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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.

Isotactic polypropylene (i-PP) is mainly produced with heterogeneous Ziegler-Natta (ZN) catalyst systems. Direct structural information on the active species is very difficult to achieve; as a matter of fact, the 13C NMR microstructure of the polymer has long been used as an indirect "fingerprint". For a long time, however, the intrinsically poor sensitivity of natural abundance 13C NMR spectroscopy hampered a complete evaluation of the microstructure, and many rare albeit important details escaped detection with standard tools and methods. The most prominent case is that of the regiodefects, which are specially important because they dictate ZN catalyst response to molecular hydrogen used as a chain transfer agent. In recent papers we have reported on the quantification of regioirregular 2,1 units in ZN i-PP by means of high-temperature cryoprobe 13C NMR. Here we shift the focus onto the stereochemical environment of such units, which was highlighted by comparing the spectra of suitable ZN PP fractions with those of model PP samples made with molecular catalysts of different structures and symmetries. The results lend support to our three-site model of ZN catalytic species, originally based on the stereochemistry of regioregular PP chains/blocks.

Olefin polymerization using Ziegler–Natta catalysts (ZNCs) is an important industrial process. Despite this, fundamental insight into the inner working mechanisms of these catalysts remains scarce. Here, we focus on the low-γ nuclei 25Mg and 35Cl for an in-depth solid-state NMR and density functional theory (DFT) study of the catalyst’s MgCl2 support in binary adducts prepared by ball-milling. Besides the bare MgCl2 support and a MgCl2–TiCl4 adduct, samples containing donors that are part of the families of 2,2-dialkyl-1,3-dimethoxypropanes and phthalates used in fourth- and fifth-generation ZNCs are studied. DFT calculations indicate that the quadrupolar coupling parameters of the chlorines differ significantly between bulk and surface sites. As a result, the NMR visibility of the chlorine sites correlates with the particle size except for the adduct with 2,2-dimethyl-1,3-dimethoxypropane donor. The DFT calculations furthermore show that the surface sites are fairly insensitive to binding of different donor molecules, making it difficult to identify specific binding motives. The surface sites with large 35Cl NMR line widths can be observed using high radio frequency field strengths. For the 2,2-dimethyl-1,3-dimethoxypropane donor, we observe additional surface sites with intermediately high quadrupolar couplings, suggesting a different surface structure for this particular adduct compared to the other systems. For 25Mg, pronounced effects of donor binding on the quadrupole interaction parameters are observed, both computationally and experimentally. Again the adduct with the 2,2-dimethyl-1,3-dimethoxypropane donor shows a different behavior of the surface sites compared to the other adducts, which display more asymmetric coordinations of the surface Mg sites. Identifying specific binding motives by comparing 25Mg NMR results to DFT calculations also proves to be difficult, however. This is attributed to the existence of many defect structures caused by the ball-milling process. The existence of such defect structures both at the surface and in the interior of the MgCl2 particles is corroborated by NMR relaxation studies. Finally, we performed heteronuclear correlation experiments, which reveal interactions between the support and Mg–OH surface groups, but do not provide indications for donor–surface interactions.

The typical activation of a fourth generation ZieglerNatta catalyst TiCl4/MgCl2/phthalate with triethyl aluminum generates Ti3+ centers that are investigated by multi-frequency continuous wave and pulse EPR methods. Two families of isolated, molecule-like Ti3+ species have been identified. A comparison of the experimentally derived g tensors and 35,37Cl hyperfine and nuclear-quadrupole tensors with DFT-computed values suggests that the dominant EPR-active Ti3+ species is located on MgCl2(110) surfaces (or equivalent MgCl2 terminations with tetra-coordinated Mg). O2 reactivity tests show that a fraction of these Ti sites is chemically accessible, an important result in view of the search for the true catalyst active site in olefin polymerization.

Olefin polymerization using Ziegler–Natta catalysts (ZNCs) is an important industrial process. Despite this, fundamental insight into the inner working mechanisms of these catalysts remains scarce. Here, we focus on the low-γ nuclei 25Mg and 35Cl for an in-depth solid-state NMR and density functional theory (DFT) study of the catalyst’s MgCl2 support in binary adducts prepared by ball-milling. Besides the bare MgCl2 support and a MgCl2–TiCl4 adduct, samples containing donors that are part of the families of 2,2-dialkyl-1,3-dimethoxypropanes and phthalates used in fourth- and fifth-generation ZNCs are studied. DFT calculations indicate that the quadrupolar coupling parameters of the chlorines differ significantly between bulk and surface sites. As a result, the NMR visibility of the chlorine sites correlates with the particle size except for the adduct with 2,2-dimethyl-1,3-dimethoxypropane donor. The DFT calculations furthermore show that the surface sites are fairly insensitive to binding of different do...

Ziegler–Natta catalysis is a very important industrial process for the production of polyolefins. However, the catalysts are not well-understood at the molecular level. Yet, atomic-scale structural information is of pivotal importance for rational catalyst development. We applied a solid-state NMR/density functional theory tandem approach to gain detailed insight into the interactions between the catalysts’ support, MgCl2, and organic electron donors. Because of the heterogeneity of the samples, large line widths are observed in the carbon spectra. Despite this, good agreement between experimental and computational values was reached, and this shows that 1,3-diether based donors coordinate at (110) surface sites, while phthalates are less selective and coordinate at both (104) and (110) surface sites.

MgCl2 is a vital component of Ziegler-Natta catalysts for olefin polymerization. Here we:synthesized anhydrous MgCl2 using different drying protocols and exploited H-1 NMR to quantify the proton content. We report on our study of neat and ball-milled MgCl2 samples by means of Mg-25 and Cl-35 solid-state NMR. DFT calculations of the quadrupole tensor aid in analysis of the spectra. The results show that, due to the morphology of the neat particles, a preferred Orientation is induced which manifests itself in unusual powder line shapes. Ball Milling reduces particle size, which subsequently leads to a small distribution of quadrupole parameters for the bulk. Surface sites, highly relevant for catalysis, are not directly observed, due to their broad lines of low intensity.