Chemical synthesis polypyrrole derivatives

Thiophene, pyrrole and their derivatives, in contrast to benzene, are easily oxidized electrochemically in common solvents and this has been a favourite route for their polymerization, because it allows in situ formation of thin films on electrode surfaces. Structure control in electrochemical polymerization is limited and the method is not well suited for preparing substantial amounts of polymer, so that there has been interest in chemical routes as an alternative. Most of the methods described above for synthesis of poly(p-phenylene) have been applied to synthesise polypyrrole and polythiophene, with varying success. 

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]. 

Polypyrrole and many of its derivatives can be synthesized via simple chemical or electrochemical methods [120]. Photochemically initiated and enzyme-catalyzed polymerization routes have also been described but less developed. Different synthesis routes produce polypyrrole with different forms chemical oxidations generally produce powders, while electrochemical synthesis leads to films deposited on the working electrode and enzymatic polymerization gives aqueous dispersions [Liu. Y. C, 2002, Tadros. T. H, 2005 and Wallace. G. G, 2003]. As mentioned above the electrochemical polymerization method is utilized extensively for production of electro active/conductive films. The film properties can be easily controlled by simply varying the electrolysis conditions such as electrode potential, current density, solvent, and electrolyte. It also enables control of thickness of the polymers. Electrochemical synthesis of polymers is a complex process and various factors such as the nature and concentration of monomer/electrolyte, cell conditions, the solvent, electrode, applied potential and temperature, pH affects the yield and the quality of the film... 

Poly (thiophene)s are of particular interest as electfochromic materials owing to their chemical stability, ease of synthesis and processability. For the most part, current research has been focused on composites, blends and copolymer formations of several conjugated polyheterocyclics, polythiophene and its derivatives, especially PEIX)T. In one example, poly(3,4-ethylenedioxythiophene) (PEDOT)/poly(2-acrylamido-2-methyl-l-propanesulfonate) (PAMPS) composite films were prepared by Sonmez et al. for alternative electrochromic applications [50]. Thin composite films comprised of PEDOT/PAMPS were reported to switch rapidly between oxidized and neufial states, in less than 0.4 s, with an initial optical contrast of 76% at A.max. 615 nm. Nanostructured blends of electrochromic polymers such as polypyrrole and poly(3,4-ethylenedioxythiophene) were developed via self-assembly by Inganas etal. for application as an electrochromic window [26]. Uniir etal. developed a graft-type electrochromic copolymer of polythiophene and polytetrahydrofuran for use in elecfiochromic devices [51]. Two EDOT-based copolymers, poly[(3,4-ethylenedioxythiophene)-aZ/-(2,5-dioctyloxyphenylene)] and poly[(3,4-ethylenedioxythiophene)-aft-(9,9 -dioctylfluorene)] were developed by Aubert et al. as other candidates for electrochromic device development [52],... 

The second problem associated with electrochemical synthesis of polypyrroles seems to be related to die difficulties found in correlating the polymer s properties with the conditions of synthesis. This has been derived from the widely spread idea of an overall understanding of the electrochemical mechanism. However, even the most simple electrochemical process of pyrrole electropolymerization involves different experimental variables in order to optimize polymer properties. These variables can be chemical, such as solvent or reactants (monomer and dopant salt), or physical, such as temperature, nature and shape of die electrodes, cell geometry or electrical conditions during synthesis. In addition, commonly the effects of all these variables are interdependent. 

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