A new polyphosphazene, [N=PPh(CF3)](n), has been prepared via the PCl5-induced cationic polymerization of Me3SiN=P(CF3)(Ph)Br. In addition, the cationic route has been used to produce [N=PPh(Me)](n) with controlled molecular weights and narrow polydispersities. By contrast, the new monomer Me3SiN=P(t-Bu)(Ph)F failed to polymerize under any conditions but was used as an initiator and terminator to prepare both mono- and ditelechelic polymers. This monomer was also used to prepare [F(Ph)(t-Bu)P=N(PCl2=N)P(t-Bu)(Ph)F](+), which was used as a substrate for reactions with alkoxy and aryloxy groups to yield the hydrolytically stable materials, [R(Ph)(t-Bu)P=N(PR2=N)P(t-Bu)(Ph)R-2].