CH621355A5 - Process for the preparation of ethylene/butadiene copolymers in high yield - Google Patents

Abstract

The technical problem of the preparation of ethylene/butadiene copolymers having a content of butadiene units from 0.1% to 10% and a vanadium content of less than 50 parts per million, a restricted molecular weight distribution, high linearity of the chains, high X-ray crystallinity and absence of gel and random distribution of monomer units, is solved by carrying out the copolymerisation in a hydrocarbon solvent medium with the aid of a quaternary catalyst system consisting of vanadium compounds, aluminium compounds, soluble polyhalogen compounds of elements from groups III and IV of the periodic table and/or acidic compounds capable of releasing protons, and also organic compounds having in their structure at least one -CX3 grouping, X being a halogen.

Description

The present invention relates to a process for the high yield polymerization of ethylene with butadiene to obtain copolymers having the following favorable properties:

i) high chain linearity,

ii) restricted distribution of molecular weights,

iii) substantially all 1,2 trans configuration of the butadiene units,

iv) inter and intramolecular statistical distribution of monomer units,

v) low residual vanadium content, even in the absence of a purification step,

these copolymers being vulcanizable by conventional sulfur-based recipes giving rise to cross-linked products similar to high density polyethylenes but with improved mechanical, thermal and chemical properties.

Ethylene-butadiene copolymers are known in the prior art, however none of the products described all together show the characteristics i) -v) listed above.

They are all to be considered indispensable if you want to have a product that offers substantial advantages over polyethylenes.

In fact, the linearity of the chain, i.e. the substantial absence of intermolecular cyclizations, branches and transverse bonds, guarantees a low fluidity of the molten polymer favoring its workability and extrudability.

The narrow distribution of molecular weights, associated with • chain linearity, ensures better mechanical properties for the copolymer, such as impact resistance, for the same average molecular weight and other characteristics.

Low polydispersity polymers are the most suitable in transformation techniques based on rotational molding, injection molding of large products, fiber spinning and others. The 1,4-trans addition of the butadiene units in a polyethylene chain is the only one capable of not significantly disturbing the crystalline structure of the latter polymer, at least for non-high content of copolymerized butadiene. In this way the physical properties (melting temperature, density, etc.) of the copolymers, which are able to vulcanize at high speed, do not differ significantly from those of high density polyethylene.

The statistical distribution of the monomeric units, both within the single macromolecule and between one macromolecule and the other, is a necessary condition for the homogeneous homogeneous reticulation of the whole copolymer. Only such copolymers can vulcanize with high cross-linking yields and provide products with physical-mechanical, thermal and chemical properties superior to those of the best polyethylenes.

Finally, the low vanadium content is another important characteristic of ethylene-butadiene copolymers, since, given the presence of double bonds, oxidation reactions in which vanadium acts as a catalyst are to be avoided. Furthermore, residues of vanadium salts, if present in a not negligible quantity, impart unwanted colors to the copolymers. The use of catalysts which produce high copolymerization yields, in addition to eliminating the drawbacks mentioned above, entail simplified processes in which the washing step of the polymer obtained is eliminated.

All this is required because the preparation of ethylene-butadiene copolymers is of industrial interest.

The different polymerization behavior of the two comonomers, ethylene and butadiene, justifies the objective difficulty of obtaining, directly from the polymerization of their mixtures, copolymers having the desired properties as described above.

Characteristics and properties of ethylene-butadiene copolymers are closely connected with the preparation process and therefore with the particular catalyst used. Known catalytic systems, whether based on the use of titanium or vanadium compounds or chromium as a transition element, in addition to not ensuring the formation of copolymers having all the characteristics listed above, do not provide sufficiently high polymerization yields.

The prior art relating to the preparation of ethylene-butadiene copolymers homogeneously crosslinkable with sulfur-based recipes, however shows that the catalysts

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used therein, they are not very active when relatively low molecular weight products are desired.

