Phosphazenes phosphonitrilic

H. R. Allcock, Recent Advances in Phosphazene (Phosphonitrilic) Chemistry, Chem. Rev. 72, 315 (1972). 

Allcock, H.R. (1972) Recent advances in phosphazene (phosphonitrilic) chemistry. Chem. Rev., 72, 315-356. 

Poly((2-trifluoroethoxy)( 1 -difluoro-2,3,4-hexafluoropentoxy) phosphazene) Phosphonitrilic fluoroelastomers... 

Which phosphazene would be more flexible one made by reaction of poly(phosphonitrilic chloride) with (a) sodium trifluoroethoxide or (b) sodium trifluorobutoxide ... 

IF the product were a monomer, its structure could be drawn as Cl2PsN, which is analogous to organic nitriles. R—C=N. For this reason the original names used for these compounds were phosphonitriles, phosphonitrilic chloride, etc. However, the products are actually either cyclic or linear polymers of general formula [NPC ,] . Thus, by analogy with benzene, borazine. etc., these compounds hove become known as phosphazenes. The rn jor product of the reaction in Eq. 16.47 and the easiest to... 

Phosphazenes, formerly known as phosphonitrilic compounds, are characterized by the presence of the group P=N. Known compounds, particularly those containing the —p=n— group, are very numerous and they have important potential applications. 

Summary Phosfdiazene bases represent a new class of highly active non-ionic catalysts that rapidly polymerize cyclosiloxanes with equilibrium attained in very short reaction times at very low catalyst levels. To date, phosphazene base catalysts have been considered an academic curiosity because of the complicated and hazardous synthetic protocol used to prepare them. A facile synthetic process has been developed, which yields ionic phosphazene bases in three steps with an overall yield of qrproximately 75%. This is achieved through nucleophilic substitution of ionic phosphonitrilic chloride oligomers with secondary amines, followed by anion exchange. These ionic phosphazenes were found to exhibit similar reactivity in the ring-opening polymerization of cyclosiloxanes to that of the non-ionic phosphazene base. 

The synthesis of oligomeric peralkylated polyaminophosphazenes based upon the P3 and P4 templates is an elegant (but complicated), hazardous, and expensive multi-step process. To exploit catalysts of this type in silicone synthesis on an industrial scale would require a much simplified and lower-cost synthetic protocol. One of the simplest and lowest-cost routes to a conjugated -P=N- template that forms the framework for the synthesis of phosphazene base materials is via phosphonitrilic chloride oligomers (Fig. 3). These are well-known acidic catalysts used in the silicone industry for the condensation polymerization of silanol-terminated polydimethylsiloxanes. Catalysts of this type are most commonly prepared by the reaction of PCI5 with NH4CI or HMDZ,... 

In 1958 I gave a course of lectures on aromatic character in Coulson s Theoretical Summer School in Oxford. After one of them I met Norman Paddock (later Professor on Chemistry in the University of British Columbia), then working in the research department of Albright and Wilson. The company s commercial interests included the cyclic phosphonitrilic halides (phosphazenes) that Paddock was characterizing. They have ring systems with alternate nitrogen and phosphorus atoms. The substituents on the pentavalent phosphorus atoms may be chlorines or fluorines or other atoms or groups. 

Phosphazenes or Phosphonitrilic Compounds Phosphazenes are a group of compounds represented by the general formula (NPX2)/7 where X = F, Cl, Br, SCN, CH3, C6H5 etc. All these compounds are... 

Phosphonitrilic chlorides are important phosphazenes with the general formula (NPCl2)j where x ranges... 

A great many compounds are known with P—N and P=N bonds.47 R2N—P bonds are particularly stable and occur widely in combination with bonds to other univalent groups, such as P—R, P—Ar and P—halogen. The entire subject of P—N compounds cannot be properly outlined in the space available. Hence, only one part of it, the chemistry of phosphazenes (also called phosphonitrilic compounds), will be described in detail. 

Fig. 3. Phosphonitrilic chloride oligomer precursor to phosphazene base.

H.R. Allcock, W.J. Cook, D.P. Mack, Phosphonitrilic compounds. XV. High molecular weight poly[bis(amino)phosphazenes] and mixed-substituent poly(aminophosphazenes), Inorg. Chem. 11 (11) (1972) 2584-2590. 

Phosphonitrilic polymers are self-extinguishing or fire-retardant. This led to the development of flame- and heat-resistant polyimide composites that are prepared from maleimide-substituted phosphazenes. The maleimide group is used for crosslinking. The substituted phosphazene can be illustrated as follows ... 

The poorly reproducible polymerization is presumably initiated by traces of cationic impurities. The poly(phosphonitrile chlorides) or poly(dichloro-phosphazenes), II, mostly have PCI3 end groups. They depolymerize at higher temperatures to hexachlorocyclotriphosphazene, I, and octachlorocyclo-tetraphosphazene and hydrolyze even in moist air. The products obtained at high yield are cross-linked and exhibit all the properties of inorganic elastomers. Consequently, they are also called inorganic rubbers. 

Phosphazenes (formerly known as phosphonitrilic compounds) may also be called azaphosphenes (Section 7.1). They are characterised by the presence of the group P=N. Known compounds, particularly those of the XV variety, are very numerous. Phosphazene chemistry has received a great deal of attention over the past 40 years. Although products with very diverse and important applications have been evolved, extensive commercialisation of these has not yet taken place. 

The chemistry of phosphorus and the heavier congeners is dominated by element to element (E-E) single bonds and, particularly in the case of phosphorus, the availability of l>d orbitals to form dir—pir double bonds with a variety of other atoms such as oxygen, nitrogen, and even sulfur. Orbital participation results in expanded octets as found in compounds such as PF5, SbCl5, X3P=0 (where X=F, Cl, Br, I), the phosphorus oxoadds and oxoanions, and a class of compounds called the phosphazenes. We will take up many of these cases as we encounter them in the appropriate sections under our usual survey of the hydrides, oxides and oxoacids, and halides. For now, however, let s take a quick look at the phosphazenes, formerly called the phosphonitriles, that contain both N and P atoms in the same molecule. 

---