Pyrrole monomers

Polypyrroles. Highly stable, flexible films of polypyrrole ate obtained by electrolytic oxidation of the appropriate pyrrole monomers (46). The films are not affected by air and can be heated to 250°C with Htde effect. It is beheved that the pyrrole units remain intact and that linking is by the a-carbons. Copolymerization of pyrrole with /V-methy1pyrro1e yields compositions of varying electrical conductivity, depending on the monomer ratio. Conductivities as high as 10 /(n-m) have been reported (47) (see Electrically conductive polymers). 

Significant variations in the properties of polypyrrole [30604-81-0] ate controlled by the electrolyte used in the polymerization. Monoanionic, multianionic, and polyelectrolyte dopants have been studied extensively (61—67). Properties can also be controlled by polymerization of substituted pyrrole monomers, with substitution being at either the 3 position (5) (68—71) or on the nitrogen (6) (72—75). An interesting approach has been to substitute the monomer with a group terminated by an ion, which can then act as the dopant in the oxidized form of the polymer forming a so-called self-doped system such as the one shown in (7) (76—80). 

Cyclic voltammograms of solutions of the pyrrole monomers also show a dependence on the anion present in the electrolyte and can show multiple peaks with Epa values ranging from + 1.0 to + 1.3 V in acetonitrile [57,279]) (+ 0.8 to + 1.1 V vs. 

Figure 40 The pyrrole monomer, l-(jV-but-4-yl-pyrrol)-l,2-bis(diphenylphosphinoborane) (62), used for electropolymerization to yield polypyrroles with diphosphine ligands protected from oxidation by borane groups. (Adapted from ref. 86.)...

Figure 67 The cobaltabisdicarbollide Cs[Co(C2B9Hn)2] (109) that was doped in or covalently bound to a pyrrole monomer to produce conducting polymers (110 being one of them). (Adapted from refs. 138 and 139.)...

Figure 3.76 (a) Plot of the log of the polypyrrole film conductivity polymer potential, , fora 10mV potential difference applied across a 13.9pm-thick film in 0.1 M tetraethylammonium tetrafluoroborate/CH3CN. (b) Plot of polypyrrole conductivity vs. fractional charge per pyrrole monomer subunit (qimm) in 0.1 M tetraethylammonium tetrafluorobnrate/CH3CN From Feldman et at. (1985). Copyright 1985 American Chemical Society. 

Incidentally, oxidation data of the pyrrole monomers show an interesting increase in oxidation potentials when containing heavier substituents (Table 25). However, the ionization potential of N -methylpyrrole (7.95 V) is smaller than that of pyrrole (8.21 V). The accepted linear relationship between ionization potential and oxidation potential210 would have it the other way round. Considering, however, that trimethylsilyl and trimethylgermyl groups are weak electron donors211, it is plausible that a nonelectronic effect is responsible for the observed trend and the potential shifts are associated with steric effects. 

Fig. 5.17 Schematic of electropolymerization of CNT-polypyrrole hybrid via (a) ester linkage and (b) noncovalent pyrene linkage of pyrrole monomer with schematic of hybrid and SEM. Scale 200 nm. Reproduced with permission from [222], (2008) Elsevier.

Fig. 122. Polymerization of pyrrole monomer in a preformed ferric stearate multilayer film (black dots represent electrically conducting polypyrrole) [764]...

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... 

Vinylpyrrole and several of its derivatives have been studied. Free radical polymerization has been shown to lead to low molecular weight (2000-13 000) polymers (20) (80MI11102). Similar results were obtained for homopolymerization of pyrrole monomers in which the polymerizable group was attached at the 2-position (73MI11101), as in monomers (21) and (22). Low molecular weights can probably be attributed to chain transfer reactions involving the pyrrole nucleus. 

The pyrrole monomer porphobilinogen arises from the condensation of two molecules of S-aminolevulinate with the ions of two water molecules. This reaction is catalyzed by S-aminolevulinate dehydrase. Condensation of four porphobilinogen molecules yields the branchpoint compound in tetrapyrrole synthesis, uroporphyrinogen III. This is a complex reaction requiring two enzymes Uroporphyrinogen I synthase, which catalyzes a head-to-tail condensation... 

