Pyrrole polymerization with acid

Pyrroles react with the conjugate acids of aldehydes and ketones to give carbinols (e.g. 67) which cannot normally be isolated but which undergo proton-catalyzed loss of water to give reactive electrophiles (e.g. 68). Subsequent reaction may lead to polymeric products, but in the case of reaction of pyrrole and acetone a cyclic tetramer (69) is formed in high yield. 

Pyrrole reacts with aldehydes and ketones under acidic conditions to form polymeric compounds. In many cases these are intractable resin-like materials however, with appropriate carbonyl compounds, interesting cyclic tetramers can be formed in very good yields. 

Other electrochemical processes of organic compounds on Pb electrodes or electrodes with UPD Pb have been studied - formaldehyde [323], oxalic acid [386], trichloro- and trifluoroethane [387], 1-phenylethylamine [388], 3-hydroxychi-nuclidine [388], dichlorodifluoromethane [389], polychlorobenzenes [390], 1-propa-nol [391], pyrrole polymerization [392], and inorganic compounds - phosphine [388] and sulfate(IV) ions [393]. Simultaneous catalytic or inhibiting influence of organic solvents - acetonitrile, dimethyl-sulfoxide, and Pb + presence on electrooxidation of small organic molecules on Pt electrodes has been studied using on-line mass spectroscopy [394],... 

The effect of intrazeolite protons on pyrrole polymerization in faujasite with ferric ions was examined, in order to distinguish the relative influence of acidity and the one-electron oxidant. If water was co-adsorbed with pyrrole, the authors could prepare materials with conductivities vaiying over a wide range. It is not clear to what extent the conductivity is due to surface-adsorbed polypyrrole, because similar simthetic methods also produced pol)mier coatings on amorphous aluminosilicates. 

Mono-C-alkylation of pyrroles cannot be achieved by direct reaction with simple alkyl halides, either alone or with a Lewis-acid catalyst, for example pyrrole does not react with methyl iodide below 100 °C above about 150 °C, a series of reactions occurs leading to a complex mixture made up mostly of polymeric material together with some poly-methylated pyrroles. The more reactive aUyl bromide reacts with pyrrole at room temperature, but mixtures of mono- to tetra-allyl-pyrroles together with oligomers and polymers are obtained. 

Pyrrole reacts with A-chlorosuccinimide to give 2-chloropyrrole. However, with SO2CI2 or aq. NaOCl, one obtains 2,3,4,5-tetrachloropyrrole. A-Bromosuccinimide forms 2-bromopyrrole, and bromine forms 2,3,4,5-tetrabromopyrrole. Pyrroles are nitrated with HNO3 in acetic anhydride at -10°C to yield 2-nitropyrroles. Concentrated sulfuric acid causes polymerization of pyrroles, but at 100°C, pyridine-SO3 complex provides the corresponding pyrrole-2-sulfonic acids [35 a]. 

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. 

Deng and co-workers [171] synthesized A -(5-hydroxy-pentyl)pyrrole by reacting 5-amino-l-pentanol with 2,5-dimethoxytetrahydrofuran in refluxing acetic acid. They then attempted to react this monomer with acid groups on the surface of a polyester and to copolymerize it with pyrrole on the treated surface. They report that a smooth metallic-looking black polymeric film adhered strongly to the surface of the polyester with surface electrical resistance in the range of 0.3-20 Q. 

Another polypyrrole/polyamide composite film was obtained by the electrode coating method [84]. In this work the polyamide film was obtained in situ by coating a stainless steel electrode with a polyamic acid film followed by imidization with pyridine and acetic anhydride. The coated electrode was submitted to a constant current in a pyrrole/LiC104 solution in acetonitrile. The surface conductivity measured after detaching the composite film from the electrode showed a strong dependence on the charge density used in the synthesis, i.e. on the amount of pyrrole polymerized in the composite. The maximum value obtained was 2 S cm . Combination of polypyrrole with this polyamide was shown to increase markedly its thermal and environmental stability. 

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