using a Cr or Ti catalyst

Description:

The present invention pertains to a polymerization process for the synthesis of disentangled ultra-high molecular weight polyethylene (UHMW-PE), to

disentangled UHMW-PE produced according to the polymerization process and to moldings and elongated bodies made thereof.

Polymerization processes for the manufacture of UHMW-PE are known.

DE102009023651 describes a process to manufacture polyethylene using a stabilizer, as e.g. an antioxidant and a metallocene catalyst (e.g. a chromium cyclopentadienyl complex). However, the process differs from present invention and does not describe the production of disentangled polyethylene.

US7956140 describes a process for the preparation of olefin polymer using a catalyst, a co-catalyst and optionally a scavenger. The catalyst is an

organometallic compound comprising an amidine-containing spectator ligand, the co-catalyst can be an alkylaluminoxane. In this process an excess of scavenger is added and the process is carried out under high pressure and at high temperature.

WO201 1/054927 describes a process for the polymerization of one or more olefins by using a substituted cyclopentadienyl-metal complex as catalyst in the presence of a co-catalyst. This document describes that the order of adding and a possible pre-mixing of co-catalyst, scavenger and catalyst is not essential to the process. Furthermore, the polymerizations described in WO201 1/054927 take place under high pressure and at relatively high temperature.

Mark et al. (Angew. Chem. Int. Ed. 2010, 49, p. 8751 -8754) describe a process to obtain UHMW-PE where cyclopentadienylchromium complexes are used as catalyst and hydroboranes are added as co-catalyst modifiers. In the process described in this publication the catalyst and co-catalyst are added to the reaction vessel which contains a solvent and the modifier. Only afterwards the monomers are added to the reaction vessel.

Furthermore, the catalyst concentration used in the polymerization is relatively high at 3*10 ^"5 mol/L.

These prior art documents describe olefin polymerizations with cyclopentadienyl chromium catalysts which do not lead to the production of disentangled UHMW- PE.

Ronca et al. (/Advances in Polymer Technology 2012, 31 , 3, p. 193-204) describe a process to manufacture polyethylene with a low number of entanglements.

However, Ronca et al. use a different catalyst. Surprisingly, the described process if applied to cyclopentadienyl catalysts results in a low catalyst activity and polymer yield.

The objective of present invention is to overcome the limitations of the prior art and to provide an alternative process for the manufacture of UHMW-PE, more especially disentangled UHMW-PE wherein chromium or titanium

monocyclopentadienyl, monoindenyl, monofluorenyl or heterocyclopentadienyl complexes are used as catalysts.

Disentangled UHMW-PE of this invention refers to a polymer with a low number of entanglements between the polymer chains in solid state having a weight average molar mass greater than a million g/mol. Polymer with a low number of entanglements for the purpose of this invention is defined as a polyethylene polymer with an elastic modulus G' _t =o of at most 1 .4 MPa, preferably at most 1 .2 MPa and even more preferably at most 1 MPa. In some embodiments the elastic modulus directly after melt may be as low as at most 0.9 MPa, even more in particular at most 0.8 MPa, and even more in particular at most 0.6 MPa. The elastic modulus G' is determined as described in Pandey et al. (Macromolecules 2011, 44 (12), pp. 4952-4960) as G' _t = ₀ where t=0 is defined as the time point where the polymer reaches the measuring point which is preferably 20°C above the equilibrium melting point of the polymer. Accordingly, the elastic modulus is determined directly after reaching the measuring point.

The objective of present invention is achieved with a process for the manufacture of disentangled ultra-high molecular weight polyethylene in the presence of a co- catalyst and a catalyst which is a substituted monocyclopentadienyl, monoindenyl, monofluorenyl or heterocyclopentadienyl complex of chromium or titanium in which at least one of the substituents on the cyclopentadienyl ring carries a neutral donor function which is bonded rigidly, not exclusively via sp ³ -hybridized carbon or silicon atoms, characterized in that the process comprises the following steps in the given order:

i) a solvent and a modifier, and optionally the co-catalyst are added to a reaction vessel,

ii) ethylene and/or other monomers are added to the reaction vessel,

iii) a mixture of the catalyst and co-catalyst is added to the reaction vessel to obtain a polymerization mixture, which reacts during a reaction time of at least 30 minutes at a temperature below 50°C and at an ethylene pressure between 1 and 6.5 bars, wherein at least 90% of all the modifier used in the polymerization process is added in the step i).

