Ziegler-natta bimetallic catalysts, catalytic systems including the same and processes for polymerizing linear alpha-olefins

Abstract

The present invention relates to Ziegler-Natta bimetallic 5 catalysts supported on MgCl2, which contains: titanium in the oxidation state +3 (Ti(III)), and optionally titanium in the oxidation state +4 (Ti(IV)); and at least one metal selected from hafnium in the oxidation state +4 (Hf(IV)) and zirconium in the oxidation state +4 (Zr(IV)); wherein: the Mg / Ti molar ratio is not higher than 3.0, preferably from 0.5 to 2.7, more preferably from 1,0 to 2.5; and the (Hf and / or Zr) / Ti molar ratio is from 0.5 to 5.0, preferably from 1.0 to 4.0, more preferably from 1.5 to 3.5. The above Ziegler-Natta bimetallic catalysts supported on MgCl2 may be included into a catalytic system in combination with at least one organoaluminum co-catalyst. Said catalytic system may be used in a process for polymerizing at least one linear α-olefin.

Description

[0001] ZIEGLER-NATTA BIMETALLIC CATALYSTS, CATALYTIC SYSTEMS INCLUDING THE SAME AND PROCESSES FOR POLYMERIZING LINEAR ALPHA-OLEFINS

[0002] ★ ★ ★ ★ ★

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to Ziegler-Natta bimetallic catalysts, to catalytic systems including the same and to processes for polymerizing linear a-olefins wherein said catalytic system is used.

[0005] BACKGROUND OF THE INVENTION

[0006] Poly-a-olef ins having high molecular weights and stereoregularity are usually obtained by polymerization of C2- C10 a-olefins by means of catalytic systems based on Ziegler-Natta catalysts, which are usually made of a titanium compound and an organoaluminum compound as co-catalyst, possibly supported on MgC12.

[0007] Bimetallic Ti-Hf catalyst systems have been studied in the polymerization of ethylene and a-olefins in order to understand the synergy between the two metals and in particular the role of hafnium. Bimetallic Ti-Hf catalysts and monometallic Ti-based catalysts, both supported on MgC12, prepared according to the procedure described in EP 0 243 327 Al, give comparable polymer yields, but in the case of the bimetallic Ti-Hf catalysts higher molecular weights are obtained (see F. Masi, S . Malquori, L . Barazzoni, C . Ferrero, A. Moalli, F. Menconi, R. Invernizzi, Makromol . Chem. Suppl . 1989, 15, 147-165) .

[0008] WO 2012 / 084920 Al relates to a catalyst comprising titanium,magnesium, aluminum, chlorine and at least one metal M selected from hafnium and zirconium, and the synthesis process thereof . The catalyst is characterized by the following atomic or molar ratios : M / Ti = 0.2-5. 0; Mg / Ti = 3.0-20.0; R-COOH / (Mg + M) = 1-8, wherein M = Hf or Zr . The catalyst is pretreated with a siloxane compound, so that the ratio Si / Ti = 0. 2-2.0. The above catalyst is used in a process for the synthesis of poly-a-olef ins at temperatures above 50°C .

[0009] WO 2016 / 016355 Al relates to a process for preparing a catalyst comprising titanium, magnesium, aluminum, chlorine and optionally at least one metal M selected from hafnium and zirconium. The catalyst is characterized by the following atomic or molar ratios : M / Ti = 0. 0-5.0; Mg / Ti = 3.0-15.0; R-COOH / (Mg + M) = 1.5-8, wherein M = Hf or Zr . The above catalyst is used in a process for the synthesis of poly-a-olef ins at temperatures from 20°C to 300°C .

[0010] WO 2011 / 060958 Al relates to a catalyst for the polymerization of a-olefins, comprising titanium, magnesium, aluminum, chlorine and at least one metal M selected from hafnium and zirconium. The catalyst is characterized by the following atomic or molar ratios : M / Ti = 0.2-5. 0; Mg / Ti = 3.0-15.0; Al / Ti = 0. 1-4.0; Cl / Ti = 15.0-60.0, wherein M = Hf or Zr . The above catalyst is used in a process for the synthesis of poly-a-olef ins at temperatures from 20°C to 300°C .

[0011] WO 2000 / 58368 Al relates to a catalyst for the polymerization of a-olefins, comprising titanium, magnesium, aluminum, chlorine and at least one metal M selected from hafniumand zirconium . The catalyst is characteri zed by the following atomic or molar ratios : M / Ti = 0 . 1- 10 . 0 ; Mg / Ti = 1 . 0-20 . 0 ; Al / Ti = 0 . 01- 6 . 0 Cl / Ti = 2 . 0-70 . 0 ; R-COO / Ti = 0 . 1- 10 . 0 , wherein M = Hf or Zr . The above catalyst is used in a process for the synthesis of poly-a-olef ins at temperatures from 20 ° C to 300 ° C .

[0012] US 7 , 348 , 383 B2 relates to a process for preparing a Ziegler-Natta bimetallic catalyst comprising magnesium, titanium, another transition metal comprising hafnium and optionally silica, wherein a solution of the respective precursors in an organic solvent containing hydroxyl functionality is subj ected to spray-drying, and the product is subsequently halogenated by means of an organoaluminum and / or organoborate halide . The catalyst is characteri zed by a Ti / Hf molar ratio from 0 . 05 to 100 . 0 , preferably from 0 . 1 to 10 (which corresponds to a Hf / Ti molar ratio from 0 . 01 to 20 , preferably from 0 . 1 to 10 ) . The above catalyst , combined with a triethylaluminum as a co-catalyst , is used in a polymeri zation process of C2-C20 olefins .

[0013] WO 2023 / 204618 Al relates to a process for preparing a Ziegler-Natta catalyst comprising magnesium ( obtained in-situ by reacting dialkyl magnesium with an inorganic halide to form the support with a characteristic XRD pattern for the 5-MgC12 phase ) , titanium, a second transition metal ( zirconium and / or hafnium and / or vanadium and / or niobium and / or tantalum) , aluminum and chlorine . The catalyst is syntheti zed by reacting a titanium compound with MgC12 in the following molar ratios : 1 : 0 . 1 to 1 : 30 , 1 : 0 . 1 to 1 : 30 , 1 : 5 to 1 : 30 , 1 : 8 to 1 : 25 , 1 : 10 to 1 : 25 , 1 : 11 to1 : 22, or 1 : 12 to 1 : 21. The above ratios correspond to obtain a catalyst having a Mg / Ti molar ratio ranging from 0. 1 to 30, from 5 to 30, from 8 to 25, from 10 to 25, from 11 to 22, from 12 to 21. The above catalyst is used in a polymerization process for producing low density olefin copolymers ( from 0.91 to 0.94 g / ml) .

