Applications of Polyphosphazenes

From information presented earlier in this chapter, it will be clear that three types of molecular structures in polyphosphazenes are known to give rise to rubbery or elastomeric properties. These are summarized in Table 3.1. First, if the side groups are single atoms, such as fluorine, chlorine, or bromine, the inherent flexibility of the backbone dominates the materials flexibilty and gives rise to elastomeric character. Unfortunately, the polymers (NPF2) , (NPC12) , and (NPBr2) are slowly hydrolyzed in contact with atmospheric moisture. Hence, they are of interest as reaction intermediates but not as usable materials. 

Being themselves capable of assuming different conformations and shapes, they do not fill the available space in the most efficient manner. Thus, molecular voids of free volume exist and this space provides freedom for movement of the polymer chains. The greater the free volume the greater the material s flexibility. The elastomers in this class are, in general, stable to water. 

The first polyphosphazenes to be developed commercially were polymers with two different types of fluoroalkoxy side groups attached to the same chain.142 145a A typical example is shown in 3.73. Polymer 3.73 has a more complicated structure than is implied by the illustration. The polymer is synthesized by the reaction of a mixture of CF3CH2ONa and CFB CFj CHjONa with poly(dichlorophosphazene). This means that the two nucleophiles compete for replacement of the available chlorine atoms, a process that could lead to a random distribution of side groups, but could also generate repeating units in which the phosphorus atoms bear two of the same side groups. 

Membranes are thin polymeric films that may permit the faster diffusion of some molecules than of others. Thin films of polymers are widely used for the separation of gases and for liquid-phase separations (dialysis). Because of the ease of property tuning, polyphosphazenes are of great interest for these types of applications, although only a few examples have yet been investigated. 

Such materials are known as semipermeable membranes. They are essential components of nearly all living things, and the development of new materials of this type is an important component of biomedical research. The control of diffusion of molecules through a membrane can be accomplished by variations in the hydrophilicity of the polymer molecules that constitute the membrane. As in biological membranes, hydrophobic molecules are more likely to pass through the hydrophobic domains of a synthetic membrane than through the hydrophilic regions, and vice versa. 

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Allcock HR. Chemistry and Applications of Polyphosphazenes. Hoboken, NJ Wiley-Interscience, 2003, p. 504. 

In this section we will describe the general principles that determined the biological applications of polyphosphazenes in different domains, putting an effort into establishing their specific utilization on the basis of structure-property relationships. This argument has been covered by several different review articles in the past [400-406,626] and has been recently highlighted by H. R. Allcock [627] and E. Schacht [407]. 

Allcock HR (2002) Chemistry and applications of polyphosphazenes. Wiley Interscience, Hoboken, New Jersey... 

All cock HR (2002) Chemistry and applications of polyphosphazenes. Wiley Interscience, Hoboken, New Jersey, p 690, Appendix 2, Table II.A Cowie JMG, Sadaghianizadeh K (1988) Polym Commun 29 126 Cowie JMG, Sadaghianizadeh N (1988) Makromol Chem, Rapid Commun 9 387 Allcock HR, Olmeijer DL, O Connor JM (1998) Macromolecules 31 753... 

Applications of polyphosphazenes as flame retardants, electrolytes for special batteries, and biomaterials have been described. Some cyclophosphazene derivatives can serve as hosts for small molecule Clathration. ... 

The interest in polyphosphazenes may be reflected by the number of papers that have appeared on this subject. Review papers deal with general features of polyphosphazenes or describe a specific subject, as application of polyphosphazenes to form ceramic materials through the sol-gel technique, medical application and polyphosphazene electrolytes. Short reviews have been presented as contributions for symposia or conferences. ... 

Diffusion coefficients for methanol (concentration range 1.0-5.0 M) in ion-exchange membranes of UV-crosslinked sulfonated poly(bis-3-methoxyphenoxyphosphazene) (134) have been reported to be much smaller than those in Nafion perfluorosulfonic ion-exchange membranes. Application of polyphosphazene-based membranes in methanol-based fuel cells has been reported. ... 

Polyphosphazenes as Carriers for Active Groups.— The increasing interest in the application of polyphosphazenes as carrier-polymers for (re)active agents is reflected by a growing number of papers dealing with the preparation of such systems. In all cases the cyclic trimeric system has been used as a small-molecule model to gain information about reaction procedures. 

The biomedical applications of polyphosphazene derivatives and phos-phoiylated polymers will not be covered in this review, although... 

As already reflected by the number of review papers, the interest in biomedical applications of polyphosphazenes continues to grow. The biodegradable polymer NP(NHC6H4C02H-4)i.8(NHC6H4C02H-4)o.2 n has been prepared and fully characterized. The carboxylato groups can react with Ca -ions to form crosslinks and to convert the water-soluble polymer to a hydrogel. Controlled drug release experiments have been carried out for chitosan en-... 

Although, the repeat imit in all the polyphosphazenes comprises the [N=PR2] repeating unit, the properties of the polymers can be radically changed by varying the R groups on phosphorus. In this section, we will examine the preparative procedures, the structural features and the applications of polyphosphazenes. 

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