GB1589095A - Process for producing cross-linked copolymers of ethylene and polyethylenically unsaturated polycyclic hydrocarbons - Google Patents

Abstract

For the preparation of interpolymers of ethylene with one or more polyene hydrocarbons, in which at least one unsaturation belongs to a polycyclic system, the reaction is carried out in the presence of a diluent and of a special catalytic system. The latter is prepared from 1 - one or more vanadium compounds of a valency greater than two, soluble in the reaction medium, 2 - one or more aluminium compounds of the general formula R2AlX in which R is an alkyl, cycloalkyl, alkylaryl or aryl radical having 1 to 12 carbon atoms and X is chlorine or bromine, and 3 - one or more of the following compounds: organic compounds containing the grouping -CCl3, hexachlorocyclopentadiene or thionyl chloride. The polymerisation yields relative to the vanadium compound are between 30,000 and more than 1,000,000 grams of copolymer per gram of vanadium, and the polymerisation product does not require a washing stage for removal of catalytic residues, but can be recovered from the reaction slurry by centrifuging or stripping. The unsaturated interpolymers thus obtained have a random distribution of the monomer units, a high mean molecular weight, a high crystallinity of the polyethylene type and a melting point above 120 DEG C.

Description

(54) PROCESS FOR PRODUCING CROSS-LINKED COPOLYMERS OF ETHYLENE AND POLYETHYLENICALLY UNSATURATED POLYCYCLIC HYDROCARBONS (71) We, SNAMPROGETTI S.p.A., an Italian company, of Corso Venezia 16, Milan, Italy, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for producing cross-linked copolymers of ethylene and polyethylenically unsaturated polycyclic hydrocarbons.

According to the present invention. there is provided a process for producing a cross-linked copolymer of ethylene and one or more polyethylenically unsaturated hydrocarbons in which at least one centre of unsaturation belongs to a polycyclic system, which process comprises (A) copolymerizing ethylene and the polyethylenically unsaturated hydrocarbon s) in a liquid phase diluent and in the presence of a catalytic system comprising (1) one or more vanadium compounds soluble in the reaction medium and in which the valency of vanadium is greater than two, (2) one or more aluminium compounds having the general formula AIRX wherein R is an alkyl, cycloalkyl, alkaryl, aralkyl or aryl radical having from 1 to 12 carbon atoms and X is a chlorine or bromine atom, and (3) either (a) one or more organic compounds containing the -CCI7 group, or (b) one or more of such compounds together with hexachlorocyclopentadiene and/or thionyl chloride, the molar ratio of aluminium compound(s) to vanadium compound(s) of the catalytic system being at least 50:1; and (B) cross-linking the copolymerization product.

The catalytic systems defined above have a high specific activity in the polymerization of ethylene with at least one of the polyethylenically unsaturated hydrocarbons, to obtain copolymers having a high molecular weight, a random distribution of the monomeric units, the presence of centres of unsaturation in the groups placed externally of the main chain, a high melting point and a high degree of crystallinity.

Copolymers of the aforementioned type can be vulcanized with conventional, sulphurbased compounding compositions to produce cross-linked products similar to the high-density polyethylenes but endowed with mechanical, thermal and chemical properties which are improved relative to conventional copolymers.

One advantage of the present invention is that in the copolymer produced, the residual vanadium, coming from the decomposition of the catalytic system, is present in only very slight amounts even if the conventional purification stage is omitted. This affords a considerable simplification of the process, inasmuch as it is possible to recover the copolymer from the reaction slurry, by centrifuging or stripping, and directly to dry it, without the need for the usual washing step.

The diluent employed in the process according to the present invention can be an aromatic or an aliphatic diluent.

Examples of vanadium compounds useful as component (1) of the catalytic system of the present invention are halides such as vanadium tetrachloride and vanadyl chloride; alcoholates such as vanadyl tri-isopropylate; chelates such as vanadium triacetylacetonate and vanadyl diacetylacetonate; and all complexes in general.

Examples of component (2) of the catalytic system are aluminium-diethyl monochloride, aluminium di-isobutyl monochloride and aluminium diethyl monobromide.

