WO2011144431A1

WO2011144431A1 - Process for the preparation of ultra high molecular weight polyethylene

Info

Publication number: WO2011144431A1
Application number: PCT/EP2011/056735
Authority: WO
Inventors: Diego Brita
Original assignee: Basell Poliolefine Italia Srl
Priority date: 2010-05-18
Filing date: 2011-04-28
Publication date: 2011-11-24

Abstract

The present invention relates to a process for the preparation of an ethylene polymer having intrinsic viscosity measured in tetraline at 135°C higher than 10 dl/g, characterized by (co)polymerizing ethylene in the presence of a catalyst system comprising the product obtained by contacting an aluminum alkyl compound with a solid component having particle size 5-20μm and containing an electron donor compound of specified formula.

Description

TITLE

PROCESS FOR THE PREPARATION OF ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE

The present invention relates to a process for the preparation of ultra high molecular weight polyethylene (UHMWPE) carried out in the presence of a specific catalyst system.

Said process is capable of producing polymers and copolymers of ethylene having ultra high molecular weight, in the form of powder, which can be used directly for compression molding processes, and, more generally, for the manufacturing processes typical of polymers with very high molecular weight.

UHMWPE is characterized by intrinsic viscosity [η] in tetraline at 135°C higher than, or equal to, 10 and generally from 11 to 30 dl/g. Its unique combination of mechanical (impact toughness, abrasion resistance, low coefficient of surface friction, excellent ESCR) and chemical (resistance to corrosion) properties stems from its very long chain. Accordingly, they all improve as the molecular weight increase.

In general, UHMWPE can be obtained with Ziegler Natta catalysts employed in slurry polymerization process in the substantial absence of chain transfer agent. Notwithstanding the absence of chain transfer agents (also called molecular weight regulators) reaching the extremely high molecular weight requires a residence time in the polymerization reactor which is normally higher than the average residence time necessary for producing PE with a regular molecular weight.

Due to the replica phenomenon, the polymer particle starting from the catalyst particle size gradually increases its volume upon reaching a notably grown volume in correspondence with the MW required. Due to the fact that large polymer particle size make the slurry process more difficult and less efficient to be carried out, it is advisable to polymerize ethylene using a solid polymerization catalyst originally having small particle size.

However, starting from very small size particle requires that the catalyst has a very good activity and tendency to produce high molecular weight in order reach the target polymer size in a time sufficiently rapid to lower the residence time in the reactor and therefore enhance the productivity which otherwise would be too low.

Thus, it would be advisable to have a catalyst able to polymerize ethylene with high activity and capable to produce polyethylene with a molecular weight as high as possible and preferably having an intrinsic viscosity higher than 11 dl/g.

In EP523657B1 it is described a catalyst capable to satisfy the above needs comprising the reaction products between: 1) a liquid obtained by reacting: A) a titanium compound containing at least one Ti-OR bond, where R is a C₁-C₂₀ alkyl, C3-C₂₀ cycloalkyl, or C₆- C₂₀ aryl radical; with B) a magnesium compound selected from: halides; compounds comprising at least one -OR or -OCOR group bonded to the magnesium, where R is a Ci- C₂₀ alkyl, cycloalkyl, or aryl radical; organometallic compounds; products of the reaction between the above mentioned compounds and electron-donor compounds; and 2) compound or composition capable of substituting in compound (A) at least one -OR group with a halogen atom, and optionally reducing the titanium of compound (A) to an oxidation status lower than 4, said catalyst component being in the form of particles having an average diameter smaller than 10 micrometers, generally from 1 to 8 micrometers, preferably from 2 to 6 micrometers, including the extremes, and being capable of producing, in a standard test for the polymerization of ethylene, a polymer in particle form having an average diameter smaller than 150 micrometers, usually from 20 to less than 150 micrometers, preferably from 40 to 120 micrometers including the extremes.

Such a catalyst is able to provide UHMWPE with the required particle size but with a polymerization activity not particularly high. Moreover, it does not produce UHMWPE with an intrinsic viscosity higher than 11 dl/g. This is because even when the polymerization is carried out without addition of the main chain transfer agent (hydrogen being the most commonly used) the growing chain termination and the start of a new polymer chain can, to a lower extent, take place due to the inevitable presence of certain chemicals in the polymerization reactor (see Encyclopedia of Polymer Science and Engineering, 1986 Ed. Vol 6 pages 394-395) or by effect of temperature. Each catalyst has its own behavior in this respect which is not predictable. It would be advisable to find a catalyst which, in the absence of hydrogen as a chain transfer agent, has a lower sensitivity towards the others, less effective, chain transfer agents or chain transfer conditions, and thus be able to generate ethylene polymers with higher molecular weight.

