To avoid difficulty in the synthesis of cyclic diblock copolymers, cyclic poly(oxyethylene)s with large molecular weights have been prepared by Williamson etherification reaction. Polyethylene glycol (PEG) sodium alkoxides (<(M)over bar (n)> = 8000 and 20 000) were reacted with a stoichiometric amount of 1,4-dibromobutane in N,N-dimethylformamide. No intermolecular condensation took place even at a high PEG alkoxide concentration of 10(-2) M and the conversion of cyclic polymers was substantially 100%.