Examples of organic compounds which contain in their molecule at least one -CCIX group are trichloroacetic acid and its esters, and trichlorotoluene.

The molar ratio between the aluminium compound(s) and vanadium compound(s) is at least 50:1, and is normally in the range from 50:1 to 500:1.

The molar ratio between the third component (3) of the catalytic system and the other two components may vary widely. More particularly, the ratio of the third component (3) to the second component (i.e. the aluminium compound(s)) is preferably in the range from 1:20 to 1:1.

Preferably, the catalytic system is prepared in the presence of at least one of the monomers which take part in the copolymerization.

The selection of the type of diluent can be dictated by reasons of the desired molecular weight of the copolymer (as the aliphatic solvents encourage the attainment of higher molecular weights), availability or cost.

Also, the use of a mixture of the two types of hydrocarbons may prove an advantage.

The polymerization temperature is generally in the range from 0 C to 1200C; it is usually particularly profitable and appropriate to operate in the range from 20"C to 80"C, a range in which the catalytic system attains maximum efficiency, and in which the use of a cooling system to remove the reaction heat is not required.

The polymerization yields can range from 30,000 to 1,000,000 grams, or more, of copolymer per gram of elemental vanadium employed in the catalytic system.

At least two monomers are employed in the reaction under the conditions indicated hereinabove; one is ethylene, and the other is one or more polycyclic compounds containing at least two centres of ethylenic unsaturation, one of such centres of unsaturation preferably belonging to the polycyclic system of bicyclo-(2.2. 1)-2-heptene.

The monomers of this second class react in the polymerization by cleavage of the condensed cyclopentene ring, the remaining unsaturations being left unaffected and outside the polymeric chain.

Examples of such monomers are: 5-ethylidene-bicyclo-(2.2.1)-2-heptene )ethylidenenorbornene); 8isopropylidenedicyclopentadiene; 5-(2' ,4'-dimethyl-penta-1', 3'-dienyl)-bicyclo-(2.2.1)-2heptene (dimethylpentadienyl-norbornene) and its isomers; and [bicyclo-(2.2.1)-3'- heptenyl]-3', 5', 5' -trimethyl-cyclohexa-1 ,3-dienyl)-methane (norbornenyl-trimethylcyclohexadienylmethane) and its isomers.

The copolymerization of ethylene and one or more hydrocarbons of the type referred to above, as promoted by the catalytic system and under the conditions of the present invention, produces hydrocarbonaceous copolymers which have properties of considerable practical interest.

More particularly, the copolymers having from 99.9% to 95% of units derived from ethylene and from 0.1% to 5% of units derived from the polyene of the kind referred to above, possess a few physical properties, such as the melting point and specific gravity, which differ only slightly from those of a high-density polyethylene having the same average molecular weight. On the other hand, the presence of centres of unsaturation in the side chain and their statistical distribution both in the interior of the individual macromolecule and among the several chains, permit cross-linked products to be obtained which have physico-mechanical, thermal and chemical properties which are positively improved over those of the most satisfactory conventional polyethylenes. Notwithstanding this, generally the units derived from ethylene constitute from 90 to 99.9% of the total units in the copolymers.

The copolymers the subject of the present invention are particularly reactive under conventional vulcanization conditions, which are those well known to the skilled practitioner when sulphur and accelerators are employed as the cross-linking agents. For example, very low contents of units derived from ethylidene-norbornene (from 0.2 to 0.5 molar %) in the copolymer with ethylene are sufficient to obtain vulcanizates with a gel content of more than 80%. When ethylidene-norbornene is present in an amount from 0.5 to 1 molar percent, a gel content of up to 100% may be obtained. Such a result, which is an index of a truly random distribution of the comonomer units, has not hitherto been obtained when using comonomers for ethylene other than those specified for the present invention.

In the particular case in which 8-isopropylidenedicyclopentadiene is copolymerized with ethylene, there are directly obtained from the polymerization, products which have branches of such a nature as to impart to the molten mass a high viscosity: this notwithstanding, the product can readily be milled in an open mill and is capable of expanding in a press even if no curing agents are present.

The cross-linked copolymers of the present invention are not only suitable for the production of expanded goods, but they can find a use in all those fields in which the properties of shock-resistance and resistance to stress-cracking are required concurrently with high values of mechanical stiffness, heat resistance and resistance to chemicals.

