Ethylene Polymerization Catalyzed By Late Transition Metal Catalyst Supported By Montmorillonite

Ethylene polymerization catalyzed by Nickel diimine catalysts supported by acidified Montmorillonite (MMT) under different conditions was investigated. The effect of Montmorillonite on the ethylene polymerization and structure of the polyethylene were studied systemically in this paper.Firstly, the pristine Ca-MMT was acidified by hydrochloric acid solution. In the acidifying process, K⁺, Na⁺, Ca²⁺, Mg²⁺ and other cations between the silicate layers of MMT could be transformed into soluble salt and replaced by H⁺. After this, with the interlayer force being weakened and crystal lattice of the layer being wrecked, the space between the layers of the MMT was enlarged. And then two kinds of Nickel diimine catalysts with different ligands having NH₂ function group (Ni^m & Ni^e) were fixed into MMT gallery by the interaction between H⁺ and -NH₂ function group . This kind of chemical supporting method can not only reinforce the bonding strength between the catalyst and the carrier but also increase the loading amount of catalyst.Secondly, the ethylene polymerization catalyzed by the supported catalysts under different conditions was studied and the obtained polyethylene was characterized by DSC,GPC å’Œ¹³C NMR and other analytical methods.As for the Ni^m catalytic system, the polymerization activity reached to maximum value in 5 minutes and decreased slowly as the time increased. At the early stage of the polymerization, polyethylene with high branching, low molecular weight and low melting point was produced in normal pressure, and as the polymerization continued the branching of polyethylene decreased and molecular weight of polyethylene increased. The most possible reason of the phenomena was the confinement effect of MMT. The nanoscopic space between the layers of MMT lowered the rate of chain transition and termination which made the molecular weight of polyethyleneincreased. At the same time, the confinement effect can also limit the "chain walking" during the polymerization to decrease the branching degree and increase melting point. As the polymerization carrying on, more polyethylene was generated between the MMT layers. As a result, the polymerization space became more and more confined so the confinement effect became more evidently. But at the end of polymerization, the effect was weakened because of the exfoliation of some MMT layers.The homogeneous Ni^m catalyst exhibited high activity at low temperature (0℃) but as the temperature increased, the polymerization activity reduced quickly. The polyethylene prepared by the homogeneous catalyst at 30℃ had low molecular weight and high branching. Compared to the homogeneous Ni^m catalyst, the catalyst supported by acidified MMT exhibited lower activity at low temperature. But it was extremely stable when the temperature increased. At 50℃ it exhibited higher activity than homogeneous catalyst. Moreover the polyethylene with relatively high molecular weight, melting point and crystallinity, low branching could be produced by it. The stability of the supported catalyst, especially at high temperature, was caused by the guarding effect of the MMT layers on the active centre. And because of the rate of chain transition and termination, "chain walking" being limited, the branching was reduced and molecular weight was increased.When polymerization carried out at higher pressure (4atm), the polymerization activity was improved because of increased concentration of ethylene monomer and the effect of nanoscopic confinement exhibited more evident because the layer space was filled with polymer. Compared to the homogeneous system, the polyethylene prepared by supported catalyst had higher molecular weight, lower branching.As for the Ni^e catalytic system, the effect of MMT layers on the activities of polymerizations carried out in different conditions and structure of the corresponding polymers was almost similar to the Ni^m system. Because of the larger ligand, the polyethylene prepared by Ni^e catalyst had higher molecular weight. But Ni^e catalyticsystem exhibited lower activity. It was possibly because the ethyl substituent close to the active centre contained in Ni^e catalyst had better motility than methyl substituent contained in Ni^m catalyst, so that the reaction between central metal and C-H occurred more easily leading to the deactivation of the catalyst.In conclusion, the results obtained from various systems showed that nanoscopic space between the layers of MMT has evident effect on polymerization and structure of polymer. The higher polymerization temperature or pressure was, the more evident the confinement effect exhibited.