A polymerisation catalyst comprising (1) a transition metal compound of Formula (A), and optionally (2) an activator, wherein Z is 5-membered heterocyclic containing carbon, nitrogen and at least one other selected from nitrogen, sulphur and oxygen, the remaining atoms in the ring being nitrogen and carbon; M is a metal from Group 3 to 11 or a lanthanide metal; E1 and E2 are divalent hydrocarbon, heterocyclic or heterosubstituted derivatives of these; D1 and D2 are donor atoms or groups; X is an anionic group, L is a neutral donor group; n=m=zero or 1; y and z are zero or integers so that X and L satisfy the valency/oxidation state of M, characterized in that the complex contains at least one polymerisable olefinic double bond which is present in, or substituent to, at least one of the atoms, groups or ligands represented by Z, E, D and L. The catalyst binds to the forming polymer providing product with good particle morphology.