Ethylene 2,6-naphthalenedicarboxylate cyclic oligomers (ENCs) were prepared by the ring/ chain equilibration (cyclodepolymerization) of poly(ethylene 2,6-naphthalenedicarboxylate) (PEN) in dilute solution of biphenyl and polymerized with antimony trioxide. ENCs thus prepared consisted of a distribution of oligomeric species displaying a broad melting range from 330 to 370 degrees C. ENC trimer (C-a) was recrystallized from ENCs solution in 1,4-dioxane, and its purity was assessed by the matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometric analysis. C-3 is the main component in ENCs, and its melting temperature is 370 degrees C. ENCs containing the diethylene glycol (DEG) unit were detected in the high-pressure liquid chromatography and MALDI-TOF analysis. ENCs and ENCs/ethylene terephthalate cyclic oligomers (ETCs) blends were polymerized with 0.5 mol %. antimony trioxide at temperatures ranging from 320 to 365 degrees C for 5 min under a nitrogen atmosphere. The highest molecular weight (MW) (IV = 0.41 dL/g) was obtained from the polymerization of ENCs at 355 degrees C for 5 min. In the case of ENCs/ETCs blends, although the polymerization temperatures were decreased by adding ETCs, high MW polyesters were not obtained due to polymerization temperatures that nevertheless were still too high. PEN polymerized from ENCs showed the same thermal behaviors as the commercial product. Consequently, it was found that antimony trioxide is an effective catalyst for ENCs polymerization. but the melting and polymerization temperatures of ENCs need to be decreased to obtain PEN of higher MW.