The process according to the invention, as defined in claim 1, consists in carrying out the polymerization reaction in one or more hydrocarbon solvents in the presence of a catalytic system consisting of:

a) one or more vanadium compounds soluble in the aforementioned hydrocarbons selected from the complexed tetra or penavalent vanadium halides and complexed trivalent vanadium, vanadium alcohols and vanadium and vanadyl chelates,

b) one or more aluminum compounds corresponding to the formula RìR2A1X wherein Rt and R2 equal or different from each other, are alkyl, cycloalkyl, alkylaryl and aryl radicals containing from 1 to 18 carbon or hydrogen atoms and X is halogen,

c) one or more plurlogenated compounds soluble or solubilizable in the reaction medium of the elements of the III and IV Group of the periodic system, and / or one or more acid compounds capable of freeing protons and d) one or more organic compounds containing in their molecule at least one CX3 grouping with X halogen.

By "complexes" we mean all compounds characterized by bonds between vanadium and mono- and bi-toothed organic binders, where the term "bonds" is defined as an ion or a molecule linked to metal or considered to be linked to metal and the terms "mono and bidentates "mean a molecule that has one or, respectively, two positions with which it can form covalent or coordination bonds with the metal.

Representative examples of vanadium compounds [according to (a)] are vanadium tetrachloride (preferably stabilized in SiCl4 solution), vanadyl trichloride, vanadium trichloride complexed with three molecules of tetrai-drofuran, vanadium triacetyl acetonate, the vanadyl diacetylaceto-born, vanadyl triisopropylate, and vanadium tetra (dimethylamide).

Among the components (b) of the catalytic system, aluminum dihydrocarbylhalides are exemplary, among which, for reasons of cost and availability, diethyl monochloride and aluminum diisobutyl monochloride are preferred. Similar results are obtained by using the corresponding bromides. Also the aluminum-hydrocarbyl-halide idi uri can be advantageously used.

Examples of components (c) of the catalytic system as defined in claim 1 are all the acid compounds according to Lewis and Brönsted, and include both the plurlogenated compounds of the metals of the III and IV Group of the periodic system, and the compounds capable of releasing ions H +, i.e. organic acids, inorganic acids and water. A condition to be respected is that they are sufficiently soluble in the hydrocarbon reaction medium or that they are solubilized by reaction with the other catalytic components. More specific examples of these compounds are boron trichloride, aluminum trichloride, aluminum monoethyldyclo-ruro, aluminum monoisobutyldichloride, tribro-wall aluminum, tin tetrachloride, acetic acid, benzoic acid, l water etc.

Particular examples of the component (d) of the catalytic system as defined in claim 1 are the esters of trichloroacetic acid, in addition to the free acid itself. Active are also, but to a lesser extent, trichlorotoluene, p.chloro-trichlorotoluene, trichloroacetic acid chloride, carbon tetrachloride.

The molar ratio between components (b) and (a) is normally high since the vanadium compound is used in very small quantities. Said ratio is higher than 10: 1 and normally between 50: 1 to 100: 1. However when the vanadium concentration is lower than 5. 10 "5 moles / liter,

the Al / V ratio is preferably between 100: 1 and 500: 1. While the optimal quantity of the vanadium compound used oscillates between LIO-1 and 1.10 "3 moles / liter, the quantity of the aluminum compound varies between 1 and 8. IO 3 moles / liter.

The molar ratio between components (c) and (b) is critical. In the event that (c) is a plurilogenated inorganic or organometallic compound, it can be expressed with the overall ratio between the halogen and aluminum grammiatoms contained in the reagents (b) and (c). This X / Al ratio must be between 1 and 1.25 or, preferably, between 1.05 and 1.15.

In the case where (c) is a proton acid or water, the ratio between (b) and (c) is chosen in the range between 10: 1 and 1: 1 or, preferably, between 4: 1 and 2: 1.

