Helix Inversion of Poly(propiolic esters)

Abstract

The effects of temperature and solvents on the helical conformation of stereoregular cis−transoidal poly(propiolic esters) [(CHCCO2R*) n ] with various chiral side chains, which were prepared with [(nbd)RhCl]2, were investigated using circular dichromism spectroscopy. CD effects of the polymers with short side chains, where R* = (S)-(CH2) n CH(CH3)C2H5 (n = 1−4), were only slightly amplified with decreasing temperature from +20 to −50 °C, and this behavior was independent of the solvents examined (CHCl3, THF, and n-hexane). In contrast, although the temperature dependence of CD effects for polymers with long alkyl pendants [5 and 6, where R* = (S)-(CH2)5CH(CH3)C2H5 and (S)-(CH2)2CH(CH3)(CH2)3CH(CH3)2, respectively] was similar in n-hexane to those for the other polymers, the Cotton effects of 5 and 6 in CHCl3 decreased in intensity with decreasing temperature and inverted in sign between −30 and −40 °C. This CD inversion process was proved to originate from the thermally induced screw-sense inversion, and computational semiempirical calculation suggested that the transitions from M (left-handed) to P (right-handed) and from P to M helices took place for polymers 5 and 6, respectively, with decreasing temperature. The screw-sense inversion driven by the change in solvent composition between n-hexane and CHCl3 was also achieved at +20 °C for a copolymer of hexyl with (S)-2-methylbutyl propiolates.

Amplification of macromolecular helicity of dynamic helical poly(phenylacetylene)s bearing non-racemic alanine pendants in dilute solution, liquid crystal and two-dimensional crystal

“…This hypothesis is supported by the fact that the poly-1L maintains its rigid rod-like feature in dilute benzene at 251 C because the persistent length of poly-1L estimated in dilute toluene solution, an analogous solvent to benzene, is extremely high (126 nm) at 251 C. 26 In contrast, the copolymers and homopolymer showed very intense ICDs in nonpolar CCl 4 ( Figure 3A), less polar tetrachloroethane (TCE; Figure 4A) and polar THF (Supplementary Figure S3A) even at 25 1C, and their ICD patterns in CCl 4 and benzene at 6 1C are almost mirror images to those in TCE and THF, indicating the inversion of the helicity of the polymer backbones that took place assisted by the solvent polarity, as reported previously for poly-1L, 26 poly(1L m -co-Aib n ) 19 and other dynamic helical polyacetylenes. [36][37][38][39][40][41][42][43][44] The CD and absorption spectral patterns in CCl 4 , TCE and THF were almost temperature-independent.…”

Intramolecular Versus Intermolecular Induction of Helical Handedness in Pyridinedicarboxamide Oligomers

“…For instance, a right-handed bias induced in a helical polymer by a group of given chirality may be converted to a lefthanded bias by changing the polarity of the solvent or the temperature, [10][11][12][13][14] or even by more specific methods such as the binding of nonchiral guest molecules [15] or by irradiation with unpolarized [16][17][18][19] or circularly polarized light. [20,21] Consequently, a small change in the environment of the helical chain may give rise to a large chiroptical signal inversion, a feature that may be of use in some optical devices.…”

Polymerization of Substituted Acetylenes by the Grubbs–Hoveyda Ru Carbene Complex

“…Rh catalysts can polymerize only monosubstituted acetylenes such as phenylacetylene and its ring-substituted derivatives, 6-11 N-propargylamides, 12-17 and propiolic esters. [19][20][21][22][23] The Rh-catalyzed polymerization proceeds by the insertion mechanism, and features excellent tolerance to polar subsituents in the monomer 24,25 and protic solvents. 26 The Rh-based polymers generally possess high cis stereoregularity, which is indispensable for the formation of helical structures of poly(N-propargylamide)s.…”