Soluble poly(dihalophosphazenes) are prepd. by bulk or soln. polymerisation of cyclic phosphazene oligomers of formula (NPX2)n (where X is F, Cl or Br and n is 3-15) by heating in the presence of a gp. IVA or VA metal cpd. catalyst (I). (I) is pref. a halogen free metal esp. metal cpds. contg. organic residues such as anions of organic acids, substits. of amines, organo-sulphur or -phosphorous cpds., alkoxides or alkyl metal cpds. (I) is esp. dibutyltin oxide, germanium dioxide, bismuth methane sulphonate or bismuth triacetate. (I) may also contain a gp. IVB metal cpd. esp. zirconium tetra- alkoxides. The reaction is carried out at 150-300 pref. 160-250 deg. C for 15-90 min. Good yields can be obtd. with short reaction times at not particularly high reaction temps. The catalysts are non-toxic and can be readily prepd. The polymers fermed are soluble with a mol. wt. of 1000-10000000 and contain practically no gel. The polyphosphazenes are used as elastomers, foams, films and flame retardants.
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