Process for the polymerization of cyclic polyhalophosphazenes using a catalyst composition of boron trihalide and oxygenated phosphorus compounds

A method of preparing a substantially gel free linear polydihalophosphazene polymer represented by the formula (NPX.sub.2).sub.n in which X is a halogen selected from the group consisting of F, Cl and Br and n is from 20 to 50,000;

• said method comprising polymerizing a cyclohalophosphazene represented by the formula (NPX.sub.2).sub.3 in which X is as defined above at a temperature of from 180.degree. to 270.degree. C. in the presence of a catalytically effective amount of a catalyst which comprises the reaction product of a boron compound of the formula BX.sub.3 in which X is F, Cl or Br with an oxygenated phosphorus compound of the formula ##STR21## wherein Y and Y' may be the same or different and are selected from the group consisting of F, Cl, Br, R--, RO-- and RS--, wherein R is an aryl or substituted aryl group with the proviso that not more than two of the groups represented by Y and Y' may be R-- groups.