[0014] US 9, 255, 160 B2 relates to a process for preparing a Ziegler-Natta multimetal catalyst comprising magnesium, obtained in-situ by reacting a hydrocarbon-soluble organomagnesium compound or complex thereof and a non-metallic or metallic halide to form a halogenated magnesium support . Such support is then contacted with a conditioning compound containing an element selected from the group consisting of boron, aluminum, gallium, indium and tellurium. The conditioned magnesium halide support is then contacted with a compound containing titanium, to form a supported titanium compound. The latter is then contacted with a second metal and a third metal independently selected from the group consisting of zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten, provided that the second metal and the third metal are not the same . The catalyst is characterized by a Mg / Ti molar ratio from 8.0 to 80.0, and a Mg / (Ti + 2nd metal + 3rd metal) molar ratio of from 5.0 to 30.0. The above catalyst is used to produce polyethylene having a density from 0.90 to 0.96 g / ml .

[0015] US 8, 809, 220 B2 relates to a process for preparing a Ziegler-Natta bimetallic catalyst comprising magnesium, titanium, hafnium and / or zirconium, aluminum, chlorine and a filler at neutral pH to make the catalyst workable by spray-drying technique . The catalyst is represented as MgxTiHfyZrz, where x is from 1 to 20, y is from 0 to 10 and z is from 0 to 10, with the proviso that y+z > 0. Therefore, the catalyst is characterized by a Mg / Ti molar ratio from 1.0 to 20.0, and (Hf and / or Zr) / Ti molar ratio higher than zero and not higher than 10.0. The particles of the bimetallic catalyst have a spherical shape with an average size (D50) from 10 to 70 pm. The above catalyst is used in an olefin polymerization process carried out in gas or slurry phase, in the presence of at least one cocatalyst .

[0016] US 11, 939, 417 B2 relates to a process for preparing a bimetallic heterogeneous catalyst comprising magnesium, titanium, and a second metal M selected from the group comprising vanadium, scandium, zirconium, niobium, magnesium, calcium, in the form of a metal complex of formula (L)nM (Y)m(XR2) , aluminum or silica, chlorine; X is a heteroatom and, preferably, XR2 is an alkoxide or a carboxylate . The catalyst is characterized by a Ti / Mg molar ratio from 0.005 to 0.25 (which corresponds to a Mg / Ti molar ratio from 4 to 200) . The above catalyst is used in a solution polymerization process for producing ethylene-based copolymers, in the presence of an organoaluminum compound as cocatalyst .

[0017] A particular class of poly-a-olef ins is known in the art with the term amorphous poly-a-olef ins (APAOs) , which are obtained from the polymerization of long-chain a-olefins and have a high molecular weight, e . g. a weight average molecular weight (Mw) higher than about 105g / mole . The APAOs are commonlyused as drag reducers, i . e . as additives for reducing friction in fluids conveyed in long pipelines .

[0018] It is known that the main requirement to transport a fluid through a pipeline is that the pressure at the pumping station is such as to guarantee final pressure and flow rate suitable for the intended use . When fluids are transported through a pipeline, generally in turbulent flow conditions, for example in case of transport of oil or hydrocarbons, there is usually a pressure drop of the transported fluid, due to the friction between the internal wall of the pipeline and the fluid itself . This issue is more relevant when fluids are transported over long distances .

[0019] Specific additives, called drag reducers, to be added to the transported fluid have been studied, developed, and used to solve the above problem. The quantitative definition of the phenomenon of drag reduction is calculated as a comparison of the pressure loss with and without the presence of a drag reducer, as reported in the equation ( I ) :

[0020] AP untreated- P treated . > >

[0021] %DR - - 100 ( I )

[0022] AP untreated

[0023] wherein :

[0024] - AP untreated is the pressure drop in the pipeline with no drag reducer added to the fluid;

[0025] - AP treated is the pressure drop in the pipeline with a drag reducer added to the fluid, by assuming that the flow rate of the fluid in the pipeline is constant .

[0026] The APAOs are the most employed class of products used asdrag reducing additives . See for instance US 3, 692, 676, WO 2008 / 073293 and US 2016 / 0024369.

[0027] The Applicant has faced the problem of providing Ziegler-Natta bimetallic catalysts, suitable for processes of polymerization of linear a-olefins, which allow to employ process conditions more convenient for an industrial application, to achieve a high monomer conversion .

[0028] In particular, the Applicant has faced the problem of producing amorphous poly-a-olef in (APAOs) to be used as drag reducers for fluids conveyed in pipelines, by means of bimetallic Ziegler-Natta catalysts which are able to regulate the polymerization process so as to improve the drag reducing effect .

[0029] Surprisingly, the Applicant has found that it is possible to solve the above stated technical problems by means of a Ziegler-Natta bimetallic catalyst supported on MgC12, which contains titanium in the oxidation state +3 (Ti ( III ) ) , and optionally titanium in the oxidation state +4 (Ti ( IV) ) ; and at least one metal selected from hafnium in the oxidation state +4 (Hf ( IV) ) and zirconium in the oxidation state +4 (Zr ( IV) ) ; wherein: the Mg / Ti molar ratio is from 0.5 to less than 3.0, preferably from 0.5 to 2.7, more preferably from 1.0 to 2.5; and the (Hf and / or Zr) / Ti molar ratio is from 0.5 to 5. 0, preferably from 1.0 to 4.0, more preferably from 1.5 to 3.5.

[0030] The above Ziegler-Natta bimetallic catalyst, when combined with an organoaluminum co-catalyst, forms a catalytic system which can be advantageously used in a process for polymerizing linear a-olefins . Such process may be carried out at atemperature not higher than 40°C, preferably from -30°C to 20°C, more preferably from -15°C to 10 °C, to produce poly-a-olef ins with a high monomer conversion. Moreover, in the case of APAOs, the obtained APAOs have improved properties when used as drag reducers for fluids conveyed in pipelines, particularly an improved drag reduction effect, which does not sharply decrease over time .

[0031] SUMMARY OF THE INVENTION

[0032] Therefore, in a first aspect, the present invention relates to a Ziegler-Natta bimetallic catalyst supported on MgC12, which contains :

[0033] titanium in the oxidation state +3 (Ti ( III ) ) , and optionally titanium in the oxidation state +4 (Ti ( IV) ) ; and at least one metal selected from hafnium in the oxidation state +4 (Hf ( IV) ) and zirconium in the oxidation state +4 (Zr ( IV) ) ;

[0034] wherein :

[0035] the Mg / Ti molar ratio is from 0.5 to less than 3.0, preferably from 0.5 to 2.7, more preferably from 1. 0 to 2.5; and the (Hf and / or Zr) / Ti molar ratio is from 0.5 to 5.0, preferably from 1.0 to 4.0, more preferably from 1.5 to 3.5.

