WO2007111499A1

WO2007111499A1 - Process for the preparation of a linear polyethylene

Info

Publication number: WO2007111499A1
Application number: PCT/NL2007/000058
Authority: WO
Inventors: Rubin Huang; John Chadwick
Original assignee: Stichting Dutch Polymer Institute
Priority date: 2006-03-27
Filing date: 2007-03-05
Publication date: 2007-10-04
Also published as: TW200740857A

Abstract

The invention deals with a process for the preparation of a substantially linear polyethylene, by polymerizing ethylene with a supported catalyst. This supported catalyst comprises on one support two or more catalyst components, one of which produces linear polyethylene and the other one branched polyethylene.

Description

PROCESS FOR THE PREPARATION OF A LINEAR POLYETHYLENE

The present invention relates to a process for the preparation of a substantially linear polyethylene in the form of a homopolymer of ethylene, or a copolymer of ethylene with up to 1 mol % of an α-olefin, by polymerization of ethylene under the influence of a supported catalyst.

In several publications authors have indicated the beneficial effects of heterogenizing a polymerization catalyst for the preparation of polyethylene. However, it is well known that the productivity of a heterogeneous catalyst in ethylene polymerization can be limited by mass-transfer limitations pertaining to monomer diffusion through the growing catalyst/polymer particle. This is particularly relevant in ethylene homopolymerization using, for example, titanium- and iron-based catalysts. These catalysts typically give linear polyethylene, whereas branched polyethylene can be produced using nickel-based catalysts. Surprisingly it has been found, and that is the heart of the present invention, that the above indicated deficiencies can be overcome, and a high productivity towards a substantially linear polyethylene can be obtained, by the use of a binary catalyst system, of which the most active component produces essentially linear polyethylene (also called HDPE, high density polyethylene), and of which the less active component produces branched polyethylene.

The process of the present invention is characterized in that the supported catalyst comprises on one support at least two metal components MC₁ and MC₂, the most active component MCi producing linear polyethylene, and the minor active component MC₂ producing branched polyethylene. As a result of the present invention, a process leading to a significant increase in productivity towards polyethylene is obtained as well as little change in polymer composition. The polyethylene prepared in the process of the present invention comprises only a minor amount of branched polyethylene, next to linear polyethylene. Here and hereinafter the term "linear polyethylene" is defined as a polyethylene with less than 5 branches per 1000 carbon atoms; this can be determined with ¹³C NMR (as known in the art). Here and hereinafter the term "branched polyethylene" is defined as a polyethylene with at least 10, even more at least 20 branches per 1000 C-atoms, determined by ¹³C NMR. Here and hereinafter the term "substantially linear polyethylene" is defined as a polyethylene with at most 5 wt. %, more preferred at most 2.5 wt. % of a branched polyethylene.

I) The catalyst(-svstem) and its components

The catalyst to be used in the present process can be described by the following four elements: a) a support b) a catalyst component MC₁ c) a catalyst component MC₂ d) the catalyst(-system), wherein both catalyst components MC-i and MC₂ are present on the support. This in contrast to a situation in which each catalyst component MC₁ and MC₂ is individually supported on its own support material.

a) The catalyst support The type of support to be used for the catalyst in the present process is in itself not critical. The literature is full of suitable supports applicable for heterogeneous catalysts. A preference nevertheless is present for the use of either MgCI₂ or SiO₂ as a support, as these are already well-tested and well-used supports. More specifically, the following support is preferred:

MgCI₂/AIR_n(OR)₃._n, each R being a (substituted) hydrocarbon group.

Such a support is known from EP-A-1 , 564,226, and from EP-A-1 , 568,716.

b) The catalyst component MC₁

The catalyst component MC₁ is a metal compound that is able to produce, and effectively produces in a process of the present invention, a linear polyethylene. A suitable metal compound of this nature is, in itself, already known in the art. More specific, the metal in MC-] is selected from groups 4-8 of the Periodic Table of Elements.