The molar ratio between the components (d) and (b) of the catalyst can vary widely, also in relation to the particular compound (d) and the reaction modes. It is generally less than 1 and is preferably chosen in the range from 1: 4 to 1: 1.

All the above reactions are carried out in the presence of a hydrocarbon which, according to a particularity of the present invention, is of an aromatic type. However also mixtures of aromatic hydrocarbons with aliphatic or cycloaliphatic hydrocarbons, halogenated aromatic hydrocarbons or their mixtures with the aforementioned or with other halogenated hydrocarbons are usable according to the invention. Therefore, although solvents such as benzene and toluene are preferred, it is possible to use clo-robenzene or mixtures of benzene / cyclohexane, toluene / n-heptane, chlorobenzene / tetrachlorethylene etc.

The polymerization temperature is chosen in a wide range. Generally it operates at temperatures above room temperature in order to avoid the use of expensive cycle-refrigerators.

Therefore the most suitable range is between 50 ° and 100 ° C, even if the possibility of operating at lower or higher temperatures, for example between 0C and 120 ° C, is not excluded.

The polymerizations are carried out under moderate ethylene pressures (2-20 kg / cm2).

To regulate the molecular weights, hydrogen is added to the system where necessary.

The ethylene-butadiene copolymers that are obtained with the process described above, are free of gel, branches and internal cyclizations as shown by infrared and nuclear magnetic resonance spectra and as can be deduced from their complete solubility in hydrocarbon solvents and hot chlorohydrocarbons.

The narrow distribution of molecular weights, associated with a high chain linearity, is highlighted by the measures of fluidity in the molten state, according to ASTM D1236-65T standard. The ratio between the flow indices measured by applying weights of 21.6 kg (MFI21, u) and 2.16 kg (MFI2.1 (Ì), respectively, is called "shear sensitivity" and results, for the copolymers in question, between 10 and 50. Said ratio is lower the lower the average molecular weight and, for the same amount, the more monodisperse and free of branching is the polymer under examination (R. Martinovitch, PJ Boche, RA McCord, SPE J. 16, 1335 [1971]).

The molecular weight distribution of the copolymers of the present invention expressed as M, v / Mu determined by gel permeation chromatography (Polymer Fractio-nation, MJR Cantow, Academic Press NY, 1967) is between 2 and 3. The content in units butadiene (moles of butadiene per 100 moles of the two monomers) in the ethylene-butadiene copolymers can vary in a wide range, for example between 0.1 and more than 10, and is regulated according to the intended use. The products of greatest interest for the subsequent transformation into reticular artefacts

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Iati, contain from 0.5 to 5% in. moles of co-polymerized butadiene.

Copolymers with less than 0.5% of butadiene units undergo a "re-degradation" in the presence of vulcanizing agents; copolymers with more than 5% of butadiene units may find interest, as well as for the low cost, for their compatibility with other highly unsaturated polymers. The 1,4 trans butadiene units make up, as detected by infrared spectrography, at least 98% of all copolymerized butadiene. In fact, bands attributable to eis or 1.2 units are irrelevant. The quantitative determination of the butadiene contained in the copolymer is therefore carried out based on the 965 cm-1 band according to known techniques.

Confirming the intermolecular and intramolecular statistical distribution of the monomeric units of the copolymers in question, the raw polymerization product is able to crosslink in all its fractions when subjected to vulcanization using conventional recipes based on sulfur or sulfur donors or quinone compounds, in the presence or absence of accelerating agents and inert fillers. The degree of crosslinking is measured by the maximum torque value reached at the Zwick curometer or by the insoluble part after 24 hours in boiling xylene, and the% gel is expressed. It varies according to the average molecular weight, the distribution of the molecular weights, the unsaturation content of the copolymer, as well as the vulcanizing compound used, and is generally comprised between 30 and 90% of the final product.

