A dialkyl-substituted dodecahexayne derivative of 15, 17,19,21,23,25-tetracontahexayne (THY) was found to polymerize spontaneously in the solid state at ambient temperature without irradiation. The polymerization of the THY crystals was monitored by IR and visible-near IR absorption, C-13 NMR and ESR spectra, and X-ray diffraction. The following polymerization scheme is proposed. First, a polydiacetylene substituted by alkyl and 1,3,5,7-alkatetraynyl groups is formed by 1,4-addition polymerization of the dodecahexayne moiety. Next, polymerization occurs at the 9,12-positions (on the opposite side) of the original dodecahexayne moiety to give a ladder polymer of two polydiacetylenes bonded by butadiynylene groups. Finally, allene and open shell structures are produced in the ladder polymer.