Mmt Layered Structure Of Nickel Catalyst For Ethylene Polymerization Reaction

The microenvironment of the polymerization may efficiently affect the polymerization activities and the structures of resulting polymer. Such confinement effect is a fundamental scientific issue. In this thesis, nickel complex for ethylene polymerization and layered montmorillonite(MMT) were selected as typical system to study the nanoscopic confinement effects of MMT on the olefin polymerization and structure of polymer prepared in-situ.Two kinds of nickel complexes and three types of modified MMTs were prepared and characterized, which was used to prepare three kinds of MMT intercalated catalysts. The supported catalyst(A) prepared by immobilizing nickel complex on the MMT pretreated with cocatalyst had a low load of Ni( II). The other two supported catalysts by intercalation of acidified MMT (B)or acidified and organic MMT(C) with nickel complex whose ligand containing amine group had high load of Ni( II).For the polymerization catalyzed by catalyst A, polyethylene with high branching, low molecular weight and low melting point was produced at early stage of polymerization at 0°C and normal pressure, and polyethylene with low branching, high molecular weight and high melting point was generated later on. Such results were caused by the confinement effects of layered structure of MMT. The nanoscopic space between layers of MMT lowers the "chain walking" rate during polymerization, which leads to polyethylene with lower branching. When polymerization was carried out at higher pressure, the activities of polymerization were improved because of increased concentration of ethylene monomer and the effect of nanoscopic confinement was more evident because the layer space was full of polymer. Compared with homogeneous system, the polymer prepared by catalyst A has higher molecular weight, low branching and high crystallinity.The activities of polymerization catalyzed by B at 0℃ and 30 ℃ are higher thanthose of A. At 0°C, the confinement effects of layered structure of MMT weakened as the polymerization went on because more and more MMT layers were exfoliated . The structure of polymer was similar as that prepared in homogeneous system. At 30 "C, the nanoscopic confinement had more evident effects because the nanoscopic space between layers of MMT lowered the "chain walking" rate and termination rate during polymerization. The active center was more stable under the protection of nanoscopic space between layers of MMT.While polymerization of ethylene was carried out in the presence of catalyst C at 0°C, the low temperature caused lower diffusivity and lower activity of polymerization. The variation of the structure of the resulting polymer is the same as that of the catalyst A. When the temperature rose to 30 °C, the diffusivity of monomer was improved evidently, the highest polymerization activity was obtained, and the activity reduced rapidly with exfolication of MMT and the effects of nanoscopic confinement were weakened. So the crystallinity of polyethylene decreased with polymerization time.In conclusion, the results obtained from various catalyst systems showed that the nanoscopic space between the layers of MMT has evident effects on polymerization and structure of polymer.