-1),proving that the BN as a carrier can improve the thermal stability of the tridentate iron complexes.Further experimental investigation（polymerization temperature,co-catalyst,and different loading capacity of the tridentate iron complex）again proved this conclusion.The highest activity of the supported tridentate iron complex is 20.3×10⁶g（PE）mol^-1（Fe）h^-1（90℃）,which is the highest thermal stability of the tridentate iron complex reported so far.The complex catalyst prepared by the modification of the structure of the tridentate iron complex shows excellent activity and thermal stability,which has practical significance in promoting the application of the iron complex catalyst on the road of industrialization.