Poly p-Phenylene

We have modified an NIR Fourier-transform spectrophotometer (JEOL JIR-5500) for the measurements of Raman spectra excited with the 1064-nm line of a continuous-wave Nd YAG laser (CVI YAG-MAX C-92) [60]. Raman-scattered radiation is collected with a 90° off-axis parabolic mirror in a back-scattering geometry. Collected radiation is passed through two or three long-wavelength-pass filters to reduce the Rayleigh scattering. 

Since PP/r is stable for a long time, we can conclude that the following disproportionation reaction does not occur. 

Tliis reaction is similar to the process of the intramolecular formation of a bi-polaron from two polarons. 

The observed infrared and Raman spectra of intact poly (p-phenylene) [80-84] have been analyzed by normal coordinate calculations [43, 84-87]. The factor group of a coplanar polymer is isomorphous with the point group D h- When the yz plane is taken in the phenylene-ring plane (z axis along the polymer chain) and the x axis perpendicular to the phenylene-ring plane, the irreducible representation at the zone center (k = 0) is as follows  

Syni- No. metric species Normal species C-substituted analog Perdeuterated analog  

A much more successful approach to synthesis of poly(p-phenylene) from dihalo-benzenes is via the decomposition of the corresponding Grignard reagent  

The coupling can be catalysed by the presence of Fe(III) or Co(ll) but is most efficiently induced by the dipyridyl complex of Ni(II) chloride, as described by Yamamoto et al.52). It is also efficiently promoted by l,4-dichoro-2-butene, as described by Taylor et al.53). The structures of these Yamamoto polyphenylenes are discussed in Sect. 3 at this point it is sufficient to note that they are yellow/brown infusible powders, apparently having a high degree of p-coupling but relatively short (10—12 rings) chain lengths. 

The alternative route to synthesis of poly(p-phenylene), which has been widely used, is the Scholl reaction5A), which involves direct oxidative elimination of two aryl hydrogen atoms with concomitant formation of a new carbon-carbon bond. This reaction occurs under Friedel-Crafts conditions and requires the presence of an appropriate oxidant to remove the hydrogen liberated in the coupling process. This route is typified by the Kovadk polymerization of benzene55) induced by aluminium chloride in the presence of stoichiometric amounts of Cu(II) chloride  

According to this view, the chain length of the polymer is limited by the delocalization of the reactive intermediates. The evidence for this rather unusual mechanism has been reviewed 48). 

One of the earliest attempts to synthesize heat-resistant polymers was the oxidative polymerization of benzene to poly(p-phenylene) [lUPAC poly(l,4-phenylene)] (Eq. 2-227) [Jones 

Poly(p-phenylene vinylene) [lUPAC poly(l,4-phenyleneethene-l,2-diyl)] (PPV) (LVIII) is obtained by the sequence in Eq. 2-230 involving successive base- and heat-induced 

The corresponding azomethine polymers (LIX) have been synthesized by the reaction of the appropriate diamine and dialdehyde [Earcas and Grigoras, 2001 Gutch et al., 2001 Morgan et al., 1987]. 

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Steiner U B, Cased W R, Suter U W, Rehahn M and Rau I U 1994 Self-assembled layers of substituted poly(p-phenylene)s on gold and copper Langmuir 10 1164-70... 

Polyheterocycles. Heterocychc monomers such as pyrrole and thiophene form hiUy conjugated polymers (4) with the potential for doped conductivity when polymerization occurs in the 2, 5 positions as shown in equation 6. The heterocycle monomers can be polymerized by an oxidative coupling mechanism, which can be initiated by either chemical or electrochemical means. Similar methods have been used to synthesize poly(p-phenylenes). 

Poly(arylene vinylenes). The use of the soluble precursor route has been successful in the case of poly(arylene vinylenes), both those containing ben2enoid and heteroaromatic species as the aryl groups. The simplest member of this family is poly(p-phenylene vinylene) [26009-24-5] (PPV). High molecular weight PPV is prepared via a soluble precursor route (99—105). The method involves the synthesis of the bis-sulfonium salt from /)-dichloromethylbenzene, followed by a sodium hydroxide elimination polymerization reaction at 0°C to produce an aqueous solution of a polyelectrolyte precursor polymer (11). This polyelectrolyte is then processed into films, foams, and fibers, and converted to PPV thermally (eq. 8). 

Eig. 2. Lattice distortions associated with the neutral, polaron, and bipolaron states in poly(p-phenylene). 

