WO2004050722A1

WO2004050722A1 - Ziegler natta catalyst system and polymerisation process using said zn catalyst system for producing ethylene copolymers having a broad molecular weight distribution

Info

Publication number: WO2004050722A1
Application number: PCT/EP2002/013505
Authority: WO
Inventors: Kyung-Jun Chu; Thomas Garoff; Päivi Waldvogel
Original assignee: Borealis Technology Oy
Priority date: 2002-11-29
Filing date: 2002-11-29
Publication date: 2004-06-17
Also published as: AU2002367755A1

Abstract

The application relates to a catalyst system for the polymerisation of ethylene with C3-C10 olefins and to a polymerisation process making use of said catalyst system for producing ethylene polymers having a broad molecular weight distribution as well as to the polymers obtained by such process.

Description

Ziegler Natta Catalyst system and polymerisation process using said ZN catalyst system for producing ethylene copolymers having a broad molecular weight distribution

The application relates to a Ziegler Natta catalyst system for the polymerisation of ethylene with C₃-C₁0 olefins and to a polymerisation process making use of said catalyst system for producing ethylene polymers having a broad molecular weight distribution.

Prior Art

In the state of art, numerous processes for polymerising olefins are known. Amongst such processes, ethylene polymerisation processes have been extensively studied.

Ethylene alone or with other C₃-C₁0 olefins is often polymerised in the presence of a catalyst composition, which has essentially two components, the first being a compound of a transition metal belonging to groups 4 to 6 of the Periodic Table of Elements (IUPAC 1990) which is often called a procatalyst, and the second being an organo etallic compound of a metal belonging to any of groups 1 to 3 and 13 of said Periodic Table of Elements which is the s.c. cocatalyst.

This kind of Ziegler-Natta catalyst composition has been further developed by depositing the procatalyst on a more or less inert and particulate support and by adding several additives, among others electron donating compounds, to the catalyst composition in the stages of its preparation. These compounds have improved the polymerisation activity of the catalyst, the operating life and other properties of the catalyst composition and, first of all, the properties of the polymers which are obtained by means of the catalyst composition.

For example, a procatalyst is disclosed in EP-A-0 688 794. wherein C₂-Cι₀ olefin homopolymers or copolymers having low or high molecular weight can be produced with an even and high activity. Independently of the amount of hydrogen introduced into the polymerisation reactor, a balance of the activities can in both cases be achieved by using said procatalyst. It is thus possible to carry out an ethylene polymerisation by the use of this catalyst to achieve resins at high and low melt flow rate and still have very similar high productivity.

However, certain poly-C₂-Cι₀ olefins such as some bimodal resins having a very broad molecular weight distribution prepared by the catalyst system described by EP-A-0 688 794 are inhomogenous . Even after compounding, the resins have an unacceptable high degree of inhomogeneity, which appears as white dots in black pipes, or as gels in films, which are prepared from the resins. The inhomogeneity problem is not specific to a catalyst prepared according to EP-A-0 688 794 only, but is a very typical feature for most of the existing catalysts .

There are only rare examples of catalysts which are capable of producing bimodal resins having a good homogeneity, like the one disclosed in WO 99/51646. However, there is no clear teaching how said properties can be controlled by preparing a catalyst for use in said polymerisation process.

Therefore, a need for an catalyst is existing whereby, making use of said catalyst in a olefin polymerisation process, polymers can be obtained which have a very broad molecular weight distribution and, on the other side, have a high degree of homogeneity.

The present invention is therefore directed to a catalyst system to be used in a polymerisation process and a process for polymerising ethylene and alpha-olefins, which produces copolymers of ethylene and C₃-C₁₀ alpha-olefins in high yield and independently of which molecular weight is aimed at. Further, the process produces homogenous polymers of ethylene and C3-C₁₀ olefins, which readily can be processed e.g. to pipes without having white dots when processed into black pipes or films without gel-formation when processed into film.

The inventors furthermore found out that the polymer prepared according to the inventive process has an improved melt strength, which makes it useful in blow moulding and film blowing applications.

Summary of the Invention

The invention is therefore directed, in a first embodiment, to a Ziegler-Natta-catalyst system for the polymerisation of ethylene and C₃-C₁₀ olefins, comprising a. a procatalyst which comprises an inorganic support, a magnesium compound carried on said support, a titanium compound carried on said support and optionally a chlorine compound carried on said support, wherein the chlorine compound is the same or different from the magnesium compound and/or the titanium compound, and b. a cocatalyst which is a mixture of at least two organometallic compounds of the formula R3AI where R can be the same or different and is a Ci to C₂o alkyl group or isoprenyl group, said R optionally being substituted by one or more halogen or hetero atoms.

