Structure and Properties of Polyphosphazenes

Polymer Polymer conformation Bond distance (A) P=N P-X Bond angles (°) NPN PNP  

The number of polyphosphazenes that have been studied by X-ray dif-fraetion are not very large. However, these studies have been quite informative. Two different measurements on [NPCyn revealed difference in metric parameters, although the main structural features are similar. Most polyphosphazenes seem to adopt the cis-trans-planar conformation over the trans-trans-planar conformation. Steric repulsion between the substituents present on phosphorus is minimized in the cis-trans planar conformation. Thus, if one considers a P2-N3 bond (bond labeled A, Fig. 3.52) as a reference, the disposition of the immediate back-bone substituents atoms N1 and P4 are cis with respect to each other. On the other hand, with respect to the adjacent N3-P4 bond (bond labeled B, Fig. 3.52), the disposition of the backbone substituents P2 and N5 are trans with respect to each other. Thus, the stereochemical orientation of the pol5mier chain alternately varies from cis to trans. 

In polymers such as [NP(Me)(Ph)] where the possibility of stereoregular polymers exists (recall polypropylene) atactic structure has been found. Most poly(alkyl/phenylphoshazenes) are amorphous which is consistent with their atactic structures. 

The P-N bond distances seem to be constant along the polymer chain although in the case of [NPCyn both constant and alternate P-N bond distances have been found in different studies. All the polymers are characterized by wide bond angles at nitrogen. 

The phosphorus NMR chemical shifts of a few selected polyphosphazenes are given in Table 3.7. Usually slightly broad signals are seen in the NMR spectrum of the polymers because of the inerease in viscosity when polymers are dissolved in solution. The P-NMR chemical shifts of polyphosphazenes indicate that these are about 20-30 ppm upfield shifted with respect to the cyclic trimers. The cyclic tetramers have intermediate chemical shifts (entries 1, 2, 5, 8, 11 in Table 3.7). In polyphosphazenes containing mixed substituents the P-NMR chemical shifts are diagnostic 

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