Synthesis Of Cobalt Complexes Bearing PNP Ligands And Their Catalytic Properties In Homopolymerization Of Butadiene And Styrene

This study focuses on design and application of single-site late-transition metal catalysts. A series of chelate cobalt complex bearing N-containing diphosphine PNP ligand was synthesized and characterized. Their catalytic properties were investigated mainly in1,3-butadiene and briefly in isoprene and styrene homopolymerization. Four parts are included in this dissertation as follows:Part Ⅰ:Basic concepts and industrial progress of polybutadiene rubber were summarized. The newly found single-site catalytic technology in diene polymerization was also detailedly explained in two ways. On one hand, transition metal complexes with the same ligand but diffirent metal centers showed totally different catalytic properties in butadiene polymerization. On the other hand, just taking the late-transition metal of cobalt for example, cobalt complexes bearing different ligands would also result in different activities and stereoselectivities.Part Ⅱ:Five PNP ligands were synthesized and characterized by elemental analysis, FT-IR,1H NMR and13C NMR. Then these ligands reacted with CoCl2to yield the corresponding cobalt complexes bearing N-containing diphosphine PNP ligands. In addition, CoCl2(PPH3)2was also synthesized as a reference catalyst bearing free phosphine ligands. It had been found that the nature of the ligand structure affected the binding of the ligand to the cobalt center and determined the coordination geometry of the cobalt complexes.Part III:All the complexes have been employed to catalyze the polymerization of1,3-butadiene, in combination with methylaluminoxane (MAO) or ethylaluminum sesquichloride (EASC) as the cocatalyst. Both the nature of the ligand and the type of cocatalyst had a remarkable influence on the polymerization activity, microstructure and molecular weight of the resulting polymers. The [Co]/MAO catalytic systems resulted in the relatively lower conversions of butadiene and cis-1,4content in the polymers than the corresponding [Co]/EASC catalytic systems. On treatment with MAO, cobalt complexes bearing PNP ligands produced much smaller amounts of polymers than CoCl2(PPh3)2. Besides, the microstructure of polybutadienes obtained from these two types of cobalt catalysts was different; the former yielded predominantly cis-1,4microstructure (75.7～88.8%) while the latter yielded predominantly1,2microstructure (76.9%). In contrast with the [Co]/MAO catalytic systems, the PBDs produced by all the [Co]/EASC catalytic systems had higher cis-1,4contents (96.6%～98.7%) and higher molecular weight (189000～289000g/mol), as well as narrower molecular weight distributions (1.94～2.68). And the polymerization behaviors of the catalytic systems were also affected by the reaction parameters. The conversion of butadiene and molecular weight of the resulting polymers increased gradually along with the increase of Al/Co molar ratio. The highest catalytic activity was gained at25℃with a complete conversion of butadiene. However, the catalytic activity decreased remarkably as the temperature was enhanced. Moreover, the variation of reaction temperature resulted in the important change of polymers’ microstructure. The content of cis-1,4-PBDs in the polymers decreased gradually along with the elevation of reaction temperature, but the trend of trans-1,4content reversed. The molecular weights of PBDs were also decreased from283000to23000g/mol as the temperature was increased from0℃to90℃, but the molecular weight distributions (1.65～2.27) of PBDs didn’t change greatly.Part Ⅳ:All the complexes have also been tried to catalyze the polymerization of isoprene or styrene. Complexes with PNP ligands showed poor catalytic properties towards isoprene. Very small amounts of cis-1,4-PIPs (>80%) could be acquired only when activated by EASC. Nevertheless CoCl2(PPh3)2had much higher catalytic activities when activated with both MAO and EASC, yielding polyisoprenes with a cis-1,4content of45.2%and82.4%repectively. During the solution polymerization of styrene, the [Co]/EASC catalytic systems displayed excellent catalytic activities (>3.94×105g(PS) mol-1(Co)·h-1) and the high activities could be be maintained over a wide temperature range from25℃to90℃. However, all polystyrenes obtained above were characterized with very low molecular weights (<104g/mol) and atactic microstructure.