A monofunctional maleimide-ended model compound was synthesized and purified. The thermal curing of this compound was followed by size exclusion chromatography and the molecular masses of the oligomers formed during the curing reaction were determined by mass spectroscopy. It was found that high molecular mass species were not thermally stable. At high temperature and after enough time, a strong decrease in the polydispersity of the oligomers and a significantly preferential formation of trimer were observed. From C-13-NMR characterization, a five membered ring structure was found for the trimer, which explains the thermal stability of this species.