Cationic Polymerization
Polydichlorophosphazene is a vital intermediate in the synthesis of most polyphosphazenes. Macromolecular substitution of this polymer yields polymers with a wide variety of properties, such as hydrophobicity, phydrophilicity, optical activity, biocompatibility, etc. The most fully developed and commonly used method for the synthesis of this macromolecular intermediate is via the thermal ring-opening polymerization of hexachlorocyclotriphosphazene, in the melt, at 250°C.
Thermal Ring Opening Polymerization
This process results in polymers of high molecular weight (Mw > 1,000,000), but
broad polydispersities. While the thermal process does not allow for significant
control over degree of polymerization, some molecular weight control of the
above process has been achieved through the use of initiators such as
OP(OPh)3/BCl3.
Another route towards the synthesis of poly(dichlorophosphazene) involves the
condensation polymerization of Cl3P=N-P(O)Cl2. Some molecular weight control is
achieved, but drawbacks include the necessity for high temperatures, the
production of corrosive side products and high polydispersities (PDI > 2).
Routes are also available for the direct synthesis of poly(alkyl/aryl)phosphazenes
via the condensation polymerization of N-silylphosphoranimines at 200°C,
developed by Neilson and Wisian-Neilson, which gives moderate molecular weight
polymers (Mn ~ 105) with polydispersity indices of 1.5-3.0.
Condensation Polymerization
In addition, Matyjaszewski and coworkers have reported that phosphoranimine species, such as (CF3CH2O)3P=NSiMe3, undergo cationic polymerizations at 100°C that produce [N=P(OCH2CF3)2]n with molecular weights (Mn) that approach 1.0-5.0 x 104 and with polydispersities of 1.2-2.5.
Because of the substantial number of polymers accessible through the macromolecular substitution of poly(dichlorophosphazene), improved methods for the synthesis of this polymer would be a significant development from both the scientific and industrial points of view. Moreover, the possiblity for control of the molecular weight of poly(dichlorophosphazene) is a key requirement for the further development of this branch of polymer chemistry. An ambient temperature polymerization route may also serve as an efficient method for the production of a wide variety of polymeric phosphazenes.
In 1995, our research group, in collaboration with the Ian Manners research group at the University of Toronto, reported the ambient temperature synthesis of poly(dichlorophosphazene). N-(trimethylsilyl)-trichlorophosphoranimine undergoes polymerization via a living cationic mechanism when initiated with trace amounts of phosphorus pentachloride. Molecular weight control and low PDI's are possible by controlling the ratio of PCl5 initator to the phosphoranimine monomer.
Living Cationic Polymerization
Direct synthesis of (aryl/alkyl)phosphazenes is also possible.
Star
Polyphosphazenes
Triarmed phosphazene-based star polymers have been synthesized via the cationic
polymerization method, with molecular weight control and low PDI's. Star
formation has been achieved by reacting a tridentate primary amine with N-(trimethylsilyl)-bromo-bis(2,2,2-trifluoroethoxy)phosphoranimine
in the presence of triethylamine to produce the trifunctional phosphoranimine
shown below. Reaction with phosphorus pentachoride results in the formation of
the trifunctional cationic species, which can be reacted with N-(trimethylsilyl)-trichlorophosphoranimine
in order to form the triarm star polymer. Subsequent macromolecular
substitution of the chlorine atoms yields a hydrolytically stable polymer.
Current research involves the synthesis of four or more branches in order to increase dimensional stability.
Block Copolymers
Block copolymers with controlled molecular weights and narrow PDI's have been
synthesized via the cationic polymerization method. Formation of these block
copolymers is achieved by first synthesizing a "living" poly(dichlorophosphazene)
via the monomer route as shown previously. Addition of a second phosphoranimine,
such as N-(trimethylsilyl)phenyldichlorophosphoranimine, results in the
formation of a block copolymer. Subsequent substitution of the chlorine atoms
with an appropriate sidegroup results in a hydrolytically stable polymer.
