Stereoselective coordination/insertion polymerization of the polar ortho-methoxystyrene has been achieved for the first time by using the cationic b-diketiminato rare earth-metal species. High activity and excellent isoselectivity (mmmm>99%) were acheived. The unmasked Lewis-basic methoxy group does not poison the Lewis-acidic metal center, but instead activates the polymerization through s-p chelation to the active species together with the vinyl group, thus lower the coordination and activation energies as compared with those of styrene derivatives lacking the methoxy group.