By carrying out the process as defined in claim 1, the ethylene-butadiene copolymer is obtained with such high yields that it is possible to eliminate the washing step of the polymerization crude, leaving the few catalytic residues incorporated in the polymer itself. In fact, the residual vanadium is in the majority of cases, less than 50 parts per million.

The ethylene-butadiene copolymers are highly crystalline on X-ray examination. Their crystallinity is, in the range of greatest interest, that typical of high density polyethylene. The absence of the crystallinity typical of polybutadiene 1,4 trans, also for contents greater than 10% of these structural units, constitutes further evidence of the statistical distribution of the ethylene and butadiene units.

Finally, the density of the copolymers is not significantly different from that of linear polyethylene and oscillates between 0.950 and 0.960 g / m2 (ASTM D 1505-68 standard).

The cross-linked products obtained from the ethylene-butadiene copolymers obtained according to the invention are interesting for numerous practical applications.

Among these, the use in the preparation of rigid foam materials (excellent insulators and soundproofing that burn without providing black fumes) of rigid containers of all sizes, of pipes, of cable coatings, and other similar applications stand out.

In all their applications, cross-linked copolymers offer, compared to polyethylenes, higher mechanical characteristics, such as the breaking load and impact resistance (ASTM D 256/56 standard), significantly improved stress-craking resistance (ASTM D standards 1693), modest decrease in the flexural modulus (ASTM D 747 standards) and acquired substantial inertia to oils and solvents.

The non-cross-linked ethylene-butadiene copolymers can also be used as intermediates for the preparation of bifunctional olefinic monomers following appropriate demolition of the unsaturated chain.

Some data on the properties now illustrated, as well as recipes and vulcanization conditions, are reported in the following examples, provided for the sole purpose of better illustrating the invention without in any way limiting it.

Example 1

5 In a steel autoclave with a capacity of 5 1, equipped with mechanical stirring, thermoregulation and electric detector with pressure gauge and access valves, a solution was introduced by suction, obtained by:

1800 ml of anhydrous toluene 10 - 1.35 ml of A1 (2H5) 2CI

- 0.12 g of A1 (C2H5) C12

- 40 g of butadiene.

15 The CI / Al ratio in the mixture is 1.1.

The autoclave was thermostated at 80 ° C, therefore:

- ethylene 14 at

- hydrogen 6 at.

20

By means of a piston pump, a toluene solution (200 ml) containing dissolved 17.5 g mg of V triacetylacetonate and 1.06 g of methyl trichloroacetate was injected in about twenty minutes.

25 The polymerization began as soon as the introduction of the solution began, it continued for 90 minutes during which the consumed ethylene was fed keeping the total pressure constant at 20 atm.

The test was stopped by introduction of 25 ml of 30 methyl alcohol. The polymer, recovered by filtering the suspension and dried in a vacuum oven at 50 ° C, has the appearance of a white powder and weighs 165 g in the dry state. The yield was 66,000 g per g of metallic V. 1.4 trans butadiene units contained (IR analysis) = 2.8% (moles).

35 The other characteristics of the polymer as such are as follows:

- MFLj ^ s = 0.7 g / 10 '

- MFI21i6 23 g / 10 '(shear sensitivity = 40)

- density = 0.9547 g / ml

- MÄ = 2.25

- melting temperature (under polarizing microscope) = 131 ° C.

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45 The crude copolymer was treated in a press at 180 ° for 30 minutes mixed with the following ingredients (parts per 100 parts of polymer): zinc oxide 5, stearic acid 1,2,2 'dimethyl-bis (4 methyl-terbutylphenol (l, N-oxideethylbenzothiazole - 2-sulfenamide (NOBS Special) 1,5, dibenzothia-50 zyl disulfide (Vulcacit DM) 0,5, Sulfur 3.

