| http://www.w3.org/ns/prov#value | - prepolymerization comprising bringing the said catalyst into contact with at least one C.sub.2 -C.sub.12 alpha-olefin and with the said organometallic compound, in quantities of from 0.05 to 100 g, preferably from 0.1 to 50 g, e.g. 0.5 to 20 g of the
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