The present review articulates the syntheses and properties of industrially important disulfide and tetrasulfide polymers. The diselenide and ditelluride polymers have also been reviewed, for the first time, so that a comprehensive view on the polymers containing group VIA elements can be obtained. The latter two polymers are gaining considerable current attention due to their semi-conducting properties. The emphasis has been made to sift through the developments in the last ten years or so to get the latest flavour in these rapidly developing polymers. We have also attempted to bring to the fore several contradicting results, like, for example, the crystallinity of ditelluride polymers, to clear the mist in such reports. We hope that this review will help those working in the field to assess the progress achieved in this area and that it may also provide useful orientation for those who wish to become involved.
Polymers containing group VIA derived weak links, viz. poly (styrene disulfide) (PSD), polystyrene tetrasulfide) (PST), and poly (styrene diselenide) (PSDSE), have been synthesized. The polymers PSD and PST were characterized by NMR, IR, UV, TGA, and fast atom bombardment mass spectrometric (FABMS) techniques. The presence of different configurational sequences in PSD and PST were identified by 13C NMR spectroscopy. PSDSE, being insoluble in common organic solvents, was characterized using solid-state 13C NMR (CP-MAS) spectroscopy. Thermal degradation of polymers under direct pyrolysis-mass spectrometric (DP-MS) conditions revealed that all the polymers undergo degradation through the weaklink scission. A comparative study of the pyrolysis products of these polymers with that of polystyrene peroxide) (PSP) revealed a smooth transformation down the group with no monomer (styrene or oxygen) formation in PSP to only styrene and selenium metal in PSDSE. This trend of group VIA is explained from the energetics of the C-X bond (X -O, S, and Se) which also seems to be important in addition to the weak X-X bond cleavage. In PSP and PSD, the behavior is also explained from the energetics of the alkoxy and thiyl radicals. The unique exothermic degradation in PSP compared to endothermic degradation in PSD and PSDSE is explained from the nature of the products of degradation.
This is the first report on the analysis of primary degradation products of a widely used commercial propellant binder, viz. hydroxy-terminated polybutadiene (HTPB), which has been carried out by direct pyrolysis mass spectrometry. The mechanism of degradation involves a radical process forming cyclic compounds as well as β-CH transfer reactions to form linear oligomers. On the basis of heats of formation data, it was found that the formation of 1,3-butadiene monomer is not favored during the primary chain scission process. However, its formation appears to emanate from the dissociation of high molecular weight cyclics/linear oligomers. In the presence of strong oxidizers such as ammonium perchlorate, the degradation pattern of HTPB, as observed in pyrolysis-gas chromatography/mass spectrometry analysis, is significantly affected, indicating its influence on HTPB degradation.
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