A facile free-radical ring-opening, polymerization of macrocyclic aryl ether thioether ketone oligomers is described. Polymerization of the cyclic oligomers via a transthioetherification reaction to high molecular weight linear polymers is achieved both in the melt and in solution in the presence of a catalytic amount of elemental sulfur or 2,2’-dithiobis(benzothiazole) (DTB). The free-radical nature of the polymerization reaction is indicated by electron paramagnetic resonance spectroscopy(EPR). The ring-opening polymerization reaction is unusual in that the molecular weight of the polymers obtained increases as the conversion of cyclic oligomers increases. The ring-chain equilibrium dramatically favors the formation of linear polymer, in which only low levels of cyclic materials are present after polymerization, even though the reaction is not enthalpically driven. also a facile process during polymerization. This intermolecular transthioetherification is demonstrated by using a low molecular weight arylene thioether, e.g., thiobis((phenylene-4-oxy)-4-benzoylbenzene), to control the molecular weight of the polymer formed via ring-opening polymerization.