. . . . " having 5 to 20 carbon atoms, X is a halogen atom and m is an integer from 1 to 3 and (B) organoaluminum compound having the general formula AlRn 'X3-n ' in which R' is a hydrocarbon radical having 1 to 20 carbon atoms, X' is a halogen atom and n is an integer of from 1 to 3, the concentration of (A) in the reaction medium being from 10-4 millimole per liter to 50 millimole per liter, and the mola" . . .