[0036] In a second aspect, the present invention relates to a process for preparing a Ziegler-Natta bimetallic catalyst supported on MgC12 as defined above, said process comprising:

[0037] (a) mixing at least one titanium compound, at least one metal compound selected from hafnium (Hf ) compounds and zirconium (Zr) compounds, MgC12, and at least one carboxylic acidof formula R-COOH, wherein R is a linear or branched C2-C30 hydrocarbon radical, possibly substituted with at least one halogen, e . g. fluorine or chlorine, in a hydrocarbon liquid medium at a temperature from 40°C to 200°C, preferably from 60°C to 130°C, to obtain a catalyst precursor;

[0038] (b) adding to the catalyst precursor at least one organoaluminum compound of formula AlyClxyR3y-xy, wherein R is an alkyl having from 1 to 10 carbon atoms, x is a number from 1.0 to 2.0 and y is an integer equal to 1 or 2, to obtain the bimetallic catalyst .

[0039] In a third aspect, the present invention relates to a catalytic system comprising:

[0040] (i) a Ziegler-Natta bimetallic catalyst supported on MgC12, which contains : titanium in the oxidation state +3 (Ti ( III ) ) , and optionally titanium in the oxidation state +4 (Ti ( IV) ) ; and at least one metal selected from hafnium in the oxidation state +4 (Hf ( IV) ) and zirconium in the oxidation state +4 (Zr ( IV) ) ; wherein: the Mg / Ti molar ratio is from 0.5 to less than 3.0, preferably from 0.5 to 2.7, more preferably from 1. 0 to 2.5; and the (Hf and / or Zr) / Ti molar ratio is from 0.5 to 5.0, preferably from 1.0 to 4.0, more preferably from 1.5 to 3.5;

[0041] (ii) at least one organoaluminum co-catalyst of formula AlRnX(3-n) , wherein: the R groups, equal or different from each other, are linear or branched Ci-Ce alkyl groups; X is halogen, preferably chlorine; n is 1, 2 or 3 .

[0042] In a fourth aspect, the present invention relates to a process for polymerizing at least one linear a-olefin, saidprocess being carried out at a temperature not higher than 40°C, preferably from -30°C to 20°C, more preferably from -15°C to 10°C, by using a catalytic system comprising:

[0043] (i) a Ziegler-Natta bimetallic catalyst supported on MgC12, which contains : titanium in the oxidation state +3 (Ti ( III ) ) , and optionally titanium in the oxidation state +4 (Ti ( IV) ) ; and at least one metal selected from hafnium in the oxidation state +4 (Hf ( IV) ) and zirconium in the oxidation state +4 (Zr ( IV) ) ; wherein: the Mg / Ti molar ratio is not from 0.5 to less than 3.0, preferably from 0.5 to 2.7, more preferably from 1.0 to 2.5; and the (Hf and / or Zr) / Ti molar ratio is from 0.5 to 5.0, preferably from 1.0 to 4.0, more preferably from 1.5 to 3.5;

[0044] (ii) at least one organoaluminum co-catalyst of formula AlRnX(3-n) , wherein: the R groups, equal or different from each other, are linear or branched Ci-Ce alkyl groups; X is halogen, preferably chlorine; n is 1, 2 or 3 .

[0045] Preferably, the above process for polymerizing at least one linear a-olefin is a mass polymerization process, namely a bulk polymerization process, carried out in the absence of a solvent, in which the reaction medium is substantially formed by the monomers, and the catalyst is preferably suspended in said reaction medium.

[0046] For the purpose of the present disclosure and of the claims that follow, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about" . Moreover, all numerical ranges include any single valuewithin the ranges and the extremes thereof, and also any intermediate ranges therein, which may or may not be specifically enumerated .

[0047] For the purpose of the present disclosure and of the claims that follow, except where otherwise indicated, the terms "comprise" or "include" encompass also the terms "consist of" or "consist essentially of" .

[0048] BRIEF DESCRIPTION OF THE FIGURE :

[0049] Figure 1 : it is a schematic representation of an apparatus used to determine the drag reduction effect (DR%) of an additive added to a fluid, as described in the examples reported hereinunder .

[0050] DETAILED DESCRIPTION OF THE INVENTION

[0051] Preferably, the linear a-olefin monomer has formula CH2=CH-R, wherein R is H or a linear C1-C12 alkyl group . Preferably, R is a linear C2-C10 alkyl group . Even more preferably, the linear a-olefin monomer is selected from: 1-hexene, 1-octene, 1-decene and 1-dodecene, or mixtures thereof . Even more preferably, the linear a-olefin monomer is selected from: 1-octene, 1-decene, 1-dodecene, and mixtures thereof .

[0052] According to a first preferred embodiment, the bimetallic catalyst contains : titanium in the oxidation state +3 (Ti ( III ) ) , and optionally titanium in the oxidation state +4 (Ti ( IV) ) ; and hafnium in the oxidation state +4 (Hf ( IV) ) .

[0053] According to a second preferred embodiment, the bimetallic catalyst contains : titanium in the oxidation state +3 (Ti ( III ) ) , and optionally titanium in the oxidation state +4 (Ti ( IV) ) ; andzirconium in the oxidation state +4 (Zr ( IV) ) .

[0054] Preferably, at least 70% by weight, more preferably at least 90% by weight, even more preferably at least 95% by weight, of titanium in the bimetallic catalyst is Ti ( III ) , the percentage being calculated on the total weight of Ti .

[0055] The amount of titanium present in the bimetallic catalyst preferably does not exceed 10% by weight, more preferably is from 1. 6% to 10% by weight, with respect to the total weight of the bimetallic catalyst .

[0056] In the catalytic system of the present invention, aluminum and titanium are preferably present in a molar ratio from 3 : 1 to 500 : 1, more preferably from 5 : 1 to 100 : 1, even more preferably from 10 : 1 to 40 : 1.

[0057] According to a preferred embodiment, the Ziegler-Natta bimetallic catalyst supported on MgC12 according to the present invention comprises at least one carboxylate group deriving from a carboxylic acid of formula R-COOH, wherein R is a linear or branched hydrocarbon radical having from 2 to 30 carbon atoms, possibly substituted with at least one halogen, e . g. fluorine or chlorine . Preferably, said at least one carboxylate group is present in an amount so as to have a (carboxylate group) / Ti molar ratio from 0.1 to 0.8, more preferably from 0.4 to 0. 6. The expression "at least one carboxylate group" encompasses the case that more than one carboxylate group is present . When more than one carboxylate group is present, the numerator of said molar ratio is the sum of the moles of each carboxylate group if more than one carboxylate is present in the bimetallic catalyst .According to a preferred embodiment, the Ziegler-Natta bimetallic catalyst supported on MgC12 according to the present invention has the following formula :

[0058] TiiMgxHfyAlzClw (R-COO)k

[0059] wherein :

[0060] R is a linear or branched hydrocarbon radical having from 2 to 30 carbon atoms, possibly substituted with at least one halogen, e . g. fluorine or chlorine;

[0061] x = 0.5 - 2.7 (preferably 1.0 - 2.5) ;

[0062] y = 1.0 - 4.0 (preferably 1.5 - 3.5) ;

[0063] z = 0.5 - 1.2 (preferably 0.7 - 0.9) ;

[0064] w = 13.0 - 25.0 (preferably 19.0 - 21.0) ;

[0065] k = 0.1 - 0.8 (preferably 0.4 - 0. 6) .

