Pyrrole polymerization

The polymeric pyrrolic autoxidation products probably result from the oxidized monomeric systems, which are analogous in structure to those isolated from photooxidation and peroxide oxidation reactions. Thus, for example, analysis of the products of the autoxidation of 1-methylpyrrole (Scheme 47) would suggest that 1 -methyl-A3-pyrrolin-2-one (153) is initially formed from a radical reaction of the pyrrole with triplet oxygen. This reaction sequence should be compared with that proposed for the oxidation of pyrroles with hydrogen peroxide (Scheme 50), which yields (181), (182) and (183) as the major isolable products. The acid-catalyzed reaction of a pyrrole with its oxidation product e.g. 153) also results in the formation of polymeric material and the formation of pyrrole black is probably a combination of oxidation and acid-catalyzed polymerization processes. 

Melanoidins based on polymeric pyrroles were dealt in Chapter 2. 

Concentrated emulsions have been used to prepare conductive composite polymers, either by polymerizing pyrrole or aniline within the pores of a porous material [62-64] or by preparing conductive lattices [65-67]. 

As early as 1984,65 66 it was reported that conductive polymer composites could be prepared electrochemically by polymerizing pyrrole on a working electrode coated with the support polymer (e.g., PVC). According to Wang and coworkers,67 the uniformity and conductivity of the polymer were improved if electrolyte was incorporated into the PVC before to inducing electropolymerization. 

Another has been formed by the reaction of an alternating copolymer of ethylene and carbon monoxide with 4-aminopyridine to form a polymeric pyrrole.143 Reaction of... 

Although the majority of the examples in the literature have used viral capsids to nudeate and template inorganic materials, it is also possible to template the formation of organic polymers on their surface. Niu et used the TMV capsid as a scaffold to bind and polymerize pyrrole and aniline into conductive polymer nanowires. Using the capsid as the scaffold allowed for the creation of well-defined constmcts with a low polydispersity, high processability, and a high aspect ratio. 

Some authors have proposed several modified process approaches to obtain electroactive electrospun nanofibers. One first practical and easy way is to spin a nonconductive polymeric web and after polymerize conductive polymers onto the fiber surface. For example, conductive polyamide-6 (PA-6] nanofibers were prepared by polymerizing pyrrole (Py] molecules directly on the fiber surface of PA-6. First, a solution of PA-6 added with ferric chloride in formic acid was electrospun with average diameter values around 260 nm. 

Wang, J.G., K.G. Neoh, and E.T. Kang. 2004. Comparative study of chemically synthesized and plasma polymerized pyrrole and thiophene thin films. Thin Solid Films 446 205. 

Other product concepts Hke electrolytic capacitors, as have been realised with TNCQ salts, PPy and PEDT are often approached by polymerizing pyrrole or EDT monomer directly on the substrate. 

Core-shell particles have been produced both chemically [134] and electrochemically [135]. For example, a dispersion of electrically conductive core-shell particles was obtained by polymerizing pyrrole or aniline in the presence of a dispersion of polyurethane or alkyd resin particles [134]. Coatings from these dispersions were reported to have conductivities in the range of 10 to 10 S/cm [134]. This coreshell approach, though yet to be fully exploited in the area of corrosion protection, enables formation of CP-containing films from waterborne dispersions. 

The second approach for improving the processabihty of ICPs is to prepare their colloidal dispersions in water or an appropriate solvent The colloid dispersions of ICPs can be obtained by chemical or electrochemical oxidation of the monomer in the presence of a steric stabihzer [29-31].The key parameter for such synthesis is the choice of an appropriate steric stabihzer which adsorbs or grafts onto the polymer coUoidal particles to prevent their aggregation or precipitation. Several polymers such as polyfethylene oxide) [32], poly(vinyl pyrroHdone) [33,34], poly(vinyl alcohol) [35], ethyl hydroxy cellulose [36], poly(vinyl alcohol-co-acetate) [37], poly(vinyl methyl ether) [38,39] and block copolymer stabihzer [40] have been used as steric stabihzers to produce PPy coUoidal dispersions. Surfactants are also employed for the synthesis of ICP coUoidal dispersions [41,42]. Very recently, stable PPy dispersions were prepared by Lu et al. by polymerizing pyrrole in an aqueous medium containing different anionic salts such as sodium benzoate, potassium hydrogen phthalate, and sodium succinate [43]. These authors also reported that the conductivity of PPy dispersions was enhanced when sodium benzoate was used as dopant. Chemical oxidahve polymerization in the presence of PSS in aqueous medium produces coUoidal dispersions and improves processability [44]. CoUoidal dispersions... 