The following Examples are intended to illustrate the present invention. The word "Vulkacit" used in Examples 2 and 7 is a registered Trade Mark.

Example I A 22-litre steel autoclave equipped with a mechanical stirrer and thermostatically kept at 60"C, was charged, in the order given, with the following reactants: toluene 16 litres Et2AICI (Et=ethylene) 100 millimoles (= 13.4 grams) ethylidene-norbornene 12.5 grams With the internal temperature at 600C, ethylene was introduced up to a pressure of 4.5 kilograms/sq.cm., and, subsequently, hydrogen until the total pressure was 5.5 kilograms/ sq.cm.

With the aid of two piston-pumps there were gradually fed in, over the course of 40 minutes approximately, and in parallel, two solutions prepared from: 1) toluene 200 ml vanadium tri acetylacetonate 0.02 millimole (6.96 milligrams), and CCl3COOC2H5 6 millimoles; and 2) toluene 100 ml, and ethylidene-nor bornene 100 grams.

The polymerization was stopped by the addition of CH3OH (200 ml) 45 minutes after the start of the introduction of the V catalyst, during which time the overall pressure was maintained at 5.5. kilograms/sq.cm. by feeding in ethylene as it was being consumed. The polymeric slurry was then centrifuged in a basket centrifuge and the resulting solids dried at 50"C in vacuo.

The dried copolymer exhibited the following properties: Percentage of units derived from ethylidene norbornene in copolymer 1.3% molar MFI2.16 4.48 grams/ 10 minutes MFI216 114.4 grams/ 10 minutes Melting point (DSC) 125"C MFI2,16 is the melt flow index as calculated in accordance with ASTM D-1236-65T using a load of 2.16 kg; and MFI2, 6 is similarly the melt flow index using a load of 21.6 kg.

Example 2 A 5-litre stainless steel autoclave equipped with a mechanical stirrer and kept at 600C thermostatically, was filled by vacuum suction with a solution composed by: toluene 1,300 ml, and Et2AICI 11.7 millimoles.

Ethylene was subsequently added until at a stable pressure of 4.5 kilograms/sq.cm, and then hydrogen was added until the total pressure was 5 kilograms/sq.cm. By means of two piston-pumps, there were introduced, in parallel and gradually over the course of 30 minutes, two solutions which had been prepared, respectively, from: 1) toluene 100 ml vanadium tri acetylacetonate 0.019 millimole, and CCl3COOC2H5 5.85 millimoles; and 2) toluene 100 ml and ethylidene norbornene 15 grams.

During the reaction, the temperature in the interior of the autoclave was maintained at 60"C and the total pressure of 5 kilograms/sq.cm was maintained therein by replacing the ethylene as it was consumed. After 40 minutes from the start of the actuation of the two pumps, methanol (50 ml) was introduced to neutralize the catalyst activity, and the polymer which had been formed was subsequently washed and dried in vacuo at 50do.

There were thus obtained 140 grams of a dry product, which had the following properties: Melt Flow Index with 2.16 kg load (MFI2.16 16, ASTM D- 1235-65T) ' 0.47 gram/ 10 minutes Melt Flow Index with 21.6 kg load (MFI21,6) 16.9 grams/ 10 minutes Melting point (differen tial thermal analysis) 1200C [rl (in decalin at 135"C) 1.79 dl/gram Percentage of units derived from ethylidene-norbornene in copolymer (IR analysis) 4.8% by weight The copolymer was compounded with the following compounding ingredients (parts per 100 parts of copolymer): zinc oxide 5 parts stearic acid 1 2,2' -dimethyl or methylene-bis -(4-methyl-6-tert.

butyl-phenol) 1 N-oxydiethylene benzothiazole-2 -sulphenamide (NOBS Special) 1.5 dibenzothiazyl disulphide (or N -cyclohexyl-benz othiazyl sulphenamide) (Vulkacit DM) 0.5 sulphur 3 The compounded copolymer was press cured at 1800C for 30 minutes. A completely gelled product was obtained which was 100% soluble in boiling xylene and had an impact resistance of 73.3 kilograms/cm/sq.cm.

