Inorganic backbones

This is also a further example of a spiro-polymer as well as being of the inorganic backbone type. 

Silicones are probably best known for their application as sealants and as release materials for pressure sensitive adhesives [107]. The silicone polymer combines an inorganic backbone made from silicon-oxygen bonds with organic substitution on the silicon atom. This repeating unit, shown below is called a siloxane. 

Poly(phosphazenes) are similar, partly inorganic polymers in that they consist of inorganic backbone, in this case of nitrogen and phosphorus atoms. They are separated formally by alternating single and double bonds and carry organic groups on the phosphorus atoms (10.3). 

In fact, considering the basic structure of these materials (vide supra), it can be immediately realized that the basic features of poly(organophosphazenes) are the result of two main contributions. The first one is fixed and is basically related to the intrinsic properties of the -P=N- inorganic backbone, while the second is variable and mostly connected to the chemical and physical characteristics of the phosphorus substituent groups. Skeletal properties in phos-phazene macromolecules intrinsically due to the polymer chain are briefly summarized below. 

Furthermore, polyphosphazene features are interpreted as the resultant combination of two basic contributions one coming from the properties inherently due to the polyphosphazene inorganic backbone (-P=N-), the other being due to the characteristics possessed by the exploited nucleophiUc substituents. 

The biomedical uses of polyphosphazenes mentioned earlier involve chemistry that could in principle be carried out on a classical petrochemical-based polymer. However, in their bioerosion reactions, polyphosphazenes display a uniqueness that sets them apart. This uniqueness stems from the presence of the inorganic backbone, which in the presence of appropriate side groups is capable of undergoing facile hydrolysis to phosphate and ammonia. Phosphate can be metabolized, and ammonia is excreted. If the side groups released in this process are also metabolizable or excretable, the polymer can be eroded under hydrolytic conditions without the danger of a toxic response. Thus, poljnners of this tjT are candidates for use as erodible biostructural materials or sutures, or as matrices for the controlled delivery of drugs. Four examples will be given to illustrate the opportunities that exist. 

Electrochemistry of a Conductive Organic Polymer with an Inorganic Backbone... 

Deprotection of X, and subsequent oxidation, reduction, and acetylation reactions can, with care, be carried out without decomposition of the inorganic backbone. Reactions of this type are of particular interest for the synthesis of bioactive or biocompatible polyphosphazenes. 

The primary approach to the development of main group inorganic polymer chemistry has been in the preparation, characterization and utilization of polymers with an inorganic backbone which may, or may not, be protected by organic substituents (la). Important members of this class of materials which are discussed in this volume and elsewhere include ... 

Andrianov, K. A. Polymers with Inorganic Backbones, Moscow, ANSSSR 1962, (Rqss.)... 

Allcock. H.R. Polyphosphazenes New Polymers with Inorganic Backbone Atoms, Science, 193, 1214-1219 (1976). 

The hydrophobic inorganic backbone provides an easy route to variable water-solubilizing side nonionic or ionizing groups. These are stable to hydrolysis at room temperature. 

In many respects, the polyphosphazenes are the prototype inorganic backbone polymers, that exemplify the principles of ring-opening and condensation polymerization, macromolecular substitution reactions and their potential for molecular design, and an enormous range of derivatives with the same backbone but different organic side groups. 

Andrianov KA (1962) Polymers with inorganic backbones. Ac.Sci. Moscow, pp 106-184, 202-236 [in Russian]... 

Figure 3. Some examples of inorganic backbone polymers (R, and R2 may be organic).

Andrianov K.A., Polymers With Inorganic Backbones of Macromolecules, Moscow, Izd AN SSSR, 1962, p. 144. (Rus)... 

Presently more than 300 different stable polyphosphazenes with different R and R groups have been synthesized. The combination of inorganic backbone providing particular properties not found in organic polymers and other physical, chemical, and biological properties coming from substituents make these materials very attractive for various applications. 

The properties described above and the thermal stability up to 150 °C, which is derived from the inorganic backbone, make these materials superior to common organic adhesives such as epoxy resins. 

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