In a preferred embodiment, the invention refers to a Ziegler- Natta-catalyst system wherein b) the cocatalyst is a mixture of compounds selected from the group consisting of tri-Cχ-Cιo "alkyl aluminium compounds, where one of the components contains short-chained alkyl groups having 1 - 3 carbon atoms and the other component contains long-chained alkyl groups having 4 - 10 carbon atoms.

In a more preferred embodiment, the invention refers to a Ziegler-Natta-catalyst system wherein b) the cocatalyst is a mixture of a compound selected from the group consisting of short-chained alkyl groups like trimethyl aluminium and triethyl aluminium, and a compound selected from long-chained alkyl groups like tri-n-octyl aluminium and in particular isoprenyl aluminium, all components being preferably supported by a support or carrier.

Generally, said Ziegler-Natta catalysts systems as described in the invention comprise a transition metal compound of Group 4 to 6 of the Periodic Table (IUPAC) and a compound of Group 1 to 3 of the Periodic Table (IUPAC), and additionally other additives, such as a donor. The Ziegler-Natta catalyst system of the invention may preferably comprise a procatalyst component comprising a titanium compound, a magnesium compound and optionally an internal donor compound. Alternatively, a cocatalyst and/or an external donor may be incorporated to said Ziegler-Natta procatalyst component when preparing the catalyst according to the method used here. The various possibilities of the compounds and combinations are within the skills of a person in the field. Accordingly, the procatalyst forming compounds can be reacted by contacting: 1) at least one compound of Group 1 to 3 metal, preferably Group 2, of the Periodic Table (IUPAC), such as a Mg compound, having a general formula selected from:

Me (ORi) 2-nXn , wherein Me is a Group 1 to 3 metal, preferably Group 2, of the Periodic Table (IUPAC), such as a Mg compound, each R_x is independently a Cι-₂₀ hydrocarbyl group, e.g. C2-1₅ group, preferably C₃-₁₀ group, such as C₄-s group, e.g. linear, cyclic or branched alkyl, aryl, aralkyl, or alkaryl, preferably a branched alkyl, such as 2-ethyl-l-hexanol, which may optionally be substituted with halogen; each X is independently a halogen, preferably chlorine; 0 ≤ n < 2 and n may or may not be an integer.

Me (ORi) 2-n (R2) n , wherein Me, Ri and n are as defined above in formula (I ) ; each R₂ is independently a hydrocarbyl as defined for Rj..

MeX₂.mTi (ORi) ₄ , wherein Me, X and Ri are as described above in formula (I); 0. l≤ m ≤ 3; and/or

complexes of MeX₂ with electron-donor compounds wherein Me and X are as described above in formula (I) ;

2) with at least one transition metal compound of Group 4 to 10, or a compound of lanthanide or actinide, preferably a transition compound of Group 4 to 6, more preferably of Group 4, of the Periodic Table (IUPAC), such as a Ti compound, wherein, particularly, a tetravalent Ti compound can be used examples of which compounds are TiX₄ , wherein X is defined as above, such as Cl; or Ti(0Rι)₄-_pX_p , wherein X and Ri are as defined above and p is 0, 1, 2 or 3;

and optionally with further compound(s), e.g.

3) optionally with at least one compound of Group 13 of the Periodic Table (IUPAC), preferably an aluminium compound, such as

Al(Ri)_xX₃__x , wherein Ri is as defined above and X is O≤ x <3; e.g. dimethyl aluminium chloride, diethyl aluminium chloride, diisobutyl aluminium chloride, ethyl aluminium dichloride (EADC) and methyl aluminium dichloride; and/or

4) optionally with at least one (internal) electron donor, e.g. those known in the art for (co) polymerising propylene and higher olefins including organic compounds containing oxygen, nitrogen, sulphur and/or phosporous, such as organic acids, organic acid anhydrides, organic acid esters, alcohols, ethers, aldehydes, ketones, amines, amine oxides, amides, thiols, various phosporous acid esters and amides, and the like, added as such or formed in situ (see also WO 00 08074 and WO 00 08073)

As the chlorine-containing compound used in an alternative embodiment of the invention together with a compound of Group 1 to 3 of the Periodic Table, e.g. those chlorine containing Al and/or Ti compounds, such as an aluminium alkyl compound and/or a tetravalent titanium compound each containing chlorine as listed above, can be used. As mentioned above, the solid procatalyst particles may contain further catalyst component (s) , such as cocatalysts and/or external donor (s), depending on the used polymerisation process, in a manner known in the art. As the cocatalyst, e.g. conventional activators based on compounds of Group 13 of the Periodic Table (IUPAC), e.g. organo aluminium, such as aluminium alkyl compounds (e.g. triethylaluminium) compounds, can be mentioned. Additionally, in case of the (co) polymerisation of polypropylene or higher olefins, one or more external donors can be used which may be selected e.g. from silanes or from the list of internal donor of point (4) above.

