Phosphazene Polymers

MST was founded to scale up and develop the chemistry of polyphosphazenes that originated in a major U.S. research University. Polyphosphazenes contain a polymer backbone of alternating phosphorus and nitrogen atoms and two organic, organometallic, or inorganic side groups attached to each phosphorus atom.

Polyphosphazenes have a degree of polymerization of 15,000 and molecular weights of two million or higher.  Polyphosphazenes can be synthesized with various topologies including star-shaped structures. Moreover, block copolymers of polyphosphazenes with organic polymers or polysiloxanes, cyclolinear species, and graft or comb macromolecules have also been synthesized. In the literature, more than 700 different polyphosphazenes have been described, and superior properties that cannot be obtained from classical macromolecules have been achieved.

Polyphosphazenes are synthesized using two different routes:

1) Ring-opening polymerization of cyclic trimer that is followed by macromolecular substitution  

This is the classical access route to polyphosphazenes. The ring-opening polymerization / macromolecular substitution route involves thermal polymerization of a cyclic phosphazene with chloro- side units to produce high molecular weight poly(dichlorophosphazene).  The chlorine atoms in this macromolecular intermediate are replaced with organic or organometallic groups such as alkoxy, aryloxy, amino, or organosilicon groups.  Cyclic phosphazenes with organic side groups can also be polymerized. This process has led to the synthesis of a broad range of polymers, with different side groups or combinations of two or more side groups, and often with unique combinations of properties.  The advantages of this synthesis method are the commercial availability of the starting cyclic trimer, the fact that more than 250 different reagents have been shown to undergo the substitution reaction, and the wide range of structures and properties that are accessible.  The main drawback is the lack of control over the polymer chain lengths and relatively high temperatures that are needed for the polymerization process.

2) Living cationic condensation polymerization (developed in the late 1990's)

Room temperature living cationic condensation polymerization of an N-silyl-chlorophosphoranimine at room temperature produces poly(dichlorophosphazene) which can be subsequently subjected to substitution reactions.  This method allows control of the polymer chain length.  Moreover, it enables synthesis of block, graft, and comb copolymers of polyphosphazenes with organic polymers or polysiloxanes.  Star-shaped polymers are also synthesized using this method.  The disadvantages of this process are the non-availability of a commercial source of the monomer and the somewhat lower molecular weights of polymers produced by this method.