Phosphazenes structure

The polymer is poly[bis(methoxyethoxyethoxy)phosphazene] (structure 31), also known as "MEEP" (20,40). In the solid state. 

The cyclodiphosphazanes shown in Scheme 1 were thought to have monomeric phosphazene structures, but their dimeric structure has now been established26 by several spectroscopic methods [31P n.m.r., 35C1 n.q.r. (X = Cl), i.r., mass]. Some of... 

Polyphosphazenes and cyclophosphazenes are almost unique as carrier molecules for transition metals because of the wide range of binding sites that can be incorporated into the phosphazene structure. The substitutive mode of synthesis described earlier allows a structural diversity that is not found, for example, in polystyrene, polyphenylene oxide, or other organic carrier polymers. 

Polyphosphazenes are the most important and the most thermally characterized of the phosphorus-containing inorganic polymers. Linear, cycloli-near, and cross-linked cyclomatic polymers based on phosphazene structures have been produced. The repeating units of some polyphosphazenes are as follows ... 

As discussed in Chapter 3, some other inorganic polymers also contain phosphorus atoms.47 They are derived from the basic phosphazene structure described in Chapter 3, and are obtained by the ring-opening polymerization of heterocyclic compounds in which one of the disubstituted phosphorus atoms is replaced by another moiety. Specifically, introduction of a carbon atom can yield poly(carbophosphazenes), with the repeat unit shown in 6.49. Alternatively, replacement with a sulfur atom can yield a poly(thiophosphazene) (6.50). Relatively little is known about these polymers at the present time 47... 

Approaches to phosphazene containing ring systems can be summarized into two research strategies. In the first strategy, heterocycles are attached as pendant groups onto either linear or cyclic phosphazene structures. Examples of this approach to yield materials with specific applications has been seen earlier in this chapter. In the second strategy, bidentate pendant groups are attached to a cyclotriphosphazene which then becomes part of the heterocycle. 

The inherent flexibility of the phosphazene structure leads to the development of advanced materials. A significant amount of work has been published using functionalized cyclotriphosphazenes to yield cyclomatrix-type polymers that have been fashioned into nanoparticles and other nanostructures. Papers have been previously published based on the chemistry of 4,4 -sulfonyldiphenol substituted cyclotriphosphazene (49). An idealized structure is shown. Typically, formation of the substituted trimer yields cross-links between trimer rings through the activity of the terminal hydroxyls. Techniques have been developed to control the polymerization of the these materials forming nanotubes, nanoparticles, and coatings for multi-walled carbon nanotubes, as shown below. 

Detailed instrumental characterization of phosphazene structures can give insight into the nature of these materials. Furthermore, techniques can be demonstrated that give more information concerning the chemical and physical nature of these intriguing materials. This section addresses the various techniques that have been applied to phosphazene structures and are provided to give an updated picture of what can be learned about phosphazenes. 

Other phosphonium cations of interest include [(Me2N)3P-C=P]+ and [Ph3PNMe2]+. The latter has a rather short P-N distance which may reflect a contribution from a phosphazene structure. 

The S (and Se) analogues of (7.239) fonn dimers in the solid state with P/N/P < 180° (7.240). On the other hand, the oxy analogue is monomeric and forms an internal H bond with P/N/P = 180°. In addition, the shorter P-N bond lengths in the oxy derivative indicate a considerable contribution from a phosphazene structure (7.418) below. 

Computational methods can play a role in leading to the understanding of phosphazene structure. An example is cyclotriphosphazene (58) that has been characterized using common chemical methods. Density Functional Theory (DFT) was used to further elucidate the structure of the molecule as a precursor to more complex dendrimeric structures. DFT suggested a concave structure for cyclotriphosphazene (58) with planar pendant group arms and a non-planar phosphazene core. These observations suggest that the terminal groups are spatially available for further chemistry to create a more extensive dendrimeric system. 

The combination of phosphazenes with metal ions provides a route for the synthesis of a wide variety of new materials with varied properties. Metals may coordinate with pendant groups or the electronic rich nitrogens located in the phosphazene structure. An extreme case of the former was provided by the attachment of six porphyrin rings about a eyelotriphosphazene eore (46). In this structure, the porphyrin rings were free to aet as host for double valent Co, Ni, Cu, and Zn. An additional complex was formed with Sn +(OH)2. 

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