Phosphazenes polyphosphazene

Lithiation Reactions. One of the earliest reactions of this type made use of metal-halogen exchange reactions carried out on poly[bis(p-bromophenoxy)phosphazene]. Polyphosphazenes that bear p-bromophenoxy side oups are normally unreactive. However, they can be lithiated, as shown in Scheme III, and the lithio derivatives react with a wide variety of electrophiles that range from chlorophosphines (19) to organometallic halides (42-45), This provides an access route to polymer-bound transition metal catalysts and other metallated or silylated polymers. 

Applications of various phosphazenes/polyphosphazenes/hybrid derivatives have been mentioned in the previous sections where appropriate. This section deals with some specific applications that seem to be the forte of the phosphazene derivatives. 

Properties. One of the characteristic properties of the polyphosphazene backbone is high chain dexibility which allows mobility of the chains even at quite low temperatures. Glass-transition temperatures down to —105° C are known with some alkoxy substituents. Symmetrically substituted alkoxy and aryloxy polymers often exhibit melting transitions if the substituents allow packing of the chains, but mixed-substituent polymers are amorphous. Thus the mixed substitution pattern is deUberately used for the synthesis of various phosphazene elastomers. On the other hand, as with many other flexible-chain polymers, glass-transition temperatures above 100°C can be obtained with bulky substituents on the phosphazene backbone. 

Phosphazene polymers are inherently good electrical insulators unless side-group stmctures allow ionic conduction in the presence of salts. This insulating property forms the basis for appHcations as wire and cable jackets and coatings. Polyphosphazenes also exhibit excellent visible and uv radiation transparency when chromophoric substituents are absent. 

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.

In this context, phosphoranimine compoimds (both homosubstituted with an unique group or bearing two different groups at the phosphorus) play a fundamental role because their polymerization under different experimental conditions eventually leads to fully substituted polyphosphazenes with no residual chlorines on the phosphazene skeleton. The general scheme of the phosphoranimine polymerization processes is reported in Fig. 10. 

To conclude this synthetic section, it appears very clear that the experimental approaches for preparation of POPs are very numerous and give accessibility to phosphazene polymers and copolymers with different structures and properties. Moreover, it has been recently estimated [10,383] that the total number of polyphosphazenes reported up to now in the literature is about 700, and that these materials can find potential practical application as flame- and fire-resistant polymers [44,283, 384-388] and additives [389, 390] thermally stable macromolecules [391] chemically inert compounds [392] low temper-... 

The possibility of substituting two or more different groups on the same polyphosphazene skeleton [263], even in different relative percentages, to form phosphazene substituent copolymers [399,457]... 

Considering first Table 5, it can be seen that Tg values for the reported poly(organophosphazenes) spanned from very low (-105 °C in the case of poly[bis(n-butoxy)phosphazene]) up to very high (-1-220 °C for poly[tris(2,2 -dioxy-l,T-binaphthyl)phosphazene]), covering almost all the intermediate temperatures between these two limits. Low TgS are indicative of very high torsional freedom of the polyphosphazene chain, which is manifested clearly when flexible substituents of reduced bulkiness are used in the substitution... 

Of course, not all the phosphazene polymers that have been synthesized are equally important. Many of them, in fact, have a mere academic or speculative interest, and will not be described in this article. A few other classes of POPs, however, do occupy an important place in phosphazene history, and have been seriously considered for industrial development and commercialization. These polymers are basically those in which the properties of the inorganic -P=N- skeleton overlap to the highest extent those of the phosphorus side substituents. In the successive sections of this article we will describe in some detail the most important classes of polyphosphazenes that fulfil this condition. 

When x=l and y=2,3,4,. and Z=H or F, a new class of polyphosphazene substrates is obtained, which derive from the simultaneous substitution of two different fluorinated alcohols of different lengths on the same polydichloro-phosphazene macromolecule. The general structure of the substrates is reported below. 

Poly[bis(amino)phosphazenes] XVI and a series of polyphosphazenes bearing Methoxy-ethoxyethoxy and alkylamine side groups XVII have been synthesized and complexed with LiCl04 by Chen-Yang [623,624]. 

Based on the synthesis of polyphosphazenes and of diblock copolyphosp-hazenes by the living cationic polymerization of phosphoranimines [237,241], the triblock poly(phosphazene-ethylene oxide) copolymer XVIII was synthesized by Allcock [223]. 

