| In this work, three unsymmetrical bis(imino)pyridyl iron complexes and sixα-diimine palladium complexes with different backbone structure were synthesized and characterized. The catalytic behaviors of these complexes for olefin polymerization were investigated and the resulting polymers were characterized. Furthermore, the relationship between the catalyst structure and the catalytic property were also examined. 1. The unsymmetrical bis(imino)pyridyl ligand L2 contains three E,Z-, Z,Z-, E,E-configurational isomers that can be detected by 1H NMR and 13C NMR spectroscopies. However, the corresponding iron complex 2 exhibits only one isomer in the solid state and solution. X-ray diffraction of 2 confirms that the iron complex adopts a Cs configuration. These complexes activated by methylaluminoxane (MAO) have high catalytic activities for ethylene polymerization and produce linear polyethylenes with bimodal or broad molecular weight distribution. The steric and electronic effect of the ortho-substituents on the aniline moiety distinctly affects the molecular weight of the obtained polyethylene. As compared with 1 and 3 having ortho-monosubstituent on aryl rings and typical complex 4 containing ortho-diisopropyl-substitution, complex 2 has better thermal stability and produces much higher molecular weight polyethylene. Even at 70°C, 2/MAO system still keeps high activity and relatively stable kinetics.2. Crystallographic analysis revealed that 5a, C6a and C7a adopt the square planar geometry, and the steric spaces of both complexes are crowed because of the bulky backbone structure. Homo- and copolymerization of ethylene and methyl acrylate were carried out with palladium complexes as catalysts, and backbone electronic and steric effects as well as reaction temperature onα-diimine palladium catalysts were investigated in detail. For ethylene homopolymerization, higher catalytic activity, molecular weight and branched degree were obtained with catalysts bearing more strongly electron-donating phenyl backbone ligands. Furthermore, the Pd (II) catalyst 5a and 6a containing more bulky camphyl and phenyl backbone respectively, and 2, 6-diisopropyl substituted aniline groups exhibits lower activity, molecular weight than classic Brookhart Pd (II) catalyst 9a. However, Catalyst 5a showed thermal stability for ethylene polymerization and could produce high molecular weight polymer at high temperature. Besides, living polymerization of ethylene using catalyst 1a was successfully carried out at low temperature under high pressure. For the copolymerization of ethylene with methyl acrylate, 1a shows the higher incorporation ratio than analogues while the molecular weight keeps constant. 3. Branchedα-olefin, 4-methyl-1-pentene (4MP), was polymerized with classical cationicα-diimine palladium complexes. Influences of reaction temperature and monomer concentration on 4MP polymerization were evaluated. At 0°C, 4-methyl-1-pentene was polymerized by 9a in a living polymerization manner for a long time. NMR, DEPT, HMQC and HMBC analyses showed many unique branches including short branch methyl and long"branch on branch"chain 2-methylalkyl are present in the polymers. The influence of reaction temperature and monomer concentraion on branching degree and branch type were also discussed in detail. The data show that as the polymerization temperature increases and monomer concentration decreases, initial 2, 1-insertion as major insertion pathway increases, which causes a decrease in total branching degree. Besides, short methyl branch increases while long 2-methylalkyl branch decreases with higher temperature and lower monomer concentration.
|
PDF Full Text Request