Phosphorus-nitrogen polymers polyphosphazenes

Phosphorus-Nitrogen Polymers. The most extensively investigated class of such polymers is the polyphosphazenes, which have been reviewed in regard to flame retardancy (143) and which are discussed in a separate article in this Encyclopedia. Research on polymeric phosphonamides, phosphoramides, and phosphorimides has not as yet led to commercial results. 

We might expect, therefore, to find phosphorus/nitrogen polymers, (R2PN) , analogous to the silicones, (R2SiO) , that we looked at in Section 10.3. These polymers are indeed known and are called polyphosphazenes in recent years they have found various practical applications. 

The study of open-chain polyphosphazenes has attracted Increasing attention In recent years, both from the standpoint of fundamental research and technological development. The polyphosphazenes are long chains of alternating phosphorus-nitrogen atoms with two substituents attached to phosphorus. These polymers have been the subject of several recent reviews (1-3). Interest has stemmed from the continuing search for polymers with improved properties for existing applications as well as for new polymers with novel properties. 

Ceramic-type materials that contain no organic linkage units can be prepared by the pyrolysis of cyclic or high polymeric aminophosphazenes. An example is shown in reaction (44). Under appropriate conditions, pyrolysis products that correspond to phosphorus-nitride are formed. Polyphosphazenes that contain both amino and borazine side groups yield phosphorus-nitrogen-boron ceramics following pyrolysis 94,95 The conversion of a formable polymer into a ceramic has many potential advantages for the controlled synthesis and fabrication of advanced ceramics. This principle is discussed in more detail in Chapter 9. 

Polyphosphazenes represent a new approach to the design and synthesis of side-chain liquid crystal polymers. Polyphosphazenes are inorganic main-chain polymers consisting of alternating phosphorus-nitrogen atoms in the main chain with two substituents attached to each phosphorus atom. The top of Figure 1 shows the general structure for a side chain liquid crystal polymer a polymer... 

Polyphosphazenes are of particular interest in this respect because the synthetic versatility allows different biologically-related properties to be designed into the molecule by changes in side group structure. Moreover, the phosphorus-nitrogen backbone offers the possibility that, in the presence of specific side groups, the polymer... 

The presence of phosphorus in the polymer backbone has a veiy practical consequence, quite apart from the structural issues. Phosphorus is one of the most important elements that prevent the combustion of organic materials. The presence of both phosphorus and nitrogen is synergistic. Thus, the phosphorus-nitrogen backbone in polyphosphazenes ensures that many poly(organophosphazenes) are not only nonflammable but also quench combustion of other compounds with which they are in contact. The mechanism of this fire suppression is believed to be both an interruption of the free radical processes that occur in a flame and the formation of an intumescent char that shields the material from the ingress of oxygen. 

The presence of the phosphorus-nitrogen backbone confers to these kind of polymers thermo-oxidative stability, fire resistance, very high torsional mobility (low barrier to skeletal bond twisting), high refractive index, and hydrophilicity. On the other hand, the side groups in polyphosphazenes control other properties such as solubility, secondary reaction chemistry, thermal decomposition, and resistance to hydrolysis. The possibility of tuning the properties of polyphosphazenes thanks to their synthetic flexibility has led to enormous interest in their applications in several areas of research. 

Polycarbophosphazenes possess a backbone of phosphorus, nitrogen, and carbon atoms and can be regarded as derivatives of classical polyphosphazenes (1) in which every third phosphorus atom is replaced by carbon. The first examples of these materials were discovered in 1989 (88). Thermal ROP of a cyclic carbophos-phazene was used to prepare the chlorinated polymeric species (23), which im-dergoes halogen replacement reactions with nucleophiles such as aryloxides and aniline to yield hydrolytically stable poly(aryloxycarbophosphazenes) (24) (Afw = ca 10 , Mn = 10 ) (eq. 23) (88-91). The polymer backbone in these materials was found to be less flexible than in classical polyphosphazenes. For example, the halo-genated polymer (23) possesses a Tg of —21°C compared to a value of —66°C for poly(dichlorophosphazene) (2). 

Sulfur-nitrogen-phosphorus polymers possess backbones that can be regarded as compositional hybrids of those present in sulfur-nitrogen polymers, such as the solid-state polymer poly(sulfur nitride) [SNh or polyoxothiazenes [RS(0)=N]n and classical polyphosphazenes, [R2P=N] (1) (92). Poly(sulfur nitride), [SNh, possesses remarkable properties such as electrical conductivity at room temperature and superconductivity below 0.3 K (93). [SNh is insoluble and has a polymeric structure in the solid state with interchain S - S interactions. As these interactions are crucial to the properties of the material, [SNl is best regarded as a solid-state polymer rather than a polymeric material with discrete macromolecu-lar chains of the type discussed in this article. 

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