The applicant has now found a process for the preparation of an ethylene polymer having intrinsic viscosity measured in tetrahydro naphthalene at 135°C higher than 10 dl/g, comprising (co)polymerizing ethylene, in the substantial absence of hydrogen as a chain transfer agent, and in the presence of a catalyst system comprising the product obtained by contacting (a) a solid component having average particle size 5-20 μηι comprising Ti, Mg, halogen, and an electron donor compound selected from the group consisting of 1 ,2- diethers, mono or diesters of 1 ,2-dihydroxy compounds, and monoethers monoesters of 1 ,2-dihydroxy compounds with (b) an aluminum alkyl compound..

Preferably, the electron donor is selected from those of formula (I) below

RRIC(OPM)-CR₂R₃(OR₅) (I) in which R, Ri, R₂ and R₃ are, independently, hydrogen or Ci-C₂o hydrocarbon groups which can also be condensed to form a cycle, R₄ and R5 are C1-C20 alkyl groups, or ReCO- groups where R₆ is a Ci-C₂o alkyl or aryl group, or they can be joined with R and R₃ respectively to form a cycle; said R to R6 groups possibly containing heteroatoms selected from O, Si, halogens, S, N and P.

Preferably, when R and R₄ form a cycle, R5 is C1-C20 alkyl group. Preferably, in the electron donor compound of formula (I), R, R₄ and R5 are methyl.

Preferably, in the electron donor compound of formula (I) Ri to R₃ are hydrogen. When R4 and R5 are linear, branched or cyclic alkyl groups they are preferably chosen among C1 -C5 alkyl groups and more preferably among methyl or ethyl. Preferably they are both methyl. Among ReCO groups preferred is acetyl and benzyl.

Specific electron donor compounds of formula (I) are ethylene glycol diacetate, 1 ,2- dimethoxypropane, 1 ,2-diethoxypropane, 1 ,2-diethoxyethane, methyl tetrahydro furfuryl ether, 1 ,2-dimethoxypropane being the most preferred. In the final solid catalyst component (a) the electron donor compound of formula (I) is preferably present in an amount ranging from 0.1 to 20%wt preferably from 0.5 to 15%wt more preferably from 1 to 10%wt with respect to the total weight of component (a).

The average particle size of component (a) is preferably from 6 to 15 μηι and more preferably from 7 to 1 1 μηι.

Preferably the solid catalyst component (a) is in a substantially spherical form. The term substantially in spherical form means particles in which the ratio among the longer axis and the shorter axis is equal to, or lower than, 1.5 and preferably lower than 1.3. Such values can be measured via known methods such as optical or electronic microscopy.

Particularly preferred are the solid catalyst components in which the Ti atoms derive from a titanium compound which contains at least one Ti-halogen bond and the Mg atoms derive from magnesium chloride. In a still more preferred aspect, both the titanium compound and the electron donor of formula (I) are supported on magnesium dichloride. Preferably, in the catalyst of the present invention at least 70% of the titanium atoms and more preferably at least 90% of them, are in the +4 valence state.

In a particular embodiment, the magnesium dichloride is in active form. The active form of magnesium dichloride present in the catalyst components of the invention is recognizable by the fact that in the X-ray spectrum of the catalyst component the major intensity reflection which appears in the spectrum of the non-activated magnesium dichloride (having usually surface area smaller than 3 m /g) is no longer present, but in its place there is a halo with the position of the maximum intensity shifted with respect to the position of the major intensity reflection, or by the fact that the major intensity reflection presents a half-peak breadth at least 30% greater that the one of the corresponding reflection of the non-activated Mg dichloride. The most active forms are those in which the halo appears in the X-ray spectrum of the solid catalyst component.

In the case of the most active forms of magnesium dichloride, the halo appears in place of the reflection which in the spectrum of the non-activated magnesium chloride is situated at the interplanar distance of 2.56 A.

