prov:value
| - R3 and R4 are different and are monocyclic or polycyclic hydrocarbon radicals which can form a sandwich structure together with the central atom Zr, ##STR5## ???BR6, ???AlR6, --Ge--, --Sn, --O--, --S--, ???SO, ???SO2, ???NR6, ???CO, ???PR6 or ???P(O)R6, where R6, R7 and R8 are identical or different and are a hydrogen atom, a halogen atom, a C1 -C10 -alkyl group, a C1 -C10 -fluoroalkyl group, a C6
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