An advanced process for the synthesis of polyphosphazenes with controlled architectures has been investigated. By this method, a wide range of well-defined phosphazene di- and triblock copolymers with controlled molecular weights and narrow polydispersities have been synthesized (M-n up to 4.8 x 10(4) with polydispersities of 1.06-1.39). The diblock copolymers, {[N=PCl2](n)[N=PR(R＇)](m)}, were synthesized by the cationic condensation polymerization of the phosphoranimines, PhCl2P=NSiMe3, Me(Et)ClP=NSiMe3, Me2ClP=NSiMe3, Ph2ClP=NSiMe3, and Ph2ClP=NSiMe3, at 35 degrees C initiated fi om the "living" end unit of poly(dichlorophosphazene), [Cl-(PCl2=N)(n)-PCl3+PCl6-] which was its elf formed by the polymerization of Cl3P=NSiMe3 with small amounts of PCl5 initiator in CH2Cl2 at 25 degrees C. Halogen replacement reactions through the use of NaOCH2CF3 and/or NaOCH2CH2OCH2CH2OCH3 on the diblock copolymers yielded fully organo-substituted macromolecules. In addition, the diblock copolymer {[N= PMe(Et)](n)[N-PMe(Ph)](m)} was formed by the block copolymerization of the two different organophosphoranimines. Triblock species were produced by the reaction of the "living" difunctional initiator, -{CH2NH(CF3CH2O)(2)P-N-PCl3+PCl6}(2), first with Cl3P=NSiMe3 and second with Me(Et)ClP=NSiMe3, followed by halogen replacement with NaOCH2CF3, to yield the triblock {[(Et)MeP=N](m)[(CF3CH2O)(2)P= N](n)-P(OCH2CF3)(2)NHCH2CH2NH-(CF3CH2O)(2)P-[N=P(OCH2CF3)(2)](n)[N=PMe(Et)](m)} The evidence for the formation of the di- and triblock copolymers includes NMR, GPC, elemental analysis, and solubility data.