| For some years now, there has been a great academic and industrial interest in olefin polymerization catalysts based on several imine complexes of late transition metals such as nickel, palladium. These kinds of catalysts are easier to synthesise and more tolerant to polar groups than metallocenes.A new series of iron and cobalt catalysts bearing 2,6-bis(imino)pyridyl tridentate ligands that are highly active for the polymerization of ethylene was recently reported by Gibson and Brookhart. Brookhart's group has also reported that less sterically bulky variations of the iron catalysts are extremely active and selective for the oligomerization of ethylene to linearα-olefins and that the iron-based polymerization systems are moderately active for the production of polypropylene. First of all, it was necessary to increase the catalyst activity for the propylene polymerization, because activation with MAO (methylaluminoxan) yields an unsatisfactory amount of polymer.It is known that the position and steric bulk of the substituents on the aryl rings of the bis(imino)pyridyl catalysts play a crucial role in determining the activity of the catalyst. A reduction of steric bulk at the ortho-aryl position results in increases in activity. But if both ortho- groups on each ring are hydrogen substituents, then the catalysts are even no active.The effect of substituents in the para-, meta-positions has been less studied. Our interest in dendritic molecules, their applications in catalysis and as new materials, led us to consider the possibility of functionalising late transition metal polymerization catalysts with dendritic components. This could take the form of attaching dendritic wedges to the catalysts, thus creating a catalyst at the core of a dendrimer. A catalytic site at the core of a dendrimer makes it possible to control the microenvironment around the catalytic centre and thus allows modifications of the catalytic selectivity. Incorporation of a catalytically active site in a dendritic macromolecule has the added potential advantage of enabling one to separate the catalyst from the product stream by means of ultra-filtration methods. This dissertation focuses on the exploration of novel iron catalysts for propylene polymerization. The works can be divided into two parts: (I) Synthesis and characterization of two generations of 2,6-bis(imino)pyridyl tridentate ligands with dendritic wedges. (II) Preparation and characterization of the iron catalysts with dendritic substituents, and which were evaluated as catalysts for the polymerization of propylene. The following text is the details:(A) The design and synthesis of two bis(imino)pyridines with dendritic structure (G1-Py and G2-Py).A coupling reaction of three or two equivalents of benzyl bromide to one equivalent of methyl hydroxybenzoate generated 3,4,5-G1-CH3, 2,5-G1-CH3 and 2,6-G1-CH3 in high yield. All the G1-CH3s were studied by X-ray analysis. The results show that these dendrons have sufficient volume to be used as the fine ligands for certain catalysts. The amide intermediates G1-CONH2 were obtained by reaction between ammonia and G1-CH3. Interestingly, 2,6-G1-CONH2 can not be prepared in the same condition, which may due to the large steric bulk. Sodium hypochlorite was found to be an effective oxidant to generate methyl carbamates G1-NHCO2CH3, which were hydrolyzed in alkali to liberate the amines G1-NH2. A coupling reaction of two equivalents of 3,4,5-G1-CH3 to one equivalent of 2,6-diacetylpyridine generated G1-Py in moderate yield in the presence of a catalytic amount of silica-alumina catalyst support with molecular sieves as the water adsorbent.As for G2-Py, the compound p-OH-SB was synthesized using Schiff-base condensation reaction of 2,6-diacetylpyridine and two molar equivalents of 4-aminophenol first, and then the dendritic wedge 1,2,3-tris(benzyloxy)-5-(bromo-methyl)benzene containing an alkylbromide functional group was attached to the para-hydroxyl position of p-OH-SB to generated the bigger dendron G2-Py in high yield. All organic compounds synthesized were characterized by elemental analysis, 1H NMR, 13C NMR, and mass spectrometry. (B) The preparation and characterization of the iron catalysts with dendritic substituents.The iron catalyst complexes were prepared by traditional method. Full characterization of the complexes proved to be difficult. High spin d6 iron complexes are paramagnetic, and only in some instances have 1H-NMRdata been reported. In our experience, obtaining interpretable spectra proved impossible. Many of the complexes are not soluble in readily available NMR solvents (CDCl3, benzene-d6), and the line broadening and unpredictable shifts of the peaks rendered assignment speculative at best. IR spectroscopy is of little value as a diagnostic technique for these compounds. Useful characterization techniques were therefore limited to elemental analysis and mass spectrometry.(C) Polymerization studies.At the same condition, the productivity of the two iron catalysts with dendritic substituents (4.89×105 g PP/molFe·h) is slightly higher than the traditional iron catalysts with para-substituents (3.87×105 g PP/molFe·h). The activity of the complexes shows small variations related to the size of the dendritic wedge. The conditions of the polymerization reaction influence the productivity of the catalyst system employed. To study these effects, we undertook a series of experiments under various conditions, employing traditional iron catalyst and the two new dendrtic iron catalysts.The effects of reaction temperature, Al/Fe molar ratio and the concentration of the catalysts on activity were investigated. The higher the reaction temperature, the lower catalytic activity was present. The reasonable explanation was the combination of the accelerated catalyst's deactivation and the decrease in propylene concentration in solvent due to the elevated temperature. As the molar ratio of Al/Fe increases, the catalytic activities increase rapidly at first and then decrease because of the deficient protection provided by small steric bulk of bluoro at para- and meta- position of aryl rings. The effect of the concentration of catalyst on activity is similar with that of the molar ratio of Al/Fe. |
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