Pyrroles polymerization, mechanism

The generally accepted polymerization mechanism for heterocyclic polymers such as poly(pyrrole) is shown in Figure 7.3 [23]. 

The PPy formation mechanism has been addressed in many studies. The radical cation-neutral pyrrole (PyH) coupling followed by a further oxidation and de-protonation in the early stage of the pyrrole polymerization was first proposed by Genies et al. [60] and Fletcher et al [22]. Fletcher et al. concluded that the adsorption of a neutral FyH molecule takes place prior to its electro-oxidation resulting in the generation of the radical cation, which subsequencly polymerized as a consequence of instantaneous nucleation with three-dimensional growth. This conclusion was later supported by Marcos et al. [61]. 

PPy may be prepared by either chemical or electrochemical oxidation of pyrrole monomer. The most widely accepted polymerization mechanism of PPy is the coupling between radical cations (Figure 8.4)... 

FIGURE 8.4 Polymerization mechanism of pyrrole through the coupling of two radical cations. 

FIGURE 1 The electrochemical polymerization mechanism for the formation of polypyrrole begins when a pyrrole radical cation forms from the oxidation of the monomer. Then two radical cations react to create a dimer and split off two protons. The dimer undergoes further oxidation to generate a radical, which reacts with another radical to propagate chain growth and eventually produce polypyrrole. 

To illustrate mechanisms of the polymerizations in a simple fashion, the polymerizations described above for poly(aniline) and poly(pyrrole) can be considered. In both the chemical and the electrochemical cases, the initial step is the generation of the radical cation, as illustrated for pyrrole polymerization in Eq. (1.9) below. 

Polyheterocycles. Heterocychc monomers such as pyrrole and thiophene form hiUy conjugated polymers (4) with the potential for doped conductivity when polymerization occurs in the 2, 5 positions as shown in equation 6. The heterocycle monomers can be polymerized by an oxidative coupling mechanism, which can be initiated by either chemical or electrochemical means. Similar methods have been used to synthesize poly(p-phenylenes). 

The most widely accepted mechanism for the anodic polymerization of pyrroles and thiophenes involves the coupling of radical cations produced at the electrode (Scheme l).5 The oligomers so produced, which are more easily oxidized than the monomer, are rapidly oxidized and couple with each other and with monomer radical cations. Coupling occurs predominantly at the a-positions (i.e., 2- and 5-position),5 and so pyrroles and thiophenes with substituents in either of these positions do not undergo anodic polymerization. The reaction is stoichiometric in that two... 

Anionic polymerization of lactams was shown to proceed according to what is called the activated monomer mechanism. With bischloroformates of hydroxy-terminated poly(tetramethyleneglycol) and poly(styrene glycol) as precursors for a polymeric initiator containing N-acyl lactam ends, block copolymers with n-pyrrol-idone and e-caprolactam were obtained by bulk polymerizations in vacuum at 30 and 80 °C, respectively361. ... 

Recently the mechanisms of pyrrole electropolymerization have been reviewed in Ref. [9b]. By the anodic reaction, an electron is withdrawn from the pyrrole monomers and cationic radicals are formed. The cationic radicals undergo a series of chemical-electro-chemical-chemical reactions and, as the result, the polymerization proceeds. If the cationic... 

The oxidative polymerization has been proposed to proceed via a radical coupling that involves the coupling of neutral radicals or cation radicals. The former case corresponds to the oxidative polymerization of phenols and dithiols in which the neutral radical is formed by one-electron transfer after dissociation of a hydron from the monomer, or by the elimination of a hydron after the oxidation. The latter case takes place when the cation radical formed by one-electron oxidation exists as a stable species. The cation radicals then couple with each other, and the dimer is formed through solvent-catalyzed hydron elimination from the intermediate dication. Oxidative polymerization of pyrrole and thiophene uses this mechanism [57-62]. 

Although some mechanistic details are still controversial, it has been established that the oxidative polymerization (chemically or electrochemically) of pyrrole and pyrrole derivatives proceeds via an E(CE) mechanism which involves cation-radical propagating species. The most commonly accepted mechanism of polypyrrole formation is illustrated in Fig. 57 [237,242]. The polymerization begins with the one-electron oxidation of pyrrole to produce cation radical 399. This cation radical has been... 

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). 

C. Discussion of Possible Mechanisms for the Acid-Catalyzed Polymerization of Pyrroles... 

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