In this contribution, we thoroughly investigated the ring-opening alternating copolymerization (ROAC) of cyclic anhydride and epoxide by using commercially available alkali metal carboxylates (AMCs) as the simple and green initiators. The idea of our work is based on the coordination effects of epoxide on an AMC and the formation of AMC-epoxide adducts, which will weaken the interaction between metal cation and its carboxylate counterion and therefore render the carboxylate to feasibly attack epoxides in a nucleophilic manner at high temperature. The coordination effects of epoxide on the AMC could be proved by Fourier transform infrared (FT-IR) spectroscopy and density functional theory (DFT) calculations. AMCs could effectively catalyze the copolymerization of phthalate anhydride (PA) and cyclohexene oxide (CHO) in bulk at 110 degrees C, affording polyesters with perfectly alternating structure. Potassium acetate (KOAc) was able to copolymerize some common cyclic anhydrides and epoxide, allowing for the preparation of polyesters with structural diversity. Of note, KOAc could mediate ROAC of PA with propylene oxide (PO) with a high molar feed ratio of [KOAc]/[PA]/[PO] = 1:20 000:150 000, affording poly(PA-alt-PO) with high molecular weight (>100 kDa). Finally, two different polymerization mechanisms, including anionic polymerization and cooperative catalysis, have been proposed according to the interaction strength between metal cation and carboxylate anion. In the "cooperative catalysis" mechanism, the alkali metal cation served as the Lewis acid to activate epoxide and cyclic active species were generated.