Photocationic polymerization of tetrahydrofuran(THF) in methylene chloride has been carried out in the presence of diphenyliodonium hexafluorophosphate. It has been f]unto that the living nature is lost to some extent by the addition of polar solvent. This can be explained with an assumption that the equilibrium between ion pair and free ion, responsible for the cationic propagation, is shifted to free ion by the polar solvent and the reactivity of free cationic propagating species results in the increase in chain transfer. The stability of cationic species has been explained in terms of thermodynamics.