| MgCl2-supported Ziegler-Natta catalyst is widely used for olefin polymerization, which is used in the range of 50-80 ℃. However, stability and stereospecificity for active site of Ziegler-Natta catalyst as well as molecular weight of polymer decrease when the polymerization temperature rises over 80 ℃. The polymerization at high temperature is desired industrially to reduce the burden of removing the heat of polymerization and improve the productivity in the commercial plants. In recent years a new supercritical olefin polymerization also require higher polymerization temperature( > 92 ℃) for propylene polymerization. So it is very important to improve the performance of Ziegler-Natta catalyst at high temperature.Ziegler-Natta catalyst system includes several components. Up to now, the catalyst and the donor have been extensively and deeply studied. In this paper another component, activator, is investigated in order to learn its influences on propylene polymerization and polymer structure.Four ethylaluminoxanes (EAO) were synthesized by the reaction between triethylaluminum and water under various molar ratios of H2O/AI at -78 ℃ (referred as EAOl~4from low to high ratio). The aluminoxanes and the normal activators, such as triethylaluminum (TEA), triisobutylaluminum (TBA) and MAO were used as cocatalyst for MgCl2-supported Ziegler-Natta catalyst including N and DQ catalyst in propylene polymerization at the temperatures ranging from 70 to 100℃.Activators have remarkable influence on activity at different polymerization temperatures. Although the activity with EAO as activator is not as good as that with alkylaluminum at 70℃, the activity obtained with EAO is close to or higher than that of TEA at higher temperatures. The activity is primarily affected by the reactivity of activator. The alkylation ability of EAO is weaker than TEA, so the activity of propylene polymerization with EAO as activator is lower than that with TEA at 70℃. The alkylation ability of EAO can be improved with increased temperature. The reactivity of TEA is so strong that it can reduce the active sites from Ti3+ to Ti2+, which is not active for propylene polymerization.Activators also have remarkable influence on isotacticity and its distribution of polymers. The size of EAO increase with the increment of oligomeric degree of aluminoxane. Bigger the size of EAO, higher the isospecificity of active site, and higher isotacticity of corresponding polymer. The isotacticity of polymers obtained with EAO3 and EAO4 are remarkably higher than those with other activators at 100℃. The UV experiment proved that internal donor could be extracted byactivators by different degrees, which affects the isotacticity of polymer in turn. Based on the TREF analysis of products, a lower isospecific active site was formed in TEA and EAOl systems at high temperature, producing polymer with lower isotacticity. The stereospecificity of active sites for other activator systems hardly change with the increase of temperature.Activators also have influence on molecular weight of polymer at different polymerization temperature. On one side, the weak reactivity of Et-Al bond of EAO reduce the chain-transfer reaction to aluminum site of activator; on the other side, the strong reaction between EAO and titanium site or support as well as side effect of EAO retard the chain-transfer reaction from propagation chain to monomer and activator, which results the higher molecular weight of polymer with EAO than that with TEA.In conclusions, the new activator can effectively improve the catalytic properties of Ziegler-Natta catalyst at high temperature. The studies in this paper demonstrate that it is a new and efficient approach to modify and improve the performance of Ziegler-Natta catalyst through variation of activator.
|
PDF Full Text Request