Preferred titanium compounds are the halides or the compounds of formula TiX_n(OR⁶)4__n, where l<n<3, X is halogen, preferably chlorine, and R⁶ is Ci-Cio hydrocarbon group.

Especially preferred titanium compounds are titanium tetrachloride and the compounds of formula T1CI₃OR⁶ where R⁶ has the meaning given above and in particular selected from methyl, n-butyl or isopropyl.

Preferably the said solid catalyst component (a) is characterized by a porosity (Pp) higher than 0.3 cm³/g, preferably higher than 0.4 cm³/g and preferably ranging from 0.4 to

3 3

0.9cm7e, more preferably from 0.4 to 0.7 cm /g. The surface area determined by BET method, is generally lower than 100 and preferably ranging from 30 to 80 m²/g. The porosity measured by the BET method is generally comprised between 0.1 and 0.7 m²/g.

The solid catalyst component (a) can be prepared according to various methods. According to one of them, a titanium compound having at least a Ti-Cl bond, preferably T1CI4, is reacted, in the presence of the electron donor of formula (I), with a catalyst precursor of formula MgCl_n(OR⁶)2-n, where n is from 0.5 to 1.5 and R⁶ has the meaning given above. Preferably, the titanium compound is used in excess. In this embodiment, the reaction temperature is not particularly critical and can range from room temperature up to 150°C preferably in the range 40-120°C.

Among the catalyst precursors particularly preferred are those in which R⁶ is selected among a C1-C8 hydrocarbon group, preferably ethyl, and n ranges from 0.6 to 1.4, in particular from 0.7 to 1.3 and especially from 0.8 to 1.2. The said catalyst precursors can be generated by exchange reaction between organometallic compounds of formula Cl_mMgR2-m, where m is from 0.5 to 1.5, and R is a hydrocarbon group, with an appropriate OR⁶ group source. The OR⁶ sources are for example R⁶OH alcohols or, preferably, a silicon compound of formula (R⁶0)_rSiR₄__r where r is from 1 to 4 and R⁶ has the meaning given above. In turn, as generally known in the art, organometallic compounds of formula Cl_mMgR₂-m can be obtained by the reaction between Mg metal and an organic chloride RC1, in which R is as defined above, optionally in the presence of suitable promoters. Preferably, the formation of Cl_mMgR₂-_m and the further exchange with the OR⁶ source takes place in one single step.

Another preferred way to prepare the solid catalyst components (a) is by reacting the titanium compound having at least a Ti-halogen bond with an adduct of formula MgCi₂»nROH preferably in the form of substantially spherical particles, where n is generally from 1 to 6, and ROH is an alcohol, in the presence of the electron donor of formula (I).

In particular, the MgCi₂»nROH is caused to react with an excess of liquid T1CI₄ containing electron donor of formula (I) in the optional presence of hydrocarbon solvents. The reaction temperature initially is from 0° to 25°C, and is then increased to 80-135°C. Then, the solid may be reacted once more with TiCU, separated and washed with a liquid hydrocarbon until no chlorine ions can be detected in the wash liquid. The electron donor compound of formula (I) is preferably added together with the titanium compound to the reaction system. However, it can also be first contacted with the adduct alone and then the so formed product reacted with the titanium compound. As an alternative method, the electron donor compound can be added after the completion of the reaction between the adduct and the titanium compound.

The MgCi₂»nROH adduct can be prepared in spherical form from melted adducts, by emulsifying the adducts in a liquid hydrocarbon and thereafter causing them to solidify by fast quenching. Representative methods for the preparation of these spherical adducts are reported for example in USP 4,469,648, USP 4,399,054, and WO98/44009. Another useable method for the spherulization is the spray cooling described for example in USP 5,100,849 and 4,829,034.

In a preferred aspect of the present invention, before being reacted with the titanium compound, the spherulized adducts are subjected to thermal dealcoholation at a temperature ranging from 50 and 150°C until the alcohol content is reduced to values lower than 2 and preferably ranging from 0.3 and 1.5 mo Is per mol of magnesium chloride.

Optionally, said dealcoholated adducts can be finally treated with chemical reagents capable of reacting with the OH groups of the alcohol and of further dealcoholating the adduct until the content is reduced to values which are generally lower than 0.5 mo Is.

