Kovacic PPP

Several electron diffraction studies have also been published by Pradere and Boudet [176,177,178]. The decision between a monoclinic or an orthorhombic cell cannot be made in their work, but an indexing based on a monoclinic cell is presented [178] a = 8.06 0.10 A, 6 = 5.55 0.04 A, c = 4.30 0.04A and j8=l00°. PPPs prepared by chemical and electrochemical methods are compared. The presence of lamellar crystals in Yamamoto and Fauvarque (electiochemical) PPP is attributed to the low degrees of polymerization (DP) of about 20. Kovacic PPP is the least crystalline of the PPPs investigated. Still, it is the best conductor, probably owing to its fibrillar nature. 

In their neutron diffraction studies of p-type doping of PPP, Stamm and Hocker [179] have determined the setting angle (p of the chains in the pristine polymer to be 57° 3°. In this refinement, the authors started from the data by Kovacic et al. [169], but assumed a monoclinic P2i/a structure with /i=I00°. While a number of later studies by others confirm that the value of 4> is around 57°, Stamm et al. [180] find a much lower value of 45° in their electron diffraction work. The neutron study yields lateral coherence lengths L of 60 A for Kovacic PPP and 150 A for Yamamoto PPP. 

PPP UV-vis spectra are highly dependent on the synthetic procedure used for sample preparation, and reported Amax are often above the theoretical value of 339 nm. Values as high as 395 nm have been obtained for PPP prepared from benzene in carbon disulfide [199]. Kovacic PPP exhibits an absorption maximum at 379 nm [176,199]. The UV-visible threshold was... 

G for as-grown samples and about 3.5 G for undoped samples [145,171]. EPR experiments on the Fauvarque, Yamamoto and Kovacic PPPs using a standard X-band spectrometer show fundamentally no difference between the various PPPs in terms of A//pp and the linewidth is large (around 7 G), meaning that we are dealing with spins which are not moving [160]. 

The stability of the remaining radical species has been studied for the Kovacic PPP. They are extremely stable, since no reaction with nitric oxide and oxygen occurs [208,209]. Samples at least 10 years old exhibit spin concentrations comparable to those of freshly prepared polymer. Treatment with A,A-dimethylaniline [208] reduces the spin concentration to 2 x 10 spins.g Since no nitrogen was found in the polymer after treatment, the reduction of spin concentration is related to electron transfer from the A,A-dimethylani-line to the PPP and not to the introduction of new groups onto the polymer backbone. 

Poly(p-phenylene) is a thermally stable material. It has been shown that it does not decompose below 400° C [213] in air and only 7% of the mass is lost when the material (Kovacic PPP) is heated in N2 to 900°C [214]. Oligophenyls have therefore been used as high-temperature lubricants, hydraulic fluids, heat-transfer agents, and coolants for nuclear reactors [214]. The thermal stability of the polymer is very sensitive to structural defects. In the oligophenyl series, the melting points increase with the number of phenyl units but decrease with ortho and meta linkage, as can be seen in Table 6.7. 

Direct polymerization of benzene through oxidative coupling yields polytp-phenylene) (PPP) an insoluble polymer of low molecular weight.427-429 Kovacic s original synthesis430 using a Lewis acid-oxidant combination [Eq. (13.81)] is the most widely employed and still the most effective procedure for the synthesis of PPP ... 

FIGURE 12.5 Chemical routes for the formation of the two different PPP on the basis of the Kovacic and Yamamoto methods. (Adapted from Endo, M., et al., Mol. Cryst. Liq. Cryst., 310, 353, 1998. With permission.)... 

Poly(p-phenylene) (PPP), prepared from benzene according to Kovacic method [288,420], was shown by Naarmann to turn from yellow to brown and finally to black with increasing reaction temperatures in an autoclave [421]. The conductivity increased accordingly [422]. This was attributed to a further condensation of the PPP chains to yield band graphite structures (cf. Fig. 32(b)). This kind of thermal structurization of PPP was also reported by Fitzer et al. [377, 423]. The temperature... 

The rather limited crystallinity of PPP produced by the Kovacic route, which can be increased to 60% by annealing above 400°C, has been an obstacle to the determination of the unit cell. Well-oriented diffraction patterns have only been obtained in electron diffraction studies. Nonetheless, Kovacic et al. [169] have laid down the basic aspects of the structure and its problems in their very original work, which included an x-ray study. Their indexing scheme for the reflections has been verified in several later studies. The authors propose a pseudo-orthogonal cell with two chains passing through, and dimensions o = 7.81 A, h =... 

