PURPOSE: The titled polymer, suitable for flame-resistant foamed rubber, having improved heat resistance, flame resistance and oil resistance, and prepared by substituting a dichlorophosphazene polymer with a phosphazene trimer substitution compound, and adding an aliphatic alcoholate thereto.

CONSTITUTION: Dichlorophosphazene trimter (NPCl2)3 or tetramer (NPCl2)4 is subjected to solution or bulk polymerization under heating or in the presence of a catalyst to give a linear dichlorophosphazene polymer. The resultant polymer is then dissolved in a solvent in a volume of 1W10 times that of the polymer and a phosphazene trimer substitution compound in a desired amount is added thereto in the presence of a deacididfying agent in a molar amountof 1W4 times that of the polymer. After the substitution is completed, the mixture is kept under reflux and an aliphatic alcoholate or aromatic alcoholate is added thereto and kept under reflux to give a polyorganophosphazene polymer expressed by the formula [Y is P3N3(OR2)5NH; R1 is lower alkyl or aromatic group; R2 is lower alkyl or halogenated alkyl group; n is 50W10,000].