Donor Decomposition by Lewis Acids in Ziegler–Natta Catalyst Systems: A Computational Investigation

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 estera 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.