Shirakawa method

The discovery by Shirakawa and co-workers paved the way for other synthetic PA work and changed the way in which researchers thought about the synthesis and handling of PA. To date, the majority of workers in this area still use Shirakawa s pioneering work as the source for a main method for FA synthesis. Given the ability to make free-standing PA films with high conductivity, other aspects of PA chemistry and physics have been explored and optimized. Ikble III offers a condensed synopsis of various advances in PA synthesis. 

The catalyst aging temperature affects the density and flexibility of the polymer film [9]. Aging the Ti/Al catalyst mixture at room temperature produces a low-density film ( 0.4 g/ml). Conversely, aging at a higher temperature gives a high-density PA (0.9-1.1 g/ml) that is soft and stretchable. 

The PA made by Naarmann s method was very regular, compact, crystalline, and stretchable. The Naarmann-type PA films were very dense and were made up of thin fibrils that had up 

The stretched Akagi-type PA films were doped with iodine, and their conductivities were measured as a function of catalyst aging 

The intrinsic nonsolvent (INS) polymerization protocol [34], where solvent is never employed, is a technique that is closely related to the solvent evacuation method. In a typical experiment, neat AlEt3 was dropped into neat Ti(OBu)4 in a Schlenk flask at 0°C. The catalyst was aged for 1 h at room temperature and then further aged for another hour at 150° C. The flask was degassed and rotated to coat its walls with the catalyst solution. The system was then cooled to —78°C and acetylene was added. The reaction was allowed to proceed for 30-60 min before quenching. The resulting PA film was washed with toluene at — 78°C and dried under vacuum. 

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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 most commonly used method to synthesize the PA is the Shirakawa method. In this method a smooth surface wetted by the Ziegler-Natta catalyst is exposed to the acetylene gas. A film of PA (generally c-PA) is produced on the smooth surface. The c-PA is converted to the f-PA by heating. The process of doping also converts the c-PA to the r-PA. 

FIG. 2.9. Conductivity versus dopant concentration for two iodine doped i-PA samples. (S) a sample fabricated by the Shirakawa method and (v) samples fabricated by the Tsukamoto method [27]. 

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. 

A Akagi modification of Shirakawa method for high strelchability [15]... 

In the early studies of iodine doping, isotropic samples prepared by the Shirakawa method or similar routes were often used [81,120,122,124,125,126,127]. Although the features were there, the lack of orientation severely impeded the interpretation of diffraction patterns. Modelling was partly based on analogies with other intercalation complexes. 

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. 

In order to determine the quality and porosity of films produced by the Shirakawa method, physical measurements are intrinsically important. Specimens for stress-strain measurement and for the preparation of highly stretch-aligned films can be prepared at -78°C using a Ti(0-n-Bu)4-AlEt3(Ti = 0.3 mole/l,Al/Ti = 3.3) system with a toluene-anisole (1 3) mixture as a solvent. The films of about 100 fim thickness have a cis content of 93-5% as determined by ATRIR, a method explained in detail in Section 1.3.1. Trans and various cisitrans films are obtained by thermal isomerization of the cis films [29]. 

Shirakawa method follows the basic lines laid down. For the most recent enhancements which include the non-solvent polymerization method refer to Akagi et al. [31-33] and Shirakawa et al. [34]. 

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. 

Four types of cis and tram structures are possible for PA (Fig. 1). Generally the trans-tramoid structure is simply termed tram, and the cis-tramoid structure is called cis. With the Shirakawa method either cis- or tram-YA can be synthesized. At a polymerization temperature of —78°C, ds-PA is formed exclusively. With increased temperature, a higher percentage of tram-PA forms (Ikble I). tram-PA forms exclusively at 150° C. At low temperatures a dr-structure arises predominantly from a cis insertion of the acetylene into the active site of the catalyst. Fukui and Inagaki suggest that a... 

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]. 

The photomodulation (PM) spectrum of trans- CH) at 4 K is shown in Fig. 22.8 the trans- CH).x sample used here was a thin film d - 1000 A) polymerized on a sapphire substrate by the Shirakawa method. The PM spectrum consists of two main PA bands with Ja > 0 and bleaching (J a < 0) for h(o > 1.65 eV. The low energy (LE) PA band peaks at 0.43 eV, and it originates from photoinduced charged defects this has been concluded from its correlation with the photoinduced IRA Vs (Fig. 22.6). The intensity of the LE band easily saturates with increased excitation power and is sample-dependent. Moreover, as was demonstrated in oriented (CH), films, the LE band is induced preferentially with light polarized perpendicular to the chain s direction (27J. Therefore it seems that the LE band is stabilized by extrinsic defects of the polymer chain. The LE band is now considered to be due to charged soliton 5S transitions, which are pushed away from midgap (predicted by the SSH theory) due to electron correlation (18], as discussed in Section II.A. 

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