The ring-opening reaction of 1-methylpyrrolidinium salt groups located at the end of poly(tetrahydrofuran) (poly(THF)) segment was studied with a series of p-substituted benzoate, having either methoxy, methyl, chloro or nitro substituent as a counter-anion. At 100 degrees C, such p-substituted benzoate anions as having an electron-donating methoxy or methyl substituent caused a ring-opening reaction of pyrrolidinium salt end groups, while others having an electron-withdrawing chloro or nitro substituent were poorly reactive to undergo the ring-opening of the end groups. Accordingly, the pathway of this nucleophilic reaction is primarily governed by the basicity, i.e., pK(a), of the corresponding p-substituted benzoic acid. At 120 degrees C, on the other hand, the nucleophilic substitution reaction at the N-methyl position of the pyrrolidinium salt end groups occurred predominantly.