Pyrrole polymerisation

For the amounts of Fe below x=l, the sheath-like structures form mostly (Fig. 2d). This proceeds likely so when the Fe amount is low enough, the catalyst does not get to deeper layers of onions and pyrrole polymerises already in outer layers, which hinders the access of further monomer molecules to the onions inside. Use of still smaller amounts of the Fe catalyst results in formation of carbon (e.g., OCM-.NO.25) consisting of both foam- and sheath-like structures (Fig. 2c). The XPS analysis reveals that 0.43 wt.% Si and 0.5 wt.% Fe remain in the surface layer of OCM-.NO.25. This sample as well as CMK-3N1.25 and CMK-3N2.00 do not bum up totally (Table 1, Fig. 3). 

Pyrrole polymerises under reducing conditions, Tliiophene-2-carboxylic acid is reduced in alkaline solution at -2.3 V v v. see, on a mercury cathode, to the 2,5-dihydro compoimd [61], Under these same conditions, furan-2-carboxylic acid is also reduced to the 2,5-dihydro compound [62]. 

The formation of conducting polymers within the confines of zeolite channels has been shown to depend on several factors, including the nature of the cation, reaction pressure and the presence of acid sites. From a detailed study of pyrrole polymerisation it is possible to postulate the following reaction mechanism. Interaction of pyrrole with a Cu site initially gave a radical monomer species, (which presumably became associated with AIO4 sites in the framework) and a Cu site. Further creation of radical polymers leads to a polymerisation reaction in which aromatic polymer chains were formed. As the polymer chain lengthens the energy levels for the n system are lowered and thus oxidation... 

N-Alkylpyrroles may be obtained by the Knorr synthesis or by the reaction of the pyrrolyl metallates, ie, Na, K, and Tl, with alkyl haUdes such as iodomethane, eg, 1-methylpyrrole [96-54-8]. Alkylation of pyrroles at the other ring positions can be carried out under mild conditions with allyhc or hensylic hahdes or under more stringent conditions (100—150°C) with CH I. However, unless most of the other ring positions are blocked, poly alkylation and polymerisation tend to occur. N-Alkylation of pyrroles is favored by polar solvents and weakly coordinating cations (Na", K" ). More strongly coordinating cations (Li", Mg " ) lead to more C-alkylation. 

There are reports of polymerisation of pyrrole [161, 162] and aniline [163] onto polyacetylene, to give oxygen and water stability [161], although there is some evidence for the polyacetylene acting electrocatalytically, oxidizing the pyrrole with no concomitant polymerisation. 

Pyrrole was first polymerised in 1916 [239, 240] by the oxidation of pyrrole with H202 to give an amorphous, powdery product known as pyrrole black, which was... 

In general the mechanism of polymerisation for thiophene appears to be similar to that of pyrrole (Section 4.11.2), occurring via a radical coupling mechanism [423] giving mainly a-a linkages [293,400,405], and involves oligomer as well as monomer radicals, with evidence to suggest that the polymerisation reaction occurs at a lower... 

To catalyse the cyanoethylation of pyrrole, 3 drops of the basic catalyst solution were added to the reaction mixture of pyrrole (30%) in the nitrile. An exotherm developed and base-catalysed polymerisation of the nitrile accelerated to explosion. 

A typical example of such a polymer is polypyrrole. The exact mechanism by which the electropolymerisation of pyrrole occurs remains a source of controversy however, by assuming 100% growth efficiency, then it can be calculated that 2.25 electrons are removed per monomer unit. Only two electrons are required to polymerise the monomer however, the film is formed in a highly oxidised state, corresponding to one electron per four units, see Figure 2.20. 

It was reasoned that the polymer must consist of a, a -coupled pyrrole units, with fi coupling less important, because of the fact that oc-substituted pyrroles do not polymerise whereas -substituted species do, and on the basis of magic angle spinning 13C nmr and IR techniques. The method of labelling the monomer ring positions is also shown in Scheme 3.9. 

The parent siliceous materials of the SBA-15 and MLV types were synthesised after typical procedures [2], The carbon replicas were prepared via polymerisation, catalysed by ferric chloride, of pyrrole introduced into the mesopores of matrices [3]. The products were characterised by the nitrogen adsorption, TEM, and thermal analysis (DTG, DTA). The nitrogen content in carbons was determined using elemental analysis, XPS, and EDX, while the Si and Fe contents, with XPS. The replicas of SBA-15 and MLV-0.75 are denoted as CMK-3Nx and OCM-.Nx, respectively, were x refers to the number of g of FeCl3 per 1 g of silica used for preparations. 

The quaternary groups in viologens can be derivatised to produce compounds capable of chemically bonding to a surface, especially electrode surfaces. These include symmetrical silanes, e.g. (1.93), which can bond to the oxide lattice on the electrode surface, and a viologen with pyrrole side chain (1.94) that undergoes anodic polymerisation to form a film of the viologen bearing polypyrrole on the electrode. Polymeric bipyridilium salts such as (1.95) have also been prepared for use in polymeric electrolytes. ... 

The other important electrochromic polymers are the polypyrroles and polythiophenes, obtained by polymerisation of the parent pyrrole and thiophene or, more importantly, their 3,4-substituted derivatives. The most widely studied of these two classes of polymers in electrochromic outlets are the poly thiophenes, which are readily synthesised by the reaction of the substituted monomer with FeClj in chloroform solution. The colour change properties of a variety of poly thiophenes in the presence of a counter-ion are shown in Table 1.14. ... 

The reactivity of the five-membered heterocycles pyrrole, furan, thiophen and imidazole (Fig. 8-10) is characterised by interactions with electrophilic reagents. The precise nature of these reactions depends upon the particular ring system. Thiophens undergo facile electrophilic substitution, whereas the other compounds exhibit a range of polymerisation and other Lewis acid-initiated reactions upon treatment with electrophiles. We saw a number of examples of Lewis acid-promoted reactions of furans and pyrroles in Chapter 6. Although reactions of complexes of five-membered heterocyclic ligands have not been widely investigated, a few examples will illustrate the synthetic potential. 

As far as calix[4]pyrrole-based materials are concerned, particular emphasis should be placed on the production of chelating resins. Those obtained by the condensation reaction of calix[4]pyrrole and formaldehyde in the presence of formic acid offer potential for their use as extracting agents for fluoride removal from water. Their advantage relies on the fact that a single-step procedure is required for polymerisation. However, research in this area is in a preliminary stage. Much work needs to be done to establish the full capacity of these materials to take up fluoride from water, the kinetics of the process, and the optimum experimental conditions for fluoride extraction. Although, their ability to extract fluoride reaches a value of 88% from solutions of tetra-n-butylammonium fluoride... 

Why is pyrrole aldehyde 2.44 less reactive to nucleophiles than, say, benzaldehyde Why is pyrrole alcohol 2.45 readily polymerised on exposure to acid ... 

In these examples it was not possible to visualise any chiral structure with a microscope, but when PANI was prepared using poly(acrylic acid) as an in situ template, helical microwires were visualised [65]. In an even more general sense, helical fibres of PANI, poly(ethylenedioxythiophene) (PEDOT), and poly(pyrrole) were prepared using synthetic lipids as templates [66,67]. The synthetic lipid molecules used are shown in Fig. 6 along with some of the helical fibres of PEDOT that are formed when the sulphonate salt is used to shape the fibres during the polymerisation. The procedure involves growing the fibres by electrochemical polymerisation onto an ITO electrode with the lipid molecules in the electrolyte. 

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