The polyphosphazene {[NP(O2C12H8]}(n) (1) [O2C12H8) = 2,2'-dioxy-1,1'-biphenyl] with [PO2C12H8] Cycles ill the repeating units reacts slowly with chlorine in concentrated sulfuric acid to give the 5,5'-chlorinated {[NP(O2C12H8)](1-x)[NP(O2C12H7Cl)](x)}(n) (2) (x = 0.2-1), with limiting oxygen index (LOI) in the interval 26-33 (estimated from the TGA residues), corresponding to potential self-extinguishing flame retardants. The reaction of I with [Ipy(2)]BF4 in CH2Cl2 ill the presence of CF3SO3H gave the 5,5'-iodinated {[NP(O2C12H8)(1-x)[NP(O2C12H7)(x)]}(n) (3) (x = 0.2-1) which, in contrast. are thermally unstable, leaving almost no TGA residues. The potential of 2 and 3 its precursors for the chemical derivation of I was assessed by the silylation method (reaction with LiBut at -78 degrees C followed by addition Of ClSiMe3 and it complete characterization of the resulting trimethylsilyl derivatives). The isolation of polymers {[NP(O2C12H8)](0.65)[NP(O2C12H7Cl)](0.15)[NP(O2C12H7SiMe3)](0.2)}(n) (4) and {[NP(O2C12H8)](0.8)[NP(O2C12H7SiMe3)]0.2}(n) (5) demonstrated that the lithiation was 40-50% efficient with the chlorine derivatives and approached 100% with the iodine analogues. The silylation of the available Li sites was less effective with the higher lithiated polymers and always below the levels usually observed. The reactivity of polyphosphazenes with 2,2'-dioxy-1,1'-biphenylphosphorus repeating units is therefore limited by steric and conformational effects reinforced by the proximity of the reaction centers to the main chain.