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

Abstract

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.

Internal Donors in Ziegler–Natta Systems: is Reduction by AlR3 a Requirement for Donor Clean‐Up?

“…Also, there is some uncertainty in the literature about the nature of the substitution transition state (TS): both four‐center and six‐center TSs have been considered . Recently, Vanka and co‐workers published an extensive computational study on donor decomposition by AlEt 3 in which only a six‐center TS was considered . We started a systematic exploration of these reactions by means of DFT calculations.…”

Silyl diol ester as a new selectivity control agent in MgCl2-supported Ziegler–Natta systems for propylene polymerization: catalyst structure and polymer properties

“…This conrms the reported data that silyl esters are likely to be quite resilient to decomposition by both Al 2 Et 6 and TiCl 2 Et on the MgCl 2 surface. 38 Table 3 shows that the catalyst A 3 provides polymers with an MWD ranging from 6.2-7.7, which is broader in comparison to catalyst B and catalyst D. It is also broader than polymers prepared with the maleate 47 and dibenzoylsul-de 18 internal donor and is similar to that prepared by the salicylate 23 and succinate 48 internal donor. These results could be attributed to the different active sites and/or the numbers of active sites in silyl diol ester-based catalysts.…”

“…2), resulting in lower isotacticity and productivity. Kumawat et al38 have found that the acidic titanium centre in the active catalytic species and TiCl 2 Et on the MgCl 2 support can participate in undesired, decomposing side reactions with donors. This result provides an explanation for the poisoning of the active site by the addition of excess SDE in the catalyst systems (Mg/SDE < 6).…”

Effect of Donors on the Activation Mechanism in Ziegler–Natta Catalysis: A Computational Study

“…As mentioned above,S autet et al [33] have reported that complexes such asTiCl 3 -OC 4 H 8 Cl can be formed by the decomposition of donors sucha st etrahydrofuran (THF) at the titanium center.M oreover,i narecent computational study,w eh ave shown that the decomposition of donors such as esters at the titaniumc enter can also lead to the formation of the Ti-OR specieso nt he MgCl 2 surface. [35] Therefore, to check whether such titaniums peciesf ormed by donor decomposition favor or disfavor the activation process, ac omparative study between wd(TiCl 4 )a nd wd(TiCl 3 -OC 4 H 8 Cl) (as ar epresentative example of ad onor decomposed product) were investigated. In the wd(TiCl 3 -OC 4 H 8 Cl) case, two possible conformationsw ere investigated.…”

“…Many computational investigationsi ncluding static density functional theory (DFT) [5,29,[34][35][36] and periodic DFT [37,38] calculations, as well as molecular simulations [39][40][41] have been reported in the fieldo fc hemistry to see the chemical changesd uring the reaction proceeding.Inr ecent times ab initio molecular dynamics simulations on large-scale systems have also become feasible. [39][40][41][42] The current work, however,f ocuses on static density functional theory calculations.…”