Polyphosphazenes classes

A second class of important electrolytes for rechargeable lithium batteries are soHd electrolytes. Of particular importance is the class known as soHd polymer electrolytes (SPEs). SPEs are polymers capable of forming complexes with lithium salts to yield ionic conductivity. The best known of the SPEs are the lithium salt complexes of poly(ethylene oxide) [25322-68-3] (PEO), —(CH2CH20) —, and poly(propylene oxide) [25322-69-4] (PPO) (11—13). Whereas a number of experimental battery systems have been constmcted using PEO and PPO electrolytes, these systems have not exhibited suitable conductivities at or near room temperature. Advances in the 1980s included a new class of SPE based on polyphosphazene complexes suggesting that room temperature SPE batteries may be achievable (14,15). 

Up to now, nine classes of different polyphosphazenes are known and characterized substituted with aliphatic alcohols [40,41,262-281] or phenols [41,95, 277,282-297],with aliphatic [42,298-300] or aromatic [301-304] amino groups, with di-functional spiro hydroxy (e.g. dihydroxybiphenyl [305] or di hydroxy-... 

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. 

In conclusion, polyphosphazenes containing fluoroalkoxy groups as side phosphorus substituents constitute one of the most relevant class of macromolecules of this family and have attracted remarkable interest in the past because of their outstanding properties and wide range of applicability, especially in low and high temperature domains, and have received renewed interest in more recent times [399,457]. 

Polyphosphazenes sulfonates XIX with the anion covalently attached to the polymer are a new class of cation conductors that have been synthesized by Shriver [625]. They were obtained by reaction of Na0C2H4S03Na with an excess of polydichlorophosphazene in the presence of 15-crown-5, followed by the reaction of the partially substituted product with the sodium salt of poly(ethylene glycol methyl ether). The conductivity at 80 °C of the polymer with x=1.8, m=7.22 is 1.7x10 S cm This low conductivity can be attributed to an extensive ion pair formation between the sodium and sulfonate ions. 

Another class of photochemically relevant polyphosphazenes is formed by macromolecules having chromophores able to absorb light in a selective way and to transfer it to external species, thus inducing different reactions by energy transfer processes. In some cases electron transfer processes are also involved. These situations are described by Formula below and the corresponding polymers and external reagents are reported in Table 26. 

On this basis, five classes of different polyphosphazenes are considered as outstanding examples of this type of macromolecules, in which skeletal and substituent features overlap to the highest extent. The reported materials are elastomers, flame retardants and self-extinguishing macromolecules, polymeric ionic conductors, biomaterials, and photosensitive polymeric compounds all of them based on the polyphosphazene structure. 

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. 

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. 

The polyphosphazenes are high molecular weight polymers with a wide range of novel and potentially useful properties. The large number of different pendant groups with widely varied functionality which can be attached to the P-N backbone demonstrate the unusual molecular design potential of this class of polymers. Undoubtedly, some of these will hold promise for future research and development. 

Today commercial applications for polyphosphazene specialty elastomers exist In aerospace, marine, oil exploration and Industrial fields. This paper describes the properties and applications for this unique class of elastomers. 

Allcock s discovery that stable polyphosphazenes could be prepared under controlled conditions opened the door for the commercial development of this class of polymers, and In 1970, the first polyphosphazene elastomers were synthesized and the technology subsequently developed by Firestone Tire and Rubber Company (5). Today, polyphosphazenes are commercially available, and represent... 

Although polyphosphazene chemistry is nearly 100 years old, the technological potential of this class of polymers is only now being realized. 

The final class of polymers containing carboranyl units to be mentioned here is the polyphosphazenes. These polymers comprise a backbone of alternating phosphorous and nitrogen atoms with a high degree of torsional mobility that accounts for their low glass-transition temperatures (-60°C to -80°C). The introduction of phenyl-carboranyl units into a polyphosphazene polymer results in a substantial improvement in their overall thermal stability. This is believed to be due to the steric hindrance offered by the phenyl-carborane functionality that inhibits coil formation, thereby retarding the preferred thermodynamic pathway of cyclic compound formation (see scheme 12). 

