GB1601861A - Ethylene polymerisation process - Google Patents

Description

(54) ETHYLENE POLYMERISATION PROCESS (71) We, BP CHEMICALS LIMITED, of Britannic House, Moor Lane, London, EC2Y 9BU, a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to a process for polymerising l-olefins.

It has long been known that olefins such as ethylene can be polymerised by contacting them under polymerisation conditions with a catalyst comprising (1) a transition metal compound, eg titanium tetrachloride and (2) a co-catalyst or activator, eg an organometallic compound such as triethyl aluminium. Catalysts of this type are generally referred to as Ziegler catalysts and will be referred to as such throughout this specification. It is also known that transition metal compounds can be supported on support materials, in particular magnesium - containing support materials such as magnesium oxide, magnesium hydroxide, magnesium carbonate, magnesium hydroxychloride and magnesium chloride and that such supported transition metal compounds can be used together with an activator to catalyse the polymerisation of l-olefins. These types of catalyst systems are generally referred to as supported Ziegler catalyst systems.

Polyethylene and copolymers of ethylene produced by contacting the monomer with a supported Ziegler catalyst under particle form process conditions (as hereinafter defined), normally have relatively narrow molecular weight distribution which renders them suitable for a variety of applications, for example, injection moulding, but limits their usefulness for certain applications, for example, fast cycle bottle blowing for which a broad molecular weight distribution polymer is more suitable.

It is an object of the present invention to provide a process for polymerising ethylene or a mixture of ethylene with one or more other l-olefins utilising a supported Ziegler catalyst under particle form process conditions so that the produced polyethylene or copolymer has a broadened molecular weight distribution.

Accordingly the present invention provides a process for polymerising ethylene or mixtures of ethylene with up to 40 wt Wo of one or more other l-olefins comprising contacting the monomer with a supported Ziegler catalyst under particle form process conditions as hereinafter defined at a temperature below 112"C, the contacting being conducted in a single reactor or in two or more reactors in cascade, and wherein the monomer undergoes at least some polymerisation in the presence of hydrogen, the polymerising monomer being subjected to a continuous or intermittent change in the molar ratio (R) of hydrogen to ethylene R being in the range 0 to 3 and the difference (AR) between the maximum and minimum value of R being at least either (i) 1OC/e of the maximum value of R or (ii) 0.03 whichever is the larger.

The monomer employed in the present invention is either ethylene or a mixture of ethylene with up to 40% by weight (based on total monomer) or other l-olefin(s).

Preferably the monomer consists of ethylene as the sole monomer. Examples of suitable other 1-olefins are C-C,(, 1-olefins, for example propylene, 1-butene and 1-hexene.

The supported Ziegler catalyst suitably comprises a compound of a metal of Groups IVA-VIA in the Periodic Table supported on a support material, and an organo-boron compound or an organometallic compound of a metal of Groups I to III in the Periodic Table as activator. The metal of Groups IVA-VIA is preferably titanium, vanadium or zirconium; most preferably it is titanium. The support material can be any support material which is capable of supporting the Group IVA-VIA metal compound and which does not inactivate the catalyst formed by adding the organometallic compound. Examples of support materials are silica, alumina, calcium carbonate, calcium phosphate, carbon, silicon carbide, and magnesium oxide. Magnesium-containing support materials are preferred.

Examples of these are magnesium oxide, magnesium hydroxide, magnesium chloride and magnesium hydroxychloride. Magnesium oxide having a mean particle diameter in the range 0.1 to 5001lem, a surface area in the range 1 to 300 square metres/g and containing 1.0 to 50 hydroxyl groups per 100 magnesium atoms is particularly preferred. Most preferably the mean particle diameter is in the range 1 to 125 um and the surface area in the range 10 to 250 square metres/g. Commercial grades of magnesium oxide having these properties may be made by controlled thermal decomposition of magnesium hydroxide.