It is also possible to include other catalyst component (s) than said ZN components to the catalyst of the invention.

Generally, in the final solid catalyst particles, the molar ratio of Mg:Ti can be e.g. between 10:1 to 1:10, preferably 5:1 to 1:1, e.g. 1:1. The molar ratio of Ti:Al can be e.g. between 10:1 to 1:2, e.g. 3:1 to 1:1. Furthermore, a suitable molar ratio of Mg:RιOH is between 1:1 to 1:4, e.g. 1:1 to 1:2, preferably 1:1.5 to 1:3.

According to the invention, said procatalyst is preferably obtainable by contacting an inorganic support with a. an aluminium compound of the formula (1):

R'₃-_nAlCl_n , wherein R' is a Cι-C₂o hydrocarbyl group or a Cι-C₂o hydrocarbyloxy group and can be the same or different in a single molecule and 1 < n < 2, or mixtures of aluminium compounds of the formula (1); b. a magnesium compound of the formula (2) :

R"₂Mg , wherein R" is a Cι-C₂₀ hydrocarbyl group or a Cι-C₂₀ hydrocarbyloxy group and can be the same or different in a single molecule, and c. a titanium compound having the formula (2) : (OR'")₄-χTiCl_x , wherein R'" is a C₂-C₂₀ hydrocarbyl group and x is an integer from 0 to .

In a preferred embodiment, the contacting step also comprises an additional intermediate contacting step using an aluminium compound or a mixture of aluminium compounds of the formula (1) before the contacting step c) if a sequential contacting is used.

As explained above, the invention relates to a catalyst system for the polymerisation of C₂-Cι₀ olefins, comprising said procatalyst and a separate cocatalyst. The cocatalyst is a mixture of at least two organometallic compounds based on a metal selected from Groups 1 to 3 and 13 of the Periodic Table of the Elements (IUPAC 1990) .

Finally, the invention relates to a process for the polymerisation ethylene, optionally with C₃-C10 olefins in which ethylene and optionally a C₃-C₁₀ olefin is under polymerisation conditions contacted with said catalyst system comprising said procatalyst and said cocatalyst.

Detailed description

Inorganic support

The inorganic support used in the invention is any support, which has the proper chemical and physical properties to act as a support for the active component of the procatalyst. The support material preferably has a suitable average particle size and particle size distribution, a high porosity and a large specific surface area. Especially good results are obtained if the support material has a specific surface area of between 100 and 1000 m²/g support and a pore volume of 1-3 ml/g support. The support material can optionally be chemically pretreated, e.g. by silylation or by treatment with aluminium alkyls.

According to a preferable embodiment of the invention, the inorganic support is a mono-oxide or mixed oxide of an element selected from Groups 3-6 and 13-14 of the Periodic Table of the Elements (IUPAC 1990), preferably a mono-oxide or mixed oxide of silicon, aluminium, titanium, chromium and/or zirconium. More preferably, the inorganic support is a mono-oxide or mixed oxide of silicon and/or aluminium, most preferably silica.

The inorganic support may further contain additional compounds. Especially useful is a silica support material containing magnesium halide in such an amount that the support contains 1 - 20%, preferably 2 - 6% by weight magnesium, as disclosed in WO 99/51646.

Usually, the support should be dried before impregnating it with other catalyst components. Further, the amount of hydroxyl groups, which appear on the surface of most inorganic oxides may be reduced by heat-treatment and/or chemical treatment. Good results are e.g. achieved by treating the support with heat at 100-900°C, preferably 400- 800 °C, for a sufficient time to reduce the hydroxyl groups on the surface to a lower level. Preferably, the thus treated support contains at most 2.0 mol and more preferably at most 1.0 mmol of hydroxyl groups/g of support.

Aluminium compound (a) The aluminium compound used in (a) has the formula (1) . The aluminium compound contains from 1 to 2 organic Cι-C₂o hydrocarbyl groups or Cι-C₂o hydrocarbyloxy groups which can be the same or different in a single molecule, i.e. different hydrocarbyl and/or hydrocarbyloxy substituents might present at a single aluminium atom, 2 to 1 chlorine atoms and aluminium, whereby it is assumed that it both chlorinates the surface of the support and partly acts as a cocatalyst precursor. However, it is not intended as a cocatalyst, but the catalyst system presupposes large amounts of a separate cocatalyst, as explained below.

Preferably, the compound is an alkyl aluminium dichloride or alkyl aluminium sesquichloride . Especially preferred compounds are an ethyl aluminium dichloride and ethyl aluminium sesquichloride. Of these, ethyl aluminium dichloride is particularly preferred.

