Phosphonitrile halide catalysts whi

Phosphonitrile halide catalysts which are useful in the method of the invention may be made by reacting in the presence of an aromatic or of a chlorinated aliphatic or aromatic hydrocarbon, e.g. toluene symtetrachloroethane or 1,2,4-trichlorobenzene, as inert solvent, a phosphorus pentahalide, e.g. phosphorus pentachloride, an ammonium halide, e.g. ammonium chloride and a selected Lewis acid. The reaction may suitably be carried out at a temperature between 100 and 220 C. followe by separating the reaction products from the solids and the volatile components, thus isolating the liquid reaction product. At temperatures below 100° C. the reaction does not occur or is so slow as to be economically not feasible. Mere mixtures of the reagents do not give satisfactory polymerisation rates. Temperatures above 220° C. may be used provided that pressure is applied to maintain the solvent in the system; however, temperatures above 220" C. are not recommended because of the tendency of the products to darken at elevated temperatures. Preferred temperature range is the refluxing temperature of the inert solvent used in the reaction, for example 120 to 160°