In 1980, Teraoka and Takahashi [175] present their x-ray and electron diffraction study in which they compare two Kovacic-type PPPs prepared by different catalyst systems. The ED pattern indeed suggests off-meridional positions for the (002) reflection in the better oriented one of the two samples. A monoclinic angle of 79° is proposed. 

Bolognesi et al. [181] have measured the effects of the pressure applied for the compaction of PPP powders into pellets on the dimensions of the unit cell. After compaction at 12 kbar the interchain distances are found to be reduced by 1%, for both Kovacic and Yamamoto PPP. 

Electron diffraction studies of Li-doped PPP have been carried out by Stamm et al. [180]. They use oriented films of Kovacic-type PPP prepared in a shear flow field, and Li vapour. The doped PPP shows some amorphous components, but the position of the reflections is essentially unchanged. It is therefore proposed that the lithium ions simply fill the vacancies between the chains in the crystal, along both the a and b cell edges (but with an occupancy of 0.5). The cell becomes orthorhombic upon doping. 

Kovacic s method was improved by Arnautov and Kobryanskii (2000) using an oxidative poly condensation route. The PPP obtained had a higher molecular weight (Sun et al. 2005). 

Unsubstituted PPP (1) is insoluble, and so direct synthesis by oxidation of benzene by the Kovacic method [12] or by nickel(O) coupling of 1,4-dihalobenzenes [9] (Scheme 2) gives insoluble and intractable powders with degrees of polymerisation of about 20 as determined by IR absorption meas-... 

Oxidative coupling of benzene proceeds using a Lewis-acid catalyst and an oxidant (the Kovacic method).For example, benzene is converted into PPP by treatment with CUCI2 and AICI3, as shown in Scheme 4.2. 

Example 1 PPP Synthesized with CUCI2/AICI3 (Kovacic Method cf. Scheme 4.2) ... 

Of all the chemical processes which were reviewed and described in references [1-3], it should be noted that up to now only the Kovacic and Yamamoto reactions and, to a lesser extent, the thermal dehydrogenation of poly(l,3-cyclohexadiene) have been used to synthesize polyphenylenes for usual or special applications. Only recently, a new impulse has been given to the synthesis of linear functionalized high molecular-weight PPPs with the adaptation of the Suzuki reaction, and this is currently one of the best possible ways of synthesizing soluble, functionalized PPPs for specific applications with, in particular, the realization of rigid rod-like polymers for mechanical applications, as will be shown in Section 2.1.3. 

However, a slightly different mechanism was postulated by Kovacic et al. [3,15-16] for the polymerization of benzene by catalyst-oxidant systems. According to Kovacic et al, benzene can be oxidized to a radical-cation by a variety of acid catalysts with or without oxidants, but will react on the substrate so that polymer chain growth occurs according to a stair-step mechanism, in which the radical-cation is delocalized over the whole chain (11) before forming a new chain of non-conjugated cyclohexadiene moieties (12) which after oxidation yields the PPP (Scheme 6.4)... 

Interestingly, films could be cast on quartz plates or KBr crystals from PS,-PPP solutions, and after heating at 420° C for a few minutes in nitrogen atmosphere, PS, was destroyed and PPP remained on the plate. The IR spectra indicated that the PPPs prepared by this procedure were similar to Kovacic s PPP, i.e. with DPs of not more than 10-15 [35]. 

The PPP obtained following this procedure is much less coloured than that prepared by the Kovacic method [3]. The IR spectra present the main absorption bands of PPP. Molecular weights, based upon IR analysis, were initially estimated to be comparable to that of PPP prepared by Kovacic. The X-ray diffraction spectrum of the material showed strong, sharp peaks even before the polymer was annealed, indicating high crystallinity. 

The Yamamoto procedure has been used to obtain substituted PPPs. In contrast to the Kovacic method, for which side-groups on the benzene unit induce electronic and steric perturbations that lead to a higher degree of defects in the PPP backbone, the metal-catalyzed procedure is very regiospecific and proves and interesting route to well defined all-para-linked substituted PPPs. Furthermore, it can be extended to phenyl nuclei bearing various organic substituents. 

The PPP prepared by this route has a higher molecular weight than the PPPs prepared by the usual Yamamoto and Kovacic routes, as evidenced by IR spectroscopy, and low bromine contents (2.7%). The... 

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