Phosphazene polymer

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. 

Another valuable characteristic of many phosphazene polymers is their flame-retardant behavior and low smoke generation on combustion (13). This property is utilized in commercial appHcations. 

A remarkable feature of phosphazene polymers of types (1) and (2) is that appropriate substituents (which are readily attached) can be used as toggle switches to turn several properties, such as hydrolytic stabiHty and electrical conductivity, on and off (1). 

The first phosphazene polymers containing carbon (79), sulfur (80,81), and even metal atoms (82) in the backbone have been reported. These were all prepared by the ring-opening polymerization of partially or fully chloro-substituted (or fluoro-substituted) trimers containing one hetero atom substituting for a ring-phosphoms atom in a cyclotriphosphazene-type ring. 

Phosphazene polymers are normally made in a two-step process. First, hexachlorocyclotriphosphazene [940-71 -6J, trimer (1), is polymerized in bulk to poly(dichlorophosphazene) [26085-02-9], chloropolymer (2). The chloropolymer is then dissolved and reprecipitated to remove unreacted trimer. After redissolving, nucleophilic substitution on (2) with alkyl or aryloxides provides the desired product (3). 

As expected, the TgS of the hybrid sulfanuric-phosphazene polymers are much closer to the values reported for poly(phosphazenes) than those of sulfanuric polymers (vide supra). The values for the polymers 14.10a... 

A few year later (1980) R. H. Neilson and P. Wisian-Neilson started their long term research project on the preparation of alky, aryl, and alkyl/aryl phosphazene polymers and copolymers [55,56,315-334] prepared in the same way by thermal polymerization of a variety of phosphoranimine derivatives [55, 329, 335, 336] to the corresponding phosphazene macromolecules. The polymers obtained by long heating (several days) at high temperatures (160-220 °C) showed relatively low (about 50,000) molecular weight. 

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-... 

Phosphazene polymers can act as biomaterials in several different ways [401, 402,407]. What is important in the consideration of skeletal properties is that the -P=N- backbone can be considered as an extremely stable substrate when fluorinated alcohols [399,457] or phenoxy [172] substituents are used in the substitution process of the chlorine atoms of (NPCl2)n> but it becomes highly hydrolytically unstable when simple amino acid [464] or imidazole [405-407] derivatives are attached to the phosphorus. In this case, an extraordinary demolition reaction of the polymer chain takes place under mild hydrolytic conditions transforming skeletal nitrogen and phosphorus into ammonium salts and phosphates, respectively [405-407,464]. This opens wide perspectives in biomedical sciences for the utilization of these materials, for instance, as drug delivery systems [213,401,405,406,464] and bioerodible substrates [403,404]. 

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. 

Table 9 General characteristics of fluorinated phosphazene polymers and copolymers ...

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. 

The behaviour of phosphazene polymers upon irradiation with UV-vis light has been the object of several review articles during the last 20 years [408-413, 708] and for this reason it will be not treated in detail in this paper. The short summary of the topic presented here deals with the consideration that the photochemical behaviour of POPs originates in the combination of the transparency of the skeleton of these materials (up to well inside in the UV range of the... 

In the frame of photochemical research, phosphazene polymers have also been exploited in combination with external reagents able to selectively absorb impinging light and induce reactivity on these materials. The general reaction scheme is shown below. 

Table 28 Hydrogen abstraction reactions in phosphazene polymers and copolymers photosensitized by external reagents...

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. 

Second, bromination of methyl groups attached to arylphenoxy-phosphazene polymers converted them to CH2Br units (22). These were then quaternized with triethylamine, and the quaternary sites were used for anion exchange with sodium heparin (Fig. 2). The... 

Derivatives of the type, RjP N=P(NH2)X 2, were obtained by displacement of the chlorine atoms marked with an asterisk by ammonia. The parent halides decomposed at ca. 150 °C giving, amongst other products, cyclophosphazenes, (NPR2) , and phosphazene polymers. 

Linear phosphazene polymers, obtained from the reaction of ammonium chloride with phosphorus pentachloride in chlorobenzene, may be rendered hydrolytically stable by reaction of one of the terminal chlorine atoms with, for example, sodium phenoxide ... 

This section is devoted to polymers containing open-chain phosphazenes. Cyclolinear and cyclomatrix phosphazene polymers are covered in section 3. Reviews are limited to a discussion of the... 

Because of the high bond energy of the siloxane (Si—O) and phosphazene (P — N) groups, siloxane and phosphazene polymers are particularly stable at high temperatures. The Tt values of phosphazene halides increase from 183 K (NPFj) to 207 K (NPClj), to 265 K (NPBR. ... 

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