The Study For Polymerization Of Ethylene And α-Olefins Catalyzed By Ni(Ⅱ)-α-Diimine Catalyst

Compared with the Ziegler-Natta and metallocene catalysts, the late transition metal catalysts could catalyze the polymerization of olefins to yield polymers with different microstructures. Currently, well defined catalysts based on complexes of cobalt, rhodium, nickel, palladium, platinum, and iron which catalyzed the polymerization of olefins have been reported. Especially the catalysts based upon a-diimine complexes of nickel and palladium could catalyze the polymerization of ethylene and a-olefins to high molecular mass polymer with the activity comparable to early-transition-metal systems. Dramatic differences in the microstructure and properties of the obtained polymers using these nickel- and palladium-based catalysts are observed as compared with those prepared using early metal Ziegler-Natta and metallocene technology. For example, branching in polyethylene prepared with nickel(Ⅱ)- or palladium(Ⅱ)-based catalysts can vary from highly branched to linear, and the properties thus vary from soft elastomers to rigid plastics. The polymer properties are greatly dependent on reaction conditions (temperature, pressure), ligand structure, and the metal.Toluene has been used as solvent in the polymerization of olefins catalyzed by the nickel(II)-based-a-diimine catalyst system. However, it is well known that its toxicity is much higher than that of n-hexane, which is a common solvent in the present industrial polymerization of olefins. To the best of our knowledge, n-hexane has never been used as the solvent in the nickel(II)-based-a-diimine catalyst system for the polymerization of olefins. If nickel(II)-based-a-diimine catalyst can catalyze the polymerization of olefins to yield polymers with different microstructures and properties in n-hexane, and satisfy with the existing production processes and conditions for the polymerization of olefins, it will be very important for its industrial application.Several Parts of research works had been carried out in the dissertation. First, the nickel(II)-a-diimine complex [(2,6-i-Pr)2C6H3-DAB(An)]NiBr2 was prepared based on literature procedures. And the Ni(II)-a-diimine catalyst [(2,6-i-Pr)2C6H3-DAB(An)]NiBr2 plus methylaluminoxane (MAO) was successfully used in the homopolymerization of ethylene,1-hexene and 1-octene and the copolymerization of ethylene with 1-hexene and 1-octene in n-hexane. The molecular weights, Tg, Tm, branching degree and density of the obtained (co)polymers were greatly controlled by ethylene pressure and polymerization temperature. Compared with ethylene homopolymer, the branching degree of the copolymers prepared by the copolymerization of ethylene with 1-hexene or 1-octene increased, while the molecular weight, density, Tm and catalyst efficiency all decreased. However, compared with the homopolymer of 1-hexene or 1-octene, the copolymers density, Tm and catalyst efficiency all increased, while the molecular weight and branching degree all decreased. Second, the polymerization of 1-octene was catalyzed by Ni(II)-a-diimine catalyst [(2,6-i-Pr)2C6H3-DAB(An)]NiBr2 plus methylaluminoxane (MAO) in toluene or n-hexane similarly. The polymerization behavior characterized by gel permeation chromatography (GPC) all showed a controlled polymerization process with weight-average molecular weights increasing linearly with time and the molecular weight distributions between 1.2 and 1.5. The catalyst efficiency was higher and up to 200kg mol-1 Ni h-1 when toluene is used as solvent and up to 420kg mol"1 Ni h-1 when n-hexane is used as solvent. The poly(1-octene)s obtained were all amorphous and their glass transition temperature Tg were -54℃and -60.10℃respectively. The structures of poly(1-octene)s characterized by'H NMR,13C NMR, HSQC, DEPT135 showed that when toluene and n-hexane were used as solvent respectively for the polymerization of 1-octene, the polymers had same types of branches containing isolated methyl, meso and racemic head-to-head methyl, hexyl and longer branches(>C6), The percentage of each type of branching were 37%,12%,51% respectively with toluene as solvent and 45%,13%,42% respectively with hexane as solvent, and the proportion of meso and racemic head-to-head were 1:1 and 2:3 respectively. And then, we found the poly(1-octene)s synthesized by MAO-activated Ni(II)-a-diimine complex [(2,6-i-Pr)2C6H3-DAB(An)] NiBr2 could be well soluble in tetrahydrofuran (THF). After fractional precipitation, poly(1-octene)s with narrow molecular weight distributions (Mw/Mn≤1.12) were obtained. Their weight-average molecular weights were measured by gel permeation chromatography(GPC) in conjunction with online model BI-MwA multiangle laser light scattering(MALLS), and their intrinsic viscosities were measured by maron's single-point method. The k, a value in Mark-Houwink equation [η]=kMαin THF at 40℃were 0.089 mL/g and 0.61 respectively. Finally, the Ni(Ⅱ)-α-diimine catalyst [(2,6-i-Pr)2C6H3-DAB(An)]NiBr2 plus methylaluminoxane (MAO) was immobilized to the surface of silica gels by physical adsorption, and the immobilized catalytst was used to catalyze the polymerization of ethylene and 1-octene furtherly. The molecular weight, Tg, Tm, branching degree and density of the polymers were determined by GPC,DSC,1H NMR,IR, etc. The heterogeneous catalyst efficiency and polymers properties were campared with homogeneous polymerization system. The results showed that the Al/Ni molar ratio of immobilized Ni(Ⅱ)-α-diimine catalyst prepared by physical adsorption was about 100. The immobilized Ni(Ⅱ)-α-diimine catalyst could catalyze the homopolymerization of ethylene and 1-octene in n-hexane. Compared with the homogeneous catalyst polymerization system, the immobilized catalyst efficiency and the molecular weight of polymers all decreased, while the density, Tm all increased. Because of the reduction ofβ-H elimination in the process of ethylene polymerization catalyzed by heterogeneous catalyst, the branching degree of polyethylenes decreased, and at a certain temperature, with the increase of the Al/Ni molar ratio and ethylene pressure, the branching degree of the polymers decreased, while the molecular weight, density, Tm and the immobilized catalyst efficiency all increased. The 1,2-insertion of 1-octene followed by migration of nickel center up to (ω-1) carbon were more frequent, which lead to the increase of branching degree and the formation of meso and racemic head-to-head methyl. And at a certain temperature, with the increase of the Al/Ni molar ratio, the molecular weight and branching degree of the poly(1-octene)s and the immobilized catalyst efficiency all increased, while the Tm decreased.