Abstract
Surface modification of MgCl2-supported Ziegler–Natta catalysts (ZNCs) by means of organic Lewis bases, either used as precatalyst components (“internal donors”, ID) or in combination with the AlEt3 cocatalyst (“external donors”, ED), is key for achieving a high stereoselectivity in propene polymerization. In fourth-generation systems, which are the working horses of this important catalyst class, the ID is an (ortho-)dialkyl phthalate; under polymerization conditions, this reacts with AlEt3 and must be replaced by an ED (typically an alkoxysilane) in order to obtain the desired performance. In a previous study, we investigated the molecular kinetics of the reaction between dibutyl phthalate and AlEt3 in solution by means of integrated experimental and computational protocols. A similar approach has now been applied to monitor the progress of the reaction for the complete catalyst system. Compared with solution (ΔH# ≈ 15 kcal mol–1; ΔS# ≈ −28 cal mol–1 K–1), the activation parameters in heterogeneous phase (ΔH# ≈ 10 kcal mol–1; ΔS# ≈ −49 cal mol–1 K–1) indicate that phthalate reduction is less activated, but suffers from an augmented entropic penalty. This suggests that the ID reacts with AlEt3 while still on the MgCl2 surface, rather than in the liquid phase after desorption, and that the surface is not innocent. Whether or not an alkoxysilane ED was present in the system turned out to be immaterial on reaction kinetics. High-level DFT calculations on a well-established MgCl2/dibutyphthalate model cluster reproduced the experimental data with a remarkably good agreement (ΔH# ≈ 10 kcal mol–1; ΔS# ≈ −46 cal mol–1 K–1). Experiments and calculations agree on the first ethyl transfer to the reacting ester group representing the rate-determining step, in solution as well in heterogeneous phase. In both cases, a four-membered transition state (TS) appears to be involved; in heterogeneous phase, though, the ester carbonyl is extra-activated by the interaction with a Lewis-acidic surface Mg center, rather than a second AlEt3 molecule. In our opinion, the interest of the proposed approach goes beyond the present study; indeed, further applications can be proposed for the design and engineering of novel ZNCs with tailored behaviors.
Highlights
Surface modification by the electron donors is essential for the stereoselectivity; decades of experimental[4−23] and computational modeling[24−35] studies demonstrated that ID and ED molecules, chemisorbed in the vicinity of the active Ti sites, shape the catalytic pocket to the ancillary ligands of molecular catalysts
The experimental activation parameters were nicely reproduced by density functional theory (DFT) calculations assuming the basic mechanism of Scheme 1, and step b of Scheme 1 was confirmed to be rate-limiting
It is important to note that kinetic studies in solution demonstrated that the reactivity of dibutyl phthalate and diethyl phthalate with AlEt3 under given conditions is the same within the experimental uncertainty (Supporting Information, Figure S2 and Table S4)
Summary
MgCl2-supported Ziegler−Natta catalysts (ZNCs) dominate the ever-growing production of isotactic polypropylene, thanks to an exceedingly high stereoselectivity (isotacticity index I.I. > 98%) and a productivity in the order of several tons of polymer per gram of Ti.[1−3] These systems consist of a solid precatalyst and a soluble cocatalyst: the former is typically obtained by reacting a MgCl2 precursor with TiCl4 in the presence of a first Lewis base (“internal donor”, ID), whereas the latter consists of AlEt3 and a second Lewis base (“external donor”, ED).[2,3] Surface modification by the electron donors is essential for the stereoselectivity; decades of experimental[4−23] and computational modeling[24−35] studies demonstrated that ID and ED molecules, chemisorbed in the vicinity of the active Ti sites, shape the catalytic pocket to the ancillary ligands of molecular catalysts. Gupta and Vanka reported the first purely computational molecular kinetic study of the reduction of a variety of monoesters of relevance in the modification of ZNCs, including EB and some alkoxy-substituted analogues, as well as representative aliphatic and silyl esters.[41] Considering the well-known tendency of AlEt3 to self-dimerize to Al2Et6, a sixmembered transition state (TS) with two AlEt3 molecules (Scheme 2a) was suggested for the first nucleophilic attack of Scheme 1b, identified as the rate limiting step for the reduction of all considered IDs. More recently, in our group the reduction by AlEt3 of three typical ester IDs, namely EB, dibutyl phthalate (DBP) and diethyl-2,3-diisobutylsuccinate (DiBS), was investigated by means of variable-temperature (VT) 1H NMR studies in toluene-d8 solution.[45] The experimental activation parameters were nicely reproduced by density functional theory (DFT) calculations assuming the basic mechanism of Scheme 1, and step b of Scheme 1 was confirmed to be rate-limiting. Experimental ICP-OES information on Ti and Al contents of the solid as a function of reaction time was gathered as a bonus
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