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

Abstract

Two TiCl 4 /Di/MgCl 2 type supported Ziegler-Natta catalysts were prepared by loading dibutylphthalate or dicyclopentyldimethoxysilane (DCPDMS) (internal donor, Di) and TiCl 4 on activated d-MgCl 2 in sequence, and a blank catalyst was prepared by loading TiCl 4 on the same d-MgCl 2 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) (C Ã i ), 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 TiCl 4 / phthalate/MgCl 2 catalyst, further shifting of active center distribution in favor of C Ã i took place, meanwhile reactivities of the three groups of active centers also remarkably changed in favor of C Ã i . Mutual effects of these changes led to overwhelming dominance of iPP production in the TiCl 4 /Di/MgCl 2 -TEA/De system (Di 5 phthalate, De 5 alkoxysilane). In contrast, though using alkoxysilane as Di also caused shifting of active center distribution in favor of C Ã i when De was not added, addition of alkoxysilane De caused reverse shifting of active center distribution in favor of those producing PP of lower stereoregularity. This unfavorable change largely counteracted the reactivity changes in favor of C Ã i caused by the De, rendering the catalytic system rather poor isospecificity. V C 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46605.

Mechanistic Study on Effect of Electron Donors in Propylene Polymerization Using the Ziegler–Natta Catalyst

“…Although this famous catalyst attracts much attention from both academia and industry, the mechanism of the electron donors on the active site remains elusive, presumably due to the heterogeneity and multicomponent nature of the catalyst. Fortunately, great efforts have been carried out to understand the role of electron donors on the active site by experimental and theoretical methods. − Andoni et al observed a structure directing role of electron donors in the formation of MgCl 2 crystal morphology. , The presence of 1,3-diether led to the preferential growth of MgCl 2 crystals along the (110) surface, whereas diisobutyl phthalate or ethyl benzoate was less selective and allowed the growth of MgCl 2 crystals along both (110) and (104) surfaces. Furthermore, electron donors influence the distribution and amount of TiCl 4 in the ultimate catalyst.…”

Internal donors on supported Ziegler Natta catalysts for isotactic polypropylene: a brief tutorial review

“…Phthalate IDs have affinity on both coordinately unsaturated 110 and 104 planes of δ MgCl 2 [53]. Fan et al used kinetic study for understanding the different active centres of DIBP/MgCl 2 /TiCl 4 -TEA in presence of EDdicyclopentyldimethoxysilane (DCPDMS) [54]. Their study typified that predominant role of phthalate is to change the active center distributions (C* i (isotactic), C* mi (medium isotactic) and C* a (atactic)) of PP catalyst, however the reactivity of activity centres lie in nearly unchanged fashion.…”

Nanocatalysts in Olefins and Dienes Polymerization Processes

“…Metallocene-based catalytic systems are dramatically different from Ziegler-Natta type catalysts, because they have much higher activity and lower polydispersity [51]. To solve the problems (such as the difficulty in controlling the polymer morphology, the very large amount of MAO needed, and the reactor-fouling problem) observed with the soluble homogeneous catalysts, heterogenization of the metallocene is crucial for industrial application [7,[52][53][54][55][56][57][58]. Development of supported metallocene catalysts enables their use in gas-and slurry-phase processes and prevents reactor-fouling problems.…”