The UHMW-PE produced in the process according to the invention can be a homopolymer of ethylene or a copolymer of ethylene with a co-monomer which is another alpha-olefin or a cyclic olefin both with generally between 3 and 20 carbon atoms. Examples include propene, 1 -butene, 1 -pentene, 1 -hexene, 1 -heptene, 1 - octene, cyclohexene, etc. The use of dienes with up to 20 carbon atoms is also possible, e.g., butadiene, 1 -4 hexadiene or dicyclopentadiene. The amount of (non-ethylene) alpha-olefin in the ethylene homopolymer or copolymer used in the process according to the invention preferably is at most 10 mole%, preferably at most 5 mole%, more preferably at most 1 mole%. If a (non-ethylene) alpha-olefin is used, it is generally present in an amount of at least 0.001 mol%, in particular at least 0.01 mole%, still more in particular at least 0.1 mole%. Obviously, this means that where ethylene is mentioned as a monomer that the monomer can also include at most 10 mole%, preferably at most 5 mole%, more preferably at most 1 mole% of a (non-ethylene) alpha-olefin monomer or cyclic olefin monomer based on the total amount of monomers.

The catalyst used in the process of current invention is known. The catalyst is a single-site catalyst which is a substituted monocyclopentadienyl, monoindenyl, monofluorenyl or heterocyclopentadienyl complex of chromium or titanium in which at least one of the substituents on the cyclopentadienyl ring carries a neutral donor function which is bonded rigidly, not exclusively via sp ³ -hybridized carbon or silicon atoms. Such catalysts have been described in US6437161 which reference is hereby included in its entirety. Preferably, these are cyclopentadienyl and indenyl complexes of the formula Y-M-X, where M is chromium or titanium, Y is a substituted monocyclopentadienyl or monoindenyl moiety and X can e.g. be selected from a fluorine, chlorine, bromine, iodine, hydrogen, C1 -C10-alkyl, C2- C10-alkenyl, C6-C20-aryl, alkylaryl having 1 -10 carbon atoms in the alkyl radical and 6-20 carbon atoms in the aryl radical or a bulky, non-coordinating anion.

Specific embodiments of substituted monocyclopentadienyl and monoindenyl complexes are also described in US6699948, US6787498, US6838563,

US6919412 and WO01 12641 . The complexes described in these documents are included by reference. Possible examples are 1 -(8-quinolyl)-indenylchromium(lll) dichloride and 3,4,5 trimethyl-1 -(8-quinolyl)-2-trimethylsilylcyclopentadienyl) chromium di chloride. 1 -(8-quinolyl)-indenylchromiunn(lll) dichloride is a preferred catalyst for use in present invention.

Even though polymerization processes with the catalysts as described in

US6437161 , US6699948, US6787498, US6838563, US6919412 and WO01 12641 are known, present invention provides an improved polymerization process which results for this specific class of catalysts in the production of disentangled UHMW- PE.

Preferably the catalyst is added at a concentration below 2*10 ^"5 mol/l, more preferably below 7*10 ^"6 mol/l or even below 5x10 ^"6 mol/l. The catalyst

concentration preferably is at least 1 *10 ^"7 mol/l, preferably at least 1 *10 ^"6 mol/l.

Besides the catalyst there is also a co-catalyst present in the process of present invention. The co-catalyst according to this invention is a group 13 atom

compound, preferably an aluminum compound, more preferably an

alkylaluminoxane, which is capable of ionizing the chromium or titanium catalyst used in this invention.

Aluminoxanes are mostly a mixture of different organoaluminum compounds. Aluminoxanes may be of the overall formula RAIO (RAIO) _m AIR wherein R is independently selected from d _-2 o hydrocarbyl radicals and m is from 0 to 50, preferably R is a Ci _-4 radical and m is from 5 to 30. In methylaluminoxane (MAO) the R groups in the compounds of the mixtures are methyl . MAO is the preferred aluminoxane co-catalyst according to this invention.

If an aluminoxane is used as co-catalyst the aluminoxane is preferably added at an aluminum to transition metal mole ratio of at least 800, more in particular at least 1000. The transition metal refers to the chromium or titanium of the catalyst. More preferably, the ratio is at least 1 100. As a general maximum, a value of 3000 may be mentioned. Also present in the polymerization reaction of current invention is a modifier. In one embodiment the modifier is a trialkylaluminum-scavenger. Trialkylaluminums are often present in aluminoxanes which are used as co-catalysts.

