Hideki Shirakawa

Metallic properties of transition metal halides doped polyacetylene: The soliton liquid state

Abstract Temperature dependence of d.c. conductivity and thermoelectric power (TEP) are measured for the newly processed polyacetylenes doped with FeCl 3 , MoCl 5 , NbCl 5 and ZrCl 4 . The data can be analyzed as followings. (1) The metallic polyacetylene shows quasi-one-dimensional charge transport mechanism. To support this conclusion, we compared the temperature dependent anisotropy of conductivity for the nondegenerate ground state conducting polymers such as PPV derivatives. (2) TEP data indicate that there exists an interaction between the delocalized charge carrier in the polymer chain and the dopant ions located nearby the chain. As a consistent interpretation, we propose that the metallic polyacetylene is formed by the soliton - antisoliton condensation (we call this as soliton liquid state). With this proposal, both the Pauli susceptibility and the infrared active vibrational modes (IRAV) observed in the metallic polyacetylene can be understood simultaneously.

Dependence of electrical conductivity on film thickness in polyacetylene — effects of mechanically forced parallel and in-plane alignments of fibrils

Abstract We have presented conventional analytical formulae which correlate the electrical conductivity of polyacetylene film with its macroscopic morphology represented by mechanically forced fibril alignment (parallel alignment) and in-plane alignment. Through numerical evaluations based on the present analytical formulae, we have clarified that the in-plane alignment contributes to enhance the parallel conductivity of the film in a cooperative manner with the parallel alignment. In particular, the in-plane alignment becomes crucially dominant when the film thickness is less than 1.0 μm, irrespective of whether the film is as-grown or mechanically stretched. On the other hand, although the parallel alignment expectedly decreases the conductivity perpendicular to the elongation direction along with the mechanical stretching, the decrease is partly com pensated by an increase due to the in-plane alignment.

The Soliton State of Heavily Doped Polyacetylene

Abstract We propose that the heavily doped polyacetylene is formed by the soliton-antisoliton pair condensation which is a mixed state of the delocalized charge carriers and the virtually bound states. With this proposal, the metallic conductivity and the temperature independent magnetic susceptibility as well as the infrared active vibrational modes (IRAV) observed in the heavily doped polyacetylene can be understood simultaneously. Furthermore, the persistence of the C˭C and C—C vibrational modes appeared in Raman spectra for the heavily doped polyacetylene can also be understood with this soliton-antisoliton condensed state model. The highest conductivity estimated from this phenomenological model is [sgrave] RT ˭ 35,000 S/cm, consistent with the observed (and confirmed) maximum conductivity of the heavily doped polyacetylene.

Stereospecific chlorination of polyacetylene by chemical doping

Abstract We present a novel utilization of chemical doping in stereospecific chlorination to π-conjugated polyene. It has been verified that not only prolonged FeCl 3 doping but also iodine doping followed by chlorine addition to one-dimensional conjugated polyene (polyacetylene, (CH) x ) produce a stereoregular chlorinated polyacetylene, (CHCl) x , i.e. di-syndiotactic poly(1,2-dichloroethylene). In the latter case, tacticity of the polymer, defined as the ratio of stereoregular to atactic segments in the (CHCl) x , strictly depends upon the dopant concentration of iodine before the chlorine addition. Thus chemical doping is a promising method to control the tacticity of (CHCl) x without polymerization. We demonstrate that the positively charged polyene segment generated by the chemical doping plays an essential role in governing stereospecific chlorination.

