Research Interests

 

       The major research focus in our labs is the synthesis, characterization, and applications of both cyclic and polymeric phosphazenes.   These inorganic materials consist of rings or linear chains of alternating nitrogen and phosphorus atoms.  Our work focuses on the family of phosphazenes in which there are two substituents attached to each phosphorus atoms by direct P-C bonds.  We prepare both the parent cyclic and polymeric phosphazenes from condensation reactions of N-silylphosphoranimines, Me₃SiNP(OPh)RR'.

 

Poly(phosphazenes)

 

A variety of new polyphosphazenes have been prepared from preformed polymers such as poly(methylphenylphosphazene), [Me(Ph)PN]_n, PMPP, shown below, by attaching new side groups by one of three general methods. These are (a) the deprotonation-substitution of methyl groups, (b) electrophilic aromatic substitution of phenyl groups, and (c) coordination of the basic backbone nitrogen atoms. These modifications provide a means of studying the structure-property relationships for this class of polymers. For example, incorporating alkylsilyl groups and fluoroalkyl groups alters surface wettability and/or gas permeability; co-substitution of the simple Me₃Si group and fluoroalkyl silyl groups improves gas permeation properties; and attaching ferrocene units imparts unusual electrochemical behavior. Graft copolymers, metal containing polymers, and polymers with a variety of organic functional groups have been prepared and characterized.  Investigation of the surface modification and crosslinking of poly(alkylarylphosphazenes) using the same types of chemistry are also under investigation

 

PMPP

 

 

Since our initial report that the simple polyphosphazene PMPP stabilizes gold nanoparticles (J. Am. Chem. Soc 2001, 123, 3846), we have directed our efforts toward using PMPP grafted polymers and blends to organize metal nanoparticles for potential applications as fuel cell membranes, catalysis, and isoelectronic materials.  Studies of these systems include measuring limiting oxygen index and gas permeability.

 

Cyclic Phosphazenes

 

The discovery that P-C bonded, cyclic phosphazenes with nongeminal substituents at phosphorus are accessible from N-silylphosphoranimines provides access to cis and trans isomers.  In the cis isomers the phenyl rings on one side of the nearly planar P₃N₃ ring form an aromatic basket.  We are currently using reactions similar to those discussed above for the polymers to design molecules with suitable reactivity and geometry for molecular recognition and transition metal catalysts.