Study On The Catalysis Of Novel Heterocyclic Schiff Base Metal Complexes With Different Diamines For MMA Polymerization

Efficient and economical catalysts can promote the qualitative leap of polyolefin industry,and understand that post-transition metal catalysts show high“tolerance”to heteroatoms on polar monomers in the process of polymerization,which will provide a convenient way to realize the efficient production of PMMA materials.In this study,in order to provide a stable catalytic environment,the first step is to select the asymmetric structure 2-acetylpyridine with less steric hindrance as the precursor,through careful molecular design and"tailoring".The basic goal is to synthesize pyridine quinoxaline ligands with strong rigidity.In the second step,using the innovative synthesis technology,the new ligand is chelated with the late transition metal to form a symmetrical binuclear complex,and the molecular structure of the complex is accurately judged by means of characterization.The third step is to combine the electronic effect and steric hindrance theory to study the catalytic effect of polymer material PMMA,which is mainly catalyzed by complexes,to solve the scientific problem of structure-activity relationship of this kind of catalyst in polar monomer polymerization,and to optimize the polymerization process and explore the microstructure of PMMA.The final goal of this study is to realize the market value of this polymer.The specific research contents include:（1）Synthesis of ligand system based on pyridyl quinoxaline structure.Asymmetric monoimine ligands are selected to replace the traditionalα-diimine structure,combined with atomic economy strategy,a stable and systematic pyridyl quinoxaline ligand system is designed reasonably and accurately.Using cheap 2-acetylpyridine as precursor,Schiff base condensation took place in anhydrous ethanol by using aromatic amines containing substituents（-H,-OCH₃,-F,-Cl,-Br）at meta-position and refluxing in liquid phase.Five novel ligands（D1,D2,D3,D4,D5）were synthesized in high yield by melting point,infrared spectroscopy（IR）,elemental analysis,ultraviolet spectroscopy（UV-vis）,¹H NMR and ¹³C NMR to form a complete pyridine quinoxaline structure system.The representative D2 single crystal was selected and the ligand structure was further confirmed by X-ray single crystal diffraction.It is worth noting that there are both condensation reaction and cyclization reaction in the process of ligand formation.Finally,it presents the target structure containing substituents on pyridyl quinoxaline.Based on the characterization data,the molecular spatial structure of the ligand was discussed.（2）Synthesis of post-transition metal nickel（Ni）and cobalt（Co）complexes.Overcome the obstacle of the coordination reaction between heteroatoms and metal ions,simplify the reaction steps,use the one-step reaction method instead of the traditional deprotonation process,select the reaction solvent and synthesis process,and finally choose the liquid phase room temperature stirring method with good economic benefit,high yield and convenient operation in different solvent systems.Ten expected symmetric binuclear complexes[M₂Cl₄（D）₂]（M=Ni,Co;D=D1,D2,D4,D5）and[M₂Cl₄（D3）₂（H₂O）₂]（M=Ni,Co）were directly synthesized from ligands with nickel chloride hexahydrate（Ni Cl₂·6H₂O）（solvent:V（Et OH）/V（THF）=1:1）or cobalt chloride hexahydrate（Co Cl₂·6H₂O）（solvent:THF）.The previous conjecture structure was verified by elemental analysis,IR,UV-vis,X-ray powder diffraction and X-ray single crystal diffraction.In addition,based on the phenomenon of single crystal to single crystal structure transition（SCSC）,the structure of the catalyst was systematically designed and the method of single crystal growth was optimized.The coordination environment of metal ions was changed by changing solvents,and the molecular structure was fine-tuned.For the first time,the crystallographic data of 10 complexes with complete samples,novel structure,systematic and representative were obtained at room temperature.The solid powders of primitive nickel and cobalt complexes were dissolved in V（DMF）/V（Et₂O）=1:2 solution by solvent diffusion method,and six novel asymmetric mononuclear metal complexes with DMF as the second ligand[Ni Cl₂（D3）·（DMF）₂]and[Co Cl₂（D）·（DMF）]（D=D1,D2,D3,D4,D5）were obtained by accident.The solid powder of the original cobalt complex was dissolved in V（Me OH）/V（Et₂O）=1:2 solution by solvent diffusion method.The crystal data confirmed that the molecular structures of the two target complexes[Co₂Cl₄（D1）₂]and[Co₂Cl₄（D3）₂·（H₂O）₂]were the expected structures designed.However,two novel asymmetric mononuclear cobalt complexes[Co Cl₂（D4）·（Me OH）]and[Co Cl₂（D5）·（Me OH）]were unexpectedly obtained by the same single crystal culture method.This may be related to the radius and electron absorption of the substituents on aromatic amines.In summary,the structural determination of the crystals obtained in different solvents means that the cobalt complex system in this study shows a very rare SCSC phenomenon,and the transformation of these structures mainly depends on the fracture of weak chemical bonds and the formation of strengthened chemical bonds.（3）study on the catalytic performance of methyl methacrylate（MMA）polymerization.Combining the traditional radical polymerization behavior of polar monomers with the principle of coordination polymerization,binuclear chloro-bridged nickel and cobalt complexes were creatively applied to the catalytic synthesis of polymer material PMMA.All the catalysts showed high catalytic activity,reaching the order of magnitude of 10⁵ g·mol^-1·h^-1,and the full polymerization of polar monomer MMA was realized.First of all,the relationship between the structure and performance of the catalyst was studied from the aspects of ligand skeleton structure and central metal ion type.It was found that when the aromatic amine substituted group was bromine,the MMA conversion rate of metal nickel complex was as high as 98.79%.Based on the theoretical basis of electronic effect and steric hindrance,the scientific problem of structure-activity relationship of this kind of catalyst was successfully solved.Secondly,the catalytic polymerization process was optimized to achieve efficient polymerization of MMA monomers.When the polymerization time is 6 h,the temperature is110℃,n（M）/n（Ni）=3000:1,and n（Ni）/n（AIBN）=2:1,the catalytic activity of 5/AIBN is as high as 4.66×10⁵ g·mol^-1·h^-1.Gel chromatography（GPC）analysis showed that the PDI distribution of PMMA was between 1.72 and 3.06,and the molecular weight was in the range of 5.8426～23.8565(×10⁵ g·mol^-1).In addition,¹H NMR and ¹³C NMR characterization shows that under the optimum process conditions,the content order of stereo regularity of PMMA obtained by 5/AIBN catalytic system is random（52.70%）,syndiotactic（35.55%）and isotactic（9.52%）,indicating that the obtained polymer is a random PMMA rich in syndiotactic structure,and this kind of microstructure polymer has great market application value and can be the first choice for materials with high heat resistance and light transmittance.