The epsilon-caprolactam monomer having a triphenylimidazole moiety such as DL-3-[alpha-4-(4,5 diphenyl-1H-imidazole-2-yl)-phenylacryloyl-amine]-epsilon-caprolactam (1) was synthesized by the Heck reaction of 2-(4-bromophenyl)-1-(methoxymethyl)-4,5-diphenylimidazole and the epsilon-caprolactam derivative having an acryloyl group in a good yield. The copolymerization of 1 and epsilon-caprolactam was carried out at 250 degrees C by using H2O as an initiator to give copolymer (4) in a good yield. The crosslinking reaction of the obtained 2 proceeded with K-3[Fe(CN)(6)] and KOH in a mixed solvent of ethanol and phenol. The resulting crosslinked polymer (5) was de-crosslinked under irradiation with visible light. Consequently, the thiol-capped de-crosslinked polymer (6) was obtained in a good yield.