To control the monomer sequence in copolymers of different hydroxyl acids remains a big challenge so far, which is valuable for tuning the properties of copolyesters. In this work, the perfectly alternating sequence-controlled copolymer of mandelic acid and glycolic acid, as the first example, was synthesized via the highly regioselective ring-opening polymerization (ROP) of a cyclic diester of mandelic acid and glycolic acid (3-phenyl-1, 4-dioxane 2, 5-diketone, PDD). The high molecular weight of poly(mandelate-alt-glycolate) (66.8 kg/mol) was achieved too, and the glass transition temperature of the alternating copolymer can reach 91.3 degrees C (M-n = 45.7 kg/mol). The high alternating level of this polymerization system may be interpreted by the high regioselectivity of a methyl glycolate zirconium complex as an intermediate mode complex in the propagation progress. The regioselectivity in the ROP of PDD can be changed from the initiation step to the propagation progress when the alkoxy group in zirconium complexes is methyl lactate or methyl mandelate.