Molecular Structure of Linear Polyphosphazenes

It should also be noted that, although most of the polymers discussed in this chapter are assumed to be linear open-chain structures, the possibility also exists that giant macrocyclic species or chain-linked macromolecules may also exist.1 Such species would be very difficult to distinguish from linear high polymers. Linear polymers have end groups that, in theory, might be detectable by NMR techniques, but in practice the chains are 

In the following sections, four different aspects of polyphosphazene structure will be reviewed briefly side-group disposition chain conformation skeletal bonding and skeletal flexibility. 

The properties of a polymer depend not only on the nature of the skeleton and the types of side groups attached to it, but also on the way in which different side groups are sequenced along the same chain. For example, isotactic or syndiotactic sequencing may favor efficient intermolecular chain packing and microcrystallite fromation. On the other hand, a random distribution of two or more different side groups will discourage crystallization, and may perhaps favor the appearance of elastomeric character. 

Because most poly(organophosphazenes) are synthesized by a macromolecular substitution route, the disposition of side groups usually depends on the steric and electron-directing characteristics of the incoming groups and on the side groups already present. Thus, the order in which two or more different side groups are introduced will also affect the outcome. 

The first step in the structural investigation of a polyphosphazene is to determine the side-group ratios by microanalysis and by NMR spectroscopy. The second step is to attempt to deduce the sequencing and configuration by a detailed analysis of the NMR spectra. 

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