Haloaldehyde polymers. 51. Helix-sense reversal of isotactic chloral oligomers in solution

Abstract

J. Am. Chem. SOC. termediate in MTG chemistry, but the observation that 1 can form in HZSM-5 from a known MTG intermediate removes a major objection to mechanisms such as those in Scheme I. Furthermore, with the ability to generate significant quantities of 1 in HZSM-5, it should be possible to further explore these mechanisms.We report the observation of helix-sense reversal of linear chloral oligomers (degree of polymerization, DP = 4-6) in solution by NMR spectroscopy and determination of the inversion barrier, which has been found to be dependent on DP.Some synthetic polymers such as polychloral,* poly(triphenylmethyl metha~rylate),~ and poly(tert-butyl i~ocyanide)~ are known to show optical activity arising entirely from conformational asymmetry (macromolecular asymmetry) when the polymers are prepared by helix-sense selective polymerization. The helix of the optically active polymers seems to be rigid in s o l~t i o n~~~ or slowly undergoes racemization to a nearly 1:l mixture of the rightand left-handed helices.' The helical conformation of polyisocyanates is less stable, and consequently, right-and left-handed states exist In equilibrium.* Although the one-handed helicity

Synthesis and structure determination of helical polymers

“…Their activation energies for the helix-sense inversion (DG a ) of 14 were then estimated to be 34.3-82.0 kJ mol -1 on the basis of the dynamic nuclear magnetic resonance technique. 46 The helical structure of the dodecamer of n-butyl isocyanate (15) was also determined to be approximately an 8/3 helix. 47 This helical structure is comparable in structure with the corresponding helical polyisocyanate proposed by the fiber XRD method.…”

Helical poly(isocyanides)

“…We discovered that simple nickel(n) salts (NiCl2.6H20 ; Ni(Acac)2, HAcac = acetylacetone) are very efficient catalysts for polymer izing a wide variety of aliphatic and aromatic isocyanides, including monomers with additional double or triple bonds, with metal ligating functions, with crown ether rings, with donor-acceptor groups, and with stable radical substituents [see (4)- (16) …”

Haloaldehyde Polymers LIII. Optical Resolution of Purely Isotactic Oligomers of Chloral: Optical Activity of the Chloral Oligomers Assuming One-Handed Helical Conformation in Solution

“…Recently, it was suggested that the isotactic chloral oligomers having a methyl group at both ends over the pentamer level may be optically resolved at room temperature based entirely on conformational asymmetry. 28 …”