Phosphazene properties

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. 

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). 

Allcock HR. Qrganometallic and bioactive phosphazenes. J Polym Sci Polym Symp, 1983, 70, 71-77. Allcock HR. Poly(organophosphazenes) Synthesis, unique properties and applications. Makromol Chem... 

Abstract In this paper the synthesis, properties and applications of poly(organophos-phazenes) have been highlighted. Five different classes of macromolecules have been described, i.e. phosphazene fluoroelastomers, aryloxy-substituted polymeric flame-retardants, alkoxy-substituted phosphazene electric conductors, biomaterials and photo-inert and/or photo-active phosphazene derivatives. Perspectives of future developments in this field are briefly discussed. 

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]. 

The general conclusion is that the phosphazene macromolecules synthesized and characterized up to now show a large range of properties and that they can potentially be exploited in many different applicative domains. 

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. 

The presence in these copolymers of hetero-substituted monomeric units randomly dispersed along the phosphazene skeleton brings about the extreme difficulty of the polymeric chains to be packed in regular structures. They lose, therefore, the original stereo-regularity of the parent phosphazene homopolymers (microcrystalline materials), and show only amorphous structures, with sharp decrease in the values of the Tg (collapsed up to about -90 °C) and with the onset of remarkable elastomeric properties [399,409,457]. 

The first phenomenon observed is the improved resistance of these materials to combustion, in a way that they may be classified as intrinsically self-extinguishing substrates. For instance, the LOI value for PTFEP is reported to be 48 [452], which is much higher than reported for classical organic plastics [283], while phosphazene fluoroelastomers have been considered as fire-retardant materials since the very beginning of their preparation and utilization [562]. Similarly to aryloxy- and arylamino- substituted POPs [389,390] (vide infra),it may be expected that the flame-resistance properties of phosphazene fluoroelastomers could be successively exported to stabihze organic macromolecules when blended with these materials. 

Fire retardancy is an often occurring theme in phosphazene chemistry and numerous reviews have focused on this subject over the years [ 10,44,387,393,396, 582]. In this article we will treat only aspects related to the flame-retardant properties of aryloxyphosphazene copolymers, which are the subject of the greatest number of applications. 

Table 11 Structure and properties of phosphazene-based ionic conductors ...

One of the most significant developments in phosphazene chemistry during the past year has been the reported application of alkoxycyclophos-phazenes, [NP(OR)2l3,4, as flame retardants in rayon. This development has, in turn, provided a stimulus for improvements to be made in the large-scale production of chlorocyclophosphazenes. Interest in the properties of the monophosphazenes has again increased considerably. 

The properties of fluoroalkoxyphosphazene polymers and copolymers [N=P(OR)2ln (R- = fluoroalkoxy-group) have been described. Condensation of the phosphazenes, (Cl3PNPh)2 and C3N3(N=PCl3)3, with... 

Thus, important structure/property relationships are emerging that are relevant to electronic and optical materials applications for these materials. In a different vein, side chain crystallization has resulted in the first liquid crystalline inorganic and organometallic macromolecules, viz., unusual poly(dialkoxy-phosphazenes) described by Allcock (p. 250) and Singler (p. 268). 

An overview of the synthesis and characterization of a unique class of polymers with a phosphorus-nitrogen backbone Is presented, with a focus on poly(dichloro-phosphazene) as a common Intermediate for a wide variety of poly(organophosphazenes). Melt and solution polymerization techniques are Illustrated, Including the role of catalysts. The elucidation of chain structure and molecular weight by various dilute solution techniques Is considered. Factors which determine the properties of polymers derived from poly(dichlorophos-phazene) are discussed, with an emphasis on the role that the organic substituent can play In determining the final properties. 

In the case of poly(alkoxyphosphazenes) (IV) or poly(aryloxyphos-phazenes) (V) a dramatic change in properties can arise by employing combinations of substituents. Polymers such as (NP CHjCF ) and (NP CgH,).) are semicrystalline thermoplastics (Table I). With the introduction of two or more substituents of sufficiently different size, elastomers are obtained (Figure 4). Another requirement for elastomeric behavior is that the substituents be randomly distributed along the P-N backbone. This principle was first demonstrated by Rose (9), and subsequent work in several industrial laboratories has led to the development of phosphazene elastomers of commercial interest. A phosphazene fluoroelastomer and a phosphazene elastomer with mixed aryloxy side chains are showing promise for military and commercial applications. These elastomers are the subject of another paper in this symposium (10). 

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.)... 

---