Stereoselective polymerization of polar vinyl monomers has been a long-standing challenge because the employed transition-metal catalysts are easily poisoned by polar groups of monomers. In this contribution, a series of b-diketiminato rare-earth metal complexes 1–5 (Ln=Gd–Lu, Y, and Sc) were successfully synthesized. In combination with AlⁱBu₃ and [Ph₃C][B(C₆F₅)₄], complexes 1c(Tb)–1g(Tm) exhibited high activities and excellent isoselectivities for the polymerization of ortho-methoxystyrene (oMOS), in which, the polar methoxy group of oMOS did not poison but activated the polymerization through chelation to the active species together with the vinyl group. Moreover, the large Gd-attached precursor 1b showed a higher activity, albeit with a slightly decreased isoselectivity. The small Sc-attached precursor 1i was completely inert. Meanwhile, the spatial steric arrangement and the coordination mode of the b-diketiminato ligand could clearly affect and even block oMOS polymerization. This work sheds new light on the coordination polymerization of polar monomers.