| In this study, the ethylene trimerization catalyst system, HN(C2H4SR)2·CrCl3/MAO, was studied under various reaction conditions on the selectivity, activity, and dynamic evolution. The effect of SiO2 support on the catalyst selectivity and activity was also studied. A simplified metallacyclic intermediates model was applied to describe trimerization mechanism. A tandem catalysis system composed of HN(C2H4S C12H25)2·CrCl3and Et(Ind)2ZrCl2 was used to synthesize ethylene-1-hexene copolymers. A mathematical model was developed to describe ethylene/1-hexene copolymerization with this tandem catalysis system based on a simplified trimerization model and an ethylene/a-olefin copolymerization model. The effect of reaction conditions, on trimerization catalyst selectivity, copolymerization catalyst activity, polymerization kinetics, and polymer properties were investigated both by experiment and simulation. HN(C2H4S C12H25)2·CrCl3 and Et(Ind)2ZrCl2 were supported on SiO2 and the effects of different supporting strategies on trimerization selectivity, 1-hexene incorporation efficiency and copolymerization activity were studied and compared to the homogeneous system.The selectivity of HN(C2H4SR)2·CrCl3 were high (>99%) with small amount of MAO at atmosphere pressure. The highest activity is 550 kg hexene/(mol Cr h) for HN(C2H4SC2H5)2·CrCl3 at 65℃, while 450 kg hexene/(mol Cr h) for HN(C2H4SC12H25)2·CrCl3 at 75℃. Supporting HN(C2H4SR)2·CrCl3 on SiO2 did not change the trimerization selectivity, but a decrease of activity and improved catalyst stability were found. The activity for supported HN(C2H4SC2H5)2·CrCl3 dropped about 100 kg hexene/ (mol Cr. h) at each condition compared to its homogeneous counterpart, while the activity for supported HN(C2H4SC12H25)2·CrCl3 only remained 1/4. The different effect of support on trimerization catalyst activity was due to the hindrance of the ligand imposed to the active site, which might block off ethylene coupling, coordination, and insertion.Ethylene-1-hexene copolymers were synthesized with a tandem catalysis system that consisted of HN(C2H4SC12H25)2·CrCl3/MAO (1/MAO) and Et(Ind)2ZrCl2/MAO (2/MAO) at atmosphere pressure. Catalyst 1 trimerized ethylene with high activity and excellent selectivity, while catalyst 2 incorporated the 1-hexene content and produced ethylene-1-hexene copolymer from an ethylene-only stock in the same reactor. Adjusting the Cr/Zr ratio and reaction temperature yielded various branching densities and thus melting temperatures. Only C4 side chain was found in the copolymer. However, broad DSC curves were observed when low temperatures and/or high Cr/Zr ratios were employed due to an accumulation of 1-hexene component and composition drifting during the copolymerization. It was found that applying a short time period of pre-trimerization resulted in more homogeneous materials that gave unimodal DSC curves.A simplified metallacyclic intermediate model was applied to describe trimerization mechanism of HN(C2H4SR)2·CrCl3, taking a first order catalyst activation, a first order deactivation, and a second order ethylene chelating intoaccount. The reaction rate can be described as Rp=k1.ka.CE2/ka-kd(e-kdt-e-kat).Mw.Based on this expression and an ethylene/a-olefin copolymerization kinetic model, a mathematical model was developed to describe ethylene/1-hexene copolymerization with HN(C2H4SC12H25)2·CrCl3/Et(Ind)2ZrCl2/MAO system. The model could predict the reaction rate, 1-hexene concentration in the liquid phase, copolymer composition, and molecular weight. A series of semi-batch polymerization runs were carried out to verify the model. Both experimentation and modeling showed that adjusting the Cr/Zr ratio yielded various branching densities and thus melting temperatures, as well as molecular weights and polydispersities. Broad composition distributions and thus broad DSC curves were observed at high Cr/Zr ratios. Modeling results elucidated that this is due to an accumulation of 1-hexene component and to composition drifting during the copolymerization. It was also found that applying a short time period of pre-trimerization improved homogeneity in chain microstructure and minimized broadening in DSC curves.Catalysts 1 and 2 were supported on silica particles and the effects of different supporting strategies on trimerization selectivity, 1-hexene incorporation efficiency and copolymerization activity were studied and compared to the homogeneous system. Supporting 2 resulted in a reduction in both catalytic activity and incorporation ability of 1-hexene because of a hindrance effect. At fixed Cr/Zr ratio, the activity of tandem system depended on if 2 was supported, but was not affected by 1 supporting. Homogeneous 2 yielded lower molecular weight copolymers with lower polydispersity index and having narrow DSC curves. Supported 2 gave higher molecular weight, higher polydispersity, and broader DSC curves because of its multiple-site nature. The tandem action of supported 1 and supported 2 gave high activities at a 107 g/(mol Zr h) level. The resulting copolymers contained only C4 side chains with melting temperature ranged from 95℃to 120℃, similar to commercial LLDPE. The samples had the highest molecular weight among the four catalysis systems under similar reaction conditions. This could be attributed to the lower 1-hexene concentration generated by supported 1 and the lower rate constants of chain transfer andβ-hydrogen elimination of supported 2.
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