Synthesis And Immobilization Of Iron-based Catalysts And Their Use In Ethylene Polymerization

Bimodal polyethylene with bimodal molecular weight distribution and bimodal composition distribution include low molecular species for processability and high molecular weight species for properties have gained considerable popularity due to a better balance of processability and properties. Dual reactor systems and single reactor are main bimodal technology. There are many advantages in using single reactor bimodal technology vis-a-vis dual reactor systems. Lower investment costs, intimate mixing of high and low molecular weight components (improved product property).Novel catalyst systems were developed to produce bimodal polyethylene in single reactor. The thesis focuses on the two parts as follows:1. A novel simple catalyst system, iron (III) acetylacetonate and bis(imino)pyridyl ligandmixture activated with methylaluminoxane (MAO) has been found to exhibit high activity for ethylene polymerization.Effects of polymerization temperature and Al/Fe molar ration have been systemically investigated on three iron (III) acetylacetonate (Fe(acac)3) and bis(imino)pyridyl ligand ( 2-R₁N=C(Me)-6-R₂N=C (Me)C₅H₃N ) ( L₁: R₁=R₂= 2,6-Me₂C₆H₃; L₂: R₁=R₂=2-Me-6-(i-Pr)C₆H₃; L₃: R₁=R₂=2,6- (i-Pr)₂C₆H₃) catalyst systems. Fe(acac)₃ could not catalyze ethylene polymerization without bis(imino)pyridyl ligand. However, bimodal polyethylene was produced by active species were formed in situ and when Fe(acac)₃/ bis(imino)pyridyl ligand system was activated by MAO. Bis(imino)pyridyl ligand play a major role in the catalyst system. High molecular weight polyethylene was obtained using Fe(acac)₃/ L₁～L₃ catalyst systems. As polymerization temperature increased, polyethylene with lower molecular weight was obtained. The effect of polymerization temperature on molecular weight distributions increased when decreasing the bulkiness of ligand. The chain transfer toMAO occurs more easily at higher equiv of Al/Fe, and the effect of Al/Fe molar ratio on molecular weight distributions increase when increasing the bulkiness of ligand when commercial MAO (including Al(Me)3) was used as cocatalyst.2. Supporting of soluble single-site catalysts on preferably inorganic substrates is essential to provide "drop in" catalysts for use in existing technologies for slurry or gas-phase polymerization processes.Three support method were tried to immobilize the catalyst system Fe(acac)₃/ L₃ .The first supported iron-based catalyst (Cat-A) was prepared by supporting cocatalyst MAO then Fe(acac)₃/ L₃ on activated Davision 955 silica gel . The second supported iron-based catalyst (Cat-B) was prepared by supporting Fe(acac)₃/L₃ then cocatalyst MAO on activated Davision 955 silica gel. The third supported iron-based catalyst (Cat-C) was prepared by supporting Fe(acac)₃/L₃ directly on activated Davision 955 silica gel.There was no activity in ethylene polymerization with Cat-A.The supported catalyst Cat-B has receivable activity when no more cocatalyst was added to the reactor. In all polymerization runs no reactor fouling was found with the supported catalyst. The polyethylene obtained showed high melting temperature and high molecular weight. The increase in activity is linear and demonstrates that the rate of propagation has a first-order rate dependence on ethylene pressure, in accordance with the proposed Cossee-type mechanism. The fact that molecular weight remained essentially invariant with ethylene pressure indicated that the overall rate of chain transfer must also be first order in ethylene and β-H transfer was predominant chain transfer process. Experiments in which the temperature of the polymerization reaction was varied revealed that an increase in temperature results in large decreased in activity and molecular weight. As Al/Fe molar ratio increased from 32 to 59, the activity of supported catalysts increased obviously, while molecular weight remained constant indicating β-H transfer was predominant chain transfer process.The supported catalyst Cat-C has receivable activity when cocatalyst was added to the reactor. In all polymerization runs no reactor fouling was found with thesupported catalyst. The polyethylene obtained showed high melting temperature and high molecular weight. The increase in activity is linear and demonstrates that the rate of propagation has a first-order rate dependence on ethylene pressure, in accordance with the proposed Cossee-type mechanism. Experiments in which the temperature of the polymerization reaction was varied revealed that an increase in temperature results in increased first and in large decreased in activity when temperature was more than 30 degree. Experiments in which the Al/Fe molar ratio of the polymerization reaction was varied revealed that an increase in Al/Fe molar ratio results in increased first and in large decreased in activity when Al/Fe molar ratio was more than 400. As Al/Fe molar ratio increased from 200 to 1000, the molecular weight of the polymer remained constant but the molecular weight distribution of the polymer was increased from 5.45 to 15.30. The reason for the molecular weight distribution of the polymer increasing from 5.45 to 15.30 may be appearing multi-active sites in the catalyst system with the Al/Fe molar ratio increased from 200 to 1000.