Molecular weight dependence of the optical rotation of Poly((R)-1-deuterio-n-hexyl isocyanate) in dilute solution

Poly((R)-1-deuterio-n-hexyl isocyanate) (alpha dPdHIC) in dilute solution is characterized by an extraordinarily large specific optical rotation [alpha]. Detailed [alpha] data were obtained for a number of nearly monodisperse alpha LdHIC samples in different solvents over a wide range of temperature. The results were similar to those for poly((R)-2-deuterio-n-hexyl isocyanate) (beta PdHIC) reported in a previous publication (Gu, Il., et al. Macromolecules 1995, 28, 1016). The molecular weight dependence of [alpha] was remarkable over a wide range and consistently analyzed by the statistical mechanical theory of Lifson et al. (Lifson, S., et al. J. An. Chem. Sac. 1989, 111, 8850) for a polyisoeyanate chain consisting of an alternating sequence of right-handed and left-handed helices intervened by helix reversals. It was shown that the large [alpha] 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(2 Delta G(h)). This energy accumulates to a significant amount on a given chain assisted by a very small helix reversal probability (v). This confirmed a similar conclusion an beta PdHIC reported in the previous paper. It was found that v is almost the same for the two polyisocyanates but the magnitude of 2 Delta G(h) for alpha PdHIC, though smaller than a few calories per mole, is about twice as large as that for beta PdHIC, exhibiting a remarkable hydrogen-deuterium isotope effect on chain conformation.