Polymerization of cyclic phosphazenes

The synthesis of inorganic macromolecules is a subject that has attracted sporadic attention since the 1940s. Activity in this area has waxed and waned in phase with new discoveries and seemingly insurmountable synthetic problems. Yet the long-range promise still exists that polymers based on chains of inorganic atoms offer the only solutions to many of the scientific and technological problems that will confront us during the next 15 or 20 years. One reason for the slow progress in this area has been the lack of interaction between organic polymer chemists on the one hand and small-molecular inorganic chemists on the other. The two conspicuous successes in the field so far [the poly(organosiloxanes) and the poly(organophosphazenes)] have resulted from a blending of these two disciplines. It is convenient to view the synthesis of inorganic macromolecules in terms of three different approaches: (1) the use of inorganic ring systems as polymerization 'monomers'; (2) the study of small-molecule inorganic ring systems as 'models' to test exploratory reactions that would be much more difficult to perform on a macromolecule; (3) the use of preformed inorganic macromolecules as reactive intermediates for the substitutive synthesis of stable polymeric derivatives. All three aspects will be mentioned in this review. However, the model compound concept (2) has recently reviewed in detail (Allcock, ref 1), and only an introductory description will be given here. Similarly, the use of preformed inorganic macromolecules as substrates for substitution reactions has been discussed elsewhere (Allcock, refs 2-4). In this article, the use of inorganic ring systems as polymerization monomers is reviewed.