Coordination−insertion polymerization of polar vinyl monomers has always been a challenging subject since the Lewis basic polar groups are usually poisonous to the Lewis acidic metal centers of the catalysts. In this paper, we report the coordination polymerization of the unmasked polar alkoxystyrenes (AOS), including o-methoxystyrene (oMOS), o-ethoxystyrene (oEOS), o-n-propoxystyrene (oPOS), o-n-butoxystyrene (oBuOS), o-benzyloxystyrene (oBnOS), and 5-phenyl-2-methoxystyrene (Ph-MOS), by using the cationic quinolyl anilido yttrium alkyl species that was inert to styrene polymerization. In addition, the styrenic monomer bearing two polar groups, 4-fluoro-2-methoxystyrene (FMOS), was also polymerized since the presence of the additional polar fluorine group did not block the interaction between the ortho-oxygen and yttrium center. In contrast, the o-(tert-butyldimethyl)silyloxystyrene (oSiOS), where the ortho-oxygen atom was prevented from coordinating to the yttrium center by the sterically bulky −SiMe 2 t Bu group, could not be polymerized. Thus, the o-alkoxy groups of AOS did not poison the rare-earth metal center but activated the polymerization through chelation to the active species. The stereoregularity of the resultant polymers could switch from syndiotactic to atactic by changing the ortho-oxygen substituent from methyl to benzyl, which was attributed to the weakening Y−σ−O coordination bond. The mechanism was elucidated by the DFT calculations.

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 β‐diketiminato rare‐earth metal complexes 1–5 (L1–5Ln(CH2SiMe3)2(THF)n, Ln=Gd–Lu, Y, and Sc) were successfully synthesized. In combination with AliBu3 and [Ph3C][B(C6F5)4], complexes 1 c(Tb)–1 g(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 σ–π chelation to the active species together with the vinyl group. Moreover, the large Gd‐attached precursor 1 b showed a higher activity, albeit with a slightly decreased isoselectivity. The small Sc‐attached precursor 1 i was completely inert. Meanwhile, the spatial steric arrangement and the coordination mode of the β‐diketiminato ligand could clearly affect and even block oMOS polymerization. This work sheds new light on the coordination polymerization of polar monomers.