[0066] As regards the organoaluminum co-catalyst of formula AlRnX<3- n) , the linear or branched Ci-Ce alkyl groups R are preferably selected from: methyl, ethyl, n-butyl, isobutyl, t-butyl, n-pentyl, isopentyl, n-hexyl, isohexyl . Particularly preferred are trialkylaluminum compounds, such as : triethylaluminum, tri-n-butylaluminum, triisobutylaluminum and trihexylaluminum.

[0067] The titanium, hafnium and zirconium compounds used to produce the Ziegler-Natta bimetallic catalyst according to the present invention may be selected from a wide range of inorganic or organometallic compounds comprising said metals . Preferably, such compounds are selected from: chlorides, bromides, alcoholates, hydrides, p-diketonates , p-acyl esters, amides, carbonates, carboxylates, phosphates, or mixtures thereof . Particularly preferred are titanium chlorides, hafnium chloridesand zirconium chlorides .

[0068] In the process for producing the Ziegler-Natta bimetallic catalyst according to the present invention, at least one carboxylic acid of formula R-COOH is added in step (i) for partially or completely dissolving the solids in the reaction medium.

[0069] The at least one carboxylic acid of formula R-COOH has a relatively large number of carbon atoms in the chain to promote dissolution in a hydrocarbon type liquid medium. The group R in the formula above may be a linear or branched hydrocarbon radical having from 2 to 30 carbon atoms, possibly substituted with at least one halogen, e . g. fluorine or chlorine . More preferably, R is a linear or branched hydrocarbon radical having from 3 to 16 carbon atoms .

[0070] Non-limiting examples of said group R are :

[0071] linear alkyl groups containing at least 3 carbon atoms, preferably not more than 16 carbon atoms, for example n-hexyl, n-octyl, n-nonyl, n-decyl or n-undecyl;

[0072] branched alkyl groups having a linear chain containing from 2 to 10 carbon atoms and a branching chain in the alpha or beta position of the linear chain with respect to the carbon atom of the carboxylic group, said branching chain being selected from: linear Ci-Ce alkyl groups, branched Ci-Ce alkyl groups, phenyl, cyclopentyl, cyclohexyl .

[0073] Preferably, the at least one carboxylic acid of formula R-COOH is selected from: 2-ethylhexanoic acid, 2-methylhexanoic acid, 2-propylhexanoic acid, 2-butylhexanoic acid. Particularlypreferred is 2-ethylhexanoic acid.

[0074] Preferably, the organoaluminum compound of formula AlyClxyR3y-xy , used in step (b) of the process for preparing a Ziegler-Natta bimetallic catalyst according to the present invention, is selected from: ethyl aluminum sesquichloride (EASC) , ethylaluminum dichloride (EADC) , isobutylaluminum dichloride ( IBADIC) and diethylaluminium chloride (DEAC) . More preferably, it is EASC .

[0075] Preferably, the catalytic system according to the present invention comprises up to 10% by weight, more preferably from 1 to 5% by weight, of the above carboxylic acid R-COOH, the percentage being calculated on the basis of the total weight of the catalytic system.

[0076] The catalytic system according to the present invention may be prepared in advance, outside the polymerization reactor, by mixing and reacting the Ziegler-Natta bimetallic catalyst supported on MgC12 with the at least one organoaluminum cocatalyst of formula AlRnX(3-n)

[0077] Alternatively, the catalytic system according to the present invention may be prepared in the polymerization reactor, where the bimetallic catalyst is introduced and reacted with the organoaluminum co-catalyst previously added therein.

[0078] During the preparation of the catalytic system, other additives may be added, commonly used for the production of Ziegler-Natta catalysts, such as : olefins, ethers, tertiary amines, non-polymerizable alcohols, halogenated hydrocarbons, preferably chlorinated hydrocarbons .The bimetallic catalyst or the catalytic system may be introduced into the polymerization reactor, where the polymerization process is carried out, in the form of a dispersion or of a solution in a saturated aliphatic hydrocarbon, preferably selected from: propane, pentane, hexane, decane, and mixtures thereof .

[0079] The process for polymerizing at least one linear a-olefin according to the present invention may be carried out according to known techniques . Preferably, the polymerization process is a mass polymerization carried out in an inert atmosphere, preferably in a nitrogen atmosphere having an oxygen content not higher than 1 ppm (by weight) .

[0080] The mass polymerization process may be carried out continuously or batch-wise .

[0081] The catalytic system in the polymerization reactor is used in an amount so as to obtain a titanium concentration preferably from 100 mmol / 1 to 200 mmol / 1, equal to approximately 4.5-9 g / 1, said concentration being calculated on the basis of the total monomer volume used in the polymerization.

[0082] The polymerization may be generally carried out for a reaction time from 24 to 360 hours, preferably from 60 to 300 hours, more preferably from 80 to 120 hours .

[0083] The polymerization process according to the present invention has a high monomer conversion, usually more than 40.0%, preferably more than 80.0%, even more preferably more than 85.0% .

[0084] The present invention is further described by means of some examples as reported hereinbelow, which are provided only forillustrative purposes and cannot be construed as a limitation to the claim scope .

[0085] EXAMPLES .

[0086] The following commercial reagents were used (purity degrees are reported as % according to the producer ' s technical data sheet) :

[0087] sodium hydroxide (CAS 1310-73-2, purity > 98.0%)

[0088] n-decane (CAS 124-18-5, purity > 99.0%)

[0089] magnesium chloride (CAS 7786-30-3, purity > 98.0%) hafnium tetrachloride (CAS 13499-05-3, purity 98.0%) titanium tetrachloride (CAS 7550-45-0, purity 99.9%) zirconium tetrachloride (CAS 10026-11-6, purity > 99.5%) 2-ethylhexanoic acid (CAS 149-57-5, purity 99.0%)

[0090] ethyl aluminum sesquichloride (CAS 12075-68-2, purity 99.9%)

[0091] 1-decene (CAS 872-05-9, purity 95.0%)

[0092] triisobutyl aluminum (CAS 100-99-2, purity 99.9%) xylene, mixture of isomers (CAS 1330-20-7, purity > 98.5%) magnesium stearate (CAS 557-04-0, purity 99.0%)

[0093] soybean oil (CAS 8001-22-7, purity 99.0%)

[0094] n-hexane (CAS 110-54-3, purity > 99.0%)

[0095] titanium butoxide (CAS 5593-70-4, purity > 97.0%) AICI3 (CAS 7446-70-0, purity 99.9%)

[0096] diethylaluminium chloride (CAS 96-10-6, purity > 99.9%) n-dodecene (CAS 112-41-4, purity > 99.0%)

[0097] n-octene (CAS 111-66-0, purity > 99.0%)

[0098] EXAMPLE 1 (invention)Preparation of a bimetallic catalyst Ti-Hf / MgC12.

