Molecules phosphazenes

Allcock HR. Small-molecule phosphazene rings as models for high polymeric chains. Acc Chem Res, 1979, 12, 351-358. 

Figure 3.7 P NMR spectra of cyclic trimeric and high polymeric phenyl-fluoro phosphazenes. Note (1) the shift in the whole spectrum that occurs in moving from a cyclic small-molecule phosphazene to a related high polymer, and (2) the chemical shift and splitting pattern that results from phosphorus coupling to the two fluorine atoms or to one fluorine. Spectra provided by W. D. Coggio.

Various theories have been proposed to explain this anomaly, but the most appealing qualitative idea from a ehemistry point of view was suggested for small-molecule phosphazenes by Dewar, Lucken, and Whitehead in the 1960s that the phosphorus atoms ean utilize their 3d orbitals for overlap with the adjacent nitrogen 2p orbitals, with a node at each... 

The chemistry of small molecule phosphazene ctm be traced back to tbe discovery that phosphorous pentachloride and ammonium chloride react to yield a mixture of compounds of which the main product is a volatile white solid, knowm to have the cyclic trimer structure, shown in Figure 3 (Ross, 1832 Liebich, 1834 Stokes, 1897). 

The poly(phosphazene) backbone can be made to be part of a fully inorganic polymer by replacing the organic substituents with chlorines, thus generating poly(dichlorophosphazene). However, this is a reactive molecule and is more usually employed as the starting material for the preparation of the various partially organic poly(phosphazenes). 

Allcock HR, Shawn R, and Scopelianos AG. Poly [(amino acid ester) phosphazenes] as substrates for the controlled release of small molecules. Biomaterials, 1994, 1, 5563-5569. 

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. 

FIGURE 3 Scanning electron micrograph (1200x magnification) of the surface of a porous alumina particle coated with poly(diphenoxy-phosphazene). Surface nitration, reduction, and glutaric dialdehyde coupling immobilized enzyme molecules to the surface. (From Ref. 23.)... 

D. Pseudohalogeno-derivatives.—Little work has been carried out in this area. Isocyanates of cyclic phosphazenes, previously unknown, are thought to be formed in the reaction of NgPaBrg with AgOCN in nitro-methane. They were detected by i.r. spectroscopy, and underwent ready polymerization, which precluded their isolation. On the other hand, isothiocyanates, [NP(NCS)2] (n = 3 or 4), are well known and a detailed study of their spectra has been reported. The azide, N3Pa(N3)8, has been the subject of an i.r. study which suggests that the molecule has Z)3A symmetry. 

Allcock, H. R., Fitzpatrick, R. J. and Salvati, L. 1991. Sulfonation of (aryloxy) phosphazenes and (arlyamino)phosphazenes small-molecule compounds, polymers and surfaces. Chemistry of Materials 3 1120-1132. 

Allcock s research led to the development of poly-phosphazene-based PEMs by his small molecule studies of the sulfonation of cyclic trimeric phosphazenes and the surface chemistry of polyphosphazene macromolecules. In a 1993 report, he described the sulfonation of aminophosphazenes with 1,3—propanesultone. While these specific materials are not necessarily ideal as PEMs, this study demonstrated a novel technique for creating sulfonated polyphosphazene materials that may provide more control over the sulfonated polymer product than wholesale sulfonation of a base polymer by a strong sulfonating agent. 

Cyclic and High-Polymeric Phosphazenes as Carrier Molecules for Carboranyl, Metallo, or Bioactive Side Groups... 

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. 

Spirocyclic product is formed as a result of replacement of both the chlorines on the same phosphorus atom, while the ansa product is formed as a result of the replacement of chlorines from two distinct phosphorus atoms within the same molecule. In open chain product only one end of the difunctional reagent is involved in reaction with the chlorocyclophosphazene. Finally, intermolecular bridged products result from the reaction of two chlorocyclophosphazene molecules with a difunctional reagent. This last reaction also is a model reaction for condensation polymerization involving phosphazenes. 

The addition of a molecule X—Z (often with Z = H) to a P=0, P=N, P=C or P=S double bond can always be a possibility. The reaction may occur with a distinct molecule, or it may even be intramolecular when XH is a constitutive part of the phosphorusbearing molecule. Structures such as A, A, A", C and C in Scheme 28 are representative of a large proportion of phosphorus compounds of (mono)phosphazenes, and of their cyclophosphazane dimers (over 1000 literature references ). 

Most chemists begin their training by learning about small molecules rather than polymers. The reasons for this are both traditional and practical. Small molecules are often easier to synthesize, purify, and characterize than are polymers. Moreover, in phosphazene chemistry it is easier to study small-molecule reactions, reaction mechanisms, and molecular structures than it is to obtain comparable information at the high-polymer level. 

For these reasons, small molecules have played a crucial role in the development of phosphazene high-polymer chemistry.117 In particular, the substitution reactions, reaction mechanisms, NMR spectroscopy, and X-ray diffraction analysis of small-molecule cyclic phosphazenes, such as 3.2 or 3.3 have provided information that could not be obtained directly from the high polymers. 

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