Sodium and potassium complexes supported by a bulky monophenoxy with one xanthenyl group at the ortho-position and 18-crown-6 or 15-crown-5 as an auxiliary ligand were synthesized and characterized. These complexes are highly iso-selective and active catalysts for the controlled ring-opening polymerization of rac-lactide. The best isotacticity (P-m) achieved was 0.86, which is the highest iso-selectivity reported to date for an alkali-metal complex. In addition, the corresponding polymer exhibited a high T-m of 182 degrees C. Furthermore, the polymerization looks like an anti-Arrhenius reaction, which is slower at high temperatures than at low temperatures.