[0099] TABLE 1

[0100]

[0101] All the glassware was previously dried in an oven at 100°C for 24 hours (to eliminate traces of residual humidity and oxygen, which are poisons for the reaction and for the reagents themselves) .

[0102] An apparatus was set up consisting of a 1000 ml four-necked glass flask, equipped with a 500 ml drip funnel connected to a trap containing 100 ml of a 20% sodium hydroxide solution in water (to neutralize the hydrochloric acid that developed during the reaction) , a magnetic stirrer with stir bar, a capillary for gas inlet and a thermometer .

[0103] The apparatus was cooled to room temperature in a stream of dry nitrogen and kept in an inert atmosphere until use .

[0104] 50 ml of dry n-decane were introduced into the flask (pretreated with 4A, 5A, 10A molecular sieves, to ensure drying) .Magnesium chloride and hafnium tetrachloride were added, previously stored inside the dry box, in the quantities indicated in Table 1 .

[0105] In a tailed test tube (sample tube) placed in the dry box, a titanium tetrachloride solution in n-decane ( 1.8027 g in 50 ml of solvent) was prepared. The content of the tailed test tube was transferred by siphoning into the flask, maintaining a slow magnetic stirring (about 300 rpm) . The tailed test tube was washed with 50 ml of n-decane . The quantity of 2-ethylhexanoic acid indicated in Table 1 was then added.

[0106] The mixture was heated to 90°C in the previously closed flask with a small vent to avoid pressurization of the flask (the temperature was reached in 45 minutes max) , the magnetic stirring was increased to about 800 rpm, and the stirred mixture was maintained at the indicated temperature for 2 hours (to obtain a substantially complete solubilization of the salts) . The mixture was slowly cooled to room temperature, maintaining stirring and inert atmosphere .

[0107] Ethyl aluminum sesquichloride (EASC) was then added while stirring ( 800 rpm) , maintaining the temperature at about 40 °C . The mixture was heated to 90 °C and maintained at that temperature for 2 hours . Stirring was then interrupted and the mixture was cooled to room temperature, allowing the solid to decant (about 4 hours) .

[0108] The supernatant was extracted by siphoning, being very careful not to aspirate the solid catalyst, and collected in a tailed bottle in an inert atmosphere . It was then slowly addedto a 10% sodium hydroxide aqueous solution.

[0109] About 250 ml of decane was added to the solid residue and stirring was maintained for a few minutes at room temperature and in inert atmosphere . Stirring was stopped and the solid was left to settle again. The operation was repeated three more times so as to obtain an Al / Ti ratio in the supernatant lower than 1.2 mol / mol . The solid was diluted with decane, to obtain the desired catalyst concentration.

[0110] A catalyst was obtained, whose formula was the following: TiiMgxHfyAlzClw ( 2-EHA)k

[0111] wherein :

[0112] x = 1.9

[0113] y = 3.0

[0114] z = 0.5

[0115] w = 19.9

[0116] k = 0.5.

[0117] EXAMPLE 2 (comparative)

[0118] Preparation of a Ti-Hf / MgC12 bimetallic catalyst .

[0119] The procedure described in Example 1 was repeated, but using different quantities of the various components of the catalyst, so as to obtain a catalyst whose formula was the following:

[0120] TiiMgxHfyAlzCl„ (2-EHA)k

[0121] wherein :

[0122] x = 5.2

[0123] y = 1.0

[0124] z = 0.9k 0.5.

[0125] EXAMPLE 3 (comparative)

[0126] Preparation of a Ti-Hf / MgC12 bimetallic catalyst .

[0127] The procedure described in Example 1 was repeated, but using different quantities of the various components of the catalyst, so as to obtain a catalyst whose formula was the following:

[0128] TiiMgxHfyAlzClw ( 2-EHA)k

[0129] wherein :

[0130] x = 0.2

[0131] y = 1.1

[0132] z = 0.4

[0133] w = 10.1

[0134] k = 0.5.

[0135] EXAMPLE 4 (comparative)

[0136] Preparation of a Ti-Hf / MgC12 bimetallic catalyst .

[0137] The procedure described in Example 1 was repeated, but using different quantities of the various components of the catalyst, so as to obtain a catalyst whose formula was the following:

[0138] TiiMgxHfyAlzCl„ (2-EHA)k

[0139] wherein :

[0140] x = 1.4

[0141] y = 0.3

[0142] z = 0.3

[0143] w = 7.8

[0144] k = 0.5.

[0145] EXAMPLE 5 (invention)

[0146] Preparation of a Ti-Zr / MgC12 bimetallic catalyst .The procedure described in Example 1 was repeated, with the following differences :

[0147] hafnium tetrachloride was replaced with zirconium tetrachloride ;

[0148] - different quantities of the various components of the catalyst were used, so as to obtain a catalyst whose formula was the following:

[0149] TiiMgxZryAlzClw( 2-EHA)k

[0150] wherein :

[0151] x = 1.7

[0152] y = 2.4

[0153] z = 0 . 6

[0154] w = 17.3

[0155] k = 0.1.

[0156] EXAMPLES 6 (invention) , 7-9 (comparative) and 10 ( invention)

[0157] Catalyst systems were prepared by using the catalysts of Examples 1 to 5, and then such catalyst systems were used for polymerizing 1-decene to produce APAOs . The procedure was as follows .

[0158] About 400 g of 1-decene were added into a dry box continuously fluxed with anhydrous nitrogen and molecular sieves MS-4A (Grace Davison) were added in large excess (about 60 g) ; the mixture was maintained at room temperature (23 °C) for 30 hours before use .

[0159] A previously dried polyethylene bottle (500 ml) was placed into the dry box; 333 g of dried 1-decene (about 450 ml) werepoured into the polyethylene bottle, then the bottle was closed and brought to 5°C .

[0160] 0.4 g of a co-catalyst (triisobutyl aluminum) were diluted in 1.1 g of n-decane; the solution was poured into the bottle containing 1-decene, the bottle was closed and shaken manually for one minute .