Example 3 The autoclave and the procedure were the same as described in Example 1. The initial charge was: normal hexane 1,300 ml Et2AICI 11.7 millimoles, and ethylidene norbornene 1 gram At the temperature of 60"C, the ethylene partial pressure was 5 kilograms/sq.cm and that of hydrogen was 1 kilogram/sq.cm. The solutions which were gradually introduced over the course of 20 minutes were composed of: 1) normal hexane 100 ml vanadium tri acetyl-acetonate 0.013 millimole, and CCl3COOC2H5 5.85 millimoles; and 2) normal hexane 100 moles and ethylidene norbornene 9 grams.

The reaction was conducted at 60"C and at a pressure of 6 kilograms/sq.cm for 30 minutes. There were obtained 135 grams of a dry copolymer which had the following properties: MFI2.16 1.38 grams/ 10 minutes MFI21.6 60.4 grams/ 10 minutes Percentage of units derived from ethyl idene-norbornene in copolymer 1.1% by weight [rll in decalin at 135"C 1.38 dl/gram The product was subjected to compounding with sulphur and accelerators as set forth in Example 2, whereafter it exhibited a gel content of 87% and an impact resistance of 62.7 kilograms/cm/sq.cm.

Example 4 The reaction was carried out in the autoclave described in Example 1, in which there were introduced, at the outset, by suction: toluene 1,300 ml Et2AlCl 11.7 millimoles; and norbornenyl trimethyl-cyclo hexadienyl-methane 1 gram.

While thermostatically regulating the temperature to 800C, ethylene was introduced up to a pressure of 3.5 kilograms/sq.cm and, subsequently, hydrogen until the total pressure became 4 kilograms/sq.cm. Polymerization was started by introducing into the autoclave a solution which contained: toluene 100 ml vanadium tri acetylacetonate 0.013 millimole; and CCl3COOC2H5 5.85 millimoles.

The solution was introduced over the course of 25 minutes.

Concurrently, there was fed in a solution in toluene (100 ml) which contained, dissolved therein, 9 grams of norbornenyl-trimethylcyclohexadienyl methane. After 30 minutes of polymerization, there were obtained 175 grams (after drying) of a dry copolymer (i.e.

264,000 grams of copolymer per gram of element vanadium contained in the catalystic system) which had the following properties: ] in decalin at at 135"C 1.52 dl/gram MFI216 0.281 gram/10 minutes MFI21.6 23.5 grams/1() minutes Melting point 129"C (DSC) The copolymer was cross-linked with sulphur and accelerators according to the compounding formula of Example 2, to give a product which had a gel content of 60.9% and an impact resistance of 33 kilograms/cm/sq.cm.

Example 5 This test was similar to that of Example 3, the only difference being that the toluene was replaced by an equal volume of normal hexane.

After 35 minutes of polymerization and subsequently drying, there were obtained 120 grams of a polymeric product which had the following properties: all in decalin at 135"C 1.06 dl/gram MFI2 16 2.78 gram/l0 minutes MFI21.6 140.2 gram/l0 minutes Melting point (DSC) 127"C After compounding with sulphur and accelerators and curing according to the procedure disclosed in Example 2, a product was obtained which had a gel content of 58.5%.

Example 6 In the same apparatus as described in Example 1, the copolymerization of ethylene with dimethylpentadienylnorbornene was carried out. The solution which was initially drawn into the autoclave by suction was composed of: toluene 1,300 ml Et2AICI 11.7 millimoles, and dimethylpend adienyl-norbornene 1 gram.

At the temperature of 80"C, ethylene was added until it attained a steady pressure of 3.5 kilograms/sq.cm and, subsequently, hydrogen has been introduced until the total pressure became 4 kilograms/sq.cm.

During the reaction, there were fed separately and over the course of 20 minutes, a solution containing: toluene 100 ml vanadium triacety lacetonate 0.017 millimole; and CCl3COOC2H5 5.85 millimoles; and a second solution composed of: toluene tOO ml dimethylpent adienyl-norbornene 9 grams.

