Polyacetylene Shirakawa method

The most commonly used form of polyacetylene is produced by the Shirakawa method, which involves the direct polymerisation of acetylene gas onto a substrate at... 

The problem of the nature of the conducting state in polyacetylene cannot be considered without a close investigation of the real nature of the samples, including characteristics such as morphology, crystallinity, defect concentration, chain length, and so on. In the early 1980s the studies were concerned with (CH) obtained by the Shirakawa method. The doping level y appeared to be a crucial parameter. As a function of y, basically... 

Suspensions of polyacetylene were prepared as burrs or fibers (46) by using a vanadium catalyst. When the solvent was removed, films of polyacetylene were formed with densities greater than that prepared by the Shirakawa method. These suspensions were mixed with various fillers to yield composite materials. Coatings were prepared by similar techniques. Blends of polypyrrole, polyacetylene, and phthalocyanines with thermoplastics were prepared (47) by using the compounding techniques typically used to disperse colorants and stabilizers in conventional thermoplastics. Materials with useful antistatic properties were obtained with conductivities from 10" to 10" S/cm. The blends were transparent and had colors characteristic of the conducting polymer. For example, plaques containing frans-polyacetylene had the characteristic violet color exhibited by thin films of solid trans-polyacetylene. 

As schematically shown in Figure 6.18, an unpaired 7i-electron is associated with the soliton in trans-polyacetylene. In this case, ENDOR spectroscopy can directly measure the spin density distribution of the soliton by the study of hyperfine coupling [98], according to the discussion in the preceding section. In fact ENDOR observations of the spin density distribution close to those predicted theoretically in the case of finite electron correlation have been reported independently for stretch-oriented cA-rich samples prepared by the conventional Shirakawa method [102-105] and for stretch-oriented trans samples prepared by the Durham route [99,106,107]. 

Polyacetylene is most commonly obtained by the Shirakawa method in which acetylene gas is polymerized by using a Ziegler-Natta catalyst [10,11]. Many studies have been conducted on the chemical doping of polyacetylene [12-16]. It is also one of the most widely studied conductive polymers with respect to its electrochemical properties. 

Comparing this with the research history of polyacetylene, both cases are analogously situated since both polyacetylene and polythiophene were intractable, their chemical and stmctural characterization was quite difficult in the conventional powder form [6,9], However, the polymer films of high quality prepared via either the Shirakawa method [7] or the electrochemical routes [10-12] have enabled easier characterization and the systematic research of polyacetylene and polythiophene. That is, the materials both in a thick free-standing form and in a thin on-substrate form are free from uneven particles which cause irregular scattering of light and yield contact resistance between those particles. This allows us to record sharply resolved spectra and to measure the intrinsic conductivity of the materials. 

For a long time there was no consensus of opinion about the morphology of ICPs. On the basis of scanning electron micrographs, some research groups favored a fibrillar structure for PAc (polyacetylene) produced by the Shirakawa method [15]. This would, it was thought, be an explanation for an anisotropy of electrical conductivity that was observed following orientation of the material [26]. 

As prepared by the method of Shirakawa et al. 2 3), polyacetylene is a free-standing film. On closer examination, its density is found to be around 0.4gem-3, only about 30% of the value (1.16 gem-3) predicted from X-ray analysis, and electron microscopy reveals complex morphologies. 

The random orientation of the crystalline order in typical Shirakawa polyacetylene means that diffraction studies are limited to powder methods. For such studies, and many others, it would be very useful to have much more oriented polymers and many attempts have been made to orient polyacetylene, either by mechanical treatment of the polymer or by appropriate modifications to the polymerization reaction. These have been reviewed earlier. 

Over the years, still different preparative methods have emerged, such as those based on a liquid-crystalline reaction medium, oriented by flow or magnetic field [20,21]. These developments are discussed in detail in various contributions to this Handbook that deal with polyacetylene. For an overview, see Tsukamoto [22] and Shirakawa [23]. 

The intractability of the early preparations of polyacetylene has severely hampered the establishment of clear-cut relationships between structure, morphology and (electrical) properties. An early example of an integrated approach to structure-property relations is a paper by Haberkom et al. [24], From a combination of x-ray data with NMR and IR investigations, these authors have found a relationship between the content of sp defects and crystallinity in polyacetylene prepared by the Shirakawa, Luttinger and other methods. Such defects are apparently expelled to the amorphous phase. The authors find a correlation with conductivity in both undoped and iodine-doped samples. 

When prepared according to Shirakawa s method at low temperature (—78 C), polyacetylene consists primarily of the cis confoimer (98%) while higher temperatures of polymerization result in an increasing percentage of the trans form. cw-Polyacetylene spontaneously iso-merizes to the trans form when kept at higher temperatures for some types this already occurs considerably at room temperature. (A type of polyacetylene refers to a distinct preparation method.) The trans form is the thennodynamically more stable fonn. Structural studies of the cis and trans varieties of polyacetylene have been performed separately and have been connected in later studies of the isomerization process. 

Polyacetylenc is the simplest and most studied conductive polymer which can be produced by polymerization of acetylene or by thermal treatment of a precursor polymer. Although its red sensitivity due to its band gap of 1.5 eV is ideal for photo-voltaic applications, many workers have to shift their interest to other systems, due to its instability. However, with all its drawbacks and with the development of newer and newer methods of stabilization of conductive polymers, it is still fascinating many researchers due to its useful electrical and electronic applications [4,66-68]. Studies of the oxidation of Shirakawa polyacetylene have been reviewed by Chien [10] and Pochan [52]. 

K. Akagi, K. Sakamaki, H. Shirakawa, Intrinsic non-solvent polymerization method for synthesis of highly stretchable and highly conductive polyacetylene films. Macromolecules, 25, 6725-6726 (1992). 

K. Akagi, S. Katayama, H. Shirakawa, K. Araya, A. Mukoh, T. Narahara, Highly conductive polyacetylene film prepared by the liquid crystal polymerization method under magnetic field, Synth. Met., 17, 241-246 (1987). 

Akagi, K., Sakamaki, K., and Shirakawa, H., Intrinsic nonsolvent polymerization method for synthesis of polyacetylene films, Synth. Med., 55, 779-784 (1993). Catellani, M., Destri, S., and Bolognesi, A., New catalytic systems for acetylene polymerization, MakromoL Chem., 187, 1345-1349 (1986). 

Figure 5 shows typical x-ray diffraction curves of two types of polyacetylene films. The usual Shirakawa-type film [1,23] gave a very sharp diffraction curve. The halfwidth of the x-ray diffraction peak at 2 of 23.2 (110 and 2(X) reflections) was small (A20 = 1.2 ). However, it increased to 1.8 after the heat treatment for thermal isomerization from cis to trans form, implying that the ordered crystal structure in the as-grown cis-rich film suffered some destruction in the trans film. On the other hand, the highly stretchable film synthesized by the SE method gave a little diffused diffraction curve, as shown in Fig. 5. As the mechanical stretching proceeds, the half-width of the x-ray diffraction peak (A20 = 2.0°) decreased drastically to 1.5...

The films obtained from these early studies were insoluble in all solvents and infusible, so structural characterization was not possible. The films had conductivities about 0.5 and 38 S/cm when doped with bromine and iodine, respectively (1,2,51), and 560 S/cm when doped with AsFs (52). Both the conducting and the insulating forms of these films are unstable in air. There have been numerous attempts to improve the conductivities of doped polyacetylene films and the synthetic methods leading to higher quality films. There are now several routes by which high quality polyacetylene can be prepared. Shirakawa has recently published an excellent review of synthetic approaches to polyacetylene (53). 

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