Pyrrole chemical polymerization

Ferrero, F., Napoli, L., Tonin, C., and Varesano, A. (2006) Pyrrole chemical polymerization on teictiles kinetics and operating conditions./. Appl. Polym. Sci.,... 

The electrochemical oxidation of monomers such as pyrrole,2-5 thiophene,6-9 aniline,10-13 etc., or their derivatives, initiates a polymerization process at the electrode/electrolyte interface that promotes the formation of a polymeric film that adheres to the electrode. A similar homogeneous polymerization process can be initiated by chemical oxidation or chemical polymerization.14-21 Some monomers can be polymerized as well by electrochemical or chemical reduction. 

The discovery that doped forms of polypyrroles conduct electrical current has spurred a great deal of synthetic activity related to polypyrroles [216-218], Reviews are available on various aspects of the synthesis and properties of polypyrroles [219,220]. In addition, summaries of important aspects of polypyrroles are included in several reviews on electrically conducting polymers [221-226]. Polypyrrole has been synthesized by chemical polymerization in solution [227-231], chemical vapor deposition (CVD) [232,233], and electrochemical polymerization [234-240]. The polymer structure consists primarily of units derived from the coupling of the pyrrole monomer at the 2,5-positions [Eq. (84)]. However, up to a third of the pyrrole rings in electrochemically prepared polypyrrole are not coupled in this manner [241]. 

Composites of polypyrrole and poly(vinyl chloride) have been prepared by several groups (64-67). Polythiophene-poly(vinyl chloride) composites have also been prepared (68). The electropolymerization of pyrrole on poly(vinyl chloride)-coated electrodes yielded composites with mechanical properties (tensile strength, percent elongation at break, percent elongation at yield) similar to poly(vinyl chloride) (65) but with a conductivity of 5-50 S/cm, which is only slightly inferior to polypyrrole (30-60 S/cm) prepared under similar conditions. In addition, the environmental stability was enhanced. Morphological studies (69) showed that the polypyrrole was not uniformly distributed in the film and had polypyrrole-rich layers next to the electrode. Similarly, poly(vinyl alcohol) (70) poly[(vinylidine chloride)-co-(trifluoroethylene)] (69) and brominated poly(vinyl carbazole) (71) have been used as the matrix polymers. The chemical polymerization of pyrrole in a poly(vinyl alcohol) matrix by ferric chloride and potassium ferricyanide also yielded conducting composites with conductivities of 10 S/cm (72-74). 

Figure 6.26 gives information on the effect of carbon black loading on the polymerization efficiency of pyrrole. The polymerization rate was reduced as the loading of carbon black was increased. The reduced rate is caused by the oligomer coupling on the surface of the carbon black and by the absorption of the chemical oxidant needed for polymerization."" ... 

In suspensions of carbon black in pyrrole, anodic polymerization takes advantage of the fact that carbon black particles are negatively charged on their surface which makes it possible for them to migrate to a positively charged anode where they become embedded within a growing polypyrrole matrix This production method is suitable for production of materials for sensors, supercapacitors, fuel cells, etc. The effect of carbon black on the chemical oxidation of pyrrole in carbon black suspensions is shown in Figure 6.26. ° ... 

Polypyrroles (PPy s) are formed by the oxidation of pyrrole or substituted pyrrole monomers. In the vast majority of cases, these oxidations have been carried out by either (1) electropolymerization at a conductive substrate (electrode) through the application of an external potential or (2) chemical polymerization in solution by the use of a chemical oxidant. Photochemically initiated and enzyme-catalyzed polymerization routes have also been described but are less developed. These various approaches produce polypyrrole (PPy) materials with different forms—chemical oxidations generally produce powders, whereas electrochemical synthesis leads to films deposited on the working electrode, and enzymatic polymerization gives aqueous dispersions. The conducting polymer products also possess different chemical/electrical properties. These alternative routes to PPy s are therefore discussed separately in this chapter. 

It is usually assumed that the mechanism of chemical polymerization is similar to that described earlier in electropolymerization. However, work in our own laboratories71 highlights the fact that it is difficult to duplicate the products of electropolymerization using a chemical oxidant. Studies using 3-methyl-4-carboxy-pyrrole have demonstrated that polymers obtained with both techniques are similar in chemical composition but differ markedly with respect to polymer morphology. 