Trialkylaluminums have a negative effect on the catalyst and decrease the polymerization efficiency and, mostly, lower the molecular weight of the polymer produced. In one embodiment the modifier used in the present invention modifies the co-catalyst by selectively reacting with the trialkyaluminum species present. One possible tnalkylalunninunn is trimethylaluminum (TMA) which is usually present in aluminoxanes such as methylaluminoxane (MAO). The modifier of this invention is selected from the group comprising phenols, alcohols, amines, anilines, boranes and silanes, preferably from phenols, more preferably from butylphenols. Possible modifiers or trialkylaluminum scavengers are tert-butanol, triphenylcarbinol, 2,6-di- tert-butylphenol, 4-methyl-2,6-di-tert-butylphenol (BHT) and 4-ethyl-2,6-di-tert- butylphenol. Preferably, the modifier is 4-methyl-2,6-di-tert-butylphenol (BHT).

Typically employed solvents for polyolefin polymerization are known to the skilled person. The solvent of present invention is an aromatic solvent or a mixture of an aliphatic and aromatic solvent. Solvents can comprise toluene, heptane, hexane, xylene and combinations thereof.

Surprisingly, it was found that the specific order of adding the catalyst, co-catalyst, modifier and the monomers to the reaction vessel is important for the efficiency of the polymerization and for the physical properties of the obtained UHMW-PE when using the specific class of catalysts chosen in the present invention.

According to present invention the solvent and the modifier (preferably a

trialkylaluminum-scavenger) are added first to the reaction vessel. This can be carried out in different ways: for example, the solvent can be added first to the reaction vessel and subsequently the modifier or vice versa or the solvent and modifier are pre-mixed and the mixture is added to the reaction vessel. Essentially all of the modifier that is added to the polymerization will be added in the first step. This means that 90%, preferably 95%, even more preferably 100% of the modifier is added in this step (step i), based on the total amount of modifier added to the polymerization reaction. Optionally, also some of the co-catalyst added in the polymerization reaction can be added at this stage to the reactor. In one

embodiment the trialkylaluminum-scavenger, the solvent and the co-catalyst are mixed and added to the reactor. In another embodiment the solvent is first added to the reactor and subsequently a mixture of trialkylaluminum-scavenger and co- catalyst is added to the reactor. In the embodiment where the modifier is a trialkylaluminum-scavenger, the amount of such trialkylaluminum-scavenger added in the polymerization reaction depends on the amount of trialkylaluminum present in the co-catalyst. The ratio of trialkylaluminum-scavenger to trialkylaluminum is at least 0.25 to 1 , preferably at least 0.5 to 1 , more preferably 1 to 1 and even more preferably at least 1 .5 to 1 .

For example, when using 100 ml of MAO supplied by Sigma Aldrich which contains 3.4% w/w of trimethylaluminum, 0.06 mole of trialkylaluminum-scavenger is added to the reaction resulting in a molar ratio trialkylaluminum-scavenger to trialkylaluminum of 1 .5.

The co-catalyst can be added to the reaction vessel in step i) and step iii).

In one embodiment of the current invention between 10 and 90 mole% of the co- catalyst based on the total amount used in the process according to this invention is added in step i). Preferably, 50 mole% is added in step i), even more preferably 80 mole%. However, at least 10 mole% of the co-catalyst will be pre-mixed with the catalyst and added in step iii) of the present polymerization process.

The co-catalyst can also function as scavenger for impurities present in the solvent (in particular water and oxygen). In the second step the reaction vessel is flushed with monomers. The monomer filling pressure during step ii) varies between 0.5 and 6.5 bar. Only afterwards the mixture of catalyst and co-catalyst is added to the reaction vessel.

The catalyst and the co-catalyst are pre-mixed in a reaction medium, the same solvent as used in step (i).

The mixture of catalyst and co-catalyst is introduced into the reaction vessel, e.g. by injection. After addition of the catalyst/co-catalyst mixture a continuous flow of monomers is supplied for polymerization.

The monomer pressure during polymerization is between 1 and 6.5 bars, preferably between 1 and 5 bars and more preferably between 1 and 3 bars. In an especially preferred embodiment of this invention the pressure is between 1 and 2.1 bar. The pressure can be kept constant at any value between said limits or it can be varied during polymerization, e.g. the monomer pressure during

polymerization can be increased.

Surprisingly, when using the catalyst chosen in this invention it was found that it is important that the modifier and the monomers are added to the reaction vessel before the catalyst to avoid the catalyst being in contact with the modifier in the absence of monomers.

This was especially surprising since it was noted that when carrying out the same process with a different catalyst (e.g. bis[/V-(3-te/t-butylsalicylidene)

pentafluoroanilinato] titanium(IV) dichloride, as in Ronca et al.) the specific order of adding ingredients to the reaction vessel is not as important.

