Probing Interactions between Electron Donors and the\nSupport in MgCl2‑Supported Ziegler–Natta\nCatalysts

Olefin\npolymerization using Ziegler–Natta catalysts (ZNCs) is an important industrial process. Despite this,\nfundamental insight into the inner working mechanisms of these catalysts\nremains scarce. Here, we focus on the low-γ nuclei 25Mg and 35Cl for an in-depth solid-state NMR and density\nfunctional theory (DFT) study of the catalyst’s MgCl2 support in binary adducts prepared by ball-milling. Besides the\nbare MgCl2 support and a MgCl2–TiCl4 adduct, samples containing donors that are part of the families\nof 2,2-dialkyl-1,3-dimethoxypropanes and phthalates used in fourth-\nand fifth-generation ZNCs are studied. DFT calculations indicate that\nthe quadrupolar coupling parameters of the chlorines differ significantly\nbetween bulk and surface sites. As a result, the NMR visibility of\nthe chlorine sites correlates with the particle size except for the\nadduct with 2,2-dimethyl-1,3-dimethoxypropane donor. The DFT calculations\nfurthermore show that the surface sites are fairly insensitive to\nbinding of different donor molecules, making it difficult to identify\nspecific binding motives. The surface sites with large 35Cl NMR line widths can be observed using high radio frequency field\nstrengths. For the 2,2-dimethyl-1,3-dimethoxypropane donor, we observe\nadditional surface sites with intermediately high quadrupolar couplings,\nsuggesting a different surface structure for this particular adduct\ncompared to the other systems. For 25Mg, pronounced effects\nof donor binding on the quadrupole interaction parameters are observed,\nboth computationally and experimentally. Again the adduct with the\n2,2-dimethyl-1,3-dimethoxypropane donor shows a different behavior\nof the surface sites compared to the other adducts, which display\nmore asymmetric coordinations of the surface Mg sites. Identifying\nspecific binding motives by comparing 25Mg NMR results\nto DFT calculations also proves to be difficult, however. This is\nattributed to the existence of many defect structures caused by the\nball-milling process. The existence of such defect structures both\nat the surface and in the interior of the MgCl2 particles\nis corroborated by NMR relaxation studies. Finally, we performed heteronuclear\ncorrelation experiments, which reveal interactions between the support\nand Mg–OH surface groups, but do not provide indications for\ndonor–surface interactions.