Radical copolymerizations of β‐propiolactone (denoted 2) with acrylonitrile (denoted 1) and with styrene (also denoted 1) and the structures of the resulting copolymers were studied. The bulk copolymerization with acrylonitrile by α,α′‐azobisisobutyronitrile at 50°C gave polyesteracrylonitriles of high enough molecular weight to form tough and transparent films, with the monomer reactivity ratios, r1 = 0.84, r2 = 0.00, and the structure of the copolymers was magnified image Radical copolymerization with the same initiator in N,N‐dimethylformamide gave polyesteracrylonitriles of the same structure as that of the bulk polymer, blended with β‐propiolactone homopolymer which was due to the competing anionic homopolymerization of β‐propiolactone. The reactivity ratios on the bulk copolymerization with styrene were r1 = 6.2 and r2 = 0.0 with benzoyl peroxide at 80°C, and r1 ≅ 32, r2 = 0 with α,α′‐azobisisobutyronitrile at 50°C. Polyesterstyrenes of intrinsic viscosity up to 0.83 were obtained.