Various oxidants are effective to polymerize pyrrole and pyrrole derivatives in organic media, but, as previously stated, the preferred ones are ferric salts, and especially ferric chloride, which is a cheap and readily available reagent. Ferric perchlorate is an even more effective reagent, since its redox potential in acetonitrile ( °=1.3V vs. SCE) is much higher than ferric chloride (the reason being the stabilization of Fe by chloride ions), but its high price has limited its use. Many other metal-based oxidants have been used, including Cu salts, Au and Ag species (in that case the aim was to make composites between the polymer and the noble metal). However, no other oxidant seems to be more effective than ferric chloride in the presence of sulfonate ions, which is a technique almost universally used in recent chemical PPy syntheses. 

There are some limitations on the choice of the solvents used to polymerize pyrrole. The solvent must simultaneously present a high dielectric constant to ensure the ionic conductivity of the electrolytic medium and a good electrochemical resistance against decomposition at the potentials required to oxidize the monomer. In addition, as polymerization proceeds via radical cation intermediates [46,47], the reaction is particularly sensitive to the nucleophilicity of the environment in the region near the electrode, where the radical cations are generated. 

Attempts to polymerize pyrrole dimers and trimers were not initially very successful, with evidence for the formation of short-chain oligomers rather than continuous films [119]. However, good quality films have been obtained from the dimer, particularly in the presence of moderately nucleophilic anions [107,120]. 

The most common way to polymerize pyrrole and thiophene derivatives is to use oxidative coupling. The oxidation may be done chemically or electrochemically, but the mechanisms are usually the same. The first step is one-electron oxidation of the monomer, resulting in a cation radical. The detailed mechanism will be presented in connection with electropolymerization. 

V vs Ag/Ag+, respectively (44,98). Numerous oxidizing reagents have been used to polymerize pyrrole, including hydrogen peroxide in acetic acid, lead dioxide, ferric chloride, nitrons acid, and ozone (99,100). 

There are two main methods used to synthesize polypyrrole chemical and electrochemical polymerization. The principal advantage of chemical methods is related to the possibility of mass production at low cost. This is often difficult to achieve with electrochemical methods, although the continuous synthesis of electrochemically polymerized pyrrole has also been developed [5,6]. On the other hand, electrochemical methods offer materials with better conducting properties and for some applications such as the construction of electronic devices, electrochemical polymerization offers the possibility of in-situ formation. 

These copolymers are prepared first by reacting the pendant chloromethyl groups on a styrene/p-chloro-methylstyrene (88/12) random copolymer with potassium pyrrole in refluxing THF overnight and then electrochemically polymerizing pyrrole onto the pendant pyrrole moiety [164,165]. 

Stanke et al. [166] first synthesized these copolymers because of the heterogeneous nature of composites, in terms of conductivity and transmittance. After successfully attaching a pyrrole moiety (7%) onto a copolymer of methyl methacrylate and 2-bromoethyl methacrylate, they polymerized pyrrole onto the copolymer by chemical oxidation in nitromethane and under argon. FT-IR spectra of the products show that the C—H bending vibration in A-monosubstituted pyroles at 725 cm" disappears even at the lowest pyrrole/2-A-pyrrolylethyl methacrylate (PEMA) ratio, indicating almost complete reaction. The conductivity of the 10 1 (pyrrole/PEMA) copolymer is 2 x 10" S cm" . 

Emulsion polymerization of pyrrole was also used to prepare blends of polypyrrole with a poly(alkyl methacrylate) [95]. A chloroform solution of a poly-(alkyl acrylate) and pyrrole was dispersed in an aqueous surfactant solution generating an emulsion. An aqueous solution of an oxidant was added to the emulsion with stirring, polymerizing pyrrole. The precipitated blend could be hot pressed in the form of films with conductivities of 6-7Scm . The curve for the variation of the conductivity of the blend with the oxidant/pyrrole ratio shows a maximum at a ratio of two with subsequent decrease. However, the yield increases to nearly 100% up to a ratio of four. The percolation threshold is approximately 10 wt% of pyrrole. The type and the concentration of the surfactant also affect the yield and conductivity. The mechanical properties of the blend depends on the number of carbons in the alkyl chain of the insulating polymer host. The strain at break of hot-pressed films increases and the stress at break decreases in the direction methyl, ethyl, butyl (Figure 18.3). This is an example where the mechanical properties of the conductive blend could be tailored according to the alkyl substituent in the poly(alkyl methacrylate) used in its preparation. 

Since concentrated mineral acids polymerize pyrroles (pp. 59, 83), ordinary nitration methods cannot be used in the series unless deactivating groups are present. It is possible to nitrate pyrrole carboxylic acids and their esters i 121 and nitropyrroles273 with concentrated nitric acid. 

Unsubstituted polypyrrole is also insoluble and infusible and hence cannot be processed into coatings and films. Plastic films coated with an antistatic layer of polypyrrole have been prepared by polymerizing pyrrole on the surface of the film or by use of polypyrrole grafted onto latex particles [50]. Nonetheless, polypyrroles are less attractive than polyanilines because of their high absorption at practically all wavelengths in the visible spectrum. 

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