Even more preferred, the metal in MC₁ is selected from the group comprising Ti, Cr and Fe.

c) The catalyst component MC? The catalyst component MC₂ is a metal compound that is able to produce, and effectively produces in a process of the present invention, a branched polyethylene. Another preferred characteristic of MC₂ is that it shows a decay-type activity, as a result of which this catalyst component is mainly active in the early stage of the polymerization. A suitable metal compound of this nature is, in itself, already known in the art. More specifically, the metal in MC₂ is selected from the group comprising Ni, Cr, and Pd. Even more preferred, the metal in MC₂ is Ni. Examples of palladium catalysts giving high degrees of branching in ethylene homopolymerization are those reported by LK. Johnson, CM. Killian and M. Brookhart in the Journal of the American Chemical Society (J. Am. Chem. Soc), 1995, 117, 6414-6415.

Examples of chromium catalysts capable of producing significant branching in ethylene homopolymerization are described in WO 2005/058983.

d) The catalyst(-system) As indicated above, the catalyst comprises a support, on which the two catalyst components are present. The catalyst component MC-i produces during the full time of the polymerization a linear polyethylene, whereas catalyst component MC₂ is preferably mainly, or even only, active in the early stage, producing a branched polyethylene. Depending on both the initial as well as the long-term activity of the catalyst components MCi and MC₂, the ratio between the two components is to be selected. Generally, the molar ratio of the metals in MC₂ and MC₁ respectively is between 0.01 and 10.0. Preferably, this ratio is between 0.2 and 0.8. This in order to produce the desired substantially linear polyethylene.

Next to the catalyst, generally also a co-catalyst can be used to enhance the catalyst activity. The choice of such a co-catalyst is also common knowledge of those skilled in the art. Reference can be given to the use of Al-based co- catalysts (like aluminum alkyl(-halide), or MAO) or boron-based activators (like B(C₆Fs)₃, [Ph₃C][B(C₆Fs)₄] or [PhNMe₂H][B(C₆Fs)₄].

II) The process

The conditions under which the process for the preparation of a polyethylene in the present invention is carried out are themselves known in the art. The conditions reflect those needed to produce the linear polyethylene. The temperature at which this polymerization is performed is generally between 5 and 150 ⁰C. More specifically, the polymerization is performed at a temperature of between 25 and 120 ⁰C.

The pressure under which the process of the present invention is performed is according to the generally used pressures in polyethylene preparation. This means that generally the pressure is between 0.1 and 300 MPa, more specifically between 0.5 and 10 MPa.

Preferably polymerization is performed in slurry or in gas phase. Ill) The polyethylene

The process of the present invention is directed to the preparation of a substantially linear polyethylene, which can either be a homopolymer, or a co- polymer with only a minor content of a second monomer. Said second monomer, present in an amount up to 1 mol %, and preferably less than 0.1 mol %, is an (x-olefin

(which can be linear or branched). Preferably, such a second monomer is an w-olefin having 3-8 C-atoms. Preference is given to the preparation of a homopolymer of ethylene. The polyethylene of the present invention is a substantially linear polyethylene, which means that it is a polyethylene with at most 5 wt. %, more preferred at most 2.5 wt. % of a branched polyethylene.

The invention will here below be elucidated in the form of Examples and comparative experiments, which are intended to illustrate, but not restrict, the invention.

Examples I - XXVI and comparative experiments A - BB

The supports used in this work were prepared by reaction Of AIEt₃ with adducts of composition MgCI₂ -H EtOH or MgCI₂ -2.1 EtOH, following procedures similar to those described in Macromolecular Chemistry and Physics, 2004, Vol. 205, pp 1987-1994. The product of the reaction of AIEt₃ with MgCI₂ -H EtOH contained 3.70 wt-% Al and 5.26 wt-% OEt, indicating a product composition MgCI₂ 0.16AIEt₂.i5(OEt)o.85. The product of the reaction of AIEt₃ with MgCI₂ -2.1 EtOH contained 10.72 wt-% Al and 7.54 wt-% OEt, indicating a product composition

MgCI₂ -0.73AIEt₂.58(OEt)₀.₄₂. The structures of the iron, nickel and chromium complexes used are shown in Scheme 1.

