| http://www.w3.org/ns/prov#value | - A process as set forth in claim 1 wherein the organometallic compound(s) of a metal of groups II and III of the periodic table of elements comprise organoaluminium compounds having the mean formula AlR'x Z.sub.(3-x), wherein R' represents an alkyl group containing from 1 to 16 carbon atoms, Z represents a hydrogen atom or a halogen and x is an integer or a fraction which can be of any value from 1
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