<p>The ring-opening alternating copolymerization (ROAC) of 3,4-dihydrocoumarin (DHC)/epoxides has been successfully developed using an imidazolium salt of 1-ethyl-3-methylimidazolium chloride (EMIMCl) as a catalyst. The resulting copolymer has a molecular weight of 13.7 kg·mol⁻¹, a narrow molecular weight distribution of 1.03 and a strictly alternating structure. The MALDI-TOF MS characterization and DFT calculations including electrostatic potential (ESP), hydrogen-atom abstraction (HAA), independent gradient model based on hirshfeld partition (IGMH) and atoms-in-molecules (AIM) analysis were used to investigate the metal-free catalytic process. The synergistic effect of anions and cations of EMIMCl for ROAC of DHC and epoxides was demonstrated. This study provides a metal-free catalytic system for the facile synthesis of alternating polyesters from DHC.</p>