Iron-based systems Catalyst immobilization was carried out using a support prepared by reaction Of AIEt₃ with MgCl₂ -LIEtOH. The support (100 mg) was contacted with 2 ml_ of a dichloromethane solution containing the desired quantities of the iron and nickel catalysts. It was observed that, after contacting overnight at ambient temperature, the colour of the catalyst(s) solution in dichloromethane was completely transferred to the solid support, indicating complete immobilization. The resulting slurry was, after dilution with light petroleum, used as such in the polymerizations.

Ethylene polymerization was carried out at 50, 60 or 70 ⁰C in a 1 L Premex reactor containing 500 ml_ light petroleum, at a constant monomer pressure of 5 bar, and using either AIEt₃ or AIZBu₃ as cocatalyst/scavenger. The results obtained with systems containing only Fe (0.5 μmol/100 mg support) or Fe + Ni (each 0.5 μmol/100 mg support) are shown in Table 1. The activities of systems containing only Ni (0.5 μmol/100 mg support) are given in Table 2. It is immediately apparent from Tables 1 and 2 that the "hybrid" catalyst systems containing both Fe and Ni exhibit activities which are significantly higher than those of the systems containing only Fe. Furthermore, the observed increases in activity are much higher than the activities given in Table 2 for the Ni systems, indicating a definite synergistic effect, the presence of nickel boosting the activity obtained with the iron catalyst. This is illustrated in Figure 1 , from which it can be seen that the difference in activity between the (Fe + Ni) and the Fe systems was around 3000 kg/mol Fe.bar.h, much greater than the activities of around 500 kg/mol Ni.bar.h obtained with only Ni. The effect of varying the Ni loading in the (Fe + Ni) system is given in Table 3, from, which it can be seen that a decrease in the Ni loading leads to increased activity. In other words, only a relatively low loading of Ni is sufficient to obtain a significant effect. In order to demonstrate that the observed increases in catalyst activity following the incorporation of a Ni catalyst into Fe-catalyzed ethylene polymerization resulted predominantly from increased productivity of the iron catalyst, selected samples were fractionated to separate and determine the relative amounts of linear (Fe-) and branched (Ni-) PE. This was carried out by dissolving 1 g polymer in 600 mL p-xylene at 135-140 ⁰C, after which the stirred solution was kept at this temperature for 1-2 h and then allowed to cool until turbidity was observed, which occurred between 75 and 80 ⁰C. Stirring was continued at this temperature for 4 h and the mixture was then filtered hot and the precipitate and soluble fraction isolated. The soluble fractions determined for the polymers in Table 3 prepared at Ni loadings of 0.1 and 0.5 μmol were 2.4 and 3.3 wt-%, respectively. The DSC peak melting temperatures were 124 and 120 ⁰C, as opposed to 137 and 136 ⁰C for the unfractionated polymers.

Chromium-based systems Catalyst immobilization was carried out using a support similar to that used in the above. The support (100 mg) was contacted with 2 mL of a dichloromethane solution containing the desired quantities of the chromium and nickel catalysts. It was observed that, after contacting overnight at ambient temperature, the blue colour of the Cr catalyst solution in dichloromethane was completely transferred to the solid support, indicating complete immobilization. The resulting slurry was, after dilution with light petroleum, used as such in the polymerizations. In the case of the (Cr + Ni) system, the colour after immobilization was light blue.

Ethylene polymerization was again carried out at 50, 60 or 70 °C, as described above and using 1 mmol AIEt₃ as cocatalyst/scavenger. The results obtained are shown in Table 4. It is apparent that the synergetic effect of incorporating Ni into the Cr-based system is less spectacular than was the case for the Fe-based system, and that the effects are dependent on polymerization temperature and catalyst loading. At 60 ⁰C the increase in activity for the (Cr + Ni) system was significantly greater than the ethylene polymerization activity obtained using only Ni. At 50 ⁰C the activities of the (Cr + Ni) systems were in most cases around 1000 kg/mol Cr.bar.h higher than the unmodified Cr systems, again greater than the Ni-only activities.

