Side group exchange reactions have been studied for linear high polymeric organophosphazenes, [N= P(OR2)](n) (n similar to 15 000). Specifically, the exchange behavior of polymers was examined where OR = OCH2CF3, OCH2CF2CF2CF2CF2H, OCH2Cl3, OC6H4CHO-p, OC6H4CN-p, and OC6H4NO2-p with sodium trifluoroethoxide. No aryloxy group replacement by trifluoroethoxy was detected, probably due to the well-protected reactive sites of the polyphosphazenes shielded by aryloxy side groups. For the exchange behavior of [N=P(OCH2CF3)(2)](n) and [N=P-(OCH2CF2CF2CF2CF2H)(2)](n) with NaOCH2CF2CF2CF2CF2H and NaOCH2CF3, partial substituent exchange was achieved for both reactions Furthermore, under conditions with excess nudeophile these side group exchange reactions are followed by reactions. that introduce sodium-oxo groups attached to phosphorus in place of organic substituents, and this is a mechanism for subsequent hydrolysis and molecular weight decline in the presence of water. Thus, the relative instability of high polymeric [N=P(OCH2CF3)(2)](n) in the presence of excess NaOCH2CF3 emerged as an explanation for problems with the synthesis of [NP(OCH2CF3)(2)](n) and [NP(OCH2CF3)(x)(OCH2(CF2)(m)CF2H)(2-x)](n), polymers that have attracted wide fundamental and practical interest. However, substituent exchange reactions do appear to be an alternative synthetic approach for the synthesis of some polyphosphazenes, especially mixed-substituted elastomers and the recently reported trichloroethoxy/trifluoroethoxy-containing cosubstituted polyphosphazenes.