Polymerization reactions according to this invention are carried out in the liquid phase. The polymerization temperature is set to below 50°C, preferably below 30°C, more preferably at or below room temperature. The polymerization temperature generally is above 0°C.

In a preferred embodiment the polymerization temperature is between 5 and 25°C, more specifically between 5 and 20°C, still more specifically between 5 and 15°C. In one embodiment the polymerization temperature is 10°C.

The polymerization reaction preferably continues for at least 30 minutes. This means that during at least 30 minutes monomers are incorporated into the growing polymer chains. However, in one embodiment it is possible to use a reaction time of at least 20 minutes. Preferably, the polymerization time is at least 40 minutes or at least 45 minutes, even more preferably at least 60 minutes. As a general maximum, a polymerization time of at most 24 hours may be mentioned.

Preferably, the polymerization time is at most 12 hours, more preferably at most 6 hours. A polymerization reaction time of at least 30 minutes is necessary to obtain a UHMW-PE with a low density of chain entanglements. To realize a long polymerization reaction time the modifier, especially a trialkylaluminum-scavenger should not be in contact with the catalyst in the absence of monomers because this would decrease catalyst activity. The polymerization reaction time is

influenced by a number of parameters, whereby a lower temperature and a lower pressure lead to a longer polymerization times. It is within the scope of the skilled person using routine trial and error to select the processing conditions in such a manner that the polymerization reaction continues for the desired period of time to achieve the desired yield and molecular weight.

This invention also pertains to disentangled UHMW-PE obtainable by the polymerization process according to this invention. All embodiments described for the polymerization process according to this invention are also applicable to the polymer produced by these processes and for any product comprising the polymer produced according to present process. The disentangled UHMW-PE of this invention is characterized by a number average molecular weight of at least 0.5 million g/mol, preferably at least 3 million g/mol. The weight average molecular weight ranges above 1 million g/mol and preferably ranges between 1 to 20 million g/mol.

The number average molecular weight is defined as:

The weight average molecular weight is defined as:

M — ∑f. ¾A€f

¹ "t m r where N, refers to the number of molecules of molecular weight M,.

The molar mass distribution of the UHMW-PE according to this invention is preferably at most 4 and even more preferably at most 6, where the molar mass distribution is defined as M _w /M _n . Mn and Mw may be determined as is described in WO2010/079172. Reference may also be made to S. Talebi et al. in

Macromolecules 2010, Vol. 43, pages 2780-2788. The UHMW-PE obtainable by the process according to the present invention is a polymer with a reduced number of chain entanglements. This low entanglement status is reflected in the elastic modulus (also referred to as storage modulus) of the polymer which is determined directly after melting and which is at most 1 .4 MPa, preferably at most 1 .2 MPa, more preferably at most 1 MPa. In some embodiments the elastic modulus may be as low as at most 0.9 MPa, even more in particular at most 0.8 MPa, and even more in particular at most 0.6 MPa. The elastic modulus G' _t = ₀ is determined as described in Pandey et al. (Macromolecules 2011, 44 (12), pp 4952^1960) as G' _t = ₀ where t=0 is defined as the time point where the polymer reaches the measuring point which is preferably 20°C above the equilibrium melting point of the polymer. Accordingly, the elastic modulus is determined directly after reaching the measuring point.

Polymers with a reduced number of chain entanglements can be converted into high tenacity, high modulus elongated bodies as described by Rastogi et al.

(Macromolecules 2011, 44, pp 5558-5568). Reference is also made to the processes described in WO2009/007045, WO2010/079172, and WO2010/079174, the process description of which is incorporated herein by reference.

In one embodiment, the UHMW-PE of this invention may comprise chain branches of a length of at least two carbon atoms which occur at least once in 6000 methylene units, preferably at least once in 4000 methylene units, or even at least once in 2000 methylene units. In one embodiment the branches have a length of at least 4 carbon atoms, preferably a length of at least 6 carbon atoms, which is considered long branching. Branches of such length are sufficiently long to influence the polymer chain dynamics in the molten state. Such branched polyethylene polymer is especially advantageous because it has been shown to have less creep than non-branched polymer (WO2012/139934). The branching of polyethylene polymer can be determined by FTIR (Fourier -transformed infra red spectroscopy) as has been described by Blitz & McFaddin (Journal of Applied Polymer Science, 1994, 51 , p. 13-20). The rheological response of the polymer can give an indication about the length of the branching. For this, the elastic modulus G' and the loss modulus G" of the polymer melt have to be measured.

