Full quantum chemical calculations with density functional theory (DFT) show that a principal role of donors in ZieglerNatta (ZN) olefin polymerization catalysts is to coordinate to the metal center at the active sites on the MgCl 2 surface. Thereby, the behavior of the catalyst is modulated to favor insertion over termination and, thus, polymerization occurs. This is shown to be true for a range of different donors. The calculations indicate that active sites that feature anionic
MgCl2 supported Ziegler–Natta (Z–N) catalyst have emerged as a most exciting chemical process for polyolefin technology, which is responsible for production of ~150 million ton polyolefin (polyethylene and polypropylene) per...
The development of new donors (Lewis
bases, usually containing
oxygen atoms) is one of the chief areas of research in Ziegler–Natta
(ZN) olefin polymerization systems. The addition of such donors has
led to improvement in the activity and selectivity of ZN systems.
However, in order for the donor to be effective, it has to be chemically
stable and resistant to decomposition by Lewis acidic species such
as AlEt3. Discussed in the current work is the chemical
stability of different ester donors, including aromatic benzoate donors
and the silyl estera promising new donor class in ZN systems.
Full quantum chemical calculations with density functional theory
(DFT) indicate that esters can undergo decomposition through different
pathways upon interaction with species such as the AlEt3 dimer: Al2Et6. Moreover, the studies show
that the active, supported titanium catalyst species can cause donor
decomposition and, in fact, is likely to be the greater threat to
donor decomposition than Al2Et6. This explains
why the addition of excess donors can lead to the poisoning of the
active site in ZN systems. We have also computationally investigated
means of improving the silyl ester donors in order to make them more
robust and resilient to donor decomposition by Al2Et6 and the supported active titanium species.
Full quantum chemical calculations, using density functional theory (DFT), have been conducted to explain the effect of donors on the “activation mechanism” in the Ziegler–Natta (Z–N) catalyst system. In the activation mechanism, the inactive TiIVCl4 catalyst converts into the active TiIIICl2Et catalyst with the help of the AlEt3 present in the system. The donors that have been considered in this study are: ethyl benzoate (eb), two representative diether cases, a phthalate donor, and a silyl ester donor. The results indicate that eb and the diether donor cases donor have a negative effect on the barriers for the activation mechanism. However, the eb donor can be displaced from the MgCl2 surface by AlEt3, which matches experimental observations. For the phthalate, silyl ester and TiCl3–OC4H8Cl cases, the results indicate that a significant induction period would be present in Z–N systems employing such donors or having such a catalytic center, before catalysis could commence.
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