Polypyrrole thin film doped with glucose oxidase (PPy-GOD) has been prepared on a glassy carbon electrode by the electrochemical polymerization of the pyrrole monomer in the solution of glucose oxidase enzyme in the absence of other supporting electrolytes. The cyclic voltammetry of the PPy-GOD film electrode shows electrochemical activity which is mainly due to the redox reaction of the PPy in the film. Both in situ Raman and in situ UV-visible spectroscopic results also show the formation of the PPy film, which can be oxidized and reduced by the application of the redox potential. A good catalytic response to the glucose and an electrochemical selectivity to some hydrophilic pharmaceutical drugs are seen at the PPy-GOD film electrode. 

The data show that the quadmpole hyperfine patterns of the rotational transitions are different between the two states, due to changes of the relative positions of some of the hyperfine components within the multiplet. The rotational spectrum of a pyrrole dimer is consistent with essentially a T-shaped structure, in which the planes of the two pyrrole monomers form an angle of 55.4(4)° and the nitrogen side of one ring is directed to the 7t-electron system of the other ring establishing a weak H bond <1997JCP504>. 

The utility of the ionic liquid as a recyclable medium for the polymerization was also demonstrated. More than 90% of [EMIM][OTf] after the polymerization was easily recovered simply by extracting the remaining pyrrole monomer with chloroform. The recoved [EMIM] [OTf could be reused five times without significant loss of reactivity for the polymerization. 

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]. 

Figure 58 Mechanism of polypyrrole formation via the coupling reaction of a cation radical with neutral pyrrole monomer. (From Refs. 240 and 244.)...

The first chemical oxidation of pyrrole 106 (scheme 26) was achieved as early as 1916 by using hydrogen peroxide to obtain an amorphous powder known as pyrrole black [158]. The room temperature conductivity of PPy 20 prepared with acid or peroxide are in the range of 10"to 10 " S/cm, which can be inereased by halogen doping to 10 S/cm [159]. This low conductivity is due to the high degree of saturation of the pyrrole monomer units caused by defects. In the last... 

Solubility has been Induced for polypyrroles by attaching alkyl [102,103] or alkyl sulfonate [104] groups to the pyrrole monomer prior to polymerisation. This results in markedly enhanced solubility in organic or aqueous medias, respectively. For example, we have shown that the electrochemical method [105] can be used to produce alkylated polypyrroles with high (400 g/L) solubility in organic solvents and reasonable (1-30 S cm ) conductivity. 

For organic solvent solubility, an alternative approach to solubilising polyanilines and polypyrroles, without sacrificing high electrical conductivity, is the use of surfactant-like dopant anions. With polypyrrole this has recently been achieved via oxidation of the pyrrole monomer with ammonium persulfate in the presence of dodecylbenzene sulfonate [128,129]. Similarly, the conducting emeraldine salt form of PAn.HA can be readily solubilised in a range of organic solvents via the use of camphorsulfonic acid or dodecylbenzenesulfonic acid as the dopant, HA [130,131]. 

The polymerization of pyrrole over Cu(II)-exchanged ZSM-5 zeolites was studied with resonance Raman spectroscopy. The authors found that a critical concentration of cupric ions must be exceeded to observe polymerization. Hosts with low Si/Al ratios gave partially oxidized pol5rpyrrole (having quinoidal and aromatic structures) and pyrrole monomer. The quinoidal structure was associated with the charge carriers. Residual oxygen degraded the polymer. 

Scheller et al. reported amperometric pyruvate sensors by potentiostatic co-pol5nmerization of Os(bipy)2pyridineCl-modified pyrrole monomer and thiophene on platinized glassy carbon electrodes on which pyruvate oxidase was adsorbed [78]. This pol5dhiophene based redox pol5uner was reported to have excellent electron transfer properties with significantly improved stability compared with polypyrrole as they are not affected by oxygen [79]. However, notable interference by ascorbate needs to be eliminated. 

Ultrasound was also used for the dispersion of a surfactant pyrrole, prior to electrooxidation to the conducting polymer [233]. An amphiphilic (pyrrolylalkyl) ammonium monomer dispersion was used to coat the electrode surface with monomer, subsequently electropolymerized to thin films using an aqueous electrolyte for this step. Ultrasound has also been used to assist impregnation of pyrrole monomer into, for example, a conventional polymer matrix prior to polymerization to yield a composite of the conducting and conventional polymers, but is also a pretreatment effect of ultrasound rather than a sonoelectrochemical one [234],... 

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