The produced polyethylene can be cross-linked by different methods, as e.g. by using radiation (X-ray, UV). The cross-linking can further improve the polymer properties, e.g. abrasion resistance, which is advantageous in some applications.

The UHMW-PE according to this invention can be used to form moldings and elongated bodies. Elongated bodies are e.g. films, tapes and fibers. Moldings are e.g. prostheses. In a preferred embodiment, the polymer used for making prostheses according to this invention is produced with a Titanium catalyst according to the process described in present invention.

The elongated bodies obtained by the process of current invention can have a tenacity of at least 2.8 N/tex, preferably at least 3.0 N/tex and more preferably at least 3.2 N/tex. In an especially advantageous embodiment the tenacity is higher than 3.5 N/tex.

The modulus of the elongated bodies preferably is higher than 150N/tex, more preferably higher than 175 N/tex and even more preferably higher than 200 N/tex.

The elongated bodies according to this invention can be used in the production of penetration-resistant articles, cables, slings, ropes, nets and pipes.

Penetration-resistant articles include anti-ballistic, cut-resistant and stab-resistant articles. Also, one penetration-resistant article can show a combination of any of these characteristics, e.g. anti-ballistic and cut-resistant properties. Non-limiting examples of penetration-resistant articles are gloves, vests, helmets and antiballistic plates.

The following examples and figures demonstrate the invention in more detail but by no means limit the scope of the invention.

Figure 1 shows FTIR spectra of polymer produced according to the process of current invention (upper continuous line) and of a polymer produced according to the process of Ronca et al. using a different catalyst (lower dotted line).

Figure 2 shows in panel a (Fig. 2a) the result of a rheology frequency sweep for two polymer samples: a polymer produced according to the invention (square dots) and of a polymer produced according to the process of Ronca et al. (round dots). Shown are the G' (black dots) and G" values (white dots). Panel b of figure 2 shows the increase of the elastic modulus G' over time for the polymer of the invention (square dots) and of the polymer of Ronca et al. (round dots). Methods

1 . Determination of catalyst activity

The catalyst activity is calculated by dividing the amount of polymer produced in kg by the moles of catalyst multiplied by the polymerization reaction time in hours and the monomer pressure in bar.

Catalyst activity: (Kg _PE )/(molc _r ^* h ^* bar)

2. Determination of polymerization yield

The polymerization yield is determined by weighting the amount of polymer obtained from the reaction, after filtering the material out of the reaction medium, washing it with methanol/acetone and drying it in a vacuum oven at 40°C for one night.

3. Making of tapes

Tapes were manufactured as follows: 25 g of polymer powder is poured into a mold with a cavity of 620 mm in length and 30 mm in width and compression- molded at 130 bar for 10 min to form a sheet. The sheet is preheated for at least 1 min and rolled with a Collin calender (diameter rolls: 250 mm, slit distance 0.15 mm, inlet speed 0.5 m/min). The tape is immediately stretched on a roll (speed 2.5 m/min). The rolled and stretched tape is further stretched in two steps on a 50 cm long oil heated hot plate. The tape comes in contact with the hot plate 20 cm from the entrance of the hot plate. The draw ratio is obtained by dividing the specific length of the sheet prior to deformation by the specific length of the tape after stretching. A typical processing temperature of polyethylene in the two stretching steps ranges between 135°C to 154°C. The sample is stretched to the desired initial draw ratio in the first stretching step. Parts of the drawn sample are used to measure the mechanical properties, whereas the remainder of the sample is drawn further to the final draw ratio and again the mechanical properties are determined.

4. Determination of mechanical properties of tapes

Tensile properties were measured according to standard D7744-201 1 using an Instron 5566 tensile tester at room temperature (25°C). To avoid any slippage, the side action grip clamps with flat jaw faces are used. The nominal gauge length of the specimen is 100 mm, and the test is performed at a constant rate of extension (crosshead travel rate) of 50 mm/min. The breaking tenacity (or tensile strength) and modulus (segment between 0.3 and 0.4 N/tex) are determined from the force against displacement between the jaws.

5. Rheology measurements

To perform the rheological experiments, a similar procedure as the one described in Pandey et al. (Macromolecules 2011, 44, 4952-4960) is applied. The nascent polymer powder of each sample is compressed into a plate of 50 mm diameter and a thickness of 0.6-0.7 mm at a fixed temperature of 125°C, under a maximum force of 20 tons for an average time of 20 min.

From this plate, using a punching device, 12-mm-diameter discs are cut for rheological studies.