Examples of activators suitable for use in the process of the present invention are organo compounds of boron, aluminium, lithium sodium, magnesium and zinc, for example triethyl aluminium, tri-isobutyl aluminium, diethyl aluminium chloride, triethyl boron and diethyl zinc. Preferred activators are trialkyl aluminium compounds wherein the alkyl is a C1-C6 group.

A supported Ziegler catalyst particularly preferred for use in the process of the present invention comprises the catalyst formed by mixing (A) a Ziegler catalyst activator comprising a trialkyl aluminium compound, eg triethyl aluminium and (B) a component formed by impregnating magnesium oxide with a halogen-containing titanium compound having the formula Ti(ORI)n(X)4 n wherein 0sn < 4 and when n > 0, Rl is an alkyl group containing 1-6 carbon atoms; and wherein X is halogen, preferably chlorine. Examples of preferred titanium compounds are titanium tetrachloride, Ti(OC2H5)3Cl Ti(OiPr)2Cl2 or mixtures thereof. If desired, the halogen-containing titanium compounds can be employed together with non-halogen-containing titanium compounds, for example titanium tetraethylate or titanium tetraisopropylate. In the preferred catalyst the quantity of halogencontaining titanium compound employed in the preparation of component (B) is suitably sufficient to give a concentration of titanium in the range 0.1 to 15 wt Wo, preferably 1.0 to 9 wt % based on the total weight of supported Ziegler catalyst component. The impregnation to form component (B) can be carried out using any convenient technique. The reaction temperature is preferably in the range -40"C to 1500C, most preferably between 20 and 100"C. The impregnation can be carried out, for example by mixing the magnesium oxide and the titanium compound with or without an inert diluent or a solvent for the transition metal compound; or the vapour of a volatile titanium compound (eg TiCl4) may be passed into a heated bed (eg a fluidised bed) of the magnesium oxide. It is preferred to carry out the impregnation by heating together the titanium compound and the magnesium oxide at a temperature in the range 70-100 C for to 5 hours in the presence of an inert diluent or a solvent for the titanium compound. Suitable inert diluents (which are in some cases also solvents for the titanium compound) are, for example saturated aliphatic hydrocarbons such as petroleum ether, butane. pentane, hexane, heptane, methyl cyclohexane and cyclohexane and aromatic hydrocarbons such as benzene, toluene and xylene. In a particularly preferred embodiment the halogen-containing titanium compound is formed by refluxing a mixture of titanium tetrachloride with an alcohol containing 1-6 carbon atoms (eg isopropanol) in the presence of an aliphatic or aromatic hydrocarbon diluent having a boiling point such that the reflux temperature of the mixture is in the range 50 to 1000C. The quantity of alcohol employed in this preferred embodiment is suitably in the range 2.0 to 2.5 moles per mole of Tic4. The reaction mixture obtained from the refluxing will comprise halogenated titanium compound(s) having the general formula Ti(ORl)nCl4 n wherein 0 < n < 4. Some hydrogen chloride is evolved during the refluxing and some remains in the mixture. The impregnation of the magnesium oxide with the halogen-containing titanium compound can then be conducted merely by mixing the former together with the total reaction mixture from refluxing the TiC14, alcohol and inert diluent and carrying out further heating. preferably by refluxing at a temperature in the range 50 to 100"C. The catalyst component (B) obtained by reacting the magnesium oxide with the halogen-containing titanium compound is preferably substantially free from unreacted halogen-containing titanium compound before mixing with the Ziegler catalyst activator (A). Removal of any unreacted halogen-containing titanium compound may be accomplished by techniques well known in the art, for example by washing the component (B) one or more times with a suitable solvent.

The quantity of activator employed in the supported Ziegler catalyst of the present invention is suitably in the range 1 to 200 moles per gramme atom of transition metal in the supported Ziegler catalyst and preferably 10 to 50 moles per g atom.

It is preferred to mix the supported transition metal component with at least some of the activator before the said component is allowed to contact the monomeric l-olefin.

Preferably the activator and the said supported transition metal component are mixed together and stirred or allowed to stand for a period of time in the range 10 min to 2 days, most preferably 1 to 4 hours preferably at a temperature in the range 0 to 800C, most preferably 20 to 50"C, before being contacted with the l-olefinic monomer.

