Highly Polymeric Phosphazenes

Traces of oxygen-containing materials such as water initiate cross-linking by providing bridging atoms, but poly-dichlorophosphazene (phosphonitrilic chloride polymer) decomposes slowly in contact with atmospheric moisture, and hydrolysis occurs rapidly and completely in water at 100°C. The tetramer polymerises more slowly than the trimer when heated, but eventually gives the same products. When the rubbery products are heated above about 350°C, depolymerisation begins (12.230). 

One of the unusual properties of (PNF2) is its low-tanperature elasticity, which reflects a relatively high degree of torsional mobility of the chain. Of the three polymeric phosphazene halides, the fluoride has the lowest glass transition temperature, which is in accord with it having least interchain interaction and highest torsional freedom at low tanperatures (Table 12.37). 

Poly bis-isothiocyanophosphazene, [PN(NCS)2] , can be made by direct polymerisation of the cyclic trimer or tetramer. It is partially crystalline in the unstretched state and is a little more resistant to hydrolysis than the halide polymers. 

Polymer properties are generally dependent upon factors such as chain-length distribution and inter-chain forces. The latter may be strongly influenced by the nature of the side groups moreover, in some cases chemical reaction between these groups may lead to considerable cross-linking taking place. 

Although the polyhalophosphazenes have potentially useful physical and mechanical properties, their chemical reactivity and hydrolytic instability rule out their practical use. Luckily, however, an important aspect of the phosphazene polymer system is the relative ease with which the properties can be modified by the introduction of different side groups. Useful properties of such organopoly-phosphazenes include resistance to water, solvents, oils, and so on non-inflanunability and flame retardancy stability to visible and ultraviolet radiation high thermal stability ( 200 C) and low-temperature flexibility and elasticity. 

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Allcock, H. R., and Brennan, D. J., Organosilicon derivatives of cyclic and high polymeric phosphazenes, J. Organomet. 

Cyclic and High-Polymeric Phosphazenes as Carrier Molecules for Carboranyl, Metallo, or Bioactive Side Groups... 

Metallophosphazenes with Phosphorus-Metal Bonds. Until recently, the chemistry of cyclic and high polymeric phosphazenes was essentially the chemistry of their organic derivatives (Scheme 1). However, a discovery reported in 1979 (31) opened up a new field of metallophosphazene chemistry in which transition metals form the nucleus of the side group structure and are linked to the skeleton through phosphorus-metal bonds. These species are of theoretical and potentially practical importance, and I will summarize briefly some of the main features known at this time. 

Figure 3.5 Schematic representation of the effect of steric hindrance generated by bulky side groups on a cyclic trimeric and a high polymeric phosphazene. Depolymerization of a high polymer to a cyclic trimer relieves the intramolecular crowding.

ORGANOSILICON DERIVATIVES OF CYCLIC AND HIGH POLYMERIC PHOSPHAZENES... 

Species that combine the properties of organosilicon compounds and phos-phazenes are prepared by the linkage of organosilicon side groups to a small molecule cyclic or linear high polymeric phosphazene skeleton. This is particularly important for high polymeric derivatives in which hybrid properties typical of polysiloxanes (silicones) and polyphosphazenes - may be obtained. 

The chemistry of cyclic oligomeric and high polymeric phosphazenes is dominated by substitution reactions, as is the chemistry of boron-nitrogen heterocycles. However, substitutive techniques play only a minor role in the chemistry of siloxane, silazane, phosphate or metaborate oligomers and polymers. It is not yet well developed in sulfur-nitrogen systems but may become more important in the future. 

N-Silylphosphoranimines eliminate substituted silanes when heated, to yield either cyclic or high polymeric phosphazenes ... 

Among other interesting polymers with cinnamate functional groups are high polymeric phosphazenes that bear cinnamate groups [154]. A typical polymer synthesis is a follows ... 

FIGURE 7.10 Sulfonation of poly(aminophosphazenes) with 1,3-propanesultone. (Reprinted with permission from Allcock, H.R., Klingenberg, E.H., and Welker, M.F., Alkanesulfonation of cyclic and high polymeric phosphazenes. Macromolecules, 26, 5512. Copyright 1993 American Chemical Society.)... 

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