It is recommendable to contact the aluminium compound directly or indirectly with the support in the form of a solution, which penetrates the pores of the support and reacts with as many of the surface groups or earlier deposited reagent molecules as possible. Thus, according to one embodiment, said aluminium compound is contacted with said inorganic support so that the aluminium compound is in hydrocarbon solution, preferably in a 5-25 w-% hydrocarbon solution, the viscosity of which most preferably is below 10 mPa ^x s. Suitable solvents are C₅-C₈ alkanes such as pentane, hexane and heptane. When contacting the aluminium compound with an inorganic support having surface hydroxyl groups, such as silica, the molar ratio between the aluminium compound and the surface hydroxyls of the inorganic support is preferably between 1 and 4. The preferred contacting temperature is between 0 and 110°C.

Magnesium compound (b) The compound or mixture containing hydrocarbyl and/or hydrocarbyloxy magnesium used in (b) may be any suitable mixture containing one or more magnesium compound. Thus, it may be magnesium dialkyl optionally having one or more halogen or hetero atoms, such as butyl-octyl-magnesium, butyl-ethyl magnesium, diethyl magnesium or dibutyl magnesium. If the magnesium compound used in (b) is a magnesium dialkyl, then it is preferred to use a halogen splitting compound in combination with the magnesium compound. Preferred halogen splitting compounds are silicon tetrahalides, in particular silicon tetrachloride .

The magnesium compound may also be a magnesium dialkoxy compound. Then, the compound has a general formula Mg(OR)₂, where either R is independently a C₂-Ci₆ alkyl optionally having one or more halogen or hetero atoms. Preferred such compounds are compounds where R is a C₆~Cι₂ alkyl group. In particular, the compound where R is 2-ethyl-l-hexanol or 2- methyl-1-pentanol is preferred.

One of the purposes of the invention is to obtain a high activity procatalyst despite high hydrogen concentrations. Then the magnesium component of the procatalyst can have both Mg-C bonds and Mg-O-C bonds. These bonds may be in the same magnesium compound such as RMgOR, or in different magnesium compounds. Thus mixtures of R₂Mg and Mg(OR)₂ are within the scope of the invention, as well as mixtures of R₂Mg and RMgOR and mixtures of RMgOR and Mg(0R)₂, each having optionally having one or more halogen or hetero atoms. Preferably, the compound or mixture containing hydrocarbyl and hydrocarbyl oxide linked to magnesium is hydrocarbon soluble, which gives a solution capable of effectively penetrating the voids and pores of the support.

According to one embodiment of the present invention, said compound or mixture containing hydrocarbyl and hydrocarbyl oxide linked to magnesium is a contact product of a di-Cχ-Cιo alkyl magnesium and a Cχ-C₁₂ alcohol. Preferably, the di-Cι-Cιo alkyl magnesium is dibutyl magnesium, butyl ethyl magnesium, diethyl magnesium or butyl octyl magnesium. Preferably, the 1-C12 alcohol is a branched alcohol, preferably a 2-alkyl alkanol, most preferably 2-ethyl-l-hexanol or 2-methyl-l- pentanol .

When the compound or mixture containing hydrocarbyl and hydrocarbyl oxide linked to magnesium is a contact product of a di-Ci-Cio alkyl magnesium and a C₁-C₁2 alcohol, the corresponding molar ratio di-Cι-Cιo alkyl magnesium to C1-C12 alcohol is preferably 1:1.3 - 1:2.2, more preferably 1:1.78 - 1:1.99, most preferably 1:1.80 - 1:1.98.

In the case, where the procatalyst is prepared by contacting the compound or mixture containing hydrocarbyl and hydrocarbyl oxide linked to magnesium with said first reaction product, the compound or mixture containing hydrocarbyl and hydrocarbyl oxide linked to magnesium is preferably in a nonpolar hydrocarbon solution, most preferably in a nonpolar hydrocarbon solution, the viscosity of which is below 10 mPa ^x s . The viscosity can be advantageously reduced by adding an aluminium alkyl compound to the solution. The aluminium alkyl may have the formula (1) or it may be an aluminium trialkyl. The reduced viscosity allows a thorough penetration of the magnesium component into the voids and pores of the support is attained. This improves the morphology of the procatalyst and thus the morphology of the poly-C₂-Cιo olefins prepared by it, as well.

When contacting the compound or mixture containing hydrocarbyl and hydrocarbyl oxide linked to magnesium, good deposition on the surface of the support, which may or may not be prereacted, is obtained if the volume of solution comprising the compound or mixture containing hydrocarbyl and hydrocarbyl oxide linked to magnesium is about two times the pore volume of the support material. This is achieved if the concentration of the complex in the hydrocarbon solvent is 5- 60% with respect to the hydrocarbon used as solvent. The solvent may be a C₅-C₈ alkane and/or an aromatic C₆-Cι₂ hydrocarbon, e.g. a mixture of a major amount of pentane, hexane or heptane and a minor amount of e.g. toluene.

