Self-doped polymers electrochemical polymerization

Self-doped PANI are very interesting due to their unique electrochemical behavior unlike PANI, the self-doped polymer remains in its doped state in near neutral or alkaline media [28]. Fully self-doped PANIs are not easy to synthesize due to the lower reactivity of acid-functionalized anilines. Kim et al. [29, 30] introduced an alternative approach in the template-assisted enzymatic polymerization of aniline. Previously, only polyanionic templates had been used for PANI synthesis. However, acid-functionalized anilines bear a net anionic charge in aqueous solution, and attempts to use SPS as template with carboxyl-functionalized aniline resulted in red-brown colored polymers with no polaron transitions, regardless of the synthetic conditions. The use of polycationic templates, such as those shown in Figure 8.2 allowed the synthesis of linear and electrically conductive PANIs with self-doping ability due to the doping effect of the carboxyl groups present in the polymer backbone. 

In an attempt to produce carbazole polymers soluble in aqueous solutions, oligoether groups have been attached to the carbazole unit at the iV-position (28d) and the polymer prepared by chemical polymerization and electrochemical polymerization [105]. Due to the oligoether substituents, electrochemical polymerization can occur in aqueous solutions without the need for a cosolvent. Polymer films switch between a highly transmissive state to deep green upon oxidation. The self-doped polymer, poly[3,6-carbaz-9-yl)propanesulfonate] (28e), has also been produced, which is water-soluble and switches from a transmissive neutral state to a dark green oxidized state [106]. 

Similarly, self-doped PABA can be prepared using excess of saccharide and one equivalent of fluoride to monomer. Complexation between saccharides and aromatic boronic acids is highly pH dependent, presumably due to the tetrahedral intermediate involved in complexation [25]. Because the pKa of 3-aminophenylboronic acid is 8.75, complexation requires pH values above 8.6. This pH range is not compatible with the electrochemical synthesis of polyaniline, which is typically carried out near a pH value of 0. However, Smith et al. have shown that the addition of fluoride can stabilize the complexation of molecules containing vicinal diols with aromatic boronic acids [23]. Based on this work, it was postulated that the electrochemical polymerization of a saccharide complex with 3-aminophenylboronic acid in the presence of one molar equivalent of fluoride at pH values lower than 8 is possible if a self-doped polymer is produced in the process. 

A modification of the electrochemical synthesis of P3TASs was reported by Havinga et al. [6,7]. These authors report the synthesis and electrochemical polymerization of 3 -(3-propylsulfonate) 2,2 5 2"-terthienyl (Figure 20.3). Notably, electrochemical oxidation could be achieved in the absence of interfering foreign electrolytic ions since the monomer itself acted as the supporting electrolyte. The self-doped polymer was water soluble but the resulting polymer solutions were reported to be unstable. 

Polyaniline (PANI) can be formed by electrochemical oxidation of aniline in aqueous acid, or by polymerization of aniline using an aqueous solution of ammonium thiosulfate and hydrochloric acid. This polymer is finding increasing use as a "transparent electrode" in semiconducting devices. To improve processibiHty, a large number of substituted polyanilines have been prepared. The sulfonated form of PANI is water soluble, and can be prepared by treatment of PANI with fuming sulfuric acid (31). A variety of other soluble substituted AJ-alkylsulfonic acid self-doped derivatives have been synthesized that possess moderate conductivity and allow facile preparation of spincoated thin films (32). 

It is also possible to polymerize the 3-aminobenzenesulfonic acid to produce polymetanilic acid which is also soluble. Thus, preparation of a solid polymer of this type is not possible in aqueous acidic solutions, but it may be possible in a neutral solution of aqueous-organic mixed medium. However, to exhibit the electrical and electrochemical properties, protonation of the imine nitrogen of the PANI backbone in poly(metanilic acid) is necessary, which requires an acidic solution. An acid group-substituted, self-doped PANI has better electrical and electrochemical properties over a wider pH range. 

Self-doped polypyrrole was first prepared by Reynolds et al. [42] and Havinga et al. [43] in 1987. Their approach to self-doping of polymers was based on monomers that were easier to polymerize electrochemically. Reynolds et al. prepared the N-substituted pyrrole copolymer, poly(pyrrole-co-(3-(pyrrol-l-yl)propanesulfonate)) (Figure 5.1) in acetonitrile containing tetrabutyl ammonium tetrafluoroborate as a supporting electrolyte on a platinum electrode. The monomer, potassium... 

There are many other studies of ITO surface modification [277] for the purpose of enhanced hole injection. ITO work function can be changed by treatment with UV-ozone [269, 312, 330, 331], plasma [332-334], acids or bases [335-337], and charge transfer [338] as well as by binding dipolar molecules [339, 340] using organotin (or zirconium) alkox-ides [341-343]. The work function of ITO may be changed by electrostatically controlled self-assembled polymer layers [344-349]. The hole injection was also enhanced by electrochemically prepared thin films [350], p-type doped thin films [351 - 354] and plasma polymerized fluorocarbon films [355]. 

Self-doped conducting polymers are conjugated polymers, in which at least a fraction of monomer units contain covalently attached, ionizable functional groups that can act as immobile dopants. These polymers can be synthesized in a number of ways including chemical or electrochemical modification of the parent polymer as well as chemical or electrochemical polymerization of substituted monomers to form homo- or copolymers. ... 

Electrochemical polymerization of 3-alkylsuIfonate pyrroles with various length of alkyl chain (61) has given water-soluble self-doped polypyrroles [297]. A self-doped polypyrrole based on the carboxyl group was prepared from anodic coupling of 3-carboxy-methylpyrrole (62) [298]. The polymer was subsequently tested as a proton pump electrode [299]. 

Electrochemical homopolymerization of poly(3-thienyl) acetic acid) and copolymers with thiophene were reported to result in polymers consisting of 2000 monomer units. The films were electrochromic in acetonitrile and exhibited optical transitions between golden yellow (reduced) and blue (oxidized) [26]. Electrochemical polymerization was reported to proceed via a two-dimensional layer-by-layer growth mechanism. The rate of propagation of the film was reported to be 0.008-0.14 S cm [27]. No information pertaining to self-doping was reported. 

Polymerization of the monomers was achieved by either electrochemical oxidation or oxidative coupling with ferric chloride as shown in Figure 20.35. Electrochemical polymerization was performed in acetonitrile solutions. No additional electrolyte was added to the solution other than the monomer itself. Polymerization was continued until a colorless electrolyte was obtained. The polymer formed a blue/black film. Oxidation potentials of the monomers and chemical analyses of the resultant polymers were reported. Conductivities as high as 0.5 S cm were obtained. Films prepared electrochemically were formed in their self-doped state in which the delocalized positive charge on the polymer was balanced by the covalently bound sulfonate ions. Figure 20.36 shows UV-vis-NIR spectra of selected self-doped polypyrroles indicating their oxidized polypyrrole backbone. 

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