Polymerisation under particle form process conditions is well known in the art and "particle'form process conditions" are defined to mean throughout this specification polymerisation conditions such that the monomer is contacted with the supported Ziegler catalyst suspended or fluidised in a medium that does not deleteriously affect the polymerisation and such that the produced polymer forms in the reaction mixture as solid particles. In the present invention the medium comprises a liquid diluent or a gas. The medium is preferably a C4-CIO liquid hydrocarbon and examples of suitable liquid hydrocarbons are isobutane and n-pentane. Examples of suitable gases are argon, nitrogen or helium mixed with the requisite proportion of gaseous monomer/hydrogen. Undiluted monomer or hydrogen/monomer mixtures may be employed if desired.

The quantity of supported Ziegler catalyst employed is suitably maintained in the range 10-500 milligrams (total catalyst) per litre of diluent although concentrations outside this range can be used if desired.

The polymerisation temperature must be below 112"C and is normally above 50"C.

Preferably the temperature lies in the range 75 to 1050C although care may have to be taken to avoid reactor fouling at the extreme ends of the broader temperature range. The polymerisation temperature can be varied in the single reactor or in the two or more reactors if desired and different temperatures may be employed in each of two or more reactors, provided the polymerisation temperature are not permitted to rise to 1120C.

The partial pressure of the ethylene in the polymerisation is suitably in the range 1 to 100 bars, preferably in the range 5 to 40 bars at the polymerisation temperature employed.

The contacting of the monomer with the catalyst to cause polymerisation is conducted in a single reactor or in two or more reactors in cascade. When the contacting is to be in a single reactor it is preferred to carry out the polymerisation under batch operating conditions. and when two or more reactors in cascade are employed, continuous operating conditions are preferred.

In the process of the present invention it is essential that the monomer undergoes at least some polymerisation in the presence of hydrogen (ie elemental hydrogen) and it is further essential that the difference AR between the maximum and minimum R value is at least either (i) 10% of the maximum value of R or (ii) 0.03 whichever is the larger. Preferably R is in the range 0 to 1.0 and preferably AR is at least 0.1.

By the term "R" is meant the ratio moles hydrogen/moles ethylene and AR is the difference between the maximum and minimum values of R employed in the single reactor or in the two or more reactors in cascade.

The polymerising monomer must be subjected to a continuous or intermittent change in the value of R. Under batch process conditions in a single reactor this may be achieved for example by commencing the polymerisation at an R value of 0 (ie in the substantial absence of hydrogen) and by increasing the R value by at least 0.03 during the polymerisation. An example of an alternative procedure is to commence the polymerisation with an R value of at least 0.03 and to increase or decrease the R value by at least either (i) 10% of the maximum value of R or (ii) 0.03, whichever is the larger. The change in R may be carried out, for example, in one or more steps or continuously and the change may go through any number of increases or decreases provided that AR is at least either (i) 10% of the maximum value of R or (ii) 0.03 whichever is the larger. Under continuous process conditions in two or more reactors in cascade the hydrogen is suitably present in one or more of the reactors for the full duration of the run; preferably under these circumstances the R value in the two or more reactors are maintained at substantially constant values throughout the run. The R value in one or more reactors in the cascade may be zero (ie may contain substantially no hydrogen for the duration of the run) provided that the R value in at least one other reactor is such that AR is at least 0.03 for the duration of the run.

In a preferred embodiment according to the present invention, ethylene is polymerised together with up to 40 wt % of one or more other l-olefins (of the type aforementioned), the one or more other l-olefins being present in the polymerisation when R is 0 or at a relatively low value (ie at or near its minimum value for a given polymerisation) and being removed from (or being absent from) the polymerisation when R is at a relatively high value (ie at or near its maximum value for the given polymerisation). Operation in accordance with this preferred embodiment leads to the production of polyolefins having improved stress crack resistance.

