Polyolefin production

A process for preparing polyolefins, which comprises contacting a catalyst system comprising

• (a) a metallocene compound having the general formula:
• CpAXMQ1Q2Cp′A′X′M′Q1′Q2′
• wherein Cp and Cp′ we each independently a substituted or unsubstituted cyclopentadienyl moiety wherein each said cyclopentadienyl moiety is substituted or unsubstituted indenyl;
• M and M′ are each a metal chosen from Group IV B transition metals and vanadium, and coordinate to Cp and Cp′ respectively;
• X and X′ are each independently a substituted or unsubstituted Group VA or VIA heteroatom and coordinate to M and M′ respectively;
• A and A′ are bridging groups between Cp and X and between Cp′ and X′ respectively and are independently chosen from —SiR′2—O—SiR′2—SinR′m—, —CnR′m— and —CR′2′ SiR′2—CR′2—SiR′2—, in which each R′ is independently H or hydrocarbyl having 1 to 20 carbon atoms, n is an integer in the range 1 to 4 and m=2n;
• each Q1, Q2 and Q1′ and Q2′ is independently a coordinating group which is hydrogen, halogen, or hydrocarbyl having 1 to 20 carbon atoms and each of Q1 and Q1′ is coordinated to both M and M′;
• and
• (b) an aluminum- or boron-containing cocatalyst capable of activating the metallocene compound with at least one olefin in a reaction zone under polymerization conditions.