The product thus obtained has a gel content (insoluble in boiling xylene) of 80% and shows the characteristics shown in the table (next to the values found on the polymer as it is):

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as it is crosslinked

Impact resistance (kg cm / cm2)

7.2

62.5

Yield strength (kg / cm2)

248

198

Breaking load (kg / cm2)

267

275

Elongation at break (%)

1600

578

Stress cracking (h)

0

500

Flexural modulus (kg / cm2)

7800

6700

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Example 2

In the apparatus and under the conditions described in Example 1, a test was carried out using the following reagents: anhydrous toluene ml 1800, A1 (C2H5) C1 ml 1.35, A1 (C2H5) C12 g 0.12, butadiene g 40 and, after thermostating at 80 ° C, ethylene up to 10 kg / cm2 and hydrogen up to 20 kg / cm in the order. The ratio between the two partial pressures is about 1.

Over the course of thirty minutes, a toluene solution (200 ml) containing 35 mg of V triacetylacetonate and 1.06 g of methyl trichloroacetate was gradually introduced into the autoclave. During the test, ethylene was fed in order to keep the initial pressure of 20 kg / cm2 constant.

The polymerization was stopped after 90 minutes by introduction of 25 ml of methyl alcohol. The polymer was isolated from the suspension by filtration and vacuum drying at 50 ° C. 148 g (29,000 g per g of metallic V) of finely divided white product obtained which, upon infrared analysis, reveals a content of 3.3% of 1,4 trans butadiene units and which has an MFI2, i6 of 12.5 and uni MFI21i "of 295 (shear sensitivity = 23.5).

The crude copolymer was cross-linked using the compound and the conditions already reported in example 1. The cross-linked product has a gel content of 53% and shows an impact resistance of 16.3 kg.cm/cm2 (against 2.6 kg.cm/cm2 of the polymer as it is). The flexural modulus is 7200 kg / cm2 (9900 kg / cm2 in the starting polymer).

Example 3

Using the autoclave described in example 1, a polymerization was carried out starting from the following reagents: anhydrous toluene ml 1800, butadiene g 40.1 (C2H3) 2C1 ml 1 (8.2 mM), Al (C, Hn) Cl2 g 0.096 (0.8 mM).

The autoclave was thermostated at 80 ° C and pressurized with ethylene (partial pressure 14 kg / cm2 and hydrogen (partial pressure 6 kg / cm2).

Keeping the temperature indicated constant, a toluene solution (200 cm3) containing dissolved 35 mg of V triacetylacetonate and 0.53 g of methyl trichloroacetate was gradually introduced in about thirty minutes. The consumed ethylene has been replenished maintaining the total pressure constantly at 20 kg / cm2.

The test was stopped after 45 minutes by introduction of 25 cm3 of methyl alcohol.

The polymer obtained after drying weighs 147 g, contains (IR analysis) 2.3% of 1,4 trans butadiene units, an MFI2illi of 4.8 g / 10 'and an MFI2I. Of 88.3 g / 10' (shear sensitivity = 18).

The Mw / Mn ratio determined by gel permeation is 2.12. After cross-linking in the presence of sulfur, with the recipe and the methods described in example 1, a product containing 75% of gel and having an impact resistance of 36.2 kg / cm / cm2 (compared to 3.9 kg.cm/cm2 of the non-crosslinked polymer).

Example 4

In the autoclave described in example 1, a solution obtained with:

- anhydrous toluene

- butadiene

- A1 (C2H5) 2C1

- CCljCOOH

I 1800 g 20 ml 2 g 0.654

truly hydrogen for a partial pressure of 4 kg / cm2 (total pressure 13 kg / cm2).

During 20 minutes 70 mg of V triacetylacetonate were dissolved, dissolved in 200 ml of toluene, while 5 total temperature and pressure were kept constant (the latter by feeding other ethylene).

The polymerization was stopped after 60 minutes. The polymer, treated as previously described (see example 1), weighs 137 g in the dry state (13,700 g per g of V io metallic). It contains 2.1% of 1.4 trans butadiene units and shows the following fluidity:

MFI2j16 = 7.06 g / 10 '- MFI21i6 = 184.8 g / 10' (shear sensitivity = 26.2).