Rheological experiments are performed using a 12-mm parallel plates strain controlled rheometer [TA instruments, ARES G2]. The sample is placed between parallel plates at an initial temperature of 1 10°C. To prevent oxidation, the temperature is controlled by a convection oven under a nitrogen environment. After stabilization at 1 10°C, the temperature is increased to 130°C at a rate of 30°C/min. After thermal stabilization, the sample is heated further to 160°C at 10°C/min while maintaining the compression force of 4 N.

Subsequently, an oscillatory amplitude sweep test is carried out to determine the range of oscillatory strains in the linear viscoelastic region. The test is performed at a fixed frequency of 10 rad/s. To follow changes in the moduli of the polymer melt an oscillatory time sweep (frequency sweep) is performed. The test is executed at a fixed frequency of 10 rad/s and a strain of 0.3%, well within the linear viscoelastic regime. The frequency sweep (Fig. 2a) provides information about the molecular weight and the branching of the measured polymer.

Determined are the elastic modulus G' and the loss modulus G".

The elastic modulus G' and the loss modulus G" of the polymer are defined in the textbook "Rheology: Principles, Measurements and Applications" (Macosko, Wiley- ' is defined by the formula:

and G" by the formula: where the deformation (Y) is:

7 = 7° sin ω|

and where Y° is the maximum deformation and ω is the frequency.

The build-up of the elastic modulus G' in time (Fig. 2b) provides information of the disentangled state of the polymer.

6. FTIR (Fourier transformed infra-red spectroscopy)

Approximately 2 grams of nascent polymer powder of each sample is compressed at a maximum force of 20 tons for an average time of 20 minutes at 125 °C. The resulting disc of 3.5 cm diameter and 0.8 cm thickness is deposited on a MKII Golden Gate™ Single Reflection ATR system cell. The measurements are carried out using a Shimadzu FTIR 8400s spectrometer, collecting 64 scans using a resolution of 4 cm ^"1 in the range of 600-4000 cm ^"1 . Example 1 : Five samples according to the invention

After drying overnight at 125 °C, a Buchi two litre cylindrical jacket reaction vessel having five stainless steel flanged lid equipped with a Buchi Glasuster Cyclone 075 mechanical stirrer with three 25 mm blades propeller, stainless steel feeding tube and a thermometer probe were used for polymerization. The reactor was saturated with a continuous flow of nitrogen. The reaction temperature was set and maintained at the desired value (10°C) by a Huber Unistat 425 thermostat with feedback loop control through the thermometer probe. Anhydrous toluene was introduced into the reaction vessel under nitrogen stream and the solvent was stirred continuously at 1500 rpm. In a separate vessel BHT (the modifier, a trialkylaluminum scavenger) of an amount as specified in Table 1 was dissolved in

MAO and was allowed to react for 30 minutes.

The BHT was dissolved in the following amount of MAO:

For sample 1 : in 8ml MAO

For sample 2: in 4ml MAO

For sample 3: in 4ml MAO

For sample 4: in 8ml MAO

For sample 5: in 4ml MAO

Subsequently, the reacted solution was introduced into the reactor. The nitrogen was replaced with ethylene under the pressure specified in Table 1 .

The catalyst 1 -(8-quinolyl) indenyl chromium(lll) dichloride (referred to as Cr, amount as stated in Table 1 ) was dissolved in MAO and toluene (examples 1 and 4: in 2 ml MAO and 8 ml toluene, examples 2, 3 and 5: in 1 ml MAO and 4 ml of toluene). The resulting solution was stirred for 5 minutes under magnetic stirring and then injected into the reactor. After 30, 60 or 120 minutes of polymerization time (cf. Table 1 ), the reaction was quenched by the addition of methanol. The resulting polyethylene was filtered, washed with copious amount of

methanol/acetone, then 0.7-1 .0 wt% of Irganox 1010 was added and the polymer was dried overnight in a vacuum oven at 40 °C. Table 1 : Polymerization conditions of samples according to this invention, in all samples the Cr catalyst was used

Rxn. Vol. : reaction volume

Cat (mg): amount of catalyst

Mod. (g): amount of modifier, here trialkylaluminum-scavenger present in the co-catalyst, in this case BHT in grams

Mod. (mol): amount of modifier, here trialkylaluminum-scavenger, in this case BHT in mol Cocat (ml): amount of co-catalyst, in this case MAO

TMA (mol): amount of trimethylaluminum in mol

Mod ./TMA: molar ratio between the modifier, i.e. trialkyaluminum-scavenger (BHT) and trimethylaluminum (TMA)

P (bar): monomer pressure, in this case ethylene, in bar

T (°C): reaction temperature in °C

Rxn time (min): reaction time in minutes

Yield (g): amount of polymer produced in g

Activity: activity of the catalytic system in (Kg _PE )/(mol _c ^* h ^* bar)

Samples 1 to 5 result all in good catalyst activity and a high yield of polymer.