A number of polyphosphazenes of repeat unit [-PRR N-] also exhibit liquid-crystalline phases [166-168]. It is certainly intriguing that apparently the only classes of flexible chains that extensively exhibit liquid-crystalline phases are the polysiloxane and polyphosphazene semi-inorganic polymers. 

Closely related to polyphosphazenes is the class of polymers known as polyheterophosphazenes, where one or more of the P atoms per repeat unit is substituted by an atom of a heteroelement. The first well-characterised example of such materials involved carbon as the replacement for phosphorus the resulting macromolecules, polycarbophosphazenes, were prepared via ROP, but at a dramatically lower temperature than for (NPCl2)3 [eqn (11.37)]. Subsequently, polymers with three-coordinate sulfur(IV) and four-coordinate sulfur(VI) centres were obtained and these materials were termed poly-thiophosphazenes and polythionylphosphazenes, respectively. The latter polymers [eqn (11.38)] are much more stable than the sulfur(IV) analogues after halogen replacement and several have been explored as matrices for gas sensors as a consequence of their high permeability. 

Worthy of attention are the attempts to produce LC polymers on the basis of inorganic polymers those are polyphosphazenes with mesogenic side groups (cholesterol) although the first results to have been published were not promising40. A broad class of heterocyclic compounds could have probably contributed to the synthesis of new systems. The synthetic possibilities of this approach are quite evidently far from being exhausted. 

Polyphosphazenes comprise by far the largest class of inorganic macromolecules. At least 700 different polymers of this type have been synthesized, with a range of physical and chemical properties that rivals that known hitherto only for synthetic organic macromolecules.1... 

For most polymers it is exceedingly difficult to predict the outcome of a balancing of all these influences. Hence, it is difficult to predict chain conformations. However, for polyphosphazenes, the situation is simpler than for most classes of macromolecules. 

Liquid-phase separations can be carried out with membranes produced from several different classes of polyphosphazenes. An example membrane prepared from [NP(NHC4H9)2] is shown in Figure 3.13. The polymer, [NP(OCH2CF3)2] , can be employed to concentrate alcohols, because the alcohol diffusion rate is much faster than that of water.156... 

The main class of bioerodible polyphosphazenes that have been developed so far are polymers with amino acid ester side groups. They are prepared by the reaction of poly(dichlorophosphazene) with the ethyl or propyl esters of amino acids such as glycine, alanine, phenylalanine, and so on (reaction (57)).196 The ethyl or propyl ester of the amino acid must be used as the nucleophile in this reaction for two reasons. First, a free carboxylic acid unit would provide a second nucleophilic site that could lead to... 

It is intriguing that even some flexible siloxane polymers form mesomorphic (liquid-crystalline) phases.34 139-166 Some illustrative data are given in Table 4.2. Both poly(diethylsiloxane) and pol y(di-n-propylsiloxane) show two crystalline modifications as well as a mesomorphic phase. (The other major class of semi-inorganic polymers, the polyphosphazenes, are also relatively flexible, and show similarly interesting behavior.)10167... 

Most synthetic polymers are totally organic macromolecules derived from petroleum. One of the exceptions is the broad class of polymers with an inorganic backbone - called polyphosphazenes. 

The design of single-component polymer transport materials continues to interest researchers in this field. The use of such materials will completely eliminate solvent extraction, diffusional instability, and crystallization of the small molecules. One obvious route that has not been successful to date is the design of yet another aromatic-amine-containing carbon-backbone polymer. An alternative may be to explore the large class of glassy silicon-backbone polymers, such as polysilylenes (14) and polyphosphazenes (iS). 

Specialty polymers achieve very high performance and find limited but critical use in aerospace composites, in electronic industries, as membranes for gas and liquid separations, as fire-retardant textile fabrics for firefighters and race-car drivers, and for biomedical applications (as sutures and surgical implants). The most important class of specialty plastics is polyimides. Other specialty polymers include polyetherimide, poly(amide-imide), polybismaleimides, ionic polymers, polyphosphazenes, poly(aryl ether ketones), polyarylates and related aromatic polyesters, and ultrahigh-molecular-weight polyethylene (Fig. 14.9). 

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