After 30 minutes from the start, a copolymer was discharged from the autoclave and its weight, after drying, was 125 grams. The appropriate analyses showed the following properties: [rl in decalin at 135"C 1.74 dl/gram MFI2.16 0.079 g/10 minutes MFI2l.6 10.68 g/10 minutes melting point (DSC) 129"C The copolymer, upon cross-linking with sulphur (see Example 2), had a gel content of 60.8%.

Example 7 A 2-litre stainless steel autoclave equipped with a mechanical stirrer and a thermostatic jacket was evacuated and then filled by suction with a solution composed of: normal hexane 900 millimoles Et2AICI 9 millimoles; and isopropylidene dicyclopentadiene 2 ml.

At a temperature of 90"C, ethylene was added until the pressure became 6 kilograms/ sq.cm, and then hydrogen was added until the total pressure became 8 kilograms/sq.cm.

With the aid of a piston pump there was rapidly introduced in the autoclave a solution composed of: normal hexane 210 ml, and vanadium triacety lacetonate (saturated solution in toluene) 0.05 millimole.

By means of the same pump, there was started immediately thereafter, the gradual feeding (over the course of 40 minutes) of another solution composed of: normal hexane 80 ml CCl3COOCH3 4.5 millimoles and 8-isopropylidene dicyclopentadiene 4 ml.

The reaction was carried out at 900C and at a pressure of 8 kilograms/sq.cm (maintained constant by feeding additional ethylene) and discontinued 60 minutes after the addition of the vanadium compound. The copolymer was collected on a filter from the slurry, and dried in vacuo at 500C. There were obtained 63 grams of a dry product. The appropriate analyses have given the following results: [rl in decalin at 135"C 1.25 dl/gram MFI2.l6 0.08 g/10 minutes MFI21.6 18.4 g/10 minutes melting point (DSC) 122"C gel content (insoluble in boiling xylene) 40% The raw copolymer was then mould cured at 1800C over the course of 30 minutes, after it had been compounded as follows, the parts being per 100 parts of copolymer, by weight: zinc oxide 5 stearic acid 1 2,2'-dimethyl or methylene-bis-(4 -methyl-6-tertbutyl -phenol) 1 N-oxydiethylenebenzo thiazole-2-sulphen amide (NOBS Special) 1.5 dibenzothiazyl di sulphide (or N-cyclo hexyl-benzothiazyl sulphenamide) (Vulkacit D,) 0.5 sulphur 3 The product which was obtained had a gel content of 71.3%.

Example 8 Using the same apparatus and following the same general procedure as described in Example 7, there were introduced in the autoclave, first a toluene solution (900 ml) containing Et2AICI (9 millimoles) and isopropylidenedicyclopentadiene (1 ml), then ethylene and hydrogen up to a steady ethylene partial pressure at 800C of 5 kilograms/ sq.cm, and 8 kilograms/sq.cm total pressure, and, lastly, vanadium triacetylacetonate (0.05 millimole) dissolved in toluene (20 ml).

The copolymerization was active during the subsequent 48 minutes, during which a mixture of CCl1COOCH3 (4.5 millimoles) and 8-isopropylidenedicyclopentadiene (5 ml) was fed, diluted in 80 ml toluene. 60 Minutes after the introduction of the vanadium compound, the copolymer slurry was discharged from the autoclave, collected on a filter and the solid polymer dried. 58 Grams were thus obtained.

The properties were as follows: ki in decalin 1.47 dl/gram at 135"C MFI2.16 0.0415 gram/l0 minutes MFI2, 6 16.86 gram/10 minutes Melting point (DSC) 126"C Gel content 37% The polymer could be easily processed on the open mill. When subjected to thermal treatment in the presence of the ingredients of Example 2 or 7 its gel content rose to 75%.

Attention is drawn to our Applications Nos. 7935802 (Serial No. 1589096) and 7935803 (Serial No. 1589097) which describe and claim a process similar to that of the present invention except that component (3) of the catalytic system is, in the first case, either (a) hexachlorocyclopentadiene or (b) hexachlorocyclopentadiene and thionyl chloride, and, in the second case, thionyl chloride.

We make no claim herein to a process for producing a terpolymer of ethylene, a higher a-olefin and a diene.

