A PROCESS FOR THE POLYMERIZATION OF ETHYLENICALLY UNSATURATED HYDROCARBONS, COMPRISING THE CONTACTING OF SUCH HYDROCARBONS WITH A CATALYST FORMED BY MIXING ALUMINUM TRIHYDROCARBONS (WITH THE HYDROCARBONS BEING EITHER ALKYL OR ARYL RADICALS) WITH A HEAVY METAL COMPOUND (FOR EXAMPLE A SALT, A FRESHLY PRECIPITATED OXIDE OR AN HYDROXIDE) OF THE METALS OF GROUPS IV-B, V-B AND VI-B OF THE PERIOD SYSTEM, INCLUDING THORIUM AND URANIUM.