Titanium-based systems

Catalyst immobilization was carried out using a support prepared by reaction of AIEt₃ with MgCI₂ 2.1 EtOH. For these studies catalyst immobilization was carried out in toluene. Two different methods were used. In method 1, the support (100 mg) was contacted with 1 μmol TiCI₄ in 3 mL toluene at 50 ⁰C for 4 h, and subsequently with 0.5-1.0 μmol of the Ni complex in 2.5-3 mL toluene at 50 ⁰C for 4 h. In method 2, the order of addition was reversed, the support being treated first with the Ni complex and subsequently with TiCI₄.

As above, ethylene polymerization was carried out at 50, 60 or 70 ⁰C, using 1 mmol AIEt₃ as cocatalyst/scavenger. These polymerizations, carried out with a less porous support than was used in the Examples with the Fe and Cr catalysts, exhibited acceleration kinetics, the rate of polymerization increasing during the course of each 1 hour run with both the Ti-only and the (Ti + Ni) systems. The activities obtained are shown in Table 5. The results of systems containing the same support impregnated only with Ni are given in Table 6. It is clear that very significant synergies were achieved, the catalyst activity at 50 ⁰C increasing from around 9000 kg/mol Ti.bar.h in the absence of Ni to around 14000 kg/mol Ti.bar.h in the presence of Ni, while with this support the activity with Ni alone was only 240 kg/mol Ni.bar.h. At 50 ⁰C, immobilization methods 1 and 2 were both very effective, while at 60 and 70 °C the results indicate positive synergy with method 2.

As was observed with the Fe/Ni and Cr/Ni systems, an increase in nickel loading led to a decrease rather than a further increase in overall activity, indicating that a very low nickel loading is optimal for the greatest synergetic effect. This is of course particularly advantageous with respect to the present invention, minimizing both the amount of Ni complex used and also minimizing the proportion of branched polyethylene in the final product.

Fractionation of a polymer prepared with Ti and Ni loadings of 10 and 5 μmol/g, respectively (Table 5; activity 14880 kg/mol Ti.bar.h) yielded 1.6 wt-% of a soluble fraction having a DSC peak melting temperature of 122 ⁰C (unfractionated polymer: 137 "C), indicating a very low proportion of branched polyethylene in the end product.

Ziegler-Natta catalyst systems

Various Ziegler-Natta catalysts were prepared differing in both porosity and chemical composition. Catalysts, prepared by the reaction of MgCI₂ HEtOH with excess TiCI₄ in several stages, and in the presence of an internal donor were selected. The compositions of these catalysts are shown in Table 7. Catalyst A was prepared with a porous, partially dealcoholated support of composition MgCI₂H EtOH, whereas catalysts B, C and D were prepared from supports with EtOH/MgCI₂ molar ratios in the range 2-3. Impregnation of these catalysts with the nickel catalyst according to scheme 1 was carried out at room temperature, by contacting the catalyst overnight with a solution of the nickel complex in toluene. The results of ethylene polymerizations carried out with these catalysts, in the presence and absence of the nickel complex, are given in Table 8. In every case, precontact of the catalyst with the nickel catalyst results in an increase of the catalyst productivity. The productivities are strongly dependent on both catalyst type and polymerization temperature. The highest productivities, with and without the nickel complex, were obtained with catalyst A, prepared with the porous MgCI₂-1.1 EtOH support. With this catalyst, the synergetic effect of the presence of the nickel complex was particularly large at 50 ⁰C₁ but also significant at 70 ⁰C. Much lower productivities were obtained with catalyst B, in stark contrast to propylene polymerization, where catalysts of type B typically give productivities around four times higher than those obtainable with catalysts of type A. In ethylene polymerization it is frequently observed that a rapid initial peak in activity is followed by a drop in rate, after which the rate gradually increases as fragmentation takes place during the course of polymerization. Changes in ethylene mass flow for the polymerizations carried out at 70 ⁰C are shown in Figure 2 and a drop in rate in the early stages of polymerization is indeed apparent with catalyst B but not with catalyst A. In the case of catalyst C, a small initial drop in rate was observed with the unmodified catalyst, but not when the catalyst was impregnated with the nickel catalyst . With this catalyst, the presence of Ni gave rise to a much steeper increase in rate throughout the polymerization carried out at 70 ⁰C, leading to a large increase in productivity. A Cr-based catalyst suitable as acting as an MC₂-catalyst is the catalyst of Example 4 of WO 2005/058983: 1- (diisopropyl (2-oxypyridine) silyl) -3- methylcyclopentadienyl) chromium dichloride.