[0161] 150 mg (corresponding to approximately 0.4 ml) of the bimetallic catalyst suspension obtained in the previous examples was poured into the bottle, so that the monomer / Ti weight ratio was 400 ppm, while the co-catalyst / Ti molar ratio was 20.

[0162] The entire procedure described above was carried out in the dry box as quickly as possible . The bottle was closed and shaken manually for one minute .

[0163] The bottle was then placed in a refrigerator stabilized at - 10°C and maintained at that temperature for 168 hours, so as to carry out the polymerization reaction.

[0164] The bottle was cut using a cutter and the solid was separated from the liquid by using a colander . The liquid contained the unreacted monomer . Then the solid was chopped into small pieces (4-6 mm) ; the solid pieces were dried at room temperature and pressure for 12 hours . The solid contained the polymer .

[0165] EXAMPLE 11 (invention)

[0166] Polymerization of 1-dodecene with the catalyst of Example

[0167] Example 6 was repeated with the following differences :

[0168] 1-decene was replaced with 1-dodecene, in the samequantities ;

[0169] - the polymerization temperature was set to 4 °C, instead of - 10 ° C ;

[0170] the above temperature was maintained for 192 hours, instead of 168 hours .

[0171] EXAMPLE 12 (invention)

[0172] Polymerization of 1-octene with the catalyst of Example 1.

[0173] Example 6 was repeated with the following differences :

[0174] 1-decene was replaced with 1-octene, in the same quantities ;

[0175] - the polymerization temperature was set to -4 °C, instead of -10°C;

[0176] the above temperature was maintained for 192 hours, instead of 168 hours .

[0177] EXAMPLE 13

[0178] Preparation of APAO solutions .

[0179] The APAOs prepared in Examples 6 to 12 were dissolved in xylene to produce APAO solutions as follows .

[0180] 0.292 g of APAO and 1000.0 g of commercial xylene were introduced into a Schott bottle . The bottle was closed and shaken by an orbital shaker at 180 rpm, at room temperature for 24 hours . 400.0 g of the obtained APAO solution was transferred into a smaller Schott bottle .

[0181] Preparation of APAO suspensions .

[0182] The APAOs prepared in Examples 6 to 12 were suspended in soybean oil to produce APAO suspensions as follows .

[0183] An ultracentrifugal mill (Retsch, model ZM200) was set up,assembling a 2 mm mesh filter and adjusting the speed of the vibrating conveyor to approximately 2 cm / sec. The rotor-stator assembly and the conveyor were filled with solid carbon dioxide pellets .

[0184] 100 parts of the APAO polymer were cut into small pieces (about 5-10 mm) and placed in a container . Liquid nitrogen was added to cover the APAO pieces . After 15 minutes, the APAO pieces were removed and placed in a polyethylene bag. 40 parts of magnesium stearate powder (anti-sticking agent) were added, to obtain a total of 140 parts ( 100 parts APAO + 40 parts antisticking agent) . The bag was shaken to mix the materials and then such materials (kept into the bag) were ground using a pestle in a mortar . This was done quickly to ensure that the APAO was maintained below its glass transition temperature . 100 parts solid carbon dioxide pellets were then introduced into the bag, which was shaken vigorously to mix the various materials .

[0185] The ultracentrifugal mill was started at a speed of 15, 000 rpm, and the content of the bag was gradually transferred into the mill conveyor . The ground material was collected in a container previously filled with liquid nitrogen for approximately 10 minutes . The 2 mm mesh filter in the mill was replaced with a 0.12 mm mesh filter . The mill was cooled again by filling the conveyor and the rotor-stator assembly with solid carbon dioxide pellets .

[0186] 100 parts of solid carbon dioxide pellets were introduced into a new polyethylene bag, and the ground material was quickly removed from the nitrogen-filled container . The bag wasvigorously shaken to distribute the carbon dioxide pellets . The ultracentrifugal mill was started and the material contained in the bag was poured into the mill conveyor to be ground, using the same procedure as indicated above (except for the size of the filter mesh) . The ground material was placed into a previously chilled Schott bottle, an aliquot of previously weighed soybean oil was added and mixed vigorously.

[0187] The final composition of the APAO suspension was as follows : APAO: 20.0% wt;

[0188] magnesium stearate : 8.0% wt;

[0189] soybean oil : 72.0% wt .

[0190] Measurement of the drag reduction effect (test loop) The APAOs as prepared above were tested to measure the drag reduction effect (test loop) , according to the following procedure .

[0191] The apparatus for measuring the drag reduction effect (test loop) is shown in Figure 1.

[0192] The apparatus comprised: a test section unit ( 8 ) connected to two steel tubes ( 11, 12 ) ; two pressure transducers (7, 9) inserted on said steel tubes ( 11, 12 ) , to measure the pressure drop; one Coriolis mass flow meter ( 6) , to measure mass flow rate; a progressive cavity pump (3) ; a drain valve (5) , used to drain the circuit after the test; a vent valve ( 10) , positioned at the top point of the loop, used after starting the pump, to ensure that no air bubbles were trapped in the circuit; a stirred vessel ( 1 ) having a net volume of about 6.5 1 and a heat j acket . The fluid to be tested was fed from the vessel (1 ) to the pump( 3 ) by means of the steel pipe ( 2 ) and returns to the vessel ( 1 ) by means of the steel pipe ( 11 ) .

[0193] The test section ( 8 ) consisted o f a 30-meter-long tube in perfluoroalkoxy material ( PFA AP-230 ) , having outer diameter 9 . 982 mm and wall thickness 1 . 499 mm, produced / distributed by Swagelok ( Swagelok code PFA-T10M- 1 . 5M-30M, lot #02924214- 1 ) . The PFA tube was connected both ends to steel tubes ( 11 , 12 ) , which had outer diameter 10 mm and 1 . 5 mm thickness , therefore substantially the same internal diameter of the test section ( 8 ) . On said steel tubes ( 11 , 12 ) two pressure transducers ( 7 , 9 ) were inserted j ust before the beginning and after the end of the test section ( 8 ) to measure the pressure drop of the test section alone . The pressure transducers were Rosemount model 3051 CG 4 ( Gage transducers , - 0 . 97 to 20 . 68 bar ( g) ) . The mass flow rate was measured by means of a Coriolis mass flow meter ( 6 ) , which was a Corimass Flow Meter MFC 085 Smart 10G+ , nominal rate 10 kg / min . The pump ( 3 ) was a progressive cavity pump (Nova Rotors , model DN 05K2 , s / n A1607201 ) , driven by a reducer (VARMEC RCV 191 NF160 1=4 . 71 ) , frequency converter (Motive , model NEO-WI-FI-3kW NVR) and motor (Motive , model 90S-4 , n . 2006DG0309 ) . The mass flow rate , read by flow meter ( 6 ) , was regulated by acting on the frequency set in the frequency converter of the pump, so that a constant flow rate of 2 . 8 kg / min was ensured . A drain valve ( 5 ) was used to drain the circuit after every test . A vent valve ( 10 ) , positioned at the top point of the loop, was used after starting the pump, to ensure that no air bubbles were trapped in the circuit .Vessel ( 1 ) was a stirred glass container having a net volume of about 6.5 1 and a heat j acket . A transparent silicone oil heat transfer fluid was used as the fluid for thermal regulation. The j acket temperature was kept equal to ambient temperature (23°C) , so that all the system could be considered at the constant temperature of 23°C . The stirrer was an anchor type stirrer, with fixed speed of 50 rpm. The pump suction was connected to the bottom of the vessel ( 1 ) by means of a steel pipe (2 ) , while the pump delivery was connected to the flowmeter ( 6) and drain (5) by means of steel pipes (4 ) .

