AbstractThe enthalpies of combustion of poly(ethylene oxide), ΔH oc(c)=‐1178,7 ± 1,3 kJ . mol‐1, and of a random copolymer of overall molar composition 0,41 (poly(ethylene oxide)) + 0,59 (poly(oxy‐1,4‐cyclohexylene)), ΔH oc(c)=‐2566,4 ± 1,8 kJ . mol‐1, were measured at 298,15 K by high‐precision bomb calorimetry. The enthalpy of polymerisation of ethylene oxide was derived as ΔH op(l → c) = ‐102,4 ± 1,6 kJ . mol‐1. The enthalpy of copolymerisation of ethylene oxide and 7‐oxabicyclo[2.2.1]heptane was ΔH op(l → c) = ‐63,0 ± 2,1 kJ . mol‐1, compared with ‐68,1 ± 1,6 kJ . mol‐1for producing the two homopolymers in the proportion in which the monomers were present in the copolymer. It is concluded that insertion of an ethylene oxide unit into a poly(oxy‐1,4‐cyclohexylene) chain does