Alternative Synthesis Routes to Linear Polymers

Linear poly(organophosphazenes) are now synthesized by six different methods. These are (a) The ring-opening polymerization of (NPC12)3 mentioned above, followed by macromolecular substitution (b) Ring-opening polymerization of organic substituted phosphazene cyclic trimers, usually followed by macromolecular substitution  

The earliest synthesis method, which involves a ring-opening polymerization of a cyclic chlorophosphazene followed by replacement of the chlorine atoms in the polymer by organic groups, is the most extensively developed route to the preparation of poly(organophosphazenes). It is summarized in reaction sequence (3). 

Hexachlorocyclotriphosphazene (3.19) is prepared on an industrial scale by the interaction of phosphorus pentachloride with ammonium chloride in an organic solvent such as chlorobenzene. This compound, after careful purification and protection from moisture, is heated in the molten state at temperatures between 210 and 250 °C to induce polymerization.15-18 The mechanism of this reaction is discussed in a later section. Here it is sufficient to note that the process can also be applied, with minor variations, to cyclic phosphazo-phosphazenes (3.22a, 3.23), to cyclic fluorophosphazenes (3.24,3.25), and to cyclic carbo- (3.26), thio- (3.27), and thionyl (3.28) -phosphazenes. 

Solutions of poly(dichlorophosphazene) (3.21) in benzene, toluene, or tetrahydrofuran react rapidly and completely with nucleophiles such as sodium trifluoroethoxide to yield derivative polymers as shown in reaction sequence (3). An impetus for this 

One of the few limitations to this process is the ability of certain bulky organic side groups to slow the replacement of nearby chlorine atoms by steric hindrance. Hence, the introduction of multi-ring- or ortho-substituted aryloxy units may require forcing (high temperature) conditions in a sealed reactor. 

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