Polymerization of aniline

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). 

The anodic polymerization of aniline can occur by a radical cation coupling mechanism analogous to that shown in Scheme 1, with coupling occurring between the N of one molecule and the para-position of another (Structure 4)21,22 However, a variety of other mechanisms have also been proposed,21 and it is likely that their relative rates depend upon the conditions (solvent, potential, pH, etc.) employed. The links between monomers are therefore not exclusively between the N and para-position (head-to-tail coupling). Head-head (-N=N-) and tail-tail (para-para) coupling occur more often as the pH is increased.71... 

The use of conjugated polymer as membranes to separate various liquid mixtures has been reported in the literature [19,20], From those, polyaniline (PANi) is one of the most interesting and studied conjugated polymers. Polyaniline is usually prepared by direct oxidative polymerization of aniline in the presence of a chemical oxidant, or by electrochemical polymerization on different electrode materials [21,22], The possible interconversions between different oxidation states and protonated and depronated states [23], figure 4, make this material remarkable for different purposes. Under most conditions, PANi... 

Nanocarbons can also be deposited onto surfaces via electrochemistry, such as electrophoretic deposition described earlier. A method for one-step electrochemical layer-by-layer deposition of GO and PANI has been reported by Chen et al. [199]. A solution of GO and aniline was prepared and deposited onto a working electrode via cyclic voltammetry. GO was reduced on the surface when a potential of approx. -1 V (vs. SCE) was applied compared to the polymerization of aniline which occurred at approx. 0.7 V (vs. SCE). Repeated continuous scans between -1.4 to 9 V (vs. SCE) resulted in layer by layer deposition [199]. A slightly modified method has been reported by Li et al. who demonstrated a general method for electrochemical RGO hybridization by first reducing GO onto glassy carbon, copper, Ni foam, or graphene paper to form a porous RGO coating [223]. The porous RGO coated electrode could then be transferred to another electrolyte solution for electrochemical deposition, PANI hybridization was shown as an example [223]. 

As can easily be understood, the in situ polymerization process is favored when distances between monomers in the interlamellar space are close to the monomeric repeat distance displayed by the polymer. Such a compatibility of distances was first demonstrated for the polymerization of aniline in V2O5 and FeOCl hosts with an exact matching between NH functions for PANl and the (1-30) crystallographic direction for V2O5 or the (201) direction for FeOCl [63]. 

Oxidoreductases such as HRP, obtained from natural and renewable sources, have been known to catalyze the polymerization of aniline and phenol-based monomers under benign conditions and in mixed solvent systems [39-41]. The mechanism... 

Guo ZW, Ruegger H, Kissner R, Ishikawa T, Willeke M, Walde P (2009) Vesicles as soft templates for the enzymatic polymerization of aniline. Langmuir 25(19) 11390-11405... 

The imprinting was accomplished by electrochemical co-polymerization of aniline with 1,2-phenylenediamine, in the presence of DNOC, which led to deposition of the resulting MIP film onto a carbon fibre microelectrode. After removing the DNOC template, the DNOC analyte was selectively determined, with the linear concentration range of 8 x 10 7 to 10 1 M and LOD of 0.2 mM, in the presence of interfering dinitrophenolic pesticides such as binapacryl, and dinobuton. 

Electropolymerization in acidic media affords free-standing films that are believed to contain varying degrees of cross-linking [267,292,304]. The miscibility of aniline with water allows for a variety of aqueous oxidants, such as ammonium peroxydisulfate, to be used [305]. Chemical polymerization of aniline can also be performed in chloroform through the use of tetrabutyl ammonium periodate [306]. Accordingly, a number of alkyl [301] and alkoxy-substituted [307] aniline derivatives have been chemically polymerized. Unfortunately, functionalization of the aniline nucleus often leads to a decrease in performance in the resulting polymers [308,309]. 

Poly (anil i n e - 2-c h Ion tan i line) -p -toluenesulfonic acid salt was obtained by oxidative polymerization of aniline with o-chloroaniline in solutions containing p-toluenesulfonic acid. The copolymer salt was subjected to heat treatment under nitrogen atmosphere at elevated (about 150°C) temperatures. The heat-treated samples acquired electric conductivity 2.7 X 10-2 (1 em According to ESR spectra, the heated poly(aniline-2-chloroani-line)-p-toluenesul Ionic salt exists as the poly(semiquinone cation radical), in which unpaired electrons are localized on or near the nitrogen atoms (Palaniappan 1997). 

Li et al. [93] have used l-ethylimidazolium trifluoroacetate, which is a Bronsted acidic ionic liquid, as a medium for the electropolymerization of aniline. They report that in this ionic liquid the oxidation potential of aniline is lower (0.58 V compared to 0.83 V in 0.5 M H2SO4) and that the growth rate of the polymer is increased. Further, the resultant films are smooth, strongly adhered to the Pt working electrode and are very electrochemically stable. Similar results have been reported by Liu et al. [92], who found that this was the best ionic liquid for the polymerization of aniline, compared to the unsatisfactory results observed in other protic ionic liquids 1-butylimidazolium tetrafluoroborate, 1-butylimidazolium nitrate and 1-butylimidazolium p-toluenesulfonate, as well as the l-butyl-3-methylimidazolium hydrogen sulfate and l-butyl-3-methyimidazolium dihydrogen phosphate. 

In an attempt to overcome the low infusible character and low solubility of aniline, dispersion polymerization of aniline was conducted in water-dispersible colloidal particles that can be cast as films or blended with other materials to prepare composites. HRP mediated polymerization of aniline in a mixture of phosphate buffer and organic solvent resulted in polyaniline composed of ortho-directed units and para-directed units. Increasing the pH or adopting an organic solvent with a high dielectric constant, enhanced the production of ortho-directed units [54]. These ortho-directed polyanilines were more thermally flexible and electrically conductive. 

Nagarajan R, Samuelson L, Tripathy S, Liu W, Kumar J, Bruno F (2003) Enzymatic polymerization of aniline or phenols around a template. US Patent 6569651... 

In addition to creating semiconductor or metal replicas of the channels of mesoporous materials, which are expected to display electron and hole quantum confinement effects, forming fibers of polymers could lead to materials with novel electrical, magnetic, optical or mechanical properties. To this end, oxidative polymerization of aniline within the channels of mesoporous silica has been reported [91]. Convincing spectroscopic evidence for intrachannel polymerization of aniline to poly(aniline) was provided. Extracted polymer had a molecular weight considerably smaller than that of the bulk material under similar conditions indicative of a diffusion constraint imposed upon the polymerization and growth of monomer inside the channel space of mesoporous silica. 

One proposed mechanism for the electrochemical polymerization of aniline is shown in Fig. 66 [287]. Aniline is oxidized to cation radical 445 which dimerizes to form dication dimer 446. Deprotonation (- 2H +) of 446 gives 443. The oxidation of dimer 443 gives cation radical 447 which is further oxidized to diiminium dication 448. The coupling of 448 with 445 followed by the loss of two protons gives 449. The addition of aniline units to the polymer chain continues in a similar manner by the coupling of the terminal diiminium dication group with monomer cation radical 445. 

Figure 66 Mechanism for polymerization of aniline involving the coupling of cation radicals. (From Ref. 287.)...

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