In conclusion, polymer electrolytes based on phosphazene backbone and containing ether side chains are, after complexation with alkali metal salts, among the highest ionically solvent-free polymer salt complexes, with conductivities in the order of 10" -10" S cm However, these conductivities are still below the value of 10 S cm" which is considered to be the minimum for practical applications. Therefore the design of new polyphosphazenes electrolytes with a higher conductivity and also a higher dimensional stability still remains a challenge for future researchers. 

As already reported in Table 6, the solubility of phosphazene polymers is strongly influenced by the nature of the substituent groups attached at the phosphorus atoms along the -P=N- skeleton. Water-solubility, for instance, can be induced in polyphosphazenes by using strongly polar substituents (e.g. methylamine [84], glucosyl [495], glyceryl [496], polyoxyethylene mono-methylether [273] or sulfonic acid [497,498] derivatives), or may be promoted by acids or bases when basic (amino substituents like ethylamine [499]) or acid (e.g. aryloxy carboxylate [499] or aryloxy hydroxylate [295]) substituents are exploited. 

A different approach to polyphosphazene-based drug delivery systems deals with hydrolytically unstable phosphazene substrates, able to degrade in a controlled way under physiological conditions in human body. A list of these bio-erodible substrates is reported in Table 21. 

Polyphosphazene films could also be modified very easily by grafting organic polymers onto the surface using chemical, photochemical or y-radiolytic processes. In almost all cases these studies led to the increase in the surface hy-drophilicity and biocompatibility of the phosphazene films without depressing their bulk features. 

When the substituent groups in the polyphosphazenes were azobenzene [719] or spiropyran [720] derivatives, photochromic polymers were obtained, showing reversible light-induced trans-cis isomerization or merocyanine formation, respectively. Only photocrosslinking processes by [2+2] photo-addition reactions to cyclobutane rings could be observed when the substituent groups on the phosphazene backbone were 4-hydroxycinnamates [721-723] or 4-hydroxychalcones [722-724]. 

In conclusion, all these types of light-induced reactions involving polyphosphazenes readily account for the great importance assumed by this topic in the phosphazene domain and for the remarkable application potentials of especially designed phosphazene 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. 

Cyclophosphazenes are a fascinating group of inorganic heterocyclic compounds whose chemistry is multi-faceted, well developed and reasonably well understood. They are closely related to the linear poly-phosphazenes this relationship is unlike any other existing between ring-polymer systems. Although cyclic siloxanes and polysiloxanes have a close interrelationship, the number and types of cyclophospha-zene derivatives that are known, together with their exact counterparts in polyphosphazenes, underscore the utility of cyclophosphazenes as models for the more complex polyphosphazenes. The literature on cyclophosphazenes has appeared earlier in the form of books (1,2), chapters of books (3-5), authoritative compilations of data (6,7), and several reviews (8-21). The current literature on this subject is reported annually in the Specialist Periodic Reports published by the Royal Society of chemistry (22). This review deals mostly with chlorocyclo-... 

This chapter covers the literatures of phosph(v)azenes. The general pattern of development in this area is similar to that observed in previous yearly reviews with additional interest being shown in polyphosphazenes and in a variety of molecular orbital calculations of both linear and cyclic phosphazenes. 

With this synthetic and molecular structural diversity, polyphosphazene chemistry has developed into a field that rivals many areas of organic polymer chemistry with respect to the tailored synthesis of polymers for specific experimental or technological uses. Indeed, hybrid systems are also available in which organic polymers bear phosphazene units as side groups. This is discussed in another Chapter. 

Third, metallocene units, such as ferrocene or ruthenocene, have been linked to phosphazene cyclic trimers or tetramers and these were polymerized and substituted to give polymers of the type mentioned previously (41). Polyphosphazenes with ferrocenyl groups can be doped with iodine to form weak semiconductors. Polymer chains that bear both ruthenocenyl and ferrocenyl side groups are prospective electrode mediator systems. 

The morphology of the semicrystalline polyphosphazenes is complex. Table I provides examples of phosphazenes with two first order transitions denoted by T(l) and Tm. The T(l) is an intermediate transition to a partially ordered state. Between T(l)... 

The true value of the chloropolymer (I) lies in its use as an intermediate for the synthesis of a wide variety of polytorgano-phosphazenes) as shown in Figure 1. The nature and size of the substituent attached to the phosphorus plays a dominant roll in determining the properties of the polyphosphazene. Homopolymers prepared from I, in which the R groups are the same or, if different, similar in molecular size, tend to be semi-crystalline thermoplastics. If two or more different substituents are introduced, the resulting polymers are generally amorphous elastomers. (See Figure 1.)... 

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