Pyrroles oligomers, structure

Martina et al. [149] synthesized pyrrole oligomers carrying the tert-butoxy carbonyl (BOC) protecting group on the nitrogen in an attempt to obtain structurally perfect, unprotected poly(2,5-pyrrole). They brominated these to produce the a,co-dibromo oligomers, which were then allowed to react with 1,6-diboronic acid of 2,5-didodecylbenzene in the presence of a palladium catalyst. 

Synthetic routes to structurally perfect oligomers of PPy, involving only 2,5 -couplings of the individual pyrrole repeat units (idealized structure shown in Equa-... 

The nanocoating of PPy on natural cellulose fiber was performed without disrupting the hierarchical network structure of individual fibers [245]. Since pyrrole was hardly adsorbed onto a cellulose surface, this approach was based on the adsorption of the growing polymer chain from the solution and on the subsequent immobilization to form thin film. The conformation of the PPy chain was parallel to the surface of the cellulose fiber, because the corresponding oligomer is adsorbed parallel to the surface and further polymerized in the lateral direction. 

Heteroaromatic systems, such as the widely used poly(thiophene)s can be obtained by simple oxidative polymerization of the soluble monomers or oligomers either by electrochemical means or oxidizing agent such as FeCla [50,51]. This common route is also used to synthesize a variety of mono- and dialkyl-, -alkoxy-, and -alkylsulfonic acid substituted and therefore soluble poly(thiophene)s [52-57] (Scheme 19) and can also be utilized to obtain poly(pyrrole)s. The disadvantage of this polymerization methods however is the regiorandom structure of the polymeric product with non-reproducible properties. 

Tamao K, Ohno S, Yamaguchi S (1996) Silole-pyrrole co-oligomers their synthesis, structure and UV-VIS absorption spectra. Chem Commun 1873-1874... 

Other important spectral features are the lines associated with the vibrations of the end groups, which decrease as chain length increases and are not observed in (Boc)PPy. In particular, the line near 1530 cm can be used as a marker for the determination of the chain length of the oligomers. The relevant information is that the Raman spectrum of the protected molecules arises mainly and selectively from the vibrations of the pyrrole backbone. This makes the Raman spectrum a useful structural probe of the heteroaromatic backbone, unlike the infrared spectra, which are very crowded and inextricable [173]. 

The behavior typical of an unmodified electrode is clearly apparent in the voltammetry acquired at platinum for a neat pyrrole solution, in the initial scan, higher potentials are required to oxidize the pyrrole solution. Subsequent cycles yield oxidative currents at lower potentials. Here, the electrochemical behavior is dominated by the more easily oxidized oligomers, in addition, the initial sweeps on platinum display the characteristic nucleation loop [27], in which the current on the reverse sweep is greater than the current on the forward sweep, indicating that nucleation of polymeric structures onto the electrode surface has begun to occur. Subsequent cycles produce successively larger oxidation currents, indicating an increase in the amount of polymer localized at the electrode surface. 

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