prov:value
| - mula XI ##STR17## in which M1 is titanium, zirconium, hafnium, vanadium, nioblum or tantalum,R14 and R15 are identical or different and are a hydrogen atom, a halogen atom, a C1 -C10 -alkyl group, a C6 -C10 -alkoxy group, a C6 -C10 aryl group, a C6 -C10 -aryloxy group, a C2 -C10 -alkenyl group, a C7 -C40 -arylalkyl group, a C7 -C40 -alkyl aryl group or a C8 -C40 -arylalkenyl group, R16 and R17 are
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