Phosphazenes small cyclic

A number of ab initio molecular orbital calculations have been performed on acyclic and cyclic phosphazenes. These calculations point to a phosphorus-nitrogen bond with a large degree of charge separation and a small but essential contribution from phosphorus d-orbitals. 

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. 

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. 

It is exceedingly difficult to determine the molecular structure of a synthetic macromolecule. X-ray diffraction—the ultimate structural tool for small-molecule studies—yields only limited information for most synthetic high polymers, and crucial data about bond lengths and bond angles are difficult to obtain.47 However, that same information can be obtained relatively easily from single crystal X-ray diffraction studies of cyclic trimers, tetramers, and short-chain linear phosphazene oligomers. The information obtained may then be used to help solve the structures of the high polymeric counterparts. 

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.

Species that combine the properties of organosilicon compounds and phos-phazenes are prepared by the linkage of organosilicon side groups to a small molecule cyclic or linear high polymeric phosphazene skeleton. This is particularly important for high polymeric derivatives in which hybrid properties typical of polysiloxanes (silicones) and polyphosphazenes - may be obtained. 

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). 

In the first approach, small molecule cyclic (Scheme 22) and high polymeric (Scheme 23) phosphazenes bearing bromomethylene - phenoxy side groups are treated with a sodium dialkyl phosphite, as shown in Scheme 22. 

My own interest in this field arose from the recognition (based on reaction mechanistic arguments) that the phosphorus-nitrogen backbone itself should be hydrolytically stable, and that replacement of the halogen atoms by organic residues should yield substituted macromolecules that are hydrolytically stable. These ideas were tested first with small-molecule cyclic chloro-phosphazenes, such as V. But the extension of these principles to the macromolecular analogue (VI) proved to be a serious problem. An insoluble polymer is an unsuitable material for substitution reactions. However, during the course of a series of kinetic experiments on the thermal conversion of V to VI, it was observed that the "swellability" of VI in benzene or tetrahydrofuran decreased as the.polymerization time was increased. ... 

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