Two alkyl ketene oligomers (AKOs) with almost 100% purities were prepared in high yields of 80-90% from palmitic acid chloride and alkyl ketene dimer (prepared from palmitic acid chloride) in the presence of amine promoters. Two AKOs thus obtained had quite similar molecular weights (1000-10,000), carbon/hydrogen contents, X-ray diffraction patterns, solution-state C-13-NMR spectra, and water-contact angles. However, two AKOs had different size-exclusion chromatograms, differential scanning calorimetric curves, FTIR spectra, and solution-state C-13-NMR spectra for C-13-labeled AKOs. Based on the C-13-NMR spectra of C-13-unlabeled AKOs, AKOs are likely to have cyclic structures, thus having the limited molecular weights of <10,000. The results of FTIR spectra and C-13-NMR spectra of C-13-labeled AKOs indicated that three different repeating units were linked with various molar ratios and various connection patterns and combinations, constituting the cyclic AKOs. As a result, C-13-NMR spectra of C-13-labeled AKOs showed quite a number of carbon signals due to various (CO)-C-13 and (CC)-C-13 structures present in AKOs. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3668-3673, 2013