Hydrophobic polyphosphazenes

As discussed in an earlier section, the chemistry exists to convert hydrophobic polyphosphazenes to materials with hydrophilic surfaces. Hydrogels of MEEP (3.79) have been radiation grafted to the surfaces of hydrophobic polymers to generate highly hydrophilic interfaces, and these systems have the advantage that the interior of the polymer retains its hydrophobicity and other properties, and only the surface layers are changed.187,188... 

Block copolymers (228), consisting of a hydrophilic poly(ethylene glycol) and a hydrophobic polyphosphazene residue, have been investigated with respect to their micelle formation in aqueous solution Micelle formation in water has also been observed for polymers (229) with ethyl glycinato substituents. Hydrolytic degradation of these polymers has been studied in aqueous thf. ... 

This basic structure provides for considerable flexibility in the design of biomaterials, as described in a recent review [27]. By selection of the side groups on the polymer chain, both hydrophobic and hydrophilic polymers can be produced. Hydrophobic polyphosphazenes may be useful as the basis of implantable biomaterials, such as heart valves. The hydrophilic polymers can be used to produce materials with a hydrophilic surface or, when the polymer is so hydrophilic that it dissolves in water, cross-linked to produce hydrogels or solid implants. In addition, a variety of bioactive compounds can be linked to polyphosphazene molecules allowing the creation of bioactive water-soluble macromolecules or polymer surfaces with biological activity. 

This polymer was one of the first hydrophobic polyphosphazenes to be synthesized, and it has received by far the most attention. It is produced by the reaction of sodium 2,2,2-trifluoroethoxide with poly(dichlorophosphazene) (4) (Scheme 1), itself prepared by either the ring-opening polymerization of the cyclic trimer,... 

Eig. 1. Schematic bioactive polyphosphazenes. (a) General stmcture, where X = hydrophilic /hydrophobic group that hydrolyzes with concurrent polymer breakdown, Y = difunctional group for attaching bioactive agent to polymer, and T = bioactive agent, (b) Actual example where X = —OC H, Y = and... 

The term "bioenertness" is a relative one since few if any synthetic polymers are totally biocompatible with living tissues. The terra is used here on the basis of preUminary in vitro and in vivo tests, together with chemical evaluations based on analogies with other well-tested systems. Two different types of polyphosphazenes are of interest as bioinert materials those with strongly hydrophobic surface characteristics and those with hydrophilic surfaces. These will be considered in turn. 

Some of the most hydrophobic synthetic polymers known are polyphosphazenes that bear fluoroalkoxy side groups (1,2,10-12). Examples are shown in structures 11 and 1. ... 

Such polymers are as hydrophobic as poly(tetrafluoroethylene) (Teflon), with water contact angles in the range of 107°. But, unlike Teflon, polyphosphazenes of this type are easy to fabricate, are flexible or elastomeric, and can be used as coatings for other materials. ... 

The connection between hydrophobicity and tissue compatibility has been noted for classical organic polymers (19). A key feature of the polyphosphazene substitutive synthesis method is the ease with which the surface hydrophobicity or hydrophilicity can be fine-tuned by variations in the ratios of two or more different side groups. It can also be varied by chemical reactions carried out on the organo-phosphazene polymer molecules themselves or on the surfaces of the solid materials. 

Both types are hydrophobic materials that, depending on the side group arrangements, can exist as elastomers or as microcrystalline fiber- or film-forming materials. Preliminary studies have suggested that these two classes of polyphosphazenes are inert and biocompatible in subcutaneous tissue implantation experiments. 

Figure 3.20 Plots showing the activity of the enzyme, glucose-6-phosphate dehydrogenase as a function of time for (open circles) the free enzyme in solution and (black circles) the enzyme bound to the surface of poly[W.v(phcnoxy)phosphazene]. The polyphosphazene is an excellent surface substrate because of the stability of the polymer backbone to nitration and reduction, and the ease with which the hydrophobicity or hydrophilicity of the surface can be changed. From Allcock and Kwon, reference 191.

The principal polyphosphazenes that have been used in hydrogels are those with linear or branched ethyleneoxy side chains, aryloxy groups with carboxylic acid substituents, or mixed-substituent polymers that bear hydrophilic methylamino side groups plus a hydrophobic cosubstituent such as phenoxy or trifluoroethoxy. Cross-linking is usually accomplished by gamma-ray irradiation or, in the case of the carboxylic acid functional species, by treatment with a di- or tri-valent cation. Here, we will consider another example based on MEEP (3.79), a polymer that is well suited to the clean method of radiation cross-linking. 

However, polyphosphazene micelles are known. These are formed by sonication of aqueous suspensions of amphiphilic diblock copolymers—macromolecules in which one block is hydrophilic and the other is hydrophobic. Both blocks may be phosphazene-based, or one block can be a phosphazene and the other an organic polymer. Polymers of this type are assembled via the living cationic polymerization discussed earlier. The polymers shown as 3.95-3.99 illustrate the range of structures that have been studied and the size of the micelles formed.237-240... 

Polyphosphazenes have also been prepared using modified amino acid ester side-chain substituents (Table 7). Studies show that the polymer containing the smallest hydrophobic side-chain constituent, glycine ethyl ester, exhibited the most rapid degradation. This was attributed to the lower steric hindrance that the small side-chain group could provide in... 

Pervaporation can also be used to remove organic compounds from wastewater. Elastomeric hydrophobic polymer membranes are used to remove compounds more hydrophobic than acetone present at levels of 200-50,000 ppm.217 Compounds such as 2 butanone, benzene, and trichloroethylene can be recovered. A polyphosphazene -poly(dimethylsiloxane) graft copolymer membrane was able to concentrate 7% ethyl acetate in water up to 97%. The use of silicalite in poly(dimethylsiloxane) improves both selectivity and flux rate.218... 

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