DFT Study of Lewis Base Interactions with the MgCl 2 Surface in the Ziegler-Natta Catalytic System: Expanding the Role of the Donors Kumar Vanka,* ,† Gurmeet Singh, ‡ Divya Iyer, † and Virendra K. Gupta* ,‡ Physical Chemistry DiVision, National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune, Maharashtra-411 008, India, and Reliance Technology Group - Hazira, Reliance Industries Ltd., Hazira Complex, Surat, Gujarat-394 510, India ReceiVed: July 18, 2010 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. Introduction Ziegler-Natta catalysis has played a dominant role in industrial polyolefin production 1-4 by providing high productiv- ity and stereoregularity, 4-13 thus becoming one of the most important industrial processes. The major transformation in Ziegler-Natta catalysis can be attributed to the introduction of the MgCl 2 matrix, resulting in better morphology control and multifold enhancement in the productivity. The MgCl 2 matrix, with a crystal structure similar to TiCl 3 , is composed of irregularly stacked Cl-Mg-Cl sandwich-like monolayers characterized by the presence of lateral cuts that are occupied by magnesium cations with unsaturated coordina- tion. 14-18 The lateral cuts have been designated as (100) or (104) for five-coordinated and (110) for four-coordinated magnesium cations for the R-MgCl 2 phase, in comparison to six-coordinated magnesium cations in the bulk, making these sites Lewis acidic. 19-22 In addition to R-MgCl 2 (cubic close packed (ccp) structure), -phase MgCl 2 (hexagonal close packed (hcp) structure) and the disordered δ-phase of MgCl 2 are also known to exist in the MgCl 2 matrix in Ziegler-Natta catalysts. 23-28 Figure 1 shows the structures of crystalline R and  MgCl 2 phases, with identical triple layer structures. Successive generations of the Ziegler-Natta catalysts have evolved into a multicomponent system comprising, in addition to TiCl 4 , MgCl 2 , and the alkylating agent AlR 3 (R ) Et being the most employed), (i) internal donors, aromatic esters (ben- zoates and phthalates, in particular), and recently, succinates and 1,3-diethers, and (ii) external donors, alkoxy benzoates and alkoxy silanes. The commonly accepted roles 29-32 of the internal and external donors during the polymerization appear to be to bind to the (110) lateral cuts of MgCl 2 and thereby provide (i) stability to the more acidic, four-coordinate (110) MgCl 2 surfaces and (ii) stereoregulating control to active titanium catalysts adjacently bound to the same (110) MgCl 2 surface. Therefore, the manner in which the donor can bind to the MgCl 2 surface is a question of significance. It is generally believed 35 that there are four ways in which donors containing two oxygen atoms that can bind to MgCl 2 can coordinate to the support: (i) the coordination of one oxygen of a donor molecule to one magnesium atom, the “monocoordination” mode, (ii) the coordination of both oxygen atoms of the donor molecule to the same magnesium atom, the “chelate” coordination mode, (iii) the coordination of both oxygens of the donor molecule to two magnesium atoms on the same layer, the “bridge coordina- tion” mode, and (iv) the coordination of both oxygens of the donor molecule to two magnesium atoms on different, adjacent MgCl 2 layers, the “zip”-coordination mode, which Correa et al. 40 have shown to be a viable binding option for the donor * To whom correspondence should be addressed. E-mail: k.vanka@ ncl.res.in; virendrakumar_gupta@ril.com. † National Chemical Laboratory. ‡ Reliance Industries Ltd. J. Phys. Chem. C 2010, 114, 15771–15781 15771 10.1021/jp106673b  2010 American Chemical Society Published on Web 08/30/2010