The particle form polyolefin product may be isolated from the polymerisation mixture by conventional techniques. For most purposes it is not necessary to remove the catalyst from the polymer, particularly when the productivity exceeds 3.0 kg per gram of catalyst.

It is preferred to carry out a catalyst deactivation step on the produced polymer before it comes into contact with the atmosphere. The deactivation can be carried out for example by exposing the polyolefin particles to a moist inert gas, for example moist nitrogen or to an alcohol in liquid or vapour form, or to oxygen gas diluted with an inert gas.

The process of the present invention produces polyethylene or copolymers thereof having a generally broader molecular weight distribution that are produced using conventional processes employing supported Ziegler catalysts. The melt indices of the polyethylene or copolymers can be varied by conventional techniques, for example; by altering the polymerisation temperature, higher temperatures tending to produce higher melt index polymer; or by employing generally higher partial pressures of hydrogen when higher melt index polymers are required and lower partial pressures of hydrogen when lower melt index polymers are required.

The invention is further illustrated by the following Examples 1-4. Examples Y, Z and A are by way of comparison.

EXAMPLES 1-3 and COMPARATIVE EXAMPLE Y Catalyst Preparation All operations were carried out in the absence of oxygen and water in nitrogen purged vessels.

(a) Dry cyclohexane (1 litre) was mixed in a 5 litre glass vessel with isopropanol (448 g; 7.46 mole) and then titanium tetrachloride (628 g; 3.31 mole) mixed with anhydrous stannic chloride (0.133 g: 0.512 x 10-3 mole) was added dropwise with stirring over 40 minutes.

The mixture was heated under reflux for 11/2 hours.

A slurry of BDH "AnalaR" (Registered Trade Mark) Grade magnesium oxide (100 g; 2.5 mole) in dry cyclohexane (600 ml) was added slowly and the mixture heated under reflux for a further 2 hours. The mixture was allowed to cool and settle overnight. The supernatant liquid was removed by decantation and the produced solid catalyst component washed eight times with dry cyclohexane (1500 ml aliquots) after which the washings contained less than 1 g per litre of titanium.

The produced solid catalyst component contained Ti = 3.94%; Cl = 39.3%; and Sn = 290 parts per 106.

(b) Dry cyclohexane (100 ml) and "Maglite K" (Registered Trade Mark) magnesium oxide (lOg: 0.25 mole) were stirred together and titanium tetrachloride (21.1 g; 0.111 mole) was added dropwise over 10 minutes. Isopropanol (15.0 g; 0.25 mole) was than added dropwise over 15 minutes. The mixture was stirred under reflux for 2 hours, then allowed to cool before the supernatant liquid was removed by decantation. The produced solid catalyst component was washed 3 times with dry cyclohexane (100 ml aliquots) after which the washings contained less than 1 g per litre of titanium.

Polvmerisation (Examples 1-3) The polymerisations according to the present invention were carried out in a 2 litre stainless steel reactor equipped with a stirrer. The solid catalyst component was charged in the form of a slurry in cyclohexane to the reactor which had previously been baked out at llO"C and purged with dry nitrogen. 1 litre of isobutane containing the required amount (see Table 1) of aluminium triethyl was charged to the reactor. The polymerisation was carried out in two stages, the first stage being carried out without hydrogen (ie R = 0) and the second stage at an R value greater than 0.03 (see Table 1). The reactor was heated up to reaction temperature and ethylene added to a partial pressure suitable to give smooth reaction (see Table 1). Ethylene was added continuously to maintain this partial pressure.

After the first stage of the reaction was complete, hydrogen was added and further ethylene was added to bring the total pressure of the reaction up to 41.4 bar. This total pressure is made up of the partial pressure of the ethylene, hydrogen and isobutane. The reaction temperature was also readjusted as required. Ethylene was added continuously to maintain the reactor contents at a pressure of 41.4 bar during the second stage of polymerisation.

One Stage Polymerisation for Comparison (Example Y) The same charging procedure was followed as for the two stage polymerisation above except that the first stage was omitted, hydrogen was charged immediately after isobutane and ethylene added to maintain the reactor pressure at 41.4 bar throughout the polymerisation.