As shown in sample 3, also longer polymerization times during the process of current invention result in a high yield and high catalyst activity. In contrast, the process as described in DE102009023651 which uses the same catalyst but a different process has a much lower catalyst activity at longer polymerization times. Subsequently, the polymer of samples 1 , 2, 4 and 5 was processed into tapes according to the described procedure and the tapes were tested to determine the mechanical properties.

Table 2 shows the results of the tests. Table 2: Mechanical properties of tapes produced from the polymers of samples 1 , 2, 4 and 5 of Table 1

TDR: total drawing ratio at 25°C in first and second stretching step on hot plate All four tested samples of tapes produced from polymer according to this invention show good mechanical properties and can be drawn to achieve high drawing ratios. These tapes show that the polymers of the invention can be converted into high tenacity, high modulus materials. Example 2 - comparison of polymerization conditions which differ from the process according to the invention

Three different polymerization conditions which differ from the process of present invention where applied using the same Cr catalyst as for samples 1 -5. For all three comparative samples (CE1 -CE3) the polymer yields, catalyst activity and the mechanical properties of tapes produced from the obtained polymers were determined. The results are shown in table 3 (polymerization conditions) and 4 (mechanical properties). For all reactions the same equipment was used as described in example 1 .

For comparative sample 1 (CE1 ), 0.75 I of anhydrous toluene was introduced into the reactor vessel under nitrogen stream and continuous stirring at 1500 rpm was applied. 1 .5 g of BHT was dissolved in 10 ml of MAO (co-catalyst) and was allowed to react for 30 minutes. Subsequently, 8 ml of the reacted solution was introduced into the reactor and 2 ml of the reacted solution was used to dissolve 6.0 mg of catalyst in combination with 8 ml of toluene, the resulting solution was stirred for 5 minutes under magnetic stirring and then injected into the reactor. After a polymerization time of 60 minutes at 10°C, the reaction was quenched by addition of methanol. The resulting polyethylene was filtered, washed with copious amounts of methanol/acetone, then 0.7-1 .0 wt% of Irganox 1010 was added and the polymer was dried overnight in a vacuum oven at 40 °C. The resulting yield was 3.0 grams with a catalyst activity of 166 Kg _PE /molcr ^* bar ^* hour.

In comparative sample 1 the catalyst is in contact with co-catalyst MAO and BHT, the modifier before the monomers are added. This results in a very low yield, i.e. 3.0 grams and very low catalyst activity, i.e. 166 Kg _PE /molcr ^* bar ^* hour. The polymer obtained was not suitable for mechanical processing.

Comparative sample 1 shows that the catalysts used in this invention should not be in contact with the modifier in the absence of monomers.

Table 3: polymerization conditions of comparative samples CE1 -CE3, for all samples the Cr catalyst was used

For comparative sample 2 (CE2) similar conditions were applied as for CE1 , however, the polymerization reaction time was kept similar to that used in the publication by Mark et al. {Angew. Chem. Int. Ed. 2010, 49, 8751 -8754).

This increases the catalyst activity and results in a high yield. However, the mechanical properties of the tape produced from the obtained polymer were poor (see table 4). Apparently, the increase in polymerization time is important to obtain UHMW-PE with a low entanglement status state that provides the desired mechanical properties. For comparative sample 3 (CE3) 1 ,0 I of anhydrous toluene was introduced into the reaction vessel under nitrogen steam and continued stirring at 1500 rpm. 4 ml of MAO was introduced into the reactor and the nitrogen was replaced with 1 .1 bar of ethylene pressure. Subsequently, the reaction was initiated by injecting a solution of 1 ml of MAO and 4 ml of toluene used to dissolve 2.5 mg of catalyst 1 -(8-quinolyl) indenylChromium(lll) dichloride. After 60 minutes of polymerization time, the reaction was quenched by addition of methanol. The resulting

polyethylene was filtered, washed with copious amount of methanol/acetone, then 0.7-1 .0 wt% of Irganox 1010 was added and the polymer was dried overnight in a vacuum oven at 40 °C. No BHT was added in this reaction. The yield was 41 .1 grams and the catalyst activity was 5458 Kg _PE /molcr ^* bar ^* hour (see table 3).

