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

Abstract

Alkyl benzoates, phthalates, and succinates are used as “internal donors” (ID) to modify the surface of MgCl2‐supported Ziegler–Natta (ZN) catalysts for polypropylene production. Phthalates, in particular, are the working horses of this catalysis, but a REACH ban of these compounds for other applications has generated a desire from the market for phthalate‐free ZN catalysts, which has in turn triggered a search for alternatives and revamped research in the area. It has been reported that under polymerization conditions certain ID classes undergo an extensive exchange with alkoxysilane “external donors” (ED), which represents an important opportunity for surface fine‐tuning. ID clean‐up has been attributed to ester group reduction by the AlR3 cocatalyst. We have now measured the activation parameters for the reactions of ethyl benzoate (EB), dibutyl phthalate (DBP), and diethyl 2,3‐diisobutylsuccinate (DiBS) with AlEt3 in toluene solution by means of variable‐temperature 1H NMR spectroscopy. Reduction is fast for EB and DBP, whereas it is slow for DiBS. Results of DFT calculations are in line with the observed reactivity. Therefore, we conclude that an irreversible reaction with AlEt3 is not the only option for ID/ED exchange. Possible alternatives are discussed.

Reactivity Trends of Lewis Acidic Sites in Methylaluminoxane and Some of Its Modifications

“…The computational protocol M06-2X/TZ(PCM)//TPSSTPSS/DZ has been previously benchmarked 56 , 57 and widely used for modeling of Al-species relevant to olefin polymerization 33 , 44 , 58 , 59 and other research fields, 60 and it has been shown to be suitable for studies related to MAO activators. 19 , 33 , 35 , 41 , 51 , 61 Additional inclusion of Grimme-type long-range dispersion corrections 62 , 63 does not affect computational results significantly (see Supporting Information ).…”

On the Nature of the Lewis Acidic Sites in “TMA‐Free” Phenol‐Modified Methylaluminoxane

“…Final single‐point energies were calculated at the M06‐2X level of theory employing triple‐ζ Dunning (TZ) basis sets (cc‐pVTZ quality) . This level of theory has proven suitable for studies related to MAO, , , , and other Al‐alkyl compounds , . Enthalpies and Gibbs free energies were then obtained from TZ single‐point energies and thermal corrections from the TPSSTPSS/cc‐pVDZ‐(PP) vibrational analyses; entropy corrections were scaled by a factor of 0.67 to account for decreased entropy in the condensed phase .…”

“…[84] This level of theory has proven suitable for studies related to MAO [7,29,50,56,92] and other Al-alkyl compounds. [38,[93][94][95] Enthalpies and Gibbs free energies were then obtained from TZ single-point energies and thermal corrections from the TPSSTPSS/cc-pVDZ-(PP) vibrational analyses; entropy corrections were scaled by a factor of 0.67 to account for decreased entropy in the condensed phase. [96][97][98] A model cluster for MAO cages, proposed by Zurek and co-workers, [6,99] was used.…”

Organocatalytic C−F Bond Activation with Alanes

“…Solvent influence (toluene, ϵ =2.3741) was modeled explicitly, using the polarizable continuum model (PCM) implemented in the Gaussian 09 software suite. The M06‐2X functional has been shown to yield accurate results for systems involving aluminum systems …”