A series of diacetylenic monomers containing at least one aromatic substituent were synthesized. Since the aromatic side group is a rigid, pyrimidyl ring, it is necessary that the other side group has a high degree of flexibility to permit polymerization in the monomer single crystal. Among the five monomers synthesized, only 8-[[(butoxycarbonyl)methyl]urethanyl]-1-(5-pyrimidyl)octa-1,3-diyne (BPOD) monomer crystals turned into a polymer upon gamma-irradiation with 68% monomer-to-polymer conversion. Poly(BPOD) is metallic red as polymerized and highly soluble in chloroform and undergoes a dramatic color transition from red (lambda(max) = 510 nm) to purple (lambda(max) = 599 nm) upon the addition of a nonsolvent such as hexane. This red-to-purple transition going from good to poor solvent is interpreted as a single-chain phenomenon. Aggregation may follow as the rigid chains interact with each other and crystallize. This aggregation could be prevented by increasing the side group-nonsolvent interaction by employing a nonsolvent which has high polarity and hydrogen-bonding capability.