Polyacetylenes Shirakawa-type

Figure 1 Typical diffraction patterns from undoped CPs (a) frms-polyacetylene, Shirakawa-type, from Ref. 34 (b) polyaniline, emeraldine salt form II, from Ref. 24 (c) polythiophene, from Ref. 22.

In this chapter the second moment studies are mainly reviewed for the two typical conjugated polymers Shirakawa-type (S-PA) [20-22,27,30-32] and Naar-mann and Theophilou-type (NT-PA) polyacetylenes [24] and polyparaphenylene [23,28,29]. The characteristic of NT-PA is a high degree of chain orientation attained by mechanical stretching, [33] which provides additional information on tlie polymer chain arrangement how much misorientatioii of the chains is left behind and how much of the amorphous portion exists [34]. From an analysis of the second moment M2... 

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...

Figure 1.35. Schematic view of fibrillar stmcture of Shirakawa-type polyacetylene. (Reprinted with permission from ref. 191)...

Figure 3.4 illustrates two types of polyacetylene. The standard Shirakawa type (Figure3.4(a))is cross-linked and contains an sp fraction of approximately 2%...

T. Ito, H. Shirakawa, and S. Ikeda. Simultaneous polymerization and formation of polyacetylene film on the surface of concentrated soluble ziegler-type catalyst solution. J. Polym. Sc. Polym. Chem. Edition, 12(1) 11, 1974. 

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. 

Druy et al. [29] reported that the kinetic studies on d.c. electrical conductivity decay of p-type doped Shirakawa polyacetylene in an inert atmosphere and indicated that two types, namley dopant dependent and dopant independent reactions, are involved in the degradation process of polyacetylene. It was observed... 

The unpaired electron spin in trans type polyacetylene (PA) plays an important role as a soliton for the conduction. The PA developed by Shirakawa [40] is a semiconductor, but changes to a conducting polymer by adding dopants. The distribution of the spin along the chain is symmetrical like a wave centering around the midpoint of the soliton. The neutral soliton of the excited state is not a conduction carrier. However, when dopants like As P5,12 and similar agents are introduced and abstract electrons from the solitons, the formed carbanium ion solitons are converted to conduction carriers. When dopants like Li, Na and similar substances add electrons to the solitons, carboanion solitons also change to conduction carriers. 

As shown in Figure 21.2, four steric (geometric) structures are theoretically possible for polyacetylenes, that is, cis-cisoid, cis-transoid, trans-cisoid, and trans-transoid, because the rotation of the single bond between two main chain double bonds in the main chain is more or less restricted. Polyacetylene can be obtained in the membrane form by use of a mixed catalyst composed of Ti(0-n-Bu)4 and EtsAl, the so-called Shirakawa catalyst (1) both the cis- and trans-isomers are known, which are thought to have cis-transoidal and trans-transoidal structures, respectively (Table 21.1). Phenylacetylene can be polymerized with a Ziegler-type catalyst, Fe(acac)3/Et3Al (2) (acac = acet-ylacetonate), Rh catalysts (7), and metathesis catalysts (3-5) that contain Mo and W as the central metals, to provide cis-cisoidal, cis-transoidal, cis-rich, or trans-rich polymers, respectively. 

Some years later the Penn Group [17] reported doping Shirakawa polyacetylene with the electron acceptors I2 or Br2, resulting in charge-transfer complexes with conductivities of 0.5-30 S cm . A comparison of the various types of polyacetylene [11] revealed some astonishing correlations conductivity was directly proportional to crystallinity and inversely proportional to the number of sp orbitals. This discovery was the key to the production of new polyacetylene types with fewer defects and greater stability. 

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