The polymers which have stimulated the greatest interest are the polyacetylenes, poly-p-phenylene, poly(p-phenylene sulphide), polypyrrole and poly-1,6-heptadiyne. The mechanisms by which they function are not fully understood, and the materials available to date are still inferior, in terms of conductivity, to most metal conductors. If, however, the differences in density are taken into account, the polymers become comparable with some of the moderately conductive metals. Unfortunately, most of these polymers also have other disadvantages such as improcessability, poor mechanical strength, instability of the doped materials, sensitivity to oxygen, poor storage stability leading to a loss in conductivity, and poor stability in the presence of electrolytes. Whilst many industrial companies have been active in their development (including Allied, BSASF, IBM and Rohm and Haas,) they have to date remained as developmental products. For a further discussion see Chapter 31. 

Poly-p-phenylene has been prepared in the laboratory by a variety of methods, including the condensation of p-dichlorobenzene using the Wurtz-Fittig reaction. Although the polymer has a good heat resistance, with decomposition... 

Several substituted linear polyphenylenes have also been prepared but none appear to have the resistance to thermal decomposition shown by the simple poly-p-phenylene. 

The simplest aromatic polysulphone, poly-(p-phenylene sulphone) (formula I of Table 21.3) does not show thermoplastic behaviour, melting with decomposition above 500°C. Hence in order to obtain a material capable of being processed on conventional equipment the polymer chain is made more flexible by incorporating ether links into the backbone. 

The polymers which have stimulated the greatest interest are the polymers of acetylene, thiophene, pyrrole and aniline, poly-p-phenylene, polyphenylvinylene and poly-l,6-heptadiyne. Of these materials polypyrrole has been available from BASF under the trade name Lutamer P160 since 1988. 

Fibers in which the basic chemical units have been formed by chemical synthesis, followed by fiber formation, are called synthetic fibers. Examples include nylon, carbon, boron fibers, organic fibers, ceramic fibers, and metallic fibers. Among all commercially available fibers, Kevlar fibers exhibit high strength and modulus. (Kevlar is a DuPont trademark for poly [p-phenylene diamine terephthalamide].) It is an aromatic polyamide (aramid) in which at least 85% of the... 

Poly(p-pheny lene)s, PPPs, constitute the prototype of rigid-rod polymers and are currently being intensively investigated [1]. The key role of PPPs follows from their conceptually simple and appealing molecular structure, from their chemical stability, and from their superior physical properties [2], In turn, this is the result of important advances made in aromatic chemistry over the last few years. The following section gives an overview of the most common methods to generate poly(p-phenylene)s via different synthetic approaches. 

An analogous case, of identical chain conformations as well as of similar unit cell dimensions, have been described for the two crystalline forms of poly-p-phenylene terephtalamide [33-36] (better known with the trade name of Kevlar). The projections along the c axis of the packing of the chains proposed for the two forms [36] has been sketched in Fig. 8, corresponding to the localization of the chain axes in (0,0, z) and (1/2,1/2, z) for the more common polymorph, in (0, 0, z) and (1/2,0, z) for the other polymorph. 

Example 20. Poly(p-phenylene terephthalamide) by silylated diamines.33... 

Synthesis of Poly(p-phenylene) 24 by Using Kumada Coupling58... 

Water-soluble poly(p-phenylene), 493 Water reducible materials, 237 WAXS. See Wide-angle x-ray spectroscopy (WAXS)... 

Polypyrrole shows catalytic activity for the oxidation of ascorbic acid,221,222 catechols,221 and the quinone-hydroquinone couple 223 Polyaniline is active for the quinone-hydroquinone and Fe3+/Fe2+ couples,224,225 oxidation of hydrazine226 and formic acid,227 and reduction of nitric acid228 Poly(p-phenylene) is active for the oxidation of reduced nicotinamide adenine dinucleotide (NADH), catechol, ascorbic acid, acetaminophen, and p-aminophenol.229 Poly(3-methylthiophene) catalyzes the electrochemistry of a large number of neurotransmitters.230... 

Fig. 10. Formation of the bipolaron (= diion) state in poly-p-phenylene upon reduction In the model it is assumed that the ionized states are stabilized by a local geometric distortion from a benzoid-like to a chinoid-Iike structure. Hereby one bipolaron should thermodynamically become more stable than two polarons despite the coulomb repulsion between two similar charges...

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