A polymerisation catalyst comprising a transition metal compound having the following Formula A, and an activating quantity of an activator, wherein Z and D 2 are each an imidazole-containing group of formula III R 1 and R 8 to R 11 are independently hydrogen or a monovalent (i) aliphatic hydrocarbon, (ii) alicyclic hydrocarbon, (iii) aromatic hydrocarbon, (iv) alkyl substituted aromatic hydrocarbon (v) heterocyclic groups, (vi) heterosubstituted derivatives of said groups (i) to (v), and (vii) hydrocarbyl-substituted heteroatom; M is a metal from Group 3 to 6 of the Periodic Table selected from Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W or a lanthanide metal; E 1 and E 2 are divalent groups independently selected from (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); D 1 is a donor atom or group selected from oxygen, imines, amines of formula N(R 12) or N(R 12)-R 20 where R 12 is i) aliphatic hydrocarbon, ii) aromatic hydrocarbon, iii) alkyl substituted aromatic hydrocarbon, iv) heterocyclic groups, v) heterosubstituted derivatives of said groups i) to iv), vi) hydrocarbyl-substituted heteroatoms groups and vii)further imidazole-containing groups and R 20 is a divalent group, or a phosphine; the divalent groups E 1 and E 2 are not linked other than through the donor atom D 1; 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. a transition metal compound having the following Formula A, and an activating quantity of an activator, wherein Z and D 2 are each an imidazole-containing group of formula III R 1 and R 8 to R 11 are independently hydrogen or a monovalent (i) aliphatic hydrocarbon, (ii) alicyclic hydrocarbon, (iii) aromatic hydrocarbon, (iv) alkyl substituted aromatic hydrocarbon (v) heterocyclic groups, (vi) heterosubstituted derivatives of said groups (i) to (v), and (vii) hydrocarbyl-substituted heteroatom; M is a metal from Group 3 to 6 of the Periodic Table selected from Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W or a lanthanide metal; E 1 and E 2 are divalent groups independently selected from (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); D 1 is a donor atom or group selected from oxygen, imines, amines of formula N(R 12) or N(R 12)-R 20 where R 12 is i) aliphatic hydrocarbon, ii) aromatic hydrocarbon, iii) alkyl substituted aromatic hydrocarbon, iv) heterocyclic groups, v) heterosubstituted derivatives of said groups i) to iv), vi) hydrocarbyl-substituted heteroatoms groups and vii)further imidazole-containing groups and R 20 is a divalent group, or a phosphine; the divalent groups E 1 and E 2 are not linked other than through the donor atom D 1; 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.