Cyclic phosphazenes, applications

Among inorganic heterocycles, perhalogenated cyclic phosphazenes occupy a very prominent place as the precursors for polyphosphazenes whose properties can he tuned by changing the substituents on the phosphorus sites (J). The heterocycles, as such, provide a robust framework for building a variety of novel molecules that have high thermal stability and flame retardancy, and have also been used recently to prepare a variety of dendrimers (2). The most widely studied among these are the perchlorinated cyclophosphazenes. Their syntheses reactions, properties, and application potential have been well documented a, 3-7). 

This review covers phosphazene literature over the period June 1996 to June 1997 (Chemical Abstracts Vols. 125 and 126) and comprises linear phosphazenes including compounds derived thereof (Section 2), cyclic phosphazenes (Section 3) and polyphosphazenes (Section 4). Structural data have been summarized in Section 5. Subjects in Sections 2 and 3 are arranged in the following sequence, reviews, theoretical studies, physico-chemical and chemical studies, and finally applications. For polyphosphazenes (Section 4) the greater part of literature has been arranged around central subjects. 

In contrast to the use of cyclic phosphazene trlmers, linear phosphonltrlllc chloride polymers were reported from the reaction of open-chain phosphonltrlllc chloride oligomers with ammonia or ammonium chloride (11). The polymers (II) obtained appear to have a lower MW than those described earlier, and they may be more useful for applications such as fire resistant coatings and foams which are discussed in a subsequent section of this paper. 

Although they are not elastomers, other phosphazenes also offer potential In areas requiring heat and fire resistance. Mixed fluoroalkoxy - aryloxy substituted cyclic phosphazenes are currently attracting Interest for fire resistant fluid applications (46). In other studies, cyclophosphazene matrix resins have been prepared which have potential for high temperature adhesive applications (47) and as composite matrix materials (48). These studies serve to further demonstrate the enhanced thermal stability and fire resistance which can be achieved with these phosphorus - nitrogen systems. 

Approaches to phosphazene containing ring systems can be summarized into two research strategies. In the first strategy, heterocycles are attached as pendant groups onto either linear or cyclic phosphazene structures. Examples of this approach to yield materials with specific applications has been seen earlier in this chapter. In the second strategy, bidentate pendant groups are attached to a cyclotriphosphazene which then becomes part of the heterocycle. 

An increasing number of investigators are interested in the propagation of cyclic monomers without carbon. The reasons are theoretical interest and anticipation of the future need of monomer sources other than the fossil fuels. Theoretically siloxane units can be arranged into lamellae or bundles (an analogy of mica or asbestos) yielding thermoplastic materials. Another possibility could be the application of the cyclic esters of phosphoric acid [326], phosphazenes, etc. 

The polymeric phosphazenes are treated in chapter (see Polyphosphazenes) A recent monograph covers the chemistry of polyphosphazenes (nomenclature, synthesis of cyclic monomers , ring opening polymerization, condensation polymerization, substitution, polymer properties, and applications more than 1000 literature citations). Other reviews have also been published recently. Sulfur-containing polyphosphazenes have also been described. ... 

Through steric hindrance and conjugative effects, these ionic phosphonium salts are very stable to hydrolysis. This, coupled with the lipophilic nature of the cation, results in a very soft, loosely bound ion pair, making materials of this type suitable for use as catalysts in anionic polymerization [8 - 13]. Phosphazene bases have been found to be suitable catalysts for the anionic polymerization of cyclic siloxanes, with very fast polymerization rates observed. In many cases, both thermodynamic and kinetic equilibrium can be achieved in minutes, several orders of magnitude faster than that seen with traditional catalysts used in cyclosiloxane polymerization. Exploiting catalysts of this type on an industrial scale for siloxane polymerization processes has been prevented because of the cost and availability of the pho hazene bases. This p r describes a facile route to materials of this type and their applicability to siloxane synthesis [14]. 

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