Remarkably <i>Stretched Cis–Transoid</i> Helices Generated in Solid Phase and Solution of Poly(carbazole acetylene) Prepared Using an Organorhodium Catalyst in Toluene

A helical π-conjugated poly­(<i>N</i>-isobutyl-3-carbazole acetylene), P­(<i>i</i>BCzA) was stereoregularly prepared using an [Rh­(norbornadine)­Cl]₂–triethylamine catalyst in toluene at 25 °C to determine the detailed geometrical and spatial structures in a tetrahydrofuran (THF) solution and the solid phase. P­(<i>i</i>BCzA), which has a bright orange color, was obtained in a yield of 90%. The 2D NMR spectrum of P­(<i>i</i>BCzA) in THF-<i>d</i>₈ at 50 °C indicated that an extraordinarily extended helix was produced. The WAXS pattern of P­(<i>i</i>BCzA) showed the formation of a tetragonal crystal packed with a remarkably <i>stretched cis–transoid</i> (REST^CT) helix similar to that observed in the solution state. This REST^CT helix was confirmed not to be an energetically stable but a metastable structure using MMFF94 program. The solid phase UV–vis spectrum of P­(<i>i</i>BCzA) showed the formation of a radical cation called a polaron because of the very large red shifts compared to that observed in solution without the doping

Rate Control of Helix Oscillation of Poly(arylacetylene)s Achieved by Design of Side-Group Structures

Poly­(2-ethynylnaphthalene)­s with different alkylester moieties at the 6-position (PNRs) were synthesized to observe the first helix oscillation (HELIOS) of helical poly­(arylacetylene)­s, a transition between stretched and contracted helices in solution unique to helical structures. Variable-temperature (VT) 1H NMR spectra of PNRs were measured to analyze the microstructures of the two helices. The size of the aromatic ring and the structures of the alkyl chain in the side group on the PNRs strongly affected the rate of the HELIOS mode. The temperature dependencies of the relative ratio of populations between stretched and contracted helices were estimated. The differences in microstructures between stretched and contracted helices were also discussed based on the results of molecular mechanics calculations and 1H–13C heteronuclear single-quantum correlation spectroscopy (1H–13C HSQC). Consequently, we successfully changed the rate of HELIOS of PNRs and obtained detailed information on the molecular conformation of the stretched and contracted helices

Strictly Alternating Sequences When Copolymerizing Racemic and Chiral Acetylene Monomers with an Organo-Rhodium Catalyst

A racemic mixture and two chiral monomers of 2-methyl-1-butyl propiolate, i.e., <i><b>rac</b></i><b>1</b>, <i><b>R</b></i><b>1</b>, and <i><b>S</b></i><b>1</b>, were stereoregularly polymerized with a catalyst, [Rh­(norbornadiene)­Cl]₂, in methanol at 40 °C to obtain the corresponding helical racemic and two chiral polymers, <b>P</b><i><b>rac</b></i><b>1</b>, <b>P</b><i><b>R</b></i><b>1</b>, and <b>P</b><i><b>S</b></i><b>1</b>, and a copolymer, <b>P</b><i><b>co</b></i>. The ¹H and ¹³C NMR spectra of the racemic and chiral polymers differed, although the NMR spectra of their monomers were the same. The structures of the <b>P</b><i><b>co</b></i> copolymers with different chiral monomer ratios were analyzed using 1D and 2D NMR, optical rotation, circular dichroism (CD), UV–vis, and computational methods to elucidate the stereochemical effect of the chiral monomers together with the polymerization mechanism. The temperature dependence of ¹H and ¹³C NMR spectra in line shape and intensity indicated that the helical main chain undergoes restricted rotation around the ester methylene bonds −O–CH₂– through a three-site jump exchange called an accordion-like helix oscillation (HELIOS). The energetically preferred structures of the helical-sense polymers <b>P</b><i><b>R</b></i><b>1</b> or <b>P</b><i><b>S</b></i><b>1</b> were simulated using the MMFF94 program. The dependence of the NMR spectral line shapes, optical rotations, and calculated structures on the monomer feed clearly indicated that the copolymers alternatively incorporate <i><b>R</b></i><b>1</b> and <i><b>S</b></i><b>1</b> to generate one-handed helical-sense chains. Based on these results, a polymerization mechanism is proposed, explaining a strictly alternating copolymerization that yields helical chains

Accordion-like Oscillation of Contracted and Stretched Helices of Polyacetylenes Synchronized with the Restricted Rotation of Side Chains

A chiral substituted acetylene, (<i>s</i>)-2-octyl propiolate, was stereoregularly polymerized using a catalyst, [Rh­(nbd)­Cl]₂, at 40 °C in methanol to give the corresponding helical polymer, P<i>s</i>2OcP. The changes of ¹H and ¹³C NMR spectra in line shapes and splitting patterns were consistently interpreted in terms of restricted rotation around the ester O–<b>*</b>C bond, ∼O–*C^εH^ε(R)∼, R = a branched CH^ε₃ in the ester side chains rather than the helix inversion with the aid of a 3-site jump model. Three peaks due to the branched methyl H^ε proton and its C^η carbon observed at 0 °C suggested the formation of three rotamers called A, B, and C, based on the presence of the contracted helix and stretched helix forms that have an intrinsic helical pitch. Furthermore, an accordion-like helix oscillation (HELIOS) along the main chain axis was proposed to explain the temperature dependence spectral changes observed in ¹H and ¹³C NMR, UV–vis, and circular dicromism (CD) spectra. The temperature dependence UV–vis and CD spectra of P<i>s</i>2OcP corroborate the presence of contracted and stretched one-handed helix sense polymers in solution in which the helical pitches and their persistence lengths depend on the temperature