Titanium compound (c) One of the main procatalyst components is the titanium compound, which is assumed to form the active center during the polymerisation of the C₂-Cιo olefins. Typical useful titanium compounds are the mixed alkoxy chlorides and chlorides of tetra-valent titanium. The most preferable titanium compound is titanium tetrachloride . Optional aluminium compounds (d)

The aluminium compound used optionally in the step before contacting in (c) takes place, has preferably the formula (1). It is possible, however, that when a mixture of aluminium compounds is used, that one of the aluminium compounds is an aluminium trialkyl, such as triethylaluminum, tri-isobutylaluminum and like. However, at least one of the aluminium compounds should have the formula (1) .

Preferred amounts of compounds

The amount of support, aluminium compound (s), compound or mixture containing hydrocarbyl and hydrocarbyl oxide linked to magnesium and titanium compound used in the preparation of the procatalyst may be varied and optimized in order to obtain the best results possible. However, the following amounts are preferable.

In the preparation of the procatalyst, the molar ratio between said aluminium compound (s) used in step (d) to said aluminium compound used in step (a) is preferably 0 - 5, preferably 0 - 2, most preferably 0 - 1.5.

The molar ratio of said titanium compound to said aluminium compound (s), measured as Ti/Al, is preferably 0.1 - 5, more preferably 0.1 - 2, most preferably 0.1 - 0.5. When adjusting the ratio of titanium to aluminium, the molecular weight distribution of the polymer and the hydrogen response of the catalyst can be influenced. With hydrogen response is here meant the effect hydrogen has on the average molecular weight of the polymer produced by the catalyst. A strong hydrogen response means that a change in hydrogen content has a strong effect on the average molecular weight of the polymer. A high Ti/Al ratio (of more than about 0.5) produces a catalyst with a strong hydrogen response, which in polymerisation produces a polymer with a narrow molecular weight distribution. On the other hand a low Ti/Al ratio (of less than about 0.5) produces a catalyst with a weak hydrogen response, which in polymerisation produces polymer with a broad molecular weight distribution. With a too high Ti/Al ratio it is not possible to produce the effect of this invention (i.e., the broadening of the molecular weight distribution) .

The amount of said aluminium compound relative to the mass of the inorganic support, measured as mmol Al/g, is preferably 0.1 - 100 mmol/g, more preferably 0.5 - 10 mmol/g, most preferably 1.0 - 5.0 mmol/g.

Preparation methods

The contacting of components (a) - (c) may be conducted at any suitable method producing the active procatalyst.

One suitable method is to contact the support sequentially with the compounds (a) to (c) , and optional (d) in a preferred order. Such preparation methods have been disclosed e.g. in EP 688794, WO 99/51646 or WO 00/44795.

Alternatively, the compounds (a) to (c) , and optional (d) may be contacted in solution and the resulting solution is impregnated on the support, as disclosed in WO 01/55230.

Alternatively still, the compounds (a) , (b) and optionally (d) may be contacted in solution and the resulting solution is impregnated on the support. Thereafter, the compound (c) is impregnated to the said support. Also this method is disclosed in WO 01/55230. The procatalyst may be washed after any state of the synthesis, using methods known in the art, such as filtering or decanting. Thus, a wash stage may be performed after the chlorination treatment, after the titanation treatment and/or as a last step of the synthesis. Inert hydrocarbons, such as pentane, hexane or heptane, may be used as wash liquids.

Cocatalyst

The above described procatalyst for the polymerisation of C₂- Cio olefins is combined with a cocatalyst so that it can be used in a polymerisation process. The cocatalyst is preferably a mixture of at least two organometallic compounds of formula R₃A1, where R is a Ci to C₂o alkyl group or isoprenyl group.

Preferably, the cocatalyst is a mixture of compounds selected from the group consisting of tri-Cι-Cι₀ alkyl aluminium compounds, where one of the components contains short-chained alkyl groups (1 - 3 carbon atoms) and the other component contains long-chained alkyl groups (4 - 10 carbon atoms) . Examples of suitable aluminium alkyls containing short- chained alkyl groups are trimethyl aluminium and in particular, triethyl aluminium. Examples of suitable components containing long-chained alkyl groups are tri-n- octyl aluminium and in particular isoprenyl aluminium. In particular, the cocatalyst is a mixture of isoprenyl aluminium and triethyl aluminium or isoprenyl aluminium and trimethyl aluminium.

