The catalytic polymerization of a series of 1,6-heptadiynes (1-4) by Mo(CO)(6) under simple thermal conditions produces corresponding poly(1,6-heptadiyne)s with highly conjugated polyenes. The number-average molecular weights of the polymers range from 2400 to 110,000. The structures of the polymers depend on the types of monomers employed. Namely, diethyl dipropargylmalonate (DEDPM) and 1 result in a polyene backbone consisting of only five-membered rings, whereas the remaining monomers (2-4) result in a mixture of both five-membered and six-membered ring structures. The copolymerization of DEDPM and phenylacetylene (PA) can also be effected by the same catalysis to yield a polyene backbone consisting of only five-membered rings as well as PA. The relative molar ratio of the two monomers determines the yields and molecular weights of the copolymers. Comparative studies show that Mo(CO)(6) exhibits reactivity toward DEDPM alone, thus initially catalyzing metathesis cyclopolymerization of DEDPM followed by copolymerization with PA.