Phosphonitrile Polymers
These polymers, also called polyphosphazenes, are useful materials when they are substituted with organic compounds [173]. They are prepared from hexachloro-cyclotriphosphazenes by ring opening polymerizations:
It is believed that the mechanism of polymerization involves an attack by electron-rich nitrogen of one cyclic monomer upon another one. At first, a cation forms through ionization of a phosphorus chloride bond [173]:
This is followed by an attack on another monomer molecule:
Alkyl, aryl, alkoxy, and aminocyclotriphosphazenes fail to polymerize. This is believed to be due to an absence of easily ionizable halogen to phosphorus bonds. At the same time, materials that presumably facilitate ionization of the phosphorus to halogen bonds, such as water, carboxylic acids, and metals, accelerate the polymerization [173].
The all-inorganic polymer decomposes readily at elevated temperatures and is very sensitive to hydrolytic attack. Quantitative replacements of the halogen groups, however, are possible with alkoxy, aryloxy, alkyl, aryl, or amino groups to yield much more stable materials. The replacements are achieved by refluxing the inorganic polymer in an ether solvent for several hours with sodium alkoxide or aryloxide, a metal alkyl, or aryl, or with a primary or a secondary amine. Of particular interest are substitutions with fluoroalkoxy groups, like the following:
Mixed fluoroalkoxy compounds can be used to obtain a variety of properties. The materials find application as elastomers, because they exhibit good chemical resistance and good thermal stability. In addition, many retain the useful elastomeric properties at low temperature. As a result, among other applications, they are attractive for use as sealants and as fuel lines in arctic environment.
Phosphonitrile polymers are self-extinguishing or fire retardant. This led to the development of flame- and heat-resistant polyimide composites that are prepared from maleimide-substituted phosphazenes [231, 232]. The maleimide group is used for cross-linking. The substituted phosphazene can be illustrated as follows:
A deferent approach to the synthesis of polyphosphazenes was reported [179]. It is based on condensation of suitable Si-N-P precursors:
where, R, R'=alkyl, aryl; X = OCH2CF3, O-Ph.
In this preparation, the desired substituents are introduced before the polymerization. The resultant polymers [179] are soluble in various solvents. Their molecular weight distributions vary from 1.4 to 3.5 and M from 50,000-150,000.
