Novel Approach for the Synthesis of Chlorophosphazene Cycles with a Defined Size via Controlled Cyclization of Linear Oligodichlorophosphazenes [Cl(PCl2=N)n–PCl3]+[PCl6]−

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.

Comprehensive Mechanistic Analysis of the Ring-Opening Polymerization of [PCl2N]3 Using Quantum Mechanical Calculations

“…41,49,50 Alternative synthetic methods using Cl 3 PNSiMe 3 have helped in the synthesis of polychlorophosphazenes; however, understanding the mechanism for the ROP of [PCl 2 N] 3 would greatly help this original synthesis and possibly allow for the design of new catalytic systems for a controlled polymerization. 15,24,31,41 Despite the relentless efforts, the actual ROP mechanism remains unclear. 51,52 The lack of understanding of the reaction mechanism hinders further improvements on tuning chlorophosphazenes with specific properties, while at the same time, the exceptional challenges in gathering experimental evidence to further explore the ROP of [PCl 2 N] 3 remain unresolved for several reasons.…”

“…One method of preparing the parent polymer, [PCl 2 N] n , is through a melt ring-opening polymerization (ROP) of [PCl 2 N] 3 . Subsequently, replacing the chlorine atoms with various (R) substituents, the functionalized derivatives become versatile and find wide applications in diverse fields, including elastomers, fire-resistant materials, optical materials, − protective coatings, conductors, , interpenetrating polymer networks, nanofibers, catalysts, lubricants, and biomedical materials. ,, Researchers have extensively examined this reaction and alternative reactions involving Cl 3 PNSiMe 3 through experimentation in an attempt to comprehend its underlying mechanism. − However, despite their efforts, the intricacies of the chlorophosphazene ROP reaction remain inadequately understood.…”

Formation Mechanism of the Unsubstituted Chlorophosphazene Cl3P═NH: A Theoretical Study via Quantum Mechanical Calculations

“…Though some recent reports indicated that using a Lewis acid-catalyzed living cationic polymerization can guarantee linear phosphazene yields with specific lengths at room temperature, the starting monomer, Cl 3 P�SiMe 3 , is highly reactive and can trigger spontaneous polymerization. 6,7 How then can one solve a century-old mystery to further help phosphazene chemists better understand this illusive substance? Quantum mechanical (QM) computational methods have been applied as invaluable tools to investigate highly reactive chemical substances that can be very challenging if not impossible to isolate through experiment and characterization using traditional instrumental analyses.…”

“…Hence, the presence of the suggested Cl 3 PNH compound is postulated . While there have been indications of its potential identification through 31 P NMR, − the isolation and comprehensive characterization of Cl 3 PNH using various instrumental analysis techniques have not been achieved historically. Despite the failed attempts to isolate Cl 3 PNH, Becke-Goehring et al used sym -tetrachloroethane ( s -TCE) as a solvent and successfully isolated [Cl 3 PNPCl 3 ] + [X] − as a precipitate .…”

“…It is assumed that the reaction performed in chlorobenzene produces the same final product, although most likely PCl 5 would retain its neutral form with a trigonal bipyramidal configuration. Though some recent reports indicated that using a Lewis acid-catalyzed living cationic polymerization can guarantee linear phosphazene yields with specific lengths at room temperature, the starting monomer, Cl 3 PSiMe 3 , is highly reactive and can trigger spontaneous polymerization. , …”