The polymer was processed into tapes and the mechanical properties of the resulting tapes were tested. The polymer could not be stretched and resulted in low values for the total draw ratio and tenacity.

Comparative sample 3 shows that the modifier is necessary to obtain a polymer which shows good processing behavior and mechanical properties.

Table 4: mechanical properties of tapes prepared from comparative polymer samples

Example 3: comparison of prior art process using a catalyst as described in the invention and a prior art catalyst.

The process of the present invention was compared to a prior art process which has been described for a different catalyst. Ronca et al. describe a polymerization process for a bis[N-(3-tert-butylsalicylidene) penta-fluoroanilinato] titanium(IV) (indicated as Fl catalyst).

The process described in Ronca et al. was applied to a polymerization with the Fl catalyst (sample CE4) and the 1 -(8-quinolyl) indenyl Chromium(lll) dichloride (Cr, CE5) of present invention (reference is made to the process of Ronca et al.

described as "Polymerization with BHT-modified MAO").

The same equipment as for example 1 was used, the amounts of catalyst, co- catalyst and trialkylaluminum scavenger (BHT) are indicated in table 5. In the process of Ronca et al. first toluene is added to the reactor, subsequently a mixture of co-catalyst MAO and BHT, afterwards ethylene is added and at last, a premix of the Fl catalyst, the co-catalyst MAO and BHT. In the process of the present invention the modifier (BHT) is added to the reactor before the monomers and the catalyst. Table 5: Polymerization conditions and results of processes according to Ronca et

Surprisingly, when the prior art process is applied to a catalyst used in present invention the catalyst activity and the polymer yield is very low. Therefore, this prior art process is not suitable for the catalysts used in current invention.

Example 4: comparison of polymer properties of polymer produced according to the invention and according to a prior art process

The polymer properties of a polymer manufactured according to present invention were compared to the properties of polyethylene manufactured with the Fl catalyst according to the process of Ronca et al. (the process referred to as

"Polymerization with BHT-modified MAO"). The branching frequency and the rheology behavior were determined for polymer samples produced according to both processes.

For this comparison FTIR spectra were prepared of the polymer of sample 4 (table 1 , which is according to the invention) and of CE4 (using Fl catalyst, according to process of Ronca et al., table 5).

Fig. 1 shows the comparison of the FTIR spectra overlaid in one graph. The spectrum of the polyethylene prepared according to the invention (upper, continuous line in graph) shows extra peaks as compared to the polyethylene produced according to the prior art process (lower, broken line of graph). These extra peaks show that the polymer of sample 4 is branched and that the process of present invention can result in branched polyethylene.

To receive more information on the length of the branches the rheology behavior of a polymer sample according to the invention (sample 6) and of a polymer produced with the Fl catalyst according to the process of Ronca et al. (CE6) was compared (samples are shown in table 6). Sample 6 was produced with the Cr catalyst as described for samples 1 -5 with the amounts of reactants as indicated in table 6. Sample CE6 was produced with the Fl catalyst according to the process described in Ronca et al.

Table 6: Samples for rheology behavior measurements

For both samples a frequency sweep as described above was carried out and the G' and G" values were determined (Fig. 2a). Additionally, the G' value was determined for the polymer after melt for both samples over time. Fig. 2a shows a crosspoint for the moduli G' and G" of the polymer sample CE6 (the black and white round dots). That means, that at lower frequencies which are applied to the polymer melt the loss modulus G" which describes the viscous properties is higher than the elastic modulus G' - the polymer behaves more like a viscous liquid at such frequencies. At high frequencies the elastic modulus is higher than the loss modulus so at those frequencies the polymer shows elastic behaviour.

In contrast, the moduli G' and G" of the polymer sample according to the invention (sample 6) do not show such crosspoint (the black and white square dots). The elastic modulus at all frequencies is higher than the loss modulus. This also indicates that sample 6 is a branched polymer.

In the second experiment the maximum modulus G' of the polymer after melt during cooling was determined (Fig. 2b). While CE6 (round dots) reaches an elastic modulus G' of higher than 2000000 Pa (or 2 MPa), sample 6 (square dots) at most reaches an elastic modulus of 1 .4 MPa. Fig. 2b also gives an indication of the elastic modulus directly after melt (t=0), this is also much lower for sample 6 than for sample CE6 and is correlated to the entanglement status of the polymer in solid state. The FTIR data show that the polymer is branched. This rheology indicates that these branches are relatively long because of the low maximum elastic modulus.

Branching of the polymer has been correlated with low creep of the polymer.

These data therefore show that the process of present invention results in a branched polymer which likely shows less creep than polymers of the prior art.