[0194] 3500.0 g of n-hexane (Sigma-Aldrich 1.04394 ) were weighed and added into the vessel ( 1 ) . Nitrogen was fed by the opening valve ( 13) , while keeping the vessel vent open (valve 14 ) , the same used to charge the solution) to keep atmospheric pressure inside the vessel . The nitrogen flow rate was low to avoid any stripping action. Said nitrogen flow was used only to reduce the oxygen ( from air) in the vessel . The stirrer was set to 50 rpm. The pump was started, and the pump speed was regulated by means of its frequency converter to set the measured flow rate to 4.3 1 / min (the mass flow meter measures the mass flow but also the density, so it is also possible to compute the volumetric flow rate) .

[0195] After 10 minutes, the recording of the pressure of the two pressure transducers was started. The pressure values were recorded every 200 milliseconds, for a total sampling time of 60 seconds .

[0196] The computed pressure drops ("AP") was the numeric averageof the recorded pressure values at the inlet of the test section, minus the numeric average of the recorded pressure values at the outlet of the test section. Therefore, the computed pressure drop was the one-minute average pressure drop measured between minute 10 and minute 11 after the pump started. Then, the pump was stopped.

[0197] 400 g of xylene containing 117 mg (292 ppm) of the APAO to be tested were added into vessel ( 1 ) by means of the inlet on the valve ( 14 ) .

[0198] Then, 100 ml of n-hexane were withdrawn from the purge (valve 5) and added to the Schott bottle that contained the composition (solution or suspension) comprising the APAO. The bottle was shaken to mix / dissolve the residual APAO, then the content of the bottle was fed again to the vessel . This rinsing operation was repeated 3 times, to be sure to add all the APAO was fed to the test apparatus .

[0199] The pump was started again but this time in reverse pumping (that is, the direction of pumping was reversed) . In this way, all the content of the test section was emptied, filling the vessel with the content of the tube .

[0200] The stirrer of the vessel was set to 150 rpm and the solution was kept under stirring for 24 hours .

[0201] After this period of continuous stirring, the pump was started again in normal direction (not reversed) and with flow rate regulated to 4.3 1 / min.

[0202] After 10 minutes, the recorder was started again and the values of the two pressure transducers were recorded for oneminute, as done for the solution without APAO. The computed pressure drops ("AP treated with APAO" ) was the 1-min-time average of the pressure at the inlet of the test section minus the 1-min-time average of the pressure at the outlet of the test section.

[0203] The drag reduction power was computed by the following formula :

[0204] 100

[0205]

[0206] ( I )

[0207] The results are reported in Table 2 .

[0208] TABLE 2 .

[0209]

[0210] (* ) comparative

[0211] COMMENTS ON THE RESULTS

[0212] From the results reported in Table 2, it is evident that the Ziegler-Natta bimetallic catalytic systems according to thepresent invention (molar ratio Mg / Ti from 0.5 to less than 3.0, preferably 0.5-2. 7, more preferably 1.0-2. 5, and molar ratio (Hf or Zr) / Ti 0. 5-5.0, preferably 1. 0-4.0, more preferably 1.5-3. 5) , allowed to obtain APAOs which showed a drag reduction (DR) value of 29.2% (Example 6) , when the second metal is Hf, and of 17.4% (Example 10) , when the second metal is Zr .

[0213] The APAOs obtained using catalytic systems not according to the present invention, showed a much lower DR value, in particular 0.0% (Example 7 and 9) and 9.8% (Example 8 ) .

[0214] The advantages of the catalytic systems according to the present invention were confirmed also for APAOs obtained from monomers other than 1-decene, such as 1-dodecene (Example 11 ) and 1-octene (Example 12 ) , having a DR value respectively of 42.0% and of 37.0% .

Claims

1. CLAIMS1 . A Ziegler-Natta bimetallic catalyst supported on MgC12 , which contains :titanium in the oxidation state +3 ( Ti ( I I I ) ) , and optionally titanium in the oxidation state +4 ( Ti ( IV) ) ; and at least one metal selected from hafnium in the oxidation state +4 (Hf ( IV) ) and zirconium in the oxidation state +4 ( Zr ( IV) ) ;wherein :the Mg / Ti molar ratio is not higher than 3 . 0 , preferably from 0 . 5 to 2 . 7 , more preferably from 1 , 0 to 2 . 5 ; andthe (Hf and / or Zr ) / Ti molar ratio is from 0 . 5 to 5 . 0 , preferably from 1 . 0 to 4 . 0 , more preferably from 1 . 5 to 3 . 5 .2 . The Ziegler-Natta bimetallic catalyst according to claim 1 , which contains : titanium in the oxidation state +3 ( Ti ( I I I ) ) , and optionally titanium in the oxidation state +4 ( Ti ( IV) ) ; and hafnium in the oxidation state +4 (Hf ( IV) ) .3 . The Ziegler-Natta bimetallic catalyst according to claim 1 , which contains : titanium in the oxidation state +3 ( Ti ( I I I ) ) , and optionally titanium in the oxidation state +4 ( Ti ( IV) ) ; and zirconium in the oxidation state +4 ( Zr ( IV) ) .4 . The Ziegler-Natta bimetallic catalyst according to any one of the preceding claims , wherein at least 70% by weight , preferably at least 90% by weight , more preferably at least 95% by weight , of titanium in the bimetallic catalyst is Ti ( I I I ) , the percentage being calculated on the total weight of Ti .5 . The Ziegler-Natta bimetallic catalyst according to anyone of the preceding claims, wherein the amount of titanium does not exceed 10% by weight, preferably is from 1. 6% to 10% by weight, with respect to the total weight of the bimetallic catalyst .

6. The Ziegler-Natta bimetallic catalyst according to any one of the preceding claims, which contains at least one carboxylate group deriving from a carboxylic acid of formula R-COOH, wherein R is a linear or branched hydrocarbon radical having from 2 to 30 carbon atoms, possibly substituted with at least one halogen, e . g. fluorine or chlorine .