Polymerisation and polymer data are given on the accompanying Table. It will be seen that much broader molecular weight distribution polymer was produced from the Examples 1-3 wherein R was charged during the polymerisation, than Example Y wherein R was constant throughout.

TABLE 1 Polymerisation and polymer properties Weight (mg) STAGE 1 STAGE 2 Example #R No. Catalyst AlEt3 Temp ( C) Length (min) Pressure (bar) Temp ( C) Length (min) Pressure (bar) H2 C2H4 H2 C2H4 1 (a)75.5 167 84 20 0 11.4 91 40 6.9 19.0 0.36 2 (b)98.0 251 80 25 0 6.9 91 35 6.2 19.7 0.31 3 (a)75.5 167 76 20 0 5.2 90 40 7.6 18.3 0.41 Y (b)98.0 167 91 60 5.5 20.3 NO SECOND STAGE 0 POLYMER PROPERTIES Example Productivity No. kg/kg h MI2.16 MI21.6 MIR = MI21.6/MI2.16 M.W.D. * 1 3720 0.046 5.2 110 broad 2 1930 0.21 21 100 broad 3 2850 2.4 130 53 broad Y 4220 2.6 61 24 narrow * Molecular Weight Distribution Polymer melt index (MI2.16) was determined according to BS 2782, method 105C. High load melt index (MI216) was determined by a method similar to that given in ASTM D1238, Procedure A, Condition F. The melt index ratio (MIR) is related to the molecular weight distribution of the polymer, high MIR values for a given melt index value indicating broader molecular weight distribution. The productivity is kg polymer per kg catalyst per hour.

COMPARATIVE EXAMPLES Z AND A AND EXAMPLE 4 Catalyst Preparation (c) Two litres of dry cyclohexane was mixed in a 10 litre glass vessel with 895 g (14.9 mole) of isopropanol (Fisons SLR Grade); 1256 g (6.62 mole) of TiC14 (Fisons SLR Grade) mixed with 0.271 g (1.040 x 10-3 mole) of SnCl4 (Fisons) were then added dropwise over 75 min with stirring. The mixture was heated under reflux for 2 hours.

When the temperature of the mixture had dropped to 69"C, a slurry of 200 g (5.0 mole) of Maglite K magnesia (Merck and Co) in 1 litre of dry cyclohexane was stirred in. The mixture was then heated under reflux for a further 2 hours. The produced slurry was then filtered to remove the impregnation liquor, then washed twice with 4 litre aliquots of dry cyclohexane and a third time with a 2.5 litre aliquot of dry cyclohexane. The product was transferred back into a clean 10 litre flask and reslurried with 7.5 litre dry cyclohexane. The supernatant liquor was removed by decanting and the catalyst slurry made up to 7.5 litre with fresh cyclohexane. The titanium concentration in the supernatant liquor was now 1.15 g/litre.

The produced catalyst component contained 3.57% by weight titanium, 39.4% by weight chlorine, 18.7% by weight magnesium and 186 ppm tin.

The catalyst component slurry was stirred and 1.13 litre (containing 100 g solid catalyst) was removed by decanting. This was mixed with 1 litre of a 10% w/w solution of triethyl aluminium in n-hexane. the triethyl aluminium solution being dropped into the slurry over 40 minutes. The produced catalyst was thus prereduced with 0.7 g triethyl aluminium per g catalyst.

(d) Five litres of dry cyclohexane was mixed in a 20 litre glass vessel with 2240 g (37.3 mole) of isopropanol (Fisons SLR Grade) and 0.678 g (2.60 x 10-3 mole) of SnCl4 (Fisons); 3140 g (16.6 mole) of titanium tetrachloride (Fisons SLR Grade) were then added dropwise over two hours with stirring. The mixture was heated under reflux for 2 hours.