Subject to the foregoing disclaimer, WHAT WE CLAIM IS: 1. A process for producing a cross-linked copolymer of ethylene and one or more polyethylenically unsaturated hydrocarbons in which at least one centre of unsaturation belongs to a polycyclic system, which process comprises (A) copolymerizing ethylene and the polyethylenically unsaturated hydrocarbon(s) in a liquid phase diluent and in the presence of a catalytic system comprising (1) one or more vanadium compounds soluble in the reaction medium and in which the valency of vanadium is greater than two, (2) one or more aluminium compounds having the general formula AIR7X wherein R is an alkyl, cycloalkyl, alkaryl, aralkyl or aryl radical having from 1 to 12 carbon atoms and X is a chlorine or bromine atom, and (3) either (a) one or more organic compounds containing the -CCl3 group, or (b) one or more of such compounds together with hexachlorocyclopentadiene and/or thionyl chloride, the molar ratio of aluminium compound(s) to vanadium compound(s) of the catalytic system being at least 50:1; and (B) cross-linking the copolymerization product.

2. A process according to claim 1, wherein the vanadium compound-is selected from vanadium halides, vanadium alcoholates, vanadium chelates and vanadium complexes.

3. A process according to claim 2, wherein the vanadium compound is selected from vanadium tetrachloride, vanadyl chloride, vanadyl tri-isopropylate, vanadium triacetylacetonate and vanadyl diacetylacetonate.

4. A process according to claim 1, 2 or 3, wherein the aluminium compound is selected from aluminium diethyl monochloride, aluminium di-isobutyl monochloride and aluminium diethyl monobromide.

5. A process according to any preceding claim, wherein the third component (3) of the catalytic system is selected from trichloroacetic acid, its esters and trichlorotoluene.

6. A process according to any preceding claim, wherein the molar ratio of the aluminium compound(s) to the vanadium compound(s) of the catalytic system is from 50:1 to 500:1.

7. A process according to any preceding claim, wherein the molar ratio of the third

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (24)

**WARNING** start of CLMS field may overlap end of DESC **. Example 8 Using the same apparatus and following the same general procedure as described in Example 7, there were introduced in the autoclave, first a toluene solution (900 ml) containing Et2AICI (9 millimoles) and isopropylidenedicyclopentadiene (1 ml), then ethylene and hydrogen up to a steady ethylene partial pressure at 800C of 5 kilograms/ sq.cm, and 8 kilograms/sq.cm total pressure, and, lastly, vanadium triacetylacetonate (0.05 millimole) dissolved in toluene (20 ml). The copolymerization was active during the subsequent 48 minutes, during which a mixture of CCl1COOCH3 (4.5 millimoles) and 8-isopropylidenedicyclopentadiene (5 ml) was fed, diluted in 80 ml toluene. 60 Minutes after the introduction of the vanadium compound, the copolymer slurry was discharged from the autoclave, collected on a filter and the solid polymer dried. 58 Grams were thus obtained. The properties were as follows: ki in decalin 1.47 dl/gram at 135"C MFI2.16 0.0415 gram/l0 minutes MFI2, 6 16.86 gram/10 minutes Melting point (DSC) 126"C Gel content 37% The polymer could be easily processed on the open mill. When subjected to thermal treatment in the presence of the ingredients of Example 2 or 7 its gel content rose to 75%. Attention is drawn to our Applications Nos. 7935802 (Serial No. 1589096) and 7935803 (Serial No. 1589097) which describe and claim a process similar to that of the present invention except that component (3) of the catalytic system is, in the first case, either (a) hexachlorocyclopentadiene or (b) hexachlorocyclopentadiene and thionyl chloride, and, in the second case, thionyl chloride. We make no claim herein to a process for producing a terpolymer of ethylene, a higher a-olefin and a diene. Subject to the foregoing disclaimer, WHAT WE CLAIM IS:

