The placement of selenium (Se) atoms into polymers to achieve functional materials for diverse applications in adaptive and biomedical materials has attracted broad interest. However, the difficulty in creating well-defined Se-rich polymers is a critical limitation. In this work, we report the successful alternating and regioselective copolymerization of nonstrained gamma-selenobutyrolactone with epoxides. Poly(selenide-alt-ester)s containing selenide and ester groups in each repeating unit are achieved under mild reaction conditions. The process mediated by the phosphazene base (P2)/alcohol at 25 degrees C exhibits high activity and living behavior, as evidenced by narrow molecular weight distributions (1.05-1.29) and switch on-off chain extension experiments. The nucleophilic attack of epoxides by selenol anions is the rate-determining step according to kinetic studies. The resultant Se-containing polyesters possess >99% alternating degree, >99% regioregularity, and programmable hydroxyl terminals. More importantly, these poly(selenide-alt-ester)s exhibit the initial thermal decomposition temperatures of up to 275 degrees C, low glass-transition temperatures of down to -59 degrees C, excellent solvability in common organic solvents, high refractive indexes (1.61-1.62), and good ionic conductivity (when doped with Li salt, 1.09 x 10(-6) S/cm at 30 degrees C, 1.73 x 10(-4) S/cm at 100 degrees C), indicating potential broad application prospects of these Se-rich polyesters.