7. The Ziegler-Natta bimetallic catalyst according to claim 6, wherein said carboxylate groups are present in an amount so as to have a (carboxylate group) / Ti molar ratio from 0.1 to 0.8, preferably from 0.4 to 0. 6.

8. A process for preparing a Ziegler-Natta bimetallic catalyst supported on MgC12 according to any one of the preceding claims, said process comprising:(a) mixing at least one titanium compound, at least one metal compound selected from hafnium (Hf ) compounds and zirconium (Zr) compounds, MgC12, and at least one carboxylic acid of formula R-COOH, wherein R is a linear or branched C2-C30 hydrocarbon radical, possibly substituted with at least one halogen, e . g. fluorine or chlorine, in a hydrocarbon liquid medium at a temperature from 40°C to 200°C, preferably from 60°C to 130°C, to obtain a catalyst precursor;(b) adding to the catalyst precursor at least one organoaluminum compound of formula AlyClxyR3y-xy, wherein R is analkyl having from 1 to 10 carbon atoms, x is a number from 1.0 to 2.0 and y is an integer equal to 1 or 2, to obtain the bimetallic catalyst .

9. The process for preparing a Ziegler-Natta bimetallic catalyst according to claim 8, wherein, in the at least one carboxylic acid of formula R-COOH, the group R is a linear or branched hydrocarbon radical having from 3 to 16 carbon atoms .

10. The process for preparing a Ziegler-Natta bimetallic catalyst according to claim 9, wherein, in the at least one carboxylic acid of formula R-COOH, R is selected from:linear alkyl groups containing at least 3 carbon atoms, preferably not more than 16 carbon atoms, for example n-hexyl, n-octyl, n-nonyl, n-decyl or n-undecyl;branched alkyl groups having a linear chain containing from 2 to 10 carbon atoms and a branching chain in the alpha or beta position of the linear chain with respect to the carbon atom of the carboxylic group, said branching chain being selected from: linear Ci-Ce alkyl groups, branched Ci-Ce alkyl groups, phenyl, cyclopentyl, cyclohexyl .

11. The process for preparing a Ziegler-Natta bimetallic catalyst according to any one of claims from 8 to 10, wherein the organoaluminum compound of formula AlyClxyR3y-xy is selected from: ethyl aluminum sesquichloride (EASC) , ethylaluminum dichloride (EADC) , isobutylaluminum dichloride ( IBADIC) and diethylaluminium chloride (DEAC) .

12. A catalytic system comprising:(i) a Ziegler-Natta bimetallic catalyst supported on MgC12,which contains : titanium in the oxidation state +3 (Ti ( III ) ) , and optionally titanium in the oxidation state +4 (Ti ( IV) ) ; and at least one metal selected from hafnium in the oxidation state +4 (Hf ( IV) ) and zirconium in the oxidation state +4 (Zr ( IV) ) ; wherein: the Mg / Ti molar ratio is not higher than 3.0, preferably from 0.5 to 2.7, more preferably from 1, 0 to 2.5; and the (Hf and / or Zr) / Ti molar ratio is from 0.5 to 5.0, preferably from 1.0 to 4.0, more preferably from 1.5 to 3.5;(ii) at least one organoaluminum co-catalyst of formula AlRnX(3-n) , wherein: the R groups, equal or different from each other, are linear or branched Ci-Ce alkyl groups; X is halogen, preferably chlorine; n is 1, 2 or 3 .

13. The catalytic system according to claim 12, wherein the Ziegler-Natta bimetallic catalyst supported on MgC12 is defined according to any one of claims from 2 to 7 .

14. The catalytic system according to any one of claims from 12 to 13, wherein, in the at least one organoaluminum cocatalyst of formula AlRnX(3-n) , the linear or branched Ci-Ce alkyl groups R are selected from: methyl, ethyl, n-butyl, isobutyl, t-butyl, n-pentyl, isopentyl, n-hexyl, isohexyl .

15. The catalytic system according to claim 14, wherein the at least one organoaluminum co-catalyst of formula AlRnX(3-n) is a trialkylaluminum compound, preferably selected from: triethylaluminum, tri-n-butylaluminum, triisobutylaluminum and trihexyl aluminum.

16. A process for polymerizing at least one linear a-olefin, said process being carried out at a temperature nothigher than 40°C, preferably from -30°C to 20°C, more preferably from -15°C to 10°C, by using a catalytic system comprising:(i) a Ziegler-Natta bimetallic catalyst supported on MgC12, which contains : titanium in the oxidation state +3 (Ti ( III ) ) , and optionally titanium in the oxidation state +4 (Ti ( IV) ) ; and at least one metal selected from hafnium in the oxidation state +4 (Hf ( IV) ) and zirconium in the oxidation state +4 (Zr ( IV) ) ; wherein: the Mg / Ti molar ratio is not higher than 3.0, preferably from 0.5 to 2.7, more preferably from 1.0 to 2.5; and the (Hf and / or Zr) / Ti molar ratio is from 0.5 to 5.0, preferably from 1.0 to 4.0, more preferably from 1.5 to 3.5;(ii) at least one organoaluminum co-catalyst of formula AlRnX(3-n) , wherein: the R groups, equal or different from each other, are linear or branched Ci-Ce alkyl groups; X is halogen, preferably chlorine; n is 1, 2 or 3 .

17. The process for polymerizing at least one linear a-olefin according to claim 16, wherein the linear a-olefin monomer has formula CH2=CH-R, wherein R is H or a linear C1-C12, preferably linear C2-C10, alkyl group .

18. The process for polymerizing at least one linear a-olefin according to claim 17, wherein the at least one linear a-olefin monomer is selected from: 1-hexene, 1-octene, 1-decene and 1-dodecene, or mixtures thereof .

19. The process for polymerizing at least one linear a-olefin according to any one of claims from 17 to 18, wherein said process is a mass polymerization process .

20. The process for polymerizing at least one linear a-olefin according to claim 19, wherein said process is a mass polymerization carried out in an inert atmosphere, preferably in a nitrogen atmosphere having an oxygen content not higher than 1 ppm (by weight) .

21. The process for polymerizing at least one linear a-olefin according to any one of claims from 16 to 20, wherein the catalytic system is used in an amount so as to obtain a titanium concentration from 100 mmol / 1 to 200 mmol / 1, equal to approximately 4.5-9 g / 1.

22. The process for polymerizing at least one linear a-olefin according to any one of claims from 16 to 21, wherein said process is carried out for a reaction time from 24 to 360 hours, preferably from 60 to 300 hours, more preferably from 80 to 120 hours .

23. The process for polymerizing at least one linear a-olefin according to any one of claims from 16 to 22, wherein the catalytic system is defined according to any one of claims from 12 to 15.

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