Subjected to cross-linking under the conditions and with the recept shown in Example 1, the copolymer is 58% crosslinked.

Example 5

Using the same equipment described above (see example 1), a copolymerization test was carried out with the following reagents:

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The solution thus obtained was transferred to the autoclave and thermostated at 105 ° C. Subsequently ethylene and hydrogen were added for respective partial pressures of 19 and 1 atm. The polymerization began at the introduction 35 of a toluene solution of V triacetylacetonate (52 mg in 200 ml), which took place over the course of 12 minutes. The polymerization was stopped after 75 minutes by introduction of 25 ml of methyl alcohol. 150 g of dry polymer having the following 40 characteristics were isolated with the usual technique:

- anhydrous toluene mi

1800

- butadiene g

20

- A1 (C2H5) 2C1

ml

1.25

- Al (C2H5) Cl2

g

0.12

- CClsCOOH

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trans-1,4-butadiene units mfi.

* 2.16

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MFI21, or

(shear sensitivity = 49) density

1.2% (moles) 0.21 10.4

.9556

The autoclave was thermostated at 80 ° C, then ethylene was introduced up to a pressure of 9 kg / cm2 and subsequent melting temperature (under the microscope 133 ° C polarizer)

The yield with respect to vanadium was 19.700 g per g of metal.

55 The copolymer is 78% cross-linked after treatment at 180 ° C for 30 minutes in the presence of the same ingredients described in example 1. The impact resistance of the cross-linked product was 71.2 kg.cm/cm2 (against 11.3 of the co-polymer as it is).

60 Example 6

In the autoclave described in example 1, a solution prepared starting from:

- anhydrous toluene I 1800 "5 - HzO mg 72

- butadiene 40 g

- A1 (C2H6) 2C1 ml 1.5

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The solution was thermostated at 80 ° C and saturated with ethylene up to a pressure of 14 kg / cm2. Hydrogen was then introduced for a partial pressure of 6 kg / cm2 (20 kg / cm2 total).

Maintaining the temperature at 80 ° C, a solution of vanadyl diacetyl acetate (26.5 mg in 100 ml of toluene) was first introduced in 5 minutes and then, in about 20 ', a solution of trichloroacetic acid in toluene (0.98 g in 100 ml).

During the test, which lasted 90 ', the consumed ethylene was fed maintaining the overall pressure at 20 kg / cm2. 190 g of dry copolymer were obtained which, based on the IR analysis, contained 2.7% of 1.4 trans butadiene units, MFI2 | 1 (1 = 0.38 g / 10 ', MRIo1 (; = 11 , 8 g / 10 minutes (shear sensitivity = 31) The yield referred to the vanadium used was 37.700 g per g of metal.

After crosslinking with sulfur based formulation, as described in Example 1, the polymer showed a gel content of 82%. The impact resistance of the crosslinked product was 78.5 kg.cm/cm2 (against 14.2 kg / cm2 of the copolymer as it is).

Example 7

Using the apparatus and the methods already described (see example 1), the reaction was carried out in the autoclave at 105 ° C:

- anhydrous toluene

- Al (CaHs) 2Cl

- AI (C2H5) C12

- butadiene

- CCI3COCI

ml 1800 ml 1.35 g 0.12 g 20 g 0.546

Ethylene and hydrogen were introduced until a partial pressure of 19 atm and 1 atm, respectively, was established (at 105 ° C).

The polymerization was started by the paralyzed addition of a toluene solution (200 ml) containing 35 mg of dissolved V-triacetylacetonate.

The test lasted 45 minutes, during which the temperature and total pressure were kept constant by feeding ethylene.