When this reflux stage was completed a slurry of 500 g (12.5 mole) of Maglite K magnesia (Merck and Co) in 1.5 litres of dry cyclohexane was stirred in. The mixture was heated under reflux for a further 2 hours. The supernatant liquor was removed by decanting and the catalyst washed six times with fresh, dry cyclohexane, at which stage the titanium content in the washings was 0.25 g/litre.

The catalyst so produced contained 2.16% by weight titanium, 40.7% by weight chlorine and 18.2coo by weight magnesium.

The catalyst component slurry was stirred and 1.370 litre (containing 134 g solid catalyst) was removed by decanting. This was mixed with 3.50 litre of a 10% w/w solution of triethyl aluminium in n-hexane. The catalyst thus made was prereduced with 1.83 g triethyl aluminium per g catalyst.

Polymerisation - Comparanve Example Z Catalyst c (above) was fed from a mud pot through a flush feeder system to a 90 litre capacity loop reactor at 600 psig. The reactor was run effectively liquid full with isobutane and contained ethylene at a concentration of 10 mol % and hydrogen at a concentration of 0.7 mol % (that is R = 0.07. AR = 0) throughout the run. The polymerisation was carried out at 100"C and triethyl aluminium (additional to that used for prereducing the catalyst) was fed at a rate of 0.65 g/h. Polymer was produced at a rate of 6.7 kg/h for 12 hours, the polymer being removed from the reactor continuously (along with some isobutane, ethylene and hydrogen which were made up for by a balancing feeding of these materials).

Polymer produced in this one stage polymerisation had narrow molecular weight distribution as shown in Table 2.

EXAMPLE 4 On completion of the previous polymerisation run (Comparative Example Z). the hydrogen feed to the reactor was shut off. The catalyst, triethyl aluminium, ethylene and isobutane feeds were continued to maintain the polymer production rate (ca 7 kg/h) and reactor solids content (ca 25% by weight). When the high load melt index (MI2l.6) of the polymer being produced had fallen to a value too low to measure and no residual hydrogen could be detected in the reactor contents (R = 0), hydrogen was introduced rapidly into the reactor, reaching a concentration of 1.4 mole Xc after 15 minutes.

The hydrogen:ethylene ratio (R) was then held within the range 0.10 - 0.14 for 165 minutes. The increasing melt index of the polymer being produced was monitored and the reaction killed 3 hours after admission of hydrogen. The contents of the reactor were flushed out and the polymer recovered; it has broad molecular weight distribution as shown in Table 2.

COMPARATIVE EXAMPLE A Catalyst d (above) was fed from a mud-pot through a slurry pump feeder system to a 90 litre capacity loop reactor at 600 psig. The reactor was run liquid full with isobutane and contained ethylene at a concentration of 10 mol % and hydrogen at a concentration of 0.26 mol % (that is R = 0.026) throughout the run. The polymerisation was carried out at 900C.

Polymer was produced at a rate of 6.6 kg/h for 6 hours, the polymer being removed from the reactor continuously (along with some isobutane, ethylene and hydrogen which were made up for by a balancing feeding of these materials).

Polymer produced in this one-stage polymerisation had narrow molecular weight distribution as shown in Table 2.

Claims (27)

1. TABLE 2 Polymerisation and polymer properties Polymer Properties Example Productivity ' MI21.6 No (kg/kg h) MI2.16 MI21.6 MIR = MWD * MI2.le Z 10 300 8.56 246 29 Narrow

