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Abstract
Despite a significant number of investigations in the field of phosphazene chemistry, the formation mechanism of this class of cyclic compounds is still poorly studied. At the same time, a thorough understanding of this process is necessary, both for the direct production of phosphazene rings of a given size and for the controlled cyclization reaction when it is secondary and undesirable. We synthesized a series of short linear phosphazene oligomers with the general formula Cl[PCl2=N]n–PCl3+PCl6– and studied their tendency to form cyclic structures under the influence of elevated temperatures or in the presence of nitrogen-containing agents, such as hexamethyldisilazane (HMDS) or ammonium chloride. It was established that linear oligophosphazenes are inert when heated in the absence of the mentioned cyclization agents, and the formation of cyclic products occurs only when these agents are involved in the process. The ability to obtain the desired size phosphazene cycle from corresponding linear chains is shown for the first time. Known obstacles, such as side interaction with the PCl6– counterion and a tendency of longer chains to undergo crosslinking elongation instead of cyclization are still relevant, and ways to overcome them are being discussed.
Highlights
Linear high molecular weight phosphazenes are of great interest due to their unique properties such as high flexibility of the polymer chain, low glass transition temperature, high thermal stability and high values of quadratic electro-optic effect [1,2]
The calculated amount of HMDS was added under argon flow to reaction mixture, containing 2 g of linear oligodichlorophosphazene with specified chain length
A significant step was reached in phosphazene chemistry with the development of the Lewis acid catalyzed living cationic polymerization of phosphoranimines, usually with N(trimethylsilyl)trichlorophosphoranimine Cl3 P=NSiMe3, which allowed for the obtaining, at room temperature, of linear compounds with defined lengths and narrow molecular weight distributions [40]
Summary
Linear high molecular weight phosphazenes are of great interest due to their unique properties such as high flexibility of the polymer chain, low glass transition temperature, high thermal stability and high values of quadratic electro-optic effect [1,2]. The obtained values of chemical shifts on 31 P nuclei, given, contradicted the generally accepted data, in particular, the fact that the hexamer signal turns out to be shifted to the region of a weaker field relative to the pentamer singlet Taking these data into account, the results obtained by Emsley indicate a gradual increase in the size of the formed cycles, with an increase in the duration of the reaction. Via living cationic polymerization of Cl3 P=NSiMe3 and the investigation of their ability for intramolecular cyclization under thermal treatment, or by the reaction with hexamethyldisilazane (HMDS) HN(SiMe3 ) or ammonium chloride NH4 Cl. A possible schema for the formation of cyclic and linear phosphazenes is proposed and discussed in comparison with the known literature data
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