The vast majority of chemical polymerizations of pyrrole have been carried out between 0°C and room temperature. In one of the few systematic studies of the influence of temperature, Miyata and coworkers77 examined polymerization with FeCl3... 

An important breakthrough in PPy chemistry was the discovery by Lee and coworkers95 in 1995 of a chemical polymerization route to an unsubstituted PPy that was soluble in organic solvents. They exploited the surfactant-like qualities of added dodecylbenzenesulfonate (DBSA 9) as a dopant anion to solubilize PPy formed during oxidation of pyrrole by aqueous (Nn4)2S208. The PPy/DBSA product, isolated as a black powder in 42% yield after 40 h reaction at 0°C, was very soluble in m-cresol, and could be dissolved in weakly polar solvents such as chloroform and dichloro-methane by the addition of an equimolar amount of dodecylbenzenesulfonic acid. A film cast from chloroform solution exhibited an electrical conductivity of 5 S cm-1, and its UV-visible spectrum was similar to that of electrochemically deposited PPy. 

PPy has been coated on PMMA or polyethylene spheres42 using a chemical polymerization process. The spheres were dispersed in methanol and then added to water containing FeCl3 oxidant. Pyrrole dissolved in water was subsequently added. 

Chemical polymerization onto sulfonated (dopant-containing) synthetic polymers has also been described.45 Sulfonated polyethylene-polystyrene was exposed to monomer and then the oxidant. A mixture of Fe11 and Fem led to more accurate control of the E° value of solution. These same workers also described a novel chem-ical/electrochemical method, in which pyrrole was initially polymerized using a low concentration of Fe111. The reduced Fem could then be reoxidized electrochemically to regenerate the oxidant. Using this chemical/electrochemical process, composite polymers with conductivities as high at 35 S cm-1 were obtained. 

Pyrrole has been chemically polymerized with oxidants including sulfuric acid [87], bromine and iodine [88], copper(II) perchlorate [89], and iron(III) chloride [90]. Soluble polypyrrole can be prepared by the introduction of flexible side chains [91-93]. In contrast with the progress made in the synthesis of regioregular PTs, all 3-substituted polypyrroles reported so far have a regio-random structure. 

Anionic metal complexes, such as tetrasulfophthalocyanine or disulfoferrocene, can be incorporated from solution during the oxidative eleetro-chemical polymerization of pyrrole [24-27]. After re-reduction of the polypyrrole film the phthalocyanine derivative remains in the film, and the observed electrochemical conductivity is explained by ion transport of small electrolyte cations [26]. 

There have been efforts to put scanning tips to different uses in which the scanning probe is employed neither as an electrode nor as a monomer/CP delivery mechanism, but as a nanomechanical tip to create scratches on a substrate to guide the growth or promote selective adhesion of CP nanofeatures. For example, it has been shown that a tip can be used to scratch the poly(methyl methacrylate) (PMMA) layer on top of electrolyte and catalyst underlayers to achieve selective chemical polymerization of pyrrole monomer on... 

The synthesis of conjugated polymers is highly dependent on the effective carbon-carbon single bond generation between unsaturated carbons in aromatic molecules. Aromatic units in conjugated polymers can be benzene, aniline, pyrrole, thiophene, carbazole, naphthalenediimide, perylenediimide (PDI), or their derivatives, etc. Although monomers are various, their synthetic methods can be mainly classified into chemical and electrochemical polymerizations. Chemical polymerization includes chemical oxidative polymerization and metal-catalyzed coupling condensation. 

Thus, optimization of all of the parameters in one experiment is difficult. In contrast, chemical polymerization does not require any special instruments it is a rather simple and fast process. Chemical polymerization method involves oxidative polymerization of pyrrole monomer by chemical oxidants either in aqueous or non-aqueous solvents or oxidation by chemical vapor deposition in order to produce bulk polypyrrole as fine powders. Fe(III) chloride and water are found to be the best oxidant and solvent for chemical polymerization of pyrrole respectively regarding desirable conductivity characteristics. 

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