1. A process for producing a cross-linked copolymer of ethylene and one or more polyethylenically unsaturated hydrocarbons in which at least one centre of unsaturation belongs to a polycyclic system, which process comprises (A) copolymerizing ethylene and the polyethylenically unsaturated hydrocarbon(s) in a liquid phase diluent and in the presence of a catalytic system comprising (1) one or more vanadium compounds soluble in the reaction medium and in which the valency of vanadium is greater than two, (2) one or more aluminium compounds having the general formula AIR7X wherein R is an alkyl, cycloalkyl, alkaryl, aralkyl or aryl radical having from 1 to 12 carbon atoms and X is a chlorine or bromine atom, and (3) either (a) one or more organic compounds containing the -CCl3 group, or (b) one or more of such compounds together with hexachlorocyclopentadiene and/or thionyl chloride, the molar ratio of aluminium compound(s) to vanadium compound(s) of the catalytic system being at least 50:1; and (B) cross-linking the copolymerization product.

2. A process according to claim 1, wherein the vanadium compound-is selected from vanadium halides, vanadium alcoholates, vanadium chelates and vanadium complexes.

3. A process according to claim 2, wherein the vanadium compound is selected from vanadium tetrachloride, vanadyl chloride, vanadyl tri-isopropylate, vanadium triacetylacetonate and vanadyl diacetylacetonate.

4. A process according to claim 1, 2 or 3, wherein the aluminium compound is selected from aluminium diethyl monochloride, aluminium di-isobutyl monochloride and aluminium diethyl monobromide.

5. A process according to any preceding claim, wherein the third component (3) of the catalytic system is selected from trichloroacetic acid, its esters and trichlorotoluene.

6. A process according to any preceding claim, wherein the molar ratio of the aluminium compound(s) to the vanadium compound(s) of the catalytic system is from 50:1 to 500:1.

7. A process according to any preceding claim, wherein the molar ratio of the third

component (3) to the second component (2) of the catalytic system is from 1:20 to 1:1.

8. A process according to any preceding claim, wherein the copolymerization is carried out in a diluent selected from aliphatic and aromatic hydrocarbons.

9. A process according to any preceding claim, wherein the catalytic system is prepared in the presence of at least one of the monomers which take part in the copolymerization.

10. A process according to any preceding claim, wherein the copolymerization is carried out at a temperature in the range of from 0 C to 1200C.

11. A process according to claim 10, wherein the copolymerization is carried out at a temperature in the range from 20"C to 8O0C.

12. A process according to any preceding claim, wherein the conditions of the copolymerization are such that the yield of copolymer referred to the vanadium compound is in the range from 30,000 to over 1,000.000 grams of copolymer per gram of elemental vanadium.

13. A process according to any preceding claim, wherein the copolymerization product is recovered from the reaction slurry by centrifuging or stripping, without any washing step for removing the catalyst residues.

14. A process according to any preceding claim, wherein the hydrocarbon comprises a bicyclo-(2.2.1)-2-heptene system.

15. A process according to any one of claims 1 to 13, wherein the hydrocarbon is 5-ethylidene-bicyclo-(2.2.1)-2-heptene (ethylidenenorbornene).

16. A process according to any one of claims 1 to 13, wherein the hydrocarbon is 8-isopropylidenedicyclopentadiene.

17. A process according to any one of claims 1 to 13, wherein the hydrocarbon is 5(2',4'-dimethyl-penta-1',3'-dienyl)-bicyclo-(2.2.1)-2-heptene.

18. A process according to any one of claims 1 to 13, wherein the hydrocarbon is [bicyclo-(2.2. 1)-3'-heptenyl-(3' ,5' ,5 '-trimethyl-cyclohexa-1 ' ,3'-dienyl) methane (norbor nenyl-trimethyl-cyclohexadienyl-methane).

19. A process according to any preceding claim, wherein the copolymerization is effected in a manner such that the units derived from ethylene in the copolymer constitute from 90 to 99.9% of the total units in the copolymer.

20. A process according to claim 19, wherein the units derived from ethylene in the copolymer constitute from 95 to 99.9% of the total units in the copolymer.

21. A process according to any preceding claim, wherein the copolymerization product is cross-linked by the use of sulphur and accelerators.

22. A process according to claim 1, wherein the copolymerization is a copolymerization substantially as described in either of the foregoing Examples 1 and 8.

23. A process according to claim 1, substantially as described in any of the foregoing Examples 2 to 7.

24. A cross-linked copolymer whenever produced by a process according to any preceding claim.