A full elucidation of chemisorbed donor behavior is of fundamental importance for tailoring the active species but has not yet been achieved for Ziegler-Natta catalysts. Herein, by exploiting the self-nucleation of polyhedral oligomeric silsesquioxane (POSS) molecules, we present porous microspheres assembled from rhombic MgCl2 nanocrystals with layered laterals. The as-formed POSS-MgCl2 aggregation, after immobilization of TiCl4, shows considerable catalytic activity toward ethylene polymerization, synthesizing ultrahigh molecular weight polyethylene with enhanced processability and mechanical properties as well as reduced entanglement. By tracing catalyst synthesis and titanium activation with in situ spectroscopy, surface approaches, and density functional theory calculations, we acquired fundamental insight into the active role of THF on the MgCl2 surface. We experimentally demonstrate (i) the full coverage of THF on the lateral surfaces of MgCl2 crystals; (ii) the partial desorption and migration of THF on the surfaces owing to its high mobility, resulting in the regeneration of unsaturated Mg2+ sites and the subsequent formation of TiCl4-THF complexes during titanation; and (iii) the active participation of THF in the final active sites (i.e., after activation). The whole data set clearly demonstrates that the dynamic adsorption of donors directly affects the structure and performance of the active species, offering a perspective on the interactions between donors and other components.