A process for producing a poly-olefin is disclosed, which comprises contacting at least one 1-olefin under polymerisation conditions with a polymerisation catalyst or catalyst system comprising (1) a compound having the Formula (I) wherein M is Fe[II], Fe[III], Co[I], Co[II], Co[III], Mn[I], Mn[II], Mn[III], Mn[IV], Ru[II], Ru[III], Ru[IV], V[II], V[III], V[IV], or V[V], Ti[II], Ti[III] or Ti[IV]; X represents an atom or group covalently or ionically bonded to the transition metal M; T is the oxidation state of the transition metal M and b is the valency of the atom or group X; R¹, R², R³, R⁴, R⁵, R⁶ and R⁷ are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl; and when any two or more of R¹-R⁷ are hydrocarbyl, substited hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl, said two or more can be linked to form one or more cyclic substituents; and optionally (2) an activating quantity of an activator compound comprising a Lewis acid capable of activating the catalyst for olefin polymerisation; and then subjecting the resulting polyolefin to crosslinking conditions. The polymer produced can be formed into shaped articles or films with desirable physical properties ##STR1##