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Organometallic and Inorganic–Organic Polymers 397

variety of cancers using ferrocene-containing polymers. Often the employed compounds are elabo-
rately designed with specially designed backbones and employing a ferrocene-containing unit as a
tether dangling from the polymer backbone.

11.5.2 POLYPHOSPHAZENES AND RELATED POLYMERS

Other inorganic and metal-containing polymers have been formed using the addition approach.
These include polyphosphazenes, polyphosphonitriles, and poly(sulfur nitride). Phosphonitrilic
polymers (Equation 11.41) have been known for many years, but since they lacked resistance to
water, they were not of interest as commercial polymers. However, when the pendant chlorine groups
are replaced by fluorine atoms, amino, alkoxy, or phenoxy groups, these polymers are more resistant

to hydrolysis. Allcock and coworkers have pioneered these efforts. Phosphonitrile fl uoroelastomers
o
o
are useful throughout a temperature range of –56 C to 180 C. Phosphazenes are produced by the
thermal cleavage of a cyclic trimer obtained from the reaction of phosphorus pentachloride and
ammonium chloride.
Cl Cl
P Cl
Cl N N
Cl Cl R P N
P Cl P P Cl (11.41)
Cl Cl Cl N Cl
Cl R
Poly(phosphonitrile chloride)

Amorphous elastomers are obtained when phosphazene is refluxed with nucleophiles, such as

sodium trifluoroethoxide (Equation 11.42) or sodium cresylate, and secondary amines. Difunctional

reactants such as dihydroxybenzenes (hydroquinone) produce cross-linked phosphazenes.
F
F F

R Cl F R O
N P R + O − N P R
+ F (11.42)
Na
Cl F O
F F

F

Polyphosphazenes generally exhibit very low T values consistent with the barriers to internal
g
rotation being low and indicate the potential of these polymers for elastomer applications. In fact,
theoretical calculations based on a rotational isomeric model assuming localized pi bonding pre-
dict the lowest (400 J/mol repeating unit) known polymer barrier to rotation for the skeletal bonds
for polydifluorophosphazene. Temperature intervals between T and T are unusually small and

m
g
generally fall outside the frequently cited relationship. This behavior may be related to compli-

cations in the first-order transition generally found for organo-substituted phosphazenes and not

common to other semicrystalline polymers. Two first-order transitions are generally observed for
o
o
organo-substituted phosphazenes with a temperature interval from about 150 C–200 C. The lower

first-order transition can be detected using differential scanning calorimetry (DCS), differential

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