Polycyclic Oligomers Research Articles

The First Stages of Chemical and Electrochemical Aniline Oxidation—Spectroscopic Comparative Study

There are several types of aniline oligomers that can be formed in the early stages of aniline oxidation: linear oligomers with repeating units joined in para positions, and various branched and polycyclic oligomers, being the two most important groups. The fraction of these different oligomeric groups depends upon the reaction conditions of aniline oxidation. The aim of this study was to analyze the first products of the chemical and electrochemical oxidation of aniline at the (starting) pH 1 and 7, in order to specify the conditions of the formation of phenazine-like oligomers, and to test the theory that they have an important role in polyaniline film formation. We have confirmed that phenazine-like oligomers do not form at pH 1, neither in the chemical nor the electrochemical oxidation of aniline; however, they form in both chemical and electrochemical oxidation of aniline at pH 7. Phenazine-like oligomers are thus definitely not necessary intermediates for PANI film formation, not even in the chemical polymerization of aniline. Finally, the redox behavior of phenazine-like oligomers was demonstrated in a medium at pH 1.

The First Products of Aniline Oxidation – SERS Spectroelectrochemistry

AbstractThere are different opinions on the first products of aniline oxidation throughout the scientific community. While electrochemists basically accept only linear oligomers with repeating units joint in para positions, chemists have proposed formation of various branched and polycyclic oligomers. It was also suggested that one of these structures, N‐phenyl‐phenazinium cation, is responsible for the adherence of polyaniline films to substrates. In this work, a surface enhanced Raman spectroscopic and spectroelectrochemical analysis of the species adsorbed onto gold surface in aniline‐containing solution at pH 1 and 5 is presented. The influence of the pH value on the oligomer structure is declared. The results are discussed in the context of linear and branched/phenazine‐like aniline oligomers.

Effect of the dimethylsilyloxy co-monomer “D” on the chemistry of polysiloxane pyrolysis to SiOC

The effect of the dimethylsilyloxy co-monomer “D” on the chemistry of polysiloxane pyrolysis to silicon oxycarbide (SiOC) glass was studied with the aim of its optimization for the preparation of refractory composites with ceramic fibers. Reasonably small weight losses (shrinkage), but also some temporary plasticity of the material during its pyrolysis was sought. The pyrolysis chemistry was varied by preparing precursors from methyltriethoxysilane (T; main monomer) and dimethyldiethoxysilane (D; co-monomer) in different ratios, but also by using an alternative composition based on tetraethoxysilane (Q) and D. Pyrolysis temperatures between 300 and 1000°C were studied, and the escaping gases – polar, as well as non-polar – were analysed by means of chromatography/mass spectrometry. It was demonstrated, that the co-monomer D, which undergoes thermal elimination and subsequent reactions with the siloxane skeleton, seems to be responsible for the generally useful “micro-creep” ability of the pyrolyzing material. At higher D contents, where the pyrolysis weight losses strongly increase, the pyrolysis gases were shown to contain also polycyclic oligomers, which consist not only of D, but of the branching T or Q “main” monomers as well. The SiO/SiC exchange reactions of escaping D with the skeleton were found to be highly efficient not only in T/D, but also in Q/D polysiloxanes. It was further found, that the SiOC products after completed pyrolysis at 1000°C still can release gases detectable by GC/MS upon repeated heating, although no significant weight losses are observed: this indicates the presence of small amounts of pyrolysable sediments in micro- and nano-pores of the SiOC glass.

Investigation of the structure of polyphenylironsiloxane

The main features of the polyphenylironsiloxane (PPIS) structure were established by a combination of preparative fractionation, elemental analysis, gel-permeation chromatography,1H NMR and UV spectroscopy, and destructive trimethylsilylation. Specimens of PPIS contain a large amount of oligophenylsiloxane, which is apparently closed polycyclic oligomers. PPIS macromolecules consist of oligomerc siloxane fragments linkedvia iron oxide fragments. PPIS is characterized by the high heterogeneity of macromolecules and their fragments.