Key Interactions in Heterogeneous Ziegler-Natta Catalytic Systems: Structure and Energetics of TiCl 4 -Lewis Base Complexes Luigi Cavallo,* ,† Silvia Del Piero, ‡ Jean-Marie Duce ´ re ´ , † Rosalisa Fedele, ‡ Andrea Melchior, ‡ Giampiero Morini, § Fabrizio Piemontesi, § and Marilena Tolazzi* ,‡ Dipartimento di Chimica, UniVersity of Salerno, V. Ponte don Melillo, Fisciano (SA), I-84084, Italy, Dipartimento di Scienze e Tecnologie Chimiche, UniVersity of Udine, V. del Cotonificio 108, Udine, I-33100, Italy, and Basell Poliolefine Italia, Centro Ricerche G. Natta, p. Donegani 12, Ferrara, I-44100, Italy ReceiVed: NoVember 24, 2006; In Final Form: January 10, 2007 Extensive calorimetric investigations on the interaction of TiCl 4 with some Lewis bases are presented. Some of the bases were chosen for their industrial relevance in heterogeneous Ziegler-Natta polymerization of propene (ethyl benzoate, L2, diisobutyl phthalate, L3, (2R,3S)-diethyl 2,3-diisopropylsuccinate, L6, (2S,3S)- diethyl 2,3-diisopropylsuccinate, L7, and 9,9-bis(methoxymethyl)-9H-fluorene, L13) while other bases were chosen as probe molecules to explore the electronic and steric effects on the complexation energy (ethyl acetate, L1, diethyl phthalate, L4, diethyl succinate, L5, tetrahydrofuran, L8, dimethoxyethane, L9, dimethoxypropane, L10, dimethoxybutane, L11, and 3,3-bis(methoxymethyl)-2,6-dimethylheptane, L12). 1,1,2,2,-Tetrachloroethane was selected as the solvent for its low donating properties, which allows the focus to be on the metal-donor interaction. The calorimetric data are discussed and compared with the efficiency of the derived catalysts. Further understanding is obtained by comparison of the experimental results with theoretical calculations based on density functional theory (DFT). The performance of different computational approaches was validated by comparison of the calculated and experimental complexation energies. Introduction Supported MgCl 2 /TiCl 4 Ziegler-Natta catalysts of the latest generations allow for the morphological control of the polymer particles and require low amounts of titanium and aluminum compounds, ensuring at the same time an easy control of the polymerization. Unfortunately, they are multisite catalysts, which makes their design extremely complicated. Nevertheless, the catalytic systems currently used have reached amazing perfor- mances, which allows for the design of versatile, clean, and economical industrial processes. 1 The presence of Lewis bases is essential to improve the stereoselectivity in polypropene synthesis using supported MgCl 2 /TiCl 4 Ziegler-Natta catalysts: they can be added in catalyst preparation (internal donors) or during polymerization (external donors). With the internal-external donors pair, it is possible to modulate the performance of the catalyst (activity, stereoselectivity) and to fine-tune the characteristics of the resulting polymer (molecular mass, molecular mass distribution, microtacticity, intra- and inter-comonomer distribution). Among the internal bases, ethylbenzoate 2 and phthalic esters 3 are always used in combination with an external donor and are the systems most widely studied in literature. 4 Subsequent studies led to the discovery of new Lewis bases, 1,3-diethers, that result in stereoselective catalytic systems when used in the absence of external bases. 5 Recently, a new class of donors has been proposed, 6 2,3-disubstitued succinates, which allows for the synthesis of highly stereoregular polymers with quite broad molecular mass distributions. The balance between MgCl 2 -TiCl 4 , MgCl 2 -Lewis base, and TiCl 4 -Lewis base interactions can affect both the composition and the performance of the catalyst: several efforts have been attempted to characterize them. For instance, the MgCl 2 -TiCl 4 system has been investigated by several authors. Observations of MgCl 2 microcrystals by optical microscopy 7 and by HRTEM 8 confirmed the copresence of (100) and (110) lateral cuts. 9 Raman spectra of comilled MgCl 2 -TiCl 4 samples suggested the forma- tion of stable complexes of TiCl 4 only on the (110) side surface. 10 In line with these results, theoretical studies indicated that TiCl 4 adsorbs on both the (100) and (110) cuts of MgCl 2 . However, while interaction between TiCl 4 and the (110) lateral cut is rather strong, these studies suggested that interaction between TiCl 4 and the (100) cut of MgCl 2 is extremely weak 11 and that TiCl 4 desorption is very likely to occur. 11a,b The MgCl 2 -Lewis base interaction (1,3-diethers in particular) has also been investigated. Although a semiempirical approach was used, it was concluded that 1,3-diethers coordinate strongly to both the (100) and (110) MgCl 2 lateral cuts, even if coordination on the (110) cut and is preferred. 12 Models able to explain the effect of the Lewis bases on the stereospecificity of propene polymerization have also been proposed. 11,13 These models range from a “simple” poisoning of aspecific sites 13a to direct chemical and sterical modification of the surroundings of isospecific sites with a direct control on monomer insertion. 11a,13e,f However, the exact mechanism through which Lewis bases operate and the details of the various key equilibria present in Ziegler-Natta catalytic systems are still unclear. For this reason, in the wake of the basic studies concerned with the MgCl 2 - TiCl 4 and MgCl 2 -Lewis base interaction, we focus here on the * To whom correspondence should be addressed. E-mails: lcavallo@unisa.it and marilena.tolazzi@uniud.it. † University of Salerno. ‡ University of Udine. § Basell Poliolefine Italia. 4412 J. Phys. Chem. C 2007, 111, 4412-4419 10.1021/jp0678204 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/27/2007