   4 N/A 0.11 15.4 143 Broad A 2 010 0.44 13.0 30 Narrow WHAT WE CLAIM IS: 1. A process for polymerising ethylene or mixtures of ethylene with up to 40 weight % of one or more other l-olefins comprising contacting the monomer with a supported Ziegler catalyst under particle form process conditions (as hereinbefore defined) at a temperature below 112"C, the contacting being conducted in a single reactor or in two or more reactors in cascade. and wherein the monomer undergoes at least some polymerisation in the presence of hydrogen. the polymerising monomer being subjected to a continuous or intermittent change in the molar ratio (R) of hydrogen to ethylene, R being in the range 0 to 3 and the difference (AR) between the maximum and minimum value of R being at least either (i) 10% of the maximum value of R or (ii) 0.03 whichever is the larger.
2. 2. A process as claimed in claim 1 wherein ethylene is the sole monomer.
3. 3. A process as claimed in claim 1 or 2 wherein the supported Ziegler catalyst comprises the catalyst formed by mixing (A) a Ziegler catalyst activator comprising a trialkyl aluminium compound and (B) a component formed by impregnating magnesium oxide with a halogen-containing titanium compound having the formula Ti(ORl)n(X)4n wherein 0%n < 4 and when n > 0. R' is in alkyl group containing 1 to 6 carbon atoms and wherein Xis halogen.
4. 4. A process as claimed in claim 3 wherein X is chlorine.
5. 5. A process as claimed in claim 1 or 2 wherein the supported Ziegler catalyst comprises the catalyst formed by mixing (A) a Ziegler catalyst activator comprising a trialkyl aluminium compound and (B) a component formed by impregnating magnesium oxide with the product of reacting together titanium tetrachloride and an alcohol containing 1 to 6 carbon atoms.
6. 6. A process as claimed in claim 5 wherein the alcohol is isopropanol.
7. 7. A process as claimed in claim 5 or 6 wherein the titanium tetrachloride is reacted with the alcohol in the presence of a hydrocarbon diluent.
8. 8. A process as claimed in any one of claims 3 to 7 wherein the activator component (A) and the supported transition metal component (B) are mixed together before the transition metal component is allowed to contact the monomer.
9. 9. A process as claimed in any preceding claim wherein the fluid medium in which the particle form process is conducted is a liquid.
10. 10. A process as claimed in claim 9 wherein the liquid is a C4-C10 hydrocarbon.
11. 11. A process as claimed in claim 10 wherein the liquid hydrocarbon is isobutane or normal pentane.
12. 12. A process as claimed in claim 1 wherein the fluid medium in which the particle form process is conducted is a gas.
13. 13. A process as claimed in claim 12 wherein the gas is undiluted monomer or hydrogen/monomer mixture.
14. 14. A process as claimed in claim 12 wherein the gas is monomer or monomer/hydrogen mixed with nitrogen, argon or helium.
15. 15. A process as claimed in any preceding claim wherein the polymerisation temperature is in the range below 112"C and above 50"C.
16. 16. A process as claimed in claim 15 wherein the polymerisation temperature lies in the range 75 to 105 C.
17. 17. A process as claimed in any preceding claim wherein the partial pressure of the ethylene in the polymerisation is in the range 1 to 100 bars at the polymerisation temperature employed.
18. 18. A process as claimed in claim 17 wherein the partial pressure of the ethylene is in the range 5 to 40 bars at the polymerisation temperature employed.
19. 19. A process as claimed in any preceding claim wherein R is in the range 0 to 1.0.
20. 20. A process as claimed in any preceding claim wherein AR is at least 0.1.
21. 21. A process as claimed in any preceding claim wherein the polymersation is carried out under continuous process conditions in two or more reactors in cascade and wherein the hydrogen is present in one or more of the reactors for the full duration of the run.
22. 22. A process as claimed in claim 21 wherein the R values in the two or more reactors are maintained at substantially constant values throughout the run.
23. 23. A process as claimed in any one of claims 1 or 3-22 wherein ethylene is polymerised with up to 40 wt % of one or more other l-olefins, the one or more other l-olefins being present in the polymerisation when R is 0 or at or near to its minimum value for the polymerisation and being removed from (or being absent from) the polymerisation when R is at, or near to, its maximum value for the polymerisation.
24. 24. A process as claimed in any one of claims 1 or 3-23 wherein the one or more other l-olefins are taken from C3-C10 l-olefins.
25. 25. A process substantially as hereinbefore described with reference to any one of Examples 1 to 4.
26. 26. Polyethylene or ethylene copolymers prepared by the process claimed in any one of the preceding claims.
27. 27. Articles fabricated from the polyethylene or copolymer ciaimed in claim 26.