- Polymer obtained 48 g 5 Features:

- 1,4 trans unit (in poly butadiene) 1,2%

- MFL (il6 0.52 g / 10 '10 - MFI21i6 (shear sensitivity = 40) 20.9 g / 10'

- density 0.9562

- Mw / Mn 2,18

15 The product obtained after cross-linking with sulfur (see conditions and recipe shown in example 1) 68% of gel insoluble in xylenes at 140 ° C shows an impact resistance of 65.2 kg.cm/cm2 (against 9, 5 of the copolymer as it is).

20 Example 8

In the autoclave described in example 9 they have been introduced

- anhydrous toluene 25 - A1 (C2H5) 2C1

- Al (CaHB) Cl2

- butadiene ml 1800 ml 1.4 g 0.1 g 40

30 After having thermostated at 80 ° C, ethylene was introduced first and then hydrogen until reaching pressures of 18 and 20 kg / cm2 in total.

A toluene solution of V triacetylacetonate (52 mg in 100 ml) 35 was then added over 5 minutes and subsequently, over 30 minutes, 42 ml of CC14 diluted with 100 cm3 of toluene.

The polymerization was stopped after 45 ', obtaining 48 g of dry polymer. The characteristics of the product were as follows:

40 butadiene% (moles) = 1.8 - MFl2il6 = 12.58 g / 10 '- MFI.1 .. = 226.5 (shear sensitivity = 18.1).

The crosslinking product, obtained as described in Example 1, had a gel content of 62%.

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Claims (10)

621355 1. Process for the preparation in high yield of ethylene-butadiene copolymers consisting in carrying out the polymerization reaction in one or more hydrocarbon solvents in the presence of a catalytic system consisting of: a) one or more vanadium compounds soluble in the aforementioned hydrocarbons selected from the complexed tetra or pentivalent vanadium halides and complexed trivalent vanadium, the vanadium alcohols and the vanadyl and vanadyl chelates. b) one or more aluminum compounds corresponding to the formula RjR.AlX wherein Rt and R2 equal or different from each other, are alkyl, cycloalkyl, alkylaryl and aryl radicals containing from 1 to 18 carbon or hydrogen atoms and X it's halogen, c) one or more plurlogenated compounds soluble or soluble in the reaction medium of the elements of the III and IV Group of the periodic system, and / or one or more acid compounds capable of releasing protons and d) one or more organic compounds containing in their molecule at least one grouping -CX3 with X halogen. 2. Process according to claim 1, wherein X in b) is chlorine or bromine. 2 3. Process according to claim 1, characterized in that the quantity of component a) varies from 10 to 4 mole-liter. Method according to claims 1 to 3, characterized in that the quantity of component b) varies from 1 to 8.10 ~ 3 moles / liter. 5. Process according to claim 1, characterized in that when (c) is a plurilogenated compound, the ratio between the grams of halogen and aluminum atoms varies from 1.00 to 1.25. 6. Process according to claim 1, characterized in that the molar ratio between the components b) and c) when c) is a proton acid, varies from 10: 1 to 1: 1, preferably from 4: 1 to 2: 1. 7. Process according to claim 1, characterized in that the ratio between the components d) and b) is comprised between 1: 4 and 1: 1. 8. Process according to claim 1, characterized in that the reaction is carried out in the presence of a hydrocarbon solvent selected from aromatic hydrocarbons, mixtures of aromatic hydrocarbons with aliphatic, cycloaliphatic hydrocarbons, halogenated aromatic hydrocarbons or mixtures with the aforementioned or with other halogenated hydrocarbons . 9. Process according to claim 1, characterized in that the reaction is carried out at temperatures between 0 and 120 ° C, preferably between 50 and 100 ° C and at ethylene pressures varying from 2 to 20 kg / cm2. 10. Ethylene-butadiene copolymers having a butadiene unit content ranging from 0.1 to 10%, a vanadium content of less than 50 parts per million, restricted molecular weight distribution, high chain linearity, high X-ray crystallinity and absence of gel and statistical distribution of monomeric units, produced by the process according to claim 1.