Effect of Chiral Substituents on the Helical Conformation of Poly(propiolic esters)

Abstract

Propiolic esters having various chiral substituents, [HCtCCO2R*, R* ) (S)-(CH2)nCHMeEt (n ) 0-5), (1S,2R,5R)-isomenthyl, and (1S,2S,5S)-myrtanyl], were polymerized with [(nbd)RhCl]2 or MoOCl4-n-Bu4Sn for the purposes of establishing the relationship between the helical conformations of the polymers and the structures of pendant chiral groups. In contrast to the poor stereoregularities of the polymers prepared with MoOCl4-n-Bu4Sn, [(nbd)RhCl]2-catalyzed polymerizations resulted in polymers with high cis contents. These stereoregular polymers with 1-4 alkylene spacers displayed intense CD effects and large optical rotations as well as large band gap energy (ca. 3.2 eV), indicating that they possess restricted main-chain conjugation and exist in helical conformations with an excess of a onehanded screw sense. In contrast, the polymers without alkylene spacers possessed more extended coplanarity of the main chains and exhibited poorer chiroptical properties than those bearing alkylene spacers. The introduction of bulkier substituents to the polymers enhanced the persistence length of the one-handed helix, resulting in an increase in the magnitude of Cotton effects and optical rotations. Polymer stereoregularity remarkably influenced the polymer conformation, and no distinct CD signals were detected for polymers with alkylene spacers if polymerizations were performed with the Mo catalyst.

Substituted polyacetylenes

“…89 Recently there are fairly many examples of helical substituted polyacetylenes; for example, induced helices based on the combinations of poly(p-carboxyphenylacetylene) and its analogues with low molecular weight compounds, 29,90 helical poly(p-alkoxy-m,m-dihydroxyphenylacetylene) synthesized by using chiral Rh catalyst systems, 28,91 and poly(menthyl/chiral alkyl propiolates). 92,93 The most salient feature of helical poly(Npropargylamides) developed by us is that the helix is mainly induced by the intramolecular hydrogen bonding between amide groups in the side chain.…”

Molecular Shapes of Monosubstituted Polyacetylenes in their Liquid Crystalline Phases

“…[27,47] The polymerization of acetylene monomers having a chiral substituent can result in optically active MSPAs, owing to the formation of helical conformation with an excess screw sense. [11,13,18,24,[40][41][42][43]45,[48][49][50][51] A systematic study of the cis-transoidal PPAs bearing D-or L-alanine residues and with a long n-decyl chain as the pendants (Figure 2a) by Yashima and co-workers gives an excellent demonstration of the rigid rod-like MSPA with a predominant one-handed helical conformation. [52][53][54][55] On highly oriented pyrolytic graphite (HOPG), the chains losing their helical conformation are flat in the first monolayer, probably due to the strong and epitaxial adsorption of the long alkyl chains on the basal plane of graphite.…”

Self‐assembly of supramolecular polymers into tunable helical structures

“…[38][39][40][41][42][43] Percec et al have reported a library of dendronized polyphenylacetylene (PPA)s that exhibit chiral helical conformations both in solution and in solid (Fig. 1).…”