Poly((R)-1-deuterio-n-hexyl isocyanate) (αPdHIC) in dilute solution is characterized by an extraordinarily large specific optical rotation [α]. Detailed [α] data were obtained for a number of nearly monodisperse αPdHIC samples in different solvents over a wide range of temperature. The results were similar to those for poly((R)-2-deuterio-n-hexyl isocyanate) (βPdHIC) reported in a previous publication (Gu, H., et al. Macromolecules 1995, 28, 1016). The molecular weight dependence of [α] was remarkable over a wide range and consistently analyzed by the statistical mechanical theory of Lifson et al. (Lifson, S., et al. J. Am. Chem. Soc. 1989, 111, 8850) for a polyisocyanate chain consisting of an alternating sequence of right-handed and left-handed helices intervened by helix reversals. It was shown that the large [α] arises from the helical conformation, where the left-handed helix exists in some excess over the right-handed one due to a small free energy favoring the former helix (2ΔG h). This energy accumulates to a significant amount on a given chain assisted by a very small helix reversal probability (ν). This confirmed a similar conclusion on βPdHIC reported in the previous paper. It was found that ν is almost the same for the two polyisocyanates but the magnitude of 2ΔG h for αPdHIC, though smaller than a few calories per mole, is about twice as large as that for βPdHIC, exhibiting a remarkable hydrogen−deuterium isotope effect on chain conformation.