Suitable phosphonitrile compounds f

Suitable phosphonitrile compounds for use in the method of the invention have a cationic phosphonitrile part (A) and an anionic part (B) which has been derived from a Lewis acid. The phosphonitrile part (A) is a linear oligomeric or polymeric phosphonitrile halide. It has the general formula [X(PX2-N)PX3]+(3), wherein X denotes a halide and D denotes an integer having a value of from 1 to 8. It is preferred that the halogen X is a chlorine atom. Phosphonitrile halide cationic parts with a value for n which is higher than 8 are less suitable as catalysts. Most preferred are the phosphonitrile halide parts in which the value of n is from 2 to 4. Phosphonitrile halide catalyst wherein the value of D is only 1 are very reactive, but they tend to be insoluble in the reagents and in most common sol vents. Unless a suitable commercially acceptable solvent is selected, it is preferred to avoid those catalyst species where n= 1. It is sometimes difficult to separate the polymeric phosphonitrile halides having different n values and mixtures may be used. It is particularly preferred that the amount of phosphonitrile halide polymer, in which n has a value of 2, is as high as possible as this gives the most active catalyst. Particularly preferred is a catalyst which exclusively consists of compounds according to the invention in which the value of n is 2