Polymerisation catalysts

A polymerisation catalyst comprising (1) a nitrogen-containing transition metal compound of Formula A, and (2) an organoaluminium or hydrocarbylboron activator, wherein either (a) R1 and R2 are monovalent groups or (b) R1 and R2 integrally form a divalent group R3 bridging the terminal nitrogen atoms of the triazene unit via carbon atoms; R1 and R2 and the divalent group R3 are (i) aliphatic hydrocarbon, (ii) alicyclic hydrocarbon, (iii) aromatic hydrocarbon, (iv) alkyl substituted aromatic hydrocarbon (v) heterocyclic groups and (vi) heterosubstituted derivatives of said groups (i) to (v); M is a metal from Group 3 to 11 of the Periodic Table or a lanthanide metal; X is an anionic group, L is a neutral donor group; n is 1 or 2, y and z are independently zero or integers such that the number of X and L groups satisfy the valency and oxidation state of the metal M. The catalyst is used to polymerise 1-olefins especially ethylene and propylene. High molecular weight homo- and co-polypropylene are disclosed.