Polyphosphazene

Polyphosphazenes include a wide range of hybrid inorganic-organic polymers with a number of different skeletal architectures that the backbone P-N-P-N-P-N-. In nearly all of these materials two organic side groups are attached to each phosphorus center. Linear polymers have the formula (N=PR1R2)n, where R1 and R2 are organic (see graphic). Other architectures are cyclolinear and cyclomatrix polymers in which small phosphazene rings are connected together by organic chain units. Other architectures are available, such as block copolymer, star, dendritic, or comb-type structures. More than 700 different polyphosphazenes are known, with different side groups (R) and different molecular architectures. Many of these polymers were first synthesized and studied in the research group of Harry R. Allcock. Polyphosphazenes include a wide range of hybrid inorganic-organic polymers with a number of different skeletal architectures that the backbone P-N-P-N-P-N-. In nearly all of these materials two organic side groups are attached to each phosphorus center. Linear polymers have the formula (N=PR1R2)n, where R1 and R2 are organic (see graphic). Other architectures are cyclolinear and cyclomatrix polymers in which small phosphazene rings are connected together by organic chain units. Other architectures are available, such as block copolymer, star, dendritic, or comb-type structures. More than 700 different polyphosphazenes are known, with different side groups (R) and different molecular architectures. Many of these polymers were first synthesized and studied in the research group of Harry R. Allcock. The method of synthesis depends on the type of polyphosphazene. The most widely used method for linear polymers is based on a two-step process. In the first step a cyclic small molecule phosphazene, known as hexachlorocyclotriphosphazene, with the formula (NPCl2)3, is heated in a sealed system at 250 °C to convert it to a long chain linear polymer with typically 15,000 or more repeating units. In the second step the chlorine atoms linked to phosphorus in the polymer are replaced by organic groups through reactions with alkoxides, aryloxides, amines or organometallic reagents. Because many different reagents can participate in this macromolecular substitution reaction, and because two or more different reagents may be used, a large number of different polymers can be produced, each with a different combination of properties. Variations to this process are possible using poly(dichlorophosphazene) made by condensation reactions. Another synthetic process uses a living cationic polymerization that allows the formation of block copolymers or comb, star, or dendritic architectures. Other synthetic methods include the condensation reactions of organic-substituted phosphoranimines. Cyclomatrix type polymers made by linking small molecule phosphazene rings together employ difunctional organic reagents to replace the chlorine atoms in (NPCl2)3, or the introduction of allyl or vinyl substituents, which are then polymerized by free-radical methods. Such polymers may be useful as coatings or thermosetting resins, often prized for their thermal stability. The linear high polymers have the geometry shown in the picture. More than 700 different macromolecules that correspond to this structure are known with different side groups or combinations of different side groups. In these polymers the properties are defined by the high flexibility of the backbone. Other potentially attractive properties include radiation resistance, high refractive index, ultraviolet and visible transparency, and its fire resistance. The side groups exert an equal or even greater influence on the properties since they impart properties such as hydrophobicity, hydrophilicity, color, useful biological properties such as bioerodibility, or ion transport properties to the polymers. Representative examples of these polymers are shown below. The first stable thermoplastic poly(organophosphazenes), isolated in the mid 1960s by Allcock, Kugel, and Valan, were macromolecules with trifluoroethoxy, phenoxy, methoxy, ethoxy, or various amino side groups. Of these early species, poly, n, has proved to be the subject of intense research due to its crystallinity, high hydrophobicity, biological compatibility, fire resistance, general radiation stability, and ease of fabrication into films, microfibers and nanofibers. It has also been a substrate for various surface reactions to immobilize biological agents. The polymers with phenoxy or amino side groups have also been studied in detail. The first large-scale commercial uses for linear polyphosphazenes were in the field of high technology elastomers, with a typical example containing a combination of trifluoroethoxy and longer chain fluoroalkoxy groups. The mixture of two different side groups eliminates the crystallinity found in single-substituent polymers and allows the inherent flexibility and elasticity to become manifest. Glass transition temperatures as low as -60 °C are attainable, and properties such as oil-resistance and hydrophobicity are responsible for their utility in land vehicles and aerospace components. They have also been used in biostable biomedical devices. Other side groups, such as non-fluorinated alkoxy or oligo-alkyl ether units, yield hydrophilic or hydrophobic elastomers with glass transitions over a broad range from -100 °C to 100 °C. Polymers with two different aryloxy side groups have also been developed as elastomers for fire-resistance as well as thermal and sound insulation applications.