Polyphosphazenes hydrolysis

Figure 12.30 Potential uses of polyphosphazenes (a) A thin film of a poly(aminophosphazene) sueh materials are of interest for biomedical applications, (b) Fibres of poly[bis(trifluoroethoxy)phosphazene] these fibres are water-repellant, resistant to hydrolysis or strong sunlight, and do not burn, (c) Cotton cloth treated with a poly(fluoroalkoxyphosphazene) showing the water repellaney eonferred by the phosphazene. (d) Polyphosphazene elastomers are now being manufaetured for use in fuel lines, gaskets, O-rings, shock absorbers, and carburettor eomponents they are impervious to oils and fuels, do not bum, and remain flexible at very low temperatures. Photographs by eourtesy of H. R. Allcock (Pennsylvania State University) and the Firestone Tire and Rubber Company.

The biomedical uses of polyphosphazenes mentioned earlier involve chemistry that could in principle be carried out on a classical petrochemical-based polymer. However, in their bioerosion reactions, polyphosphazenes display a uniqueness that sets them apart. This uniqueness stems from the presence of the inorganic backbone, which in the presence of appropriate side groups is capable of undergoing facile hydrolysis to phosphate and ammonia. Phosphate can be metabolized, and ammonia is excreted. If the side groups released in this process are also metabolizable or excretable, the polymer can be eroded under hydrolytic conditions without the danger of a toxic response. Thus, poljnners of this tjT are candidates for use as erodible biostructural materials or sutures, or as matrices for the controlled delivery of drugs. Four examples will be given to illustrate the opportunities that exist. 

Allcock, H. R., and Kwon, S., Glyceryl polyphosphazenes Synthesis, properties, and hydrolysis. Macromolecules. 21. 

Although the hydrolytic stability of some phosphazene polymer makes them attractive as structural materials, it is possible to create hydrolytically sensitive phosphazenes that may be useful medically as slow-rdease drugs. Steroids, antibiotics. and catecholamines (e.g., dopamine and epinephrine) have been linked to a polyphosphazene skeleton (Fig. 16.27) with the intention that slow hydrolysis would provide these drugs in a therapeutic steady state. 

Examples of known phosphazene polymer blends are those in which phosphazenes with methylamino, trifluoroethoxy, phenoxy, or oligo-ethyleneoxy side groups form blends with poly(vinyl chloride), polystyrene, poly(methyl methacrylate), or polyethylene oxide).97 100 IPNs have been produced from [NP(OCH2CH2OCH2CH2OCH3)2] (MEEP) and poly(methyl methacrylate).101-103 In addition, a special type of IPN has been reported in which a water-soluble polyphosphazene such as MEEP forms an IPN with a silicate or titanate network generated by hydrolysis of tetraethoxysilane or tetraalkoxytitanane.104 These materials are polyphosphazene/ceramic composites, which have been described as suitable materials for the preparation of antistatic layers in the manufacture of photographic film. 

The ester protection technique has also been used in the preparation of amino acid derivatives of polyphosphazenes (70, 71). For example, eAyl glycinate reacts with poly(dichlorophosphazene) to link the amino function to e phosphorus atoms of the backbone. Subs uent exposure of the protected polymer to water in a biological environment leads to hydrolysis of the ester function and breakdown of the polymer to ethanol, glycine, phosphate, and ammonia. This has been used both for the controlled delivery of drugs and for stimulating the regrowth of bone in tissue engineering experiments (72). 

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. 

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