Mechanism of internal and external electron donor effects on propylene polymerization with MgCl2-supported Ziegler-Natta catalyst: New evidences based on active center counting

摘要

Two TiCl4/Di/MgCl2 type supported Ziegler-Natta catalysts were prepared by loading dibutylphthalate or dicyclopentyldimethoxysilane (DCPDMS) (internal donor, Di) and TiCl4 on activated -MgCl2 in sequence, and a blank catalyst was prepared by loading TiCl4 on the same -MgCl2 without adding Di. These catalysts have similar specific surface area and pore size distribution, thus form a suitable base for comparative studies. Propylene polymerization with the catalysts was conducted in n-heptane slurry using triethylaluminum (TEA) as cocatalyst, and the effects of Di as well as De (external donor, in this work it was DCPDMS) on the number of active centers, the distribution of active centers among three polypropylene (PP) fractions (isotactic, medium isotactic, and atactic PP chains), and chain propagation rate constants of the PP fractions were studied by counting the number of active centers in the PP fractions using a method based on selective quench-labeling of the propagation chains by 2-thiophenecarbonyl chloride. When De was not added in the polymerization, introducing a phthalate type Di in the catalyst evidently changed the active center distribution by enhancing the proportion of active centers producing isotactic PP (iPP) (Ci), but scarcely changed reactivities of the three groups of active centers forming the three fractions. When the De was added in the polymerization system with TiCl4/phthalate/MgCl2 catalyst, further shifting of active center distribution in favor of Ci took place, meanwhile reactivities of the three groups of active centers also remarkably changed in favor of Ci. Mutual effects of these changes led to overwhelming dominance of iPP production in the TiCl4/Di/MgCl2-TEA/De system (Di=phthalate, De=alkoxysilane). In contrast, though using alkoxysilane as Di also caused shifting of active center distribution in favor of Ci when De was not added, addition of alkoxysilane De caused reverse shifting of active center distribution in favor of those