A Pd-based catalyst suitable as acting as an MC₂-catalyst is catalyst 1b in J. Am. Chem. Soc. 1995,117,6414.

Table 1. Polymerizations with iron- and iron/nickel-based systems

Example/ Catalyst(s) Cocatalyst Temperature Al/Fe Al/Ni Activity Comp.exp. ⁰C mol/mol mol/mol kg/moϊ

Fe.bar.h

A Fe AIIBu₃ 50 1000 3040

I Fe + Ni AIZBu₃ 50 1000 1000 6080

B Fe AIEt₃ 50 1000 _ 10600

Il Fe + Ni AIEt₃ 50 1000 1000 13840

C Fe AIEt₃ 60 1000 _ 11720

III Fe + Ni AIEt₃ 60 1000 1000 14680

D Fe AIEt₃ 70 1000 - 15280

IV Fe + Ni AIEt₃ 70 1000 1000 19520

Table 2. Polymerizations with a nickel-based system

Example/ Catalyst(s) Cocatalyst Temperature Al/Ni Activity

Comp.exp. ⁰C mol/mol kg/mol Ni.bar.h

E Ni AIEt₃ 50 1000 560

F Ni AIEt₃ 60 1000 400

G Ni AIEt₃ 70 1000 640 Table 3. Polymerizations with different Ni loadings in hybrid iron/nickel catalyst systems

Example/ Fe Ni Cocatalyst Temperature Al/Fe Activity

Comp.exp. μmol μmol ⁰C mol/mol Kg/mol

Fe.bar.h

B 0.5 0 AIEt₃ 50 1000 10600

V 0.5 0.1 AIEt₃ 50 1000 16480

Vl 0.5 0.3 AIEt₃ 50 1000 15160

IV 0.5 0.5 AIEt₃ 50 1000 13840

Table 4. Polymerizations with chromium- and chromium/nickel-based systems

Example/ Catalyst(s) Temperature Cr loading Ni loading Activity

Comp.exp. ⁰C μmol/g μmol/g kg/mol Cr. bar. h

H Cr 50 20 _ 3100

J Cr 50 20 - 2950

VII Cr + Ni 50 20 5 3900

VIII Cr + Ni 50 20 10 4110

IX Cr + Ni 50 20 20 3680

K Cr 60 20 _ 3970

Xl Cr + Ni 60 20 10 4640

L Cr 70 20 - 6560

Xl Cr + Ni 70 20 10 6850 Table 5. Polymerizations with titanium- and titanium/nickel-based systems

Example/ Catalyst(s) Temperature Ti loading Ni Activity

Comp.exp. loading

⁰C μmol/g μmol/g kg/mol Ti.bar.h

M Ti 50 10 - 9360

XII Ti + Ni^a) 50 10 5 14880

XIII Ti + Ni^b) 50 10 5 13960

XIV Ti + Ni^a) 50 10 10 12340

N Ti 60 10 _ 13400

XV Ti + Ni^a) 60 10 5 12620

XVI Ti + Ni^b) 60 10 5 14420

O Ti 70 10 _ 14340

XVII Ti + Ni^a) 70 10 5 14180

XVIII Ti + Ni^b) 70 10 5 15900

^a) Immobilization method 1 ^b) Immobilization method 2

Table 6. Polymerizations with nickel-based systems

Example/ Temperature Ni loading Activity Comp.exp. ⁰C μmol/g kg/mol Ni.bar.h

P 50 5 240

Q 60 5 160

R 70 80

2,6-bis(1-(2,4,6-trimethylphenylimino)ethyl)pyridine iron(ll) dichloride

(R = /-Pr)

