An ion-coupling reaction of a monofunctional poly(tetrahydrofuran) [poly(THF)] having a tetrahydrothiophenium salt end group (1) with poly(sodium acrylate) (2) was performed by a simple precipitation of a THF solution of 1 into an aqueous solution containing an excess amount of 2. The coupling reaction took place efficiently to produce poly(THF) polymacromonomer through the ion-exchange reaction between 1 and 2 and the subsequent ring-opening reaction by the nucleophilic attack of the carboxylate anion toward the moderately strained cyclic onium salt end group of 1 at ambient condition.