Imidazole was employed as an anionic initiator for the homopolymerization of maleic anhydride in various solvents. Polymerization was very rapid and the obtained products have different colours varying from white to dark brown. The colour of the product was found to depend on the quantity of imidazole added, and the solvent of polymerization. Darkness of polymers are thought to be due to cross-conjugation displaying paramagnetic character. The ESR signals of dark brown polymers were more intense than the lighter ones whereas the white products had no ESR signal.