We report here the effect of size and type of substituents upon the fine and conformational structure and unusual optical absorption behavior for the substituted poly(1,6-heptadiynes). 4-(Carboxy-3-(trimethylsilyl)-1-propyl)-1,6-heptadiyne (1a), 4,4-bis(3-(trimethylsilyl)-1-propyl) dipropargylmalonate (1b), 4,4-bis[(acetoxy)methyl]-1,6-heptadiyne (2a), 4,4-bis[(ethylcarboxy)methyl]-1,6-heptadiyne (2b), 4,4-bis[(tert-butyldimethylsiloxy)methyl]-1,6-heptadiyne (3a), and 4,4-bis[(tert-butyldiphenylsiloxy)methyl]-1,6-heptadiyne (3b) were all prepared and polymerized to give organo-soluble polymers in high yield with MoCl5-based catalysts. From a series of lambda(max) values of the polymers, we have found that incorporation of a bulkier substituent in the 4-position of 1,6-heptadiyne forces the conjugated polyene into a more planar structure, as indicated by the bathochromic shift of maximum absorption bands resulting from the pi-pi* transition of the conjugated polymer backbone. It appears that chain twisting of the substituted poly(1,6-heptadiynes) is determined primarily by the bulk of the substituent group at the 4-position rather than by the point of attachment to the polymer chain which usually decreases the effective conjugation length of the polymer pi-system. When an extremely bulky substituent was introduced at the 4-position, the resulting polymer had predominantly a symmetric single repeat unit with a five-membered ring and a trans vinylene group. The polymer 3b (lambda(max) = 606 nm) with a nearly 100% five-membered ring structure is environmentally very stable : there is no observable change in IR, NMR, and UV-visible spectra upon exposure to air for 6 weeks. This stability of backbone to air oxidation is thought to be due to effective shielding of the highly conjugated backbone by the bulky substituents.