4.1. Syntheses Methods of Linear Polyphosphazenes

A brief comprehensive effort has been shown in Figure ​Figure77 where we can locate the possible substituents for the reaction of polyphosphazenes. This procedure is derived from the HCCP ring-opening polymerization to synthesize linear polyphosphazenes, also known as PDCPs. This reaction follows the nucleophilic substitution of all chlorine atoms attached to phosphorus atoms and replaced by bifunctional organic or organometallics species all along the chain length.^44,45 This synthesis pathway is controlled by the main component PDCP which is completely inorganic in nature and used by various kinds of polyphosphazene materials known in the literature, such as polymeric intermediates, macromolecular reactants, parent compounds, as well as polymeric sources.

Broad stepwise reaction summary for the synthesis of the polyphosphazenes.^44,45

Additionally, controlled polymerization of PDCP is also possible through a living cationic polymerization of trichlorophosphoranimine (Cl₃PNSiMe₃). This procedure can be carried out simply in solution at room temperature via reaction of Cl₃PNSiMe₃ with 2 equiv of PCl₅, giving a cationic species [Cl₃PNPCl₃]⁺ with PCl₆^– as the counterion.⁴⁶ This species can start the polymerization with the addition of further equivalents of Cl₃PNSiMe₃, leading to polymer chains with a “living” cationic end group.⁴⁷ The formation of 1 equiv of ClSiMe₃ as a side product with each monomer molecule added to the polymer makes this polymerization a rare example of a polycondensation reaction. The living chain growth mechanism, with one cationic initiator per propagating chain, not only permits the use of molecular weight via the feed monomer to the initiator ratio but also takes the lead for poly(dichloro)phosphazenes with narrow polydispersities. Reaction times may vary depending on the desired polymer chain length, the monomer concentration, and the nature of the counterion,⁴⁷ but Cl₃PNSiMe₃ is noted to be consumed completely relatively quickly within a few hours in the preferred solvent dichloromethane.⁴⁸

All of these key features indicate the excellent reactivity of chlorine atoms attached to skeletal phosphorus present in polyphosphazene chains.^37,49 The key objective of this review is to highlight cyclophosphazene chemistry, specifically HCCP derivatives, so the maximum weight will be positioned on cyclophosphazene chemistry instead of linear polyphosphazenes.