The spontaneous copolymerization of isobutoxyallene (1) with 4-phenyl-1,2,4-triazoline-3,5-dione (2) was carried out to afford a copolymer with a number-average molecular weight of 5900-10,300. The copolymer consisted of a 2,3-polymerization unit of 1 and a -N=N- polymerization unit of 2, maintaining an alternating character regardless of the monomer feed ratio. The corresponding copolymerization of 1 with 2 in the presence of methanol afforded the adduct of the compounds without the polymer, indicating the generation of a zwitterion of 1 and 2.