2,3-bis(2,6-diisopropylphenylimino)butane nickel(ll) dibromide

1-(8-quinolyl)indenyl chromium(lll) dichloride Scheme 1. Catalysts used.

Polymerization temperature (⁰C)

Figure 1. Comparison of binary (iron + nickel) and iron catalyst activities

Table 7. Ziegler-Natta catalyst compositions

Catalyst Ti Mg Internal donor

wt% wt% Type wt%

A 2.0 19.0 diisobutyl phthalate 6.8

B 2.9 16.9 diisobutyl phthalate 16.6

C 4.0 14.6 9,9-bis(methoxymethyl)fluorene 13.1

D 3.7 17.5 ethyl benzoate 14.8

Table 8. Ethylene polymerization using Ziegler-Natta catalysts in presence and absence of the nickel catalyst according to scheme 1 ^a

Entry Catalyst Ni Temperature Time Productivity type mg μmol ⁰C h g/g cat.h

S A 50 0 50 1 220

XIX A 50 0.5 50 1 1054

T A 50 0 70 1 1926

XX A 50 0.5 70 1 2524

U B 50 0 50 1 58

XXI B 50 0.5 50 1 72

W B 50 0 70 1 664

XXII B 50 0.5 70 1 764

Y C 50 0 50 1 32

XXIII C 50 0.5 50 1 36

Z C 30 0 70 1 943

XXlV C 30 0.5 70 1 2483

AA D 100 0 50 2 46

XXV D 100 0.5 50 2 101

BB D 100 0 70 2 771

XXVI D 100 0.5 70 2 960

^a Polymerization conditions: 500 ml. of light petroleum, AIEt₃ 1 mmol, ethylene pressure 0,5 MPa.

Time (mm) Time (mm)

(catalyst A) (catalyst B)

(catalyst C) (catalyst D)

Figure 2. Rate-time profiles for polymerizations carried out at 70 ⁰C with Ziegler-Natta catalysts A-D in the presence (•) and absence (A) of the nickel catalyst according to scheme 1.

Claims

I . Process for the preparation of a substantially linear polyethylene in the form of a homopolymer of ethylene, or a copolymer of ethylene with up to 1 mol% of an α-olefin, by polymerization of ethylene under the influence of a supported catalyst, wherein the supported catalyst comprises on one support at least two metal components MC₁ and MC_2, the most active component MCi producing linear polyethylene, and the minor active component MC₂ producing branched polyethylene.

2. Process according to claim 1 , wherein the metal in MCi is selected from groups 4-8 of the Periodic Table of Elements.

3. Process according to claim 2, wherein the metal in MCi is selected from the group comprising Ti, Cr, and Fe.

4. Process according to anyone of claims 1-3, wherein the metal in MC₂ is selected from the group comprising Ni, Cr, and Pd.

5. Process according to claim 4, wherein the metal in MC₂ is Ni.

6. Process according to anyone of claims 1-5, wherein the support is selected from either MgCI₂ or SiO₂.

7. Process according to claim 6, wherein the support is MgCI₂/AIR_n(OR)₃._n, each R being a (substituted) hydrocarbon group.

8. Process according to anyone of claims 1-7, wherein the polymerization is performed at a temperature of between 25 and 120 ⁰C.

9. Process according to anyone of claims 1-7, wherein the polymerization is performed in slurry or in gas phase.

10. Process according to anyone of claims 1 -9, wherein the molar ratio of the metals in MC₂ and MC₁ is between 0.2 and 0.8.

I 1. Process according to anyone of claims 1 -10, wherein the prepared polyethylene is a homopolymer.