Poly radical cation structure

Another view has recently been proposed by Wegner.Naphthalene and other simple aromarics can be oxidize electrochemi-cally to form monomelic radial cat n salts (Ar. X ) which have conductivities of 10 to 10 s/cm. The crystal structures of these reveal that the aromatic moieties form stacks, along which the charges and the electrons are presumably delocalized. The structure is formally analogous to that deduced for oxidized (doped) polyacetylene in which the polyene chains are arranged in stacks. This leads to the idea that intermolecular delocalization is the important feature which leads to high conductivity. Other data are consistent with this rationale. Biphenyl and terphenyl radical cation salts have crystal structures very similar to that of oxidized (doped) poly(p-phenylene lO). In the older literature oligoanilines (26) are reported upon iodine treatment to yield conductivities up to 1 s/cm the aniline moieties are stacked in these materials as well. Poly(N-vinyl-carbazole) (27) forms radical cation structures by oxidation with... 

Figure 2.30 Representation of the asymmetric unit of [Nd(L )4(H20)][(TTF—CH=CH—Py+)] 2-The radical cation donors are drawn as balls and sticks the paramagnetic anionic coordination complexes of Nd(III) are drawn as capped sticks [23d], (Reprinted with permission from F. Pointillart, O. Maury, Y. Fe Gal, S. Golhen, O. Cador and F. Ouahab, 4-(2-Tetrathiafulvalenyl-ethenyl)pyridine (TTF—CH=CH—py) radical cation salts containing poly(P-diketonate) rare earth complexes synthesis, crystal structure, photoluminescent and magnetic properties, Inorganic Chemistry, 48, 7421-7429, 2009. 2009 American Chemical Society.)...

Electrochemical (cyclovoltametric) investigations of the ladder-type poly-(para-phenylene) species 71 support the results of the chemical oxidation (doping) experiments both in solution and in the solid state (film). A reversible oxidation takes place and it is well-separated into two waves especially in the solid-state experiment. These are assigned to the formation of radical cationic (79) and dicationic species (80), respectively. The halfwave potential (E1/2) for the first oxidation wave lies between 0.75 V (solution experiment) and 0.95 V (solid state - film) - versus a standard calomel electrode SCE) [106]. Consequently, one has to search for an alternative synthetic process to generate the ladder-type poly(phenylenemethide)s 77 or polymers containing extended segments of the fully unsaturated structure desired. The oxidation of polymeric carbanions appeared suitable, but it proved necessary to work under conditions which completely exclude water and air. 

Radical-Cation Salts as Models for Conducting Polymers. Polymers that have an extended Tr-electron system in their backbones, for example, polyacetylene (PA) and poly(p-phenylene) (PPP), can be transformed by oxidation or reduction in the solid state (doping) to derivatives that exhibit metallike conductivity (24, 25). These materials are usually insoluble and infusible and exhibit a very complicated morphology that cannot be changed by subsequent treatment. The lack of knowledge about the structure and state of order is the cause of the current controversy about the conduction mechanism in doped polymers. 

The first method reported for the synthesis of poly(paraphenylene) is the coupling of benzene by Lewis acid catalysis in the presence of an oxidant. In 1963, poly(paraphenylene) was synthesized by stirring benzene, aluminum chloride, and copper(II) chloride yielding an insoluble light brown powder [98]. The most favored mechanism is shown in Scheme 29.13. Benzene is oxidized to its radical cation that then propagates cationically. A second oxidation step produces the bis-cation, which looses two protons rearomatizing the terminal rings. Further oxidation of the dihydro structures finally affords the polymer [99]. 

Larger proportions of head-to-head or tail-to-tail structures must always be expected when steric effects are small and the resonance stabilization of the growing end (free radicals, cations, anions) is low. The atomic radius of the fluorine atom is less than that of the hydrogen atom, which helps to explain the high proportion of head-to-head structures in poly-(vinyl fluoride). Similar effects lead to the 40% head-to-head structures that occur in the polymerization of propylene oxide with diethyl zinc and water as initiator. 

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