The weight ratio of the two compounds forming the cocatalyst may range from 10:90 to 90:10. Preferably, the ratio of the components in the cocatalyst mixture is adjusted to obtain the desired broadness of the molecular weight distribution. In the C₂-Cιo olefin polymerisation catalyst system according to the present invention, the molar ratio between the aluminium in said cocatalyst and the titanium of said procatalyst is preferably 1:1 - 100:1, more preferably 2:1 - 50:1 and most preferably 3:1 - 20:1.

It has been found that the method of the invention allows the control of the molecular weight distribution of the polymer produced by the catalyst system, at least in the case where one of the components is isoprenyl aluminium. By increasing the fraction of isoprenyl aluminium in the cocatalyst the molecular weight distribution can be broadened. Thus, the process makes it possible to produce resins for different end-use applications requiring different molecular weight distribution using the same solid catalyst component and modifying the composition of the cocatalyst. It is then no more necessary to change the solid catalyst component and thus the transitions between different polymer grades can be simplified.

The method of the invention is especially useful in two-stage or multi-stage polymerisation process producing polyethylene having a bimodal molecular weight distribution. In such a process one alternative is to feed the cocatalyst comprising the mixture of two aluminium alkyl compounds into the first polymerisation stage, so that the broadening of the molecular weight distribution is obtained in all polymerisation stages.

However, in some cases it may be desired to produce the low molecular weight component having a narrow molecular weight distribution and a high molecular weight component having a broad molecular weight distribution. In such a case it would be advantageous to feed one component of the cocatalyst (triethyl aluminium) into the first polymerisation stage producing the low molecular weight polymer and the second component of the cocatalyst into the second polymerisation stage, where the thus introduced second component and the first component carried over from the first polymerisation stage form the mixture of the invention.

As discussed above, the broadness of the molecular weight distribution of the polymer produced by the solid catalyst component can be influenced by the Ti/Al - ratio in the catalyst. Thus, a choice of a solid component having a suitable Ti/Al - ratio and an adjustment of the fraction of isoprenyl aluminium in the cocatalyst to an appropriate level allows the production of polymers having the desired properties with respect to the average molecular weight and molecular weight distribution. Especially if the catalyst is used in a multi-stage process, the processability of the polymer (as indicated by melt strength and melt elasticity, which depend on the molecular weight distribution) can be adjusted at the desired level.

Polymerisation process

Most preferably, the polymerisation is a multi-step process, in the steps of which different amounts of hydrogen are present as molecular weight regulating agent (so called chain transfer agent) . This leads to different molecular weight fractions of the ethylene/C₃-Cι₀-olefin copolymer. The advantage of the catalyst system of the invention is that the molecular weight distribution of the polymer produced in each stage can be controlled according to the needs of the end use application of the polymer. Thus, a polymer used for blow moulding, which needs to have a certain melt strength and thus a broad molecular weight distribution in high molecular weight polymer fraction can be produced with a relatively large fraction of isoprenyl aluminium (50 - 90%) in the cocatalyst.

This type of polymer is also useful for film applications producing a material, which easily can be processed on a film line, and producing films with good homogeneity. On the other hand pipe resins, which need to have good mechanical properties and for which a narrow molecular weight distribution in the high molecular weight component is preferred, can be produced with a low fraction of isoprenyl aluminium (less than 50%) in the cocatalyst.

A typical multi-step polymerisation process is a two-stage process, in which the hydrogen pressures deviate considerably from each other. A broad molecular weight distribution is usually obtained. One such process has been described in EP- B-517 868.

It should be understood that the multi-step process described above may include additional precontacting or prepolymerisation stages, where the catalyst is pretreated or prepolymerised before it is introduced into the first polymerisation stage. A process including a prepolymerisation stage has been described in WO-A-96/18662.

In the polymerisation process, said C₂-Cιo olefin monomer can be any monomer having from two to ten carbon atoms, such as ethylene, propylene, 1-butene, isobutene, 3-methyl-l-butene, 1-hexene, 4-methyl-l-pentene, 4, 4-dimethyl-l-pentene, vinylcyclohexane, cyclopentene, cyclobutene and norbornene. It is also suitable e.g. for the copolymerisation of ethylene/propylene, ethylene/1-butene, ethylene/1-hexene, ethylene/1, 3-butadiene, ethylene/4-methyl-l-pentene, ethylene/-α-ω-diolefins such as α-ω-octadiene, propylene/cyclopentene, ethylene/norbornene, ethylene/dimethanooctahydronaphthalene and ethylene/propylene/ethylidene norbornene. Most preferably, said C₂-Cιo olefin is ethylene or, optionally, ethylene together with no more than 20 w-% of a C₃-Cι₀- -olefin .

Examples

In the following examples, the procatalyst is prepared, recovered and analyzed. Further, it is tested in the polymerisation of ethylene, whereby the obtained ethylene polymer is processed and analysed.

