A series of diynes bearing flexible aliphatic, electron-donating, or electron-withdrawing groups between the acetylenes moieties or as lateral groups was prepared and subjected to the polymerization with (eta(5)-cyclopentadienyl)bis(triphenylphosphine)cobalt (1). As a result, organocobalt polymers bearing various substituents in the side or the main chain were obtained in high yields. From the detailed structural elucidation of polymers or from the model experiments, some of the organocobalt polymers were found to be contaminated with (eta(5)-cyclopentadienyl)(eta(4)-cyclobutadiene) units. The regiochemistry of the main chain of the obtained polymers was also studied from the model experiments, from which 2,5-linkage of the main chain was found to be predominant in the case of diynes bearing less sterically hindered lateral substituents. The polymer obtained from 1 and a diyne having electron-withdrawing groups between the acetylenes and lateral aliphatic moieties (i.e., 1,4-bis( 1-oxo-2-undecynyl)benzene, 2I) was found to have 100% of the cobaltacyclopentadiene moieties in the repeating units and the 2,5-selective (ca. 90%) main chain linkage.