AbstractPropylene oxide and tetrahydrofuran were polymerized cationically by anin situcatalytic system composed of antimony pentachloride and 1,2‐propanediol. The rates of polymerization were measured by vapor‐phase chromatography in the temperature range from −20°C to +20°C. The Arrhenius parameters pertaining to the reaction of each of the comonomers were evaluated and compared with data published earlier for other catalytic systems. The present catalyst system was incapable of initiating a homo‐polymerization of tetrahydrofuran in the absence of propylene oxide, while the latter was readily homopolymerized. In a copolymerization system, the rates of consumption of both monomers were first‐order in respect to the catalyst, but the reaction ceased when all of the propylene oxide had been consumed. The relative reactivity of the two monomers as characterized by the copolymerization parametersr1(PO) = 1.15 andr2(THF) = 0.70 suggests that in the copolymerization system, tetrahydrofuran is capable of a reaction with its own active center. This is discussed in terms of a possible mechanism involving the effects of penultimate units and extensive chain transfer. The latter is well evident from the molecular weights of resulting copolyethers, which do not exceed o