Catalyst Examples

Example I (Preparation of Complex) 7.9 g (60.8 mmol) of 2-ethyl-l-hexanol was added slowly to 27.8 g (33.2 mmol) of 19.9% butyl-octyl-magnesium. The reaction temperature was kept under 35°C. This complex was used in the following catalyst preparations. 2-ethyl-l- hexanol/butyl-octyl-magnesium molar ratio is 1.83:1.

Example 1 (Preparation of the Solid Catalyst Component) Onto 1 gram of silica support SP9-275 sold and manufactured by Grace, having an average particle size of 16 μm and which had been activated at 600°C for 6 hours, 1.6 mmol of EADC was introduced. The mixture was stirred for 2 hours at 20°C.

Then, an amount of magnesium complex prepared according to Example I, corresponding to 1.4 mmol of magnesium, was introduced onto the support. The mixture was stirred for 3.5 hours at 20 - 40°C. Then, 0.7 mmol TiCl₄ was introduced onto the support. The mixture was stirred overnight at 45°C. The catalyst was then dried for 2.5 hours at 45 - 70°C and recovered.

Example 2

The catalyst was prepared according to the description of

Example 1, except that the amount of EADC was 2.8 mmol.

Example 3

Onto 1 gram of silica support SP9-275 sold and manufactured by Grace, having an average particle size of 16 μm and which had been activated at 600°C for 6 hours, 1.6 mmol of EADC was introduced. The mixture was stirred for 2 hours at 20°C. Then, an amount of magnesium complex prepared according to Example I, corresponding to 1.4 mmol of magnesium, was introduced onto the support. The mixture was stirred for 3.5 hours at 20 - 40°C. After this, 1.2 mmol of EADC was added and the mixture was stirred for 2 hours at 20 °C. Then, 0.9 mmol TiCl₄ was introduced onto the support. The mixture was stirred overnight at 45°C. The catalyst was then dried for 2.5 hours at 45 - 70°C and recovered.

Polymerisation Examples

Example 4 (comparative)

The solid catalyst component prepared in Example 1 was used in ethylene polymerisation. Ethylene was polymerised in pentane slurry in following conditions:

Reactor volume 3 1

Amount of n-pentane 1.8 1

Amount of catalyst 100 mg Al/Ti-ratio (Cocatalyst/catalyst ) 15

Polymerisation temperature 90°C

Amount of hydrogen (in 500 ml vessel at 25°C) 500 kPa Partial pressure of pentane at polymerisation temp. 440 kPa Total pressure kept with continuous ethylene feed 1440 kPa Polymerisation time 1 hour

The measured amount of n-pentane was filled into the reactor and the temperature was increased to 90°C. Catalyst components were added and ethylene was taken through the hydrogen-charging vessel. Triethyl aluminium was used as cocatalyst .

Example 5 (comparative) The procedure of Example 4 was repeated except that the solid catalyst component was produced according to Example 2.

Example 6

The procedure of Example 5 was repeated, except that a mixture containing 75% of triethyl aluminium and 25% of isoprenyl aluminium was used as cocatalyst.

Example 7

The procedure of Example 5 was repeated, except that a mixture containing 25% of triethyl aluminium and 75% of isoprenyl aluminium was used as cocatalyst.

Example 8 (comparative)

The procedure of Example 4 was repeated, except that the solid catalyst component was produced according to Example 3. Example 9

The procedure of Example 8 was repeated, except that a mixture containing 75% of triethyl aluminium and 25% of isoprenyl aluminium was used as cocatalyst.

Example 10

The procedure of Example 8 was repeated, except that a mixture containing 25% of triethyl aluminium and 75% of isoprenyl aluminium was used as cocatalyst.

Polymerisation results

Table 2 - Laboratory polymerisation results

The examples show that the molecular weight distribution (as shown by shear thinning index, SHI i_/ioo) is broadened, or the high molecular weight fraction of the polymer is increased, when the fraction of isoprenyl aluminium in the cocatalyst is increased.

Claims

1. A Ziegler-Natta-catalyst system for the polymerisation of ethylene and C₃-Cι₀ olefins, comprising a. a procatalyst which comprises an inorganic support, a magnesium compound carried on said support, a titanium compound carried on said support and optionally a chlorine compound carried on said support, wherein the chlorine compound is the same or different from the magnesium compound and/or the titanium compound, and b. a cocatalyst which is a mixture of at least two organometallic compounds of the formula R₃AI where R can be the same or different and is a Ci to C₂o alkyl group or isoprenyl group, said R optionally being substituted by one or more halogen or hetero atoms.

