Electrochemical Polymerization of Anilines

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. 

Instead of chemical oxidative polymerization, electropolymerization can also be considered. A recent study shows that slow but efficient electropolymerization is possible if anilinium-exchanged zeolite Y is subjected to oxidative treatment at the electrode-electrolyte interface. Cyclic voltammetric signatures of the polymerization suggest that it occurs mostly through one dimer (p-aminodiphenylamine) which imdergoes oxidative polymerization. Electrochemical polymerization of aniline in zeolite molecular sieves was studied. A zeolite-modified electrode showed shape-selectivity for 12-molybdophosphoric acid. 

A significant development in PAn chemistry was our report26,27 of the first optically active PAn s, prepared through the electrochemical polymerization of aniline in the presence of the chiral dopant acids (+)- or (-)-10-camphorsulfonic acid (HCSA). Films of the emeraldine salts PAn/(+)- HCSA and PAn/(-)-HCSA can be... 

Another route for the preparation of composites based on PANI and SWNTs is electrochemical synthesis [26,30,144]. A common method for the electrochemical polymerization of aniline is cychc voltammetry. The aqueous acidic medium frequently used is HCl or H2SO4. It is well known that cyclic voltammograms of polyaniUne recorded in aqueous acidic media usually show two redox couples associated with the inteconversion between leucoemeraldine and emeraldine (first couple) and between emeraldine and pernigraniline (the second couple) [ 151 ]. At the potential of the second oxidation peak, PANI is unstable in... 

Similarly, perturbations such as those described above have also been reported in the case of PANI covalently functionalized SWNT samples prepared by electrochemical polymerization of aniline on a CNT film deposited on a Pt electrode. The covalent functionalization of SWNTs with PANI was explained, on the basis of the Raman and FTIR spectra shown in Figures 5.10 and 5.11, respectively, as taking place in two successive steps [26]. The first corresponds to the electrochemical polymerization of aniline on a SWNT film from which one obtains a composite of PANI-leucoemeraldine salt (LS)-functionalized... 

Figure 5.10 Raman spectra (Kexc= 1064 nm) of PANI/SWNT composites obtained by electrochemical polymerization of aniline on a SWNT film in HCI 0.5 M. Curve 1 corresponds to the SWNT film Raman spectrum. Curves 2-4 and 6-8 show the evolution of the Raman spectrum after 25, 50, 75, 100, 150, and 300 cycles, respectively, carried out in the potential range (-200 +700) mV vs. SCE with a sweep rate of 100 mV s The dedoping of the PANI-salt-functionalized SWNT films (curves 4 and 8), as a result of the chemical reaction with NH4OH 1M solution, is illustrated in curves 5 and 9. (Reprinted with permission from Carbon, Covalent functionalization of single-walled carbon nanotubes by aniline electrochemical polymerization by M. Baibarac, I. Baltog, S. Lefrant et ah, 42, 15, 3143-3152. Copyright (2004) Elsevier Ltd)...

L. M. Santos, J. Ghilane, C. Fave, P.-C. Lacaze, H. Randriamahazaka, L. M. Abrantes, and J.-C. Lacroix, Electrografting polyaniline on carbon through the electroreduction of diazonium salts and the electrochemical polymerization of aniline, J. Phys. Chem. C, 112, 16103-16109 (2008). 

Kan s researeh group also investigated the effect of alcohols on the electrochemical polymerization of aniline at a constant potential of 0.8 V (vs. SCE) [61]. They found that the presence of alcohols favors formation of nanofibers. This is attributed to hydrogen bonding between alcohols and polyaniline chains, which results in the polymer chains being pushed apart [62]. 

A. L. Briseno, S. B. Han, I. E. Rauda, F. M. Zhou, C. S. Toh, E. J. Nemanick, N. S. Lewis, Electrochemical polymerization of aniline monomers infiltrated into well ordered truncated eggshell structures of polyelectrolyte multilayers, Langmuir 2004, 20, 219. 

Ext ive investigations on polyaniline (PAn) and its derivatives have be carried out (i) since they possess a moderate conductivity upon doping with protonic acid and an excellent stability under ambient conations (2,3). PAn is simply prepared by the chemical and electrochemical oxidation of aniline or its derivatives in aqueous solution. In general, however, the chemical and electrochemical polymerization of aniline monomer lead merely to an insoluble powder and a thin brittle film, respectively. Hence, it is very difficult to process PAn for a practical use. In order to deal well with this problem, the improvement of processability of PAn has been studied by preparing polymer composites (4) and soluble PAn (5,6) and using plasma polymerization (7) and postsulfonation of PAn (8,9). Another approadi to the preparation of processible PAn is to apply a precursor polymer, e.g., PAn can be produced by the thermal treatment of poly(anthranilic acid) 0 ANA) (10). This mefliod is particularly useful for the preparation of processible PAn or its composites with other insulating polymers since it does not use external dopants that often cause an inconvenient situation associated with a practical use of the conducting polymer. 

W/o microemulsions of water, aniline, light petroleum, and nonionic surfactant Empilan NP-5 have been utilized for electrochemical polymerization of aniline to polyaniline [48]. Improved homogeneity and conductivity was achieved compared with polyaniline grovm in water. The heterogeneous nature of the microemulsion directed the mode of polymer growth and improved conductivity and structural features. 

Example 2 Homopolymer of Sulfonated Aniline - Electrochemical Polymerization of Aniline-2,5-Disulfonic Acid (ADSA)... 

The electrochemical polymerization of aniline-2,5-disulfonic acid (ADSA) is performed by applying a constant current density of 0.3 mA cm on a Pt plate electrode in 0.1 M monomer containing 0.5 M HCl electrolyte. The polymerization is carried out at room temperature under inert atmosphere. After polymerization, ethanol is added to precipitate the polymer from solution, and the polymer is vacuum dried at 70 °C for 24 h. The molecular weight of the poly(ADSA) obtained varies from 5000 to 12000 depending on synthetic conditions, and the conductivity ranges in between 0.04 and 0.3 S cm , which is also independent up to pH 10." ... 

Chen and Fang [161] have reported the preparation of polyvinyl alcohol and polyaniline by polymerizing aniline on polyvinyl alcohol-coated electrodes by chemical and electrochemical methods. Their report indicates that the electrochemical method yields a better composite. Various morphological aspects show that electrochemical polymerization of aniline on PVA surfaces leads to the formation of a composite having different gradients of polyaniline content. 

Other sulfonated polymeric electrolytes were also used in combination with polyaniline [136]. Sodium poly(styrene sulfonate), poly(2-acrylamido-2-methyl-l-propane sulfonate) and poly(vinyl sulfonate) were used as electrolytes in the electrochemical polymerization of aniline. Perchloric acid was added to lower the pH. The polymerization of aniline was performed by potential sweep using a platinum foil as working electrode. According to the authors, the polyelectrolyte is incorporated into the polyaniline film in a ratio of one sulfonate group for every four aniline repeating units. 

Polyaniline" has been described in many papers [1-6] during the past 100 years or so. It has been reported as existing in various, usually ill-defined forms such as "aniline black", "emeraldine", "nigraniline", etc., synthesized by the oxidative chemical or electrochemical polymerization of aniline. Certain of these materials have been shown to have an unexpectedly high conductivity [2-6]. 

Homma T, Kondo M, Kuwahara T, Shimomura M (2012) Electrochemical polymerization of aniline in the presence of poly(acrylic acid) and characterization of the resulting films. Polymer 53 223-228... 

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