The anionic part of the catalyst is

The anionic part of the catalyst is derived from a Lewis acid and preferably has the formula [MX(v-i+1)R2t] -(4), wherein M denotes an element with a value of electronegativity on Pauling's scale of from 1.0 to 2.0, R2 is an alkyl group having up to 12 carbon atoms, v is the valence or oxidation state of the element M and 0Although it is preferred that the value of t is zero alkyl groups may be included.Preferably the Lewis acid based anion contains a halide X which is the same as the halide of the phosphonitrile cationic part, i.e. most preferably a chlorine. The element M of the Lewis acid part is an electropositive element having an electronegativity value according to Pauling's scale of from 1 to 2, preferably from 1.2 to 1.9, most preferably 1.5 to 1.9. Suitable elements are found in Groups Ib, Ila, IIb, IIla, IVa, IVb, Va, Vb, VIb, VIlb and VIll of the Periodic Table. They include Al,B,Mg, Sb and Si.