2. The Ziegler-Natta-catalyst system according to claim 1 wherein b) the cocatalyst is a mixture of compounds selected from the group consisting of tri-Cι-Cι₀ alkyl aluminium compounds, where one of the components contains short-chained alkyl groups having 1 - 3 carbon atoms and the other component contains long-chained alkyl groups having 4 - 10 carbon atoms.

3. The Ziegler-Natta-catalyst system according to claim 1 or 2 wherein b) the cocatalyst is a mixture of a compound selected from the group consisting of short-chained alkyl groups like trimethyl aluminium and triethyl aluminium, and a compound selected from long-chained alkyl groups like tri-n-octyl aluminium and in particular isoprenyl aluminium.

4. The Ziegler-Natta-catalyst system according to any of claims 1 to 3 wherein the procatalyst is obtainable by contacting said inorganic support with a. an aluminium compound of the formula (1):

RVnAlCln , wherein R' is a Cι-C₂₀ hydrocarbyl group or a Cι-C₂o hydrocarbyloxy group and can be the same or different in a single molecule and 1 < n < 2, or mixtures of aluminium compounds of the formula (1); b. a magnesium compound of the formula (2) : R"₂Mg , wherein R" is a Cι-C₂₀ hydrocarbyl group or a Cι-C₂o hydrocarbyloxy group and can be the same or different in a single molecule, and c. a titanium compound having the formula (3) :

(OR^'")₄._xTiCl_x , wherein R^'" is a C₂-Co hydrocarbyl group and x is an integer from 0 to .

5. The Ziegler-Natta-catalyst system according to claim 4, wherein the procatalyst is obtainable by contacting of components a) to c) in a sequential order.

6. The Ziegler-Natta-catalyst system according to claim 5, wherein the contacting also comprises an additional intermediate contacting step using an aluminium compound or a mixture of aluminium compounds of the formula (1) before the contacting with component c) .

7. The Ziegler-Natta-catalyst system according to any of claims 4 to 6, wherein said aluminium compound of the formula (1) is preferably an alkyl aluminium dichloride or alkyl aluminium sesquichloride, and, particularly preferred, ethyl aluminium dichloride and ethyl aluminium sesquichloride .

8. The Ziegler-Natta-catalyst system according to any of claims 4 to 7, wherein said aluminium compound is contacted with said inorganic support in a hydrocarbon solution, preferably in a 5-25 w-% hydrocarbon solution, the viscosity of which most preferably is below 10 mPa ^x s.

9. The Ziegler-Natta-catalyst system according to any of claims 4 to 8, wherein said magnesium compound having hydrocarbyl and/or hydrocarbyloxy substituents is any suitable compound or mixture of compounds selected from hydrocarbyl and/or hydrocarbyloxy magnesium, optionally having one or more halogen or hetero atoms.

10. The Ziegler-Natta-catalyst system according to any of claims 4 to 9, wherein said magnesium compound having hydrocarbyl and/or hydrocarbyloxy substituents is any suitable compound or mixture of compounds selected from magnesium dialkyls, preferably butyl-octyl-magnesium, butyl-ethyl magnesium, diethyl magnesium and/or dibutyl magnesium.

11. The Ziegler-Natta-catalyst system according to claim 10, wherein said magnesium compound is a magnesium dialkyl which is used in combination with a halogen splitting compound, preferably silicon tetrahalides, in particular silicon tetrachloride.

12. The Ziegler-Natta-catalyst system according to any of claims 4 to 11, wherein said titanium compound is selected from the group selected from mixed C₂-C₂o hydrocarbyloxy titanium chlorides and chlorides of tetravalent titanium, titanium tetrachloride being preferred.

13. Use of the catalyst system of any of the foregoing claims in a process for the polymerisation of ethylene with C₃- Cio olefins.

14. Process for producing polyethylene copolymers having a bimodal molecular weight distribution in a at least two- stage polymerisation process wherein the ZN catalyst system of any of claims 1 to 12 is used and the cocatalyst comprising the mixture of two aluminium alkyl compounds is fed into the first polymerisation stage, so that the broadening of the molecular weight distribution is obtained in all polymerisation stages.

15. Process for producing polyethylene copolymers having a bimodal molecular weight distribution in a at least two- stage polymerisation process wherein the ZN catalyst system of any of claims 1 to 12 is used to produce the low molecular weight component by feeding the aluminium compound of the cocatalyst having short-chained alkyl groups having 1 - 3 carbon atoms into the first polymerisation stage producing the low molecular weight polymer and by feeding the other aluminium compound of the cocatalyst containing long-chained alkyl groups having 4 - 10 carbon atoms into the second polymerisation stage. Polyolefin , obtainable by a polymerisation process making use of the ZN catalyst system of any of claims 1 to 12 for the polymerisation of ethylene with C₃-C10 olef ins .