Design, synthesis, and uses of phosphazene high polymers

Polyphosphazenes form one of the most diverse classes of macromolecules. They have a backbone of alternating phosphorus and nitrogen atoms and bear two organic or organometallic side groups linked to each phosphorus. Most of the known examples are prepared via the replacement of chlorine atoms in the macromolecular intermediate (NPCl₂)_n by the use of organic or organometallic nucleophiles. The different polymers generated by the introduction of various side groups span the range of properties from elastomers to glasses, and from hydrogels to bioerodible polymers. Uses are being developed in biomedicine, aerospace technology, fire-resistance, fuel cell membranes, and in solid polymer electrolyte batteries.

However, until recently, the only access route to (NPCl₂)_n was via the thermal ring-opening polymerization of the cyclic trimer, (NPCl₂)₃, which yields polymers with broad molecular weight distributions, and little or no means for molecular weight control or access to block copolymers. A new method for the synthesis of polyphosphazenes is described here. It involves the room-temperature, living cationic condensation polymerization of Me₃SiN=PCl₃ catalyzed by PCl₅, which gives (NPCl₂)_n with precise molecular weight control and access via living mechanisms to block copolymers, stars, and telechelic systems. The recent synthesis of phosphazene-organic block copolymers by these methods is a major advance toward the commercialization of polyphosphazenes. The impact of these developments on the properties and uses of polyphosphazenes is also mentioned.