Organometallic Polyphosphazenes

Unique combinations of properties continue to be discovered in inorganic and organometallic macromolecules and serve to continue a high level of interest with regard to potential applications. Thus, Allcock describes his collaborative work with Shriver (p. 250) that led to ionically conducting polyphosphazene/salt complexes with the highest ambient temperature ionic conductivities known for polymer/salt electrolytes. Electronic conductivity is found via the partial oxidation of unusual phthalocyanine siloxanes (Marks, p. 224) which contain six-coordinate rather than the usual four-coordinate Si. 

Second, as a logical development of the first approach, polyphosphazenes have been synthesized that bear phosphine units connected to aryloxy side groups (37). The phosphine units bind organometallic compounds, such as those of iron, cobalt, osmium, or ruthenium (38). In several cases, the catalytic activity of the metal is retained in the macromolecular system (39). A similar binding of transition metals has been accomplished through nido carboranyl units linked to a polyphosphazene chain (40). 

Amino, alkoxy, and aryloxy polyphosphazenes are typically prepared by nucleophilic displacement reactions of poly(dihalophosphazenes). Analogous reactions with organometallic reagents, however, result in chain degradation and cross linking rather than in linear, alkyl, or aryl substituted poly(phosphazenes). The thermolysis of appropriate silicon-nitrogen-phosphorus compounds can be used to prepare fully P—C bonded poly(organophosphazenes). The synthesis of two of these materials and their Si—N—P precursors is described here. 

Polyelectrolyte complexes can be used as implants for medical use, as microcapsules, or for binding of pharmaceutical products, including proteins. In recent years, a new class of organometallic polymers, polyphosphazenes, has become available. Synthetic flexibility of polyphosphazenes makes them a suitable material for controlled-release technologies. Desirable characteristics of a polymeric system used for drug delivery are as follows ... 

Polyphosphazenes are synthetic macromolecules with a backbone of alternating phosphorus and nitrogen atoms and with two organic, inorganic, or organometallic side groups linked to each phosphorus (1). 

Lithiation Reactions. One of the earliest reactions of this type made use of metal-halogen exchange reactions carried out on poly[bis(p-bromophenoxy)phosphazene]. Polyphosphazenes that bear p-bromophenoxy side oups are normally unreactive. However, they can be lithiated, as shown in Scheme III, and the lithio derivatives react with a wide variety of electrophiles that range from chlorophosphines (19) to organometallic halides (42-45), This provides an access route to polymer-bound transition metal catalysts and other metallated or silylated polymers. 

It is reported that this polydimethyl phosphazene can lead to exchange reactions with Li to produce anionic species that could be reacted with organic or organometallic halides to produce pending alkyl polyphosphazenes. 

FIGURE 11.1 General structure of polyphosphazenes. R can be organic or organometallic or a combination of different functional groups. 

This was followed up by Allcock in 1966, where they reported the synthesis of the first hydrolytically stable, soluble, and high-molecular-weight polyphosphazene by replacing the chlorine atoms with organic or organometallic nucleophiles, e.g., with alkoxide or aryloxide intermediates... 

The main method of synthesis for polyphosphazenes [4-6] involves the thermal ring opening polymerization of monomer 5 to uncrosslinked high polymer 6, followed by solution state nucleophilic replacement of the chlorine atoms in 6 by organic or organometallic side groups. The overall process is illustrated in Scheme I. 

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