Intrinsic non-solvent polymerization method for synthesis of polyacetylene films

Abstract We have developed a novel solvent-free polymerization method for synthesis of highly stretchable and highly conducting polyacetylene film. In this procedure, we prepare a catalyst by mixing tetraalkoxytitanium, Ti(OR)4, with trialkylaluminum, AIR3, without solvent. The catalyst is aged at room temperature for 1 hour followed by an aging at high temperature of 150°C for 1 hour, and then it is subjected to an acetylene polymerization at −78°C. Since the procedure is completely free from solvent until the end of polymerization, it can be called an ‘intrinsic non-solvent’ polymerization method. The method enables us to prepare an ultimately concentrated catalyst that is promising to synthesize polyacetylene films with high density and high stretchability. A prefereble Al/Ti ratio is found to be 4 – 5, but not 2 that has been optimized for high-temperature aged catalyst. This result suggests that the co-catalyst, AIR3, might play a role of solvent towards catalytically active complexes formed between Ti(OR)4 and AIR3.

Chemical and Electrochemical Syntheses of Soluble Electroconducting Poly(2,5-Thienylene) Derivatives

Abstract Soluble electroconducting poly[3 -(ω-phenylalkyl)-2,5 -thienylene)]s are synthesized by electrochemical and chemical polymerizations of 3-(2-phenylethyl)thiophene and 3-(3-phenylpropyl)thiophene, which are synthesized by a Grignard cross-coupling reaction of l-bromo-3-phenylalkane with 3-bromothiophene in the presence of dichloro[1,3-bis(diphenylphosphino)propane]-nickel(II). The chemical, electrochemical, and electrical properties of these polymers are reported, including initial characterization, spectroscopy, and electrochemical cyclic voltammetry.

Synthesis of Highly Conducting Polyacetylene Thin Films

Preparative condition of a Ti(O-n-Bu) 4 -AlEt 3 catalyst and polymerization methods were investigated for synthesis of highly conducting polyacetylene films. One of the most important factors is thermal aging of the catalyst. Those films prepared by a catalyst aged over 80°C have good stretchability, high bulk density and high electrical conductivity. On the contrary, those films synthesized by a catalyst aged at room temperature below 80°C are porous and not stretchable. An aging at 150°C for 5 minutes is enough to produce a stretchable highly conducting film. Composition of the catalyst system is also an important factor determining properties of the films

Mechanism of stereospecific chlorination of polyene via prolonged chemical doping

Abstract Stereospecific chlorination of one-dimensional conjugated polyacetylene, (CH) x , through chemical doping with FeCl 3 was investigated. Based on the step-wise chlorination mechanism proposed, molecular orbital calculations were carried out using finite polyene models. The central region in the polyene segment of (CH) x is confirmed to be the more preferable doping site compared with the terminal region. The cation centre of the positively charged polyene segment affords the first chlorination site for the chloride anion, Cl − , dissociated from the dopant species as FeCl 4 − . The π-electron charge polarization on the chlorinated polyene enables the 1,2 addition of Cl 2 to occur repeatedly The steric hindrance, mainly due to the nuclear repulsion between the chlorine atoms, determines the energetically favourable trans-cis structure as a repeating unit of C 4 H 4 Cl 4 , resulting in a stereoregular chlorinated polyacetylene, (CHCl) x .

AC conductivity of hs-PA and S-PA films

Abstract The frequency (f) and temperature dependence of the complex conductivity σ ∗ were measured for highly stretchable polyacetylene (hs-PA) and “Shirakawa” polyacetylene (S-PA). When σ ∗ and f were reduced by using the dc conductivity σ(0), all the experimental data of σ ∗ of the lightly doped hs-PA were on a master curve whereas those of the lightly doped S-PA did not show such universality. This suggests the difference between hs-PA and S-PA in the intermolecular conduction mechanism.

Morphology dependence of DSC curves for cis-trans isomerization of polyacetylene films

Abstract The exothermic peak due to cis-trans isomerization of polyacetylene (PA) films synthesized by a non-solvent method is a broad single peak (around 145 °C) for highly stretchable PA, and a broad peak (around 170 °C) with a shoulder at lower temperature (around 140 °C) for PA films synthesized by the conventional Shirakawas method. This difference is discussed based on the morphology change by mechanical drawing and rolling. The peak position of cis-trans isomerization depends strongly on the order of crystallites.