摘要
The catalytic activity and stereospecificity of olefin polymerization by using heterogeneous TiCl_(4)/MgCl_(2)Ziegler-Natta(Z-N)catalysts are determined by the structure and nature of active centers,which are mysterious and fairly controversial.In this work,the propylene polymerization kinetics under different polymerization temperatures by using Z-N catalysts were investigated through monitoring the concentration of active centers[C*]with different tacticity.SEM was applied to characterize the catalyst morphologies and growing polypropylene(PP)particles.The lamellar thickness and crystallizability of PP obtained under different polymerization conditions were analyzed by DSC and SAXS.The PP fractions and active centers with different tacticity were obtained with solvent extraction fractionation method.The catalytic activity,active centers with different tacticity and propagation rate constant kp,fragmentation of the catalyst,crystalline structure of PP are correlated with temperature and time for propylene polymerizations.The polymerization temperature and time show complex influences on the propylene polymerization.The higher polymerization temperature(60℃)resulted higher activity,kp and lower[C*],and the isotactic active centers Ci*as the majority ones producing the highest isotactic polypropylene(iPP)components showed much higher kp when compared with the active centers with lower stereoselectivity.Appropriate polymerization time provided full fragmentation of the catalyst and minimum diffusion limitation.This work aims to elucidate the formation and evolution of active centers with different tacticity under different polymerization temperature and time and its relations with the fragmentation of the PP/catalyst particles,and provide the solutions to the improvement of catalyst activity and isotacticity of PP.
基金
financially supported by the Major Scientific and Technological Innovation Project of Shandong Province(No.2019JZZY010352)
Natural Science Foundation of Shandong Province(No.ZR2019MB072)
Taishan Scholar Program。
