DFT Study of Lewis Base Interactions with the MgCl₂ Surface in the Ziegler−Natta Catalytic System: Expanding the Role of the Donors

Abstract

We present a computational study, using density functional theory, of the role of internal and external donors in supported heterogeneous Ziegler-Natta polymerization catalyst systems. The focus of the investigation is the ability of the donors to stabilize the MgCl 2 support through the zip mode of coordination. Phthalates and alkoxy benzoates have been considered as representative internal and external donors, respectively. Models for the R (ccp) and the (hcp) phases of the (110) MgCl 2 lateral cut have been considered. Studies were first done with a "Fully Fixed Model", where the atoms in the MgCl 2 lateral cuts were kept fixed. The studies indicated that the phthalate donors would preferentially stabilize the R phase of MgCl 2 , while the alkoxy benzoate donors would stabilize the phase more, corroborating experimental results. A comparison of the zip coordination mode with other modes of coordination (mono, chelate, and bridge) indicates that it would be preferred over the mono and chelate coordination modes and be competitive with the bridge coordination mode. The validity of the Fully Fixed Model was then tested by doing calculations with the Partially Relaxed (some of the magnesium and chlorine atoms unfixed) and the Fully Relaxed (all atoms free) Models. The results from the latter two models corrected some of the discrepancies that had been observed for the Fully Fixed Model in comparison to previous experimental and computational investigations but were in general agreement with the Fully Fixed Model, indicating that the overall conclusions drawn with the earlier model are correct. Insertion studies done on an octahedrally coordinated titanium site flanked by two zip-coordinated phthalate donor molecules indicated that the zip-coordinating donors can confer exceptional regioselectivity to the titanium center. A list of potential donors that could serve as external donors, with di-iso-butyl phthalate (DIBP) as the internal donor, was investigated, and it was found that 1,3-diethers would serve as the best external donors to DIBP.

Roles of malonate donor on activity and stereoselectivity of Ziegler–Natta catalyzed propylene polymerization

“…From the relative barriers, we could conclude that the ZN catalyzed propylene insertion in the absence of the electron donors is regioselective in preference of the primary (1,2) insertion and nonstereoselective (no preference for one of the two enantiofaces). These theoretical results are in agreement with other theoretical results [18,26,55,56]. Additionally, it was notable that TS structures of all types of insertion exhibit a strong a-agostic interaction which is obvious from the elongated C a -H a bond distance (1.14 Å) as compared with the normal CeH bond distance (1.10 Å).…”

“…4. The presence of the malonate donor in the chelate mode favors the primary (1,2)-si insertion similarly to cases for the zip [26] and the bridge [18] coordination modes. Thus, coadsorption of the di-n-butyl 2-cyclopentyl malonate converts the aspecific (nonstereoselective) Ti mononuclear species into isospecific (stereoselective) one by sterically controlling the orientation of the growing chain at the transition state.…”

“…Assuming that atoms on the surface present a structure close to that in the bulk of the crystal, the MgeCl distances and all the CleMgeCl angles were set to X-ray values of 2.49 Å and 90 , respectively [45]. In addition, the a-MgCl 2 (110) surface has been selected for the studies, with the distance of 5.9 Å between the layers as suggested by Vanka and co-worker [26]. Many theoretical studies have reported that the interaction of TiCl 4 with the MgCl 2 (110) face is strong [46,47] while that with MgCl 2 (100) and (104) faces are weak [20,48].…”

“…Most of the works have been focused on ethylbenzoates [10,16,23e25], phthalates [26], succinates [18], alkoxysilanes [19,27], and 1,3-diethers [28e30]. Malonate compounds as internal electron donor have been reported to be very potent electron donors which show good activity and high isotacticity [31,32].…”

Structural Characterization of the EtOH–TiCl4–MgCl2 Ziegler–Natta Precatalyst

“…For this purpose, several experimental [29][30][31][32][33][34][35] and computational 13,15,[36][37][38][39][40][41][42][43][44][45][46] studies were performed. Many features of the catalyst, as the structure of the precatalyst, the activation process, the oxidation state(s) of the active titanium species during the polymerization process were investigated; notwithstanding, there are still several open questions.…”