Polyphosphazenes hydrolytic stability

The purpose of this chapter is to introduce a new class of polymers for both types of biomedical uses a polymer system in which the hydrolytic stability or instability is determined not by changes in the backbone structure, but by changes in the side groups attached to an unconventional macromolecular backbone. These polymers are polyphosphazenes, with the general molecular structure shown in structure 1. 

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. 

RS(0)=N]n and classical polyphosphazenes, [R2P=N]n 1 [20,21]. The first well-characterized examples of these materials, polythiophosphazenes, were also reported by Allcock et al. [22]. These polymers were prepared via the thermal ROP of a cyclothiophosphazene. This yielded the hydrolytically sensitive polythio-phosphazene 12 with a backbone of three-coordinate sulfur(IV), nitrogen, and phosphorus atoms. Although reaction of 12 with nucleophiles such as aryloxides yielded materials 13 with improved hydrolytic stability, degradation in the presence of moisture was still rapid except where very bulky substituents such as o-phenylphenoxy were present ... 

Applications. Polymers with small alkyl substituents, particularly (13), are ideal candidates for elastomer formulation because of quite low temperature flexibility, hydrolytic and chemical stability, and high temperature stability. The ability to readily incorporate other substituents (in addition to methyl), particularly vinyl groups, should provide for conventional cure sites. In light of the biocompatibility of polysiloxanes and P—O- and P—N-substituted polyphosphazenes, poly(alkyl/arylphosphazenes) are also likely to be biocompatible polymers. Therefore, biomedical applications can also be envisaged for (3). A third potential application is in the area of solid-state batteries. The first steps toward ionic conductivity have been observed with polymers (13) and (15) using lithium and silver salts (78). 

Polyphosphazenes may provide particular advantages over their organic counterparts in the field of biomedical applications. For artificial organ research, materials can be synthesized that have specific surface properties, extreme stability under hydrolytic or oxidative conditions, and minimal interactions with blood or living tissues. Polymers that possess fluoroalkoxy or aryloxy side groups... 

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