Poly dichlorophosphazene synthesis

Figure 1. Synthesis of poly(dichlorophosphazene) and poly(organophosphazenes).

Synthesis-Structure-Properties. Poly(dichlorophosphazene) is important as an intermediate for the synthesis of a wide range of poly(organophosphazenes) (Figure I). The nature and size of the substituent attached to phosphorus plays a dominant role in determining the properties of the polyphospha-... 

The grafting from methodology was also utilized for the synthesis of poly(4-methylphenoxyphosphazene-g-2-methyl-2-oxazoline) graft copolymers [187]. The synthetic approach involved the thermal polymerization of hexachlorophosphazene, in the presence of aluminum chloride, to give low molecular weight poly(dichlorophosphazene). The chloro groups were subsequently replaced by 4-methylphenoxy groups, followed by partial bromi-... 

C. H. Honeyman, I. Manners, C. T. Morrissey, and H. R. Allcock, Ambient temperature synthesis of poly(dichlorophosphazene) with molecular weight control. J. Am. Chem. Soc. 117, 7035-7036 (1995). 

Synthesis, The synthesis of poly(dichlorophosphazene). Ihe parent polymer to over 300 macromolecules of types 111 and (2). is carried out via controlled, ring-opening polymerization of the corresponding cyclic trimer. 

As discussed, condensation reactions form the basis of the synthesis of the cyclic trimer, (NPC12)3. The reaction between phosphorus pentachloride and ammonia or ammonium chloride proceeds in a stepwise fashion, as shown in reaction sequence (18), by elimination of hydrogen chloride first to form a monomer (3.40), then a linear dimer (3.41), trimer (3.42), tetramer, and so on. Cyclization could occur to give cyclic chlorophosphazenes at any stage beyond the dimer, but in practice is less likely as the chains grow beyond a certain length. Several authors have extended this process to produce relatively low molecular weight poly(dichlorophosphazene).36-39... 

In general, the synthesis of polyphosphazene polymers is unique in that, in theory, an infinite number of polymers with a variety of properties can be derived from the common polymeric intermediate, poly(dichlorophosphazene) (PNCI2), by replacing the chlorines with different nucleophiles. If the polydichlorophosphazene precursor is reacted with the sodium salts of trifluoroethanol and a mixed fluorotelomer alcohol, a poly(fluoroaIkoxyphosphazene) elastomer (FZ elastomer) is obtained. It contains a small amount of an unsaturated substituent as a curing site. The polymer is a soft gum, which can be compounded with carbon blacks and fillers and cured with sulfur or peroxides or by radiation. 

The synthesis of MEEP involves the reaction of poly(dichlorophosphazene) with the sodium salt of methoxy ethoxy ethanol. The byproduct in this reaction is sodium chloride which has to be separated from the polymer completely, since even traces of the ionic impurities would lead to spurious results. However, unfortunately MEEP is also soluble in water and therefore separation from sodium chloride is rendered extremely difficult. A cumbersome and lengthy dialysis procedure is required to effect the separation and purification of the polymer. Further MEEP is also hydrophilic and residual water in the polymer is an undesirable feature for a solid electrolyte particularly when involved with alkali metal salt complexes. Additionally the dimensional stability of MEEP is poor and has been commented upon above. 

The most obvious synthesis route to such polymers is the use of a difunctional reagent in the macromolecular substitution step. However, this method cannot be employed. For example, the reaction of poly(dichlorophosphazene) with NaOCH2CH2NH2 leads to crosslinking of the chains (Scheme II). Even a small amount of crosslinking in the initial stages of a macromolecular substitution will result in precipitation of the polymer, and this will prevent further halogen replacement. Avoidance of this comphcation requires die use of two alternative strategies. 

FIGURE 11.2 Synthesis of the prepolymer poly(dichlorophosphazene) (b) via thermal ring-opening polymerization of trimer hexachlorocyclotriphosphazene (a) or living cationic polymerization of a phosphoranamine monomer (c). 

Scheme 1. Reaction sequences involved in polyphosphazene synthesis. The key reaction intermediate is poly(dichlorophosphazene) (4) which serves as a reaction platform for replacement of P-Cl bonds by P-organic side group structures. Tunability of properties comes through substitution after...

SYNTHESIS TECHNIQUES (a) Stokes thermally polymerized hexachlorocyclotriphosphazene via a ring-opening process to provide a cross-linked elastomer, but it was not until 1965 that a high molecular poly(dichlorophosphazene) was isolated and subsequently transformed, via nucleophilic substitution, into thermally stable semicrystailline homopolymers. This procedure was used widely to synthesize a variety until a few years ago, but it suffered from relatively low conversions (<70% so as to avoid cross-linking), unknown chain-end groups and lack of molecular weight control of the product. These difficulties obstructed its commercialization. 

Because of the importance of the linear poly(dichlorophosphazene) towards the synthesis of other poly(organophosphazenes), as we will see in the subsequent sections, there have been many efforts to find alternative synthetic procedures for this polymer. These are discussed in the following sections. 

The phosphoranimine most commonly used in the aforementioned condensation route to poly(dichlorophosphazene) is ClaP NSiCCHala. The synthesis of this compound has been described (21) and is based on a modification of an earlier procedure (26) involving the reaction of PCI5 with LiN(Si(CH3)3)2 at —78°C in hexane. However, the purification required by this method is a challenge and leads to an overall yield of <40%. 

Another method of direct synthesis of poly(dichlorophosphazene) developed by Carriedo and co-workers (33) has been disclosed. NH4CI and PCI5 react in the presence of CaS04-H20 and NH2HSO3 as promoters, leading to polyphosphazenes of molecular weight on the order of 2 x 10 . The yields are 30-40% on the basis ofPCls. 

---