prov:value
| - group, a C6 -C18 -aryl group or hydrogen, and p is an integer of from 2 to 50, wherein the metallocene is a compound of the formula I ##STR14## wherein M1 is a metal of group IVb, Vb or VIb of the Periodic Table,R1 and R2 are identical or different and are a hydrogen 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
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