Linear phosphazene oligomers

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. 

The synthesis of linear phosphazene oligomers has been studied in some detail from the reaction of PCI5 with ammonium chloride in tetrachloroethane or chlorobenzene The following chain growth mechanism is believed to operate ... 

A number of recently reported synthetic methods include the formation of organic polymers with pendent cyclic or linear phosphazene side groups (reactions 7-9) and a process for the preparation of linear polymers in which phosphazene rings are linked together by organic oligomer chains using acyclic diene metathesis (ADMET) techniques (reaction 10). ... 

In contrast to the use of cyclic phosphazene trlmers, linear phosphonltrlllc chloride polymers were reported from the reaction of open-chain phosphonltrlllc chloride oligomers with ammonia or ammonium chloride (11). The polymers (II) obtained appear to have a lower MW than those described earlier, and they may be more useful for applications such as fire resistant coatings and foams which are discussed in a subsequent section of this paper. 

It was later noticed that the hexachlorocyclotriphosphazene can be polymerized by heating in the melt to yield an uncrosslinked linear high polymer, poly[(dichloro)-phosphazene] (Allcock et al, 1965-1966 Rose, 1968). Further heating results in the formation of an insoluble crosslinked material. This leads in both cases to a transparent, rubbery elastomer which hydrolyzes slowly, when exposed to moisture, forming phosphate, ammonia and hydrochloric acid. At temperatures above 350°C, the polymer depolymerizes to cyclic oligomers. The uncrosslinked species serve as highly reactive polymeric precursor. 

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