Polymerisation and oligomerisation catalysts

A polymerisation catalyst comprising

• (1) a transition metal compound having the following Formula A, and optionally
• (2) an activating quantity of a suitable activator,
• wherein Z comprises a five-membered heterocyclic group, the five membered heterocyclic group containing at least one carbon atom, at least one nitrogen atom and at least one other hetero atom selected from the group consisting of nitrogen, sulphur and oxygen, the remaining atoms in said ring being selected from the group consisting of nitrogen and carbon;
• M is a metal from Group 3 to 7 of the Periodic Table or a lanthanide metal;
• E1 and E2 are divalent groups independently selected from the group consisting of (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);
• D1 and D2 are donor atoms or groups;
• the divalent groups E1 and E2 are not linked other than through the donor atom or group D1;
• X is an anionic group, L is a neutral donor group;
• n=m=zero or 1;
• 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.