prov:value
| - , phenyl or benzyl, and n is an integer from 2 to 50, and a metallocene of the formula XI ##STR22## in which M1 is titanium, zirconium, hafnium, vanadium, niobium or tantalum,R14 and R15 are identical or different and are a hydrogen atom, a halogen atom, a C1 -C10 -alkyl group, a C1 -C10 -alkoxy group, a C6 -C10 -aryl group, a C6 -C10 -aryloxy group, a C2 -C10 -alkenyl group, a C7 -C40 -arylalkyl
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