The MgC^/electron donor of formula (I) molar ratio used in the reactions indicated above preferably ranges from 7: 1 to 40: 1 , preferably from 8: 1 to 35: 1.

The particle size of the catalyst components obtained with this method is easily controllable and allows the preparation of components with an average particle size in the range of 5-20 μηι which are needed to prepare UHMWPE

Also the particle size distribution is narrow being the SPAN of the catalyst particles comprised between 0.7 and 1.3 preferably from 0.8 to 1.2. The SPAN being defined as the value of the ratio , wherein P90 is the value of the diameter such that 90% of

P50

the total volume of particles have a diameter lower than that value; P10 is the value of the diameter such that 10% of the total volume of particles have a diameter lower than that value and P50 is the value of the diameter such that 50% of the total volume of particles have a diameter lower than that value.

The alkyl-Al compound (b) can be preferably selected from the trialkyl aluminum compounds such as for example trimethylaluminum (TMA), triethylaluminum (TEAL), triisobutylaluminum (TIBA), tri-n-butylaluminum, tri-n-hexylaluminum, tri-n- octylaluminum. Also alkylaluminum halides and in particular alkylaluminum chlorides such as diethylaluminum chloride (DEAC), diisobutylalumunum chloride, Al- sesquichloride and dimethylaluminum chloride (DMAC) can be used. It is also possible to use, and in certain cases preferred, mixtures of trialkylaluminum's with alkylaluminum halides. Among them mixtures between TEAL and DEAC are particularly preferred. The use of TEAL and TIBA, alone or in mixture is also preferred.

An external electron donor compound can also be used particularly if needed in order to prepare polymer having narrower molecular weight distribution. It can be selected from the group consisting of ethers, esters, amines, ketones, nitriles, silanes and mixtures of the above. In particular, it can advantageously be selected from the C2-C20 aliphatic ethers and in particulars cyclic ethers preferably having 3-5 carbon atoms cyclic ethers such as tetrahydrofurane, dioxane.

In addition, the external electron donor compound can also be advantageously selected from silicon compounds of formula R_a ⁵R_b ⁶Si(OR⁷)_c, where a and b are integer from 0 to 2, c is an integer from 1 to 3 and the sum (a+b+c) is 4; R⁵, R⁶, and R⁷, are alkyl, cycloalkyl or aryl radicals with 1-18 carbon atoms optionally containing heteroatoms. Particularly preferred are the silicon compounds in which a is 0, c is 3, R⁶ is a branched alkyl or cycloalkyl group, optionally containing heteroatoms, and R⁷ is methyl. Examples of such preferred silicon compounds are cyclohexyltrimethoxysilane, t-butyltrimethoxysilane and thexyltr imethoxys ilane .

The above mentioned components (a), (b) and optionally (c) can be fed separately into the reactor where, under the polymerization conditions can exploit their activity. It may be advantageous the pre- contact of the above components, optionally in the presence of small amounts of olefins, for a period of time ranging from 0.1 to 120 minutes preferably in the range from 1 to 60 minutes. The pre-contact can be carried out in a liquid diluent at a temperature ranging from 0 to 90°C preferably in the range of 20 to 70°C.

Although it is in principle possible to produce UHMWPE with several types of processes including gas-phase polymerization, the preferred polymerization technique is the slurry type polymerization which is carried out using as suspension medium, one can use an aliphatic, cycloaliphatic, or aromatic hydrocarbon solvent, such as n-heptane, pentane, hexane, or toluene, for example.

Preferably, the ethylene pressure ranges from 5 to 20 atm, while the polymerization temperature is from 50-100°C in correspondence with residence time in the polymerization reactor ranging from 1 to 5 hours.

Examples of ultra high molecular weight ethylene polymers which can be obtained with the catalysts obtained from the catalyst components of the present invention are, besides the homopolymers, the ethylene copolymers with small quantities of C3-C₁₀ alpha -olefins, such as propylene, 1-butene, 1-hexene, 4- methyl- 1-pentene, 1-octene.

As previously stated, said polymers and copolymers with a very high molecular weight are characterized by an intrinsic viscosity [ eta ] in tetraline at 135 °C, which is higher than or equal to 10 dl/g, preferably from 1 1 to 25 dl/g, and more preferably from 12 to 20 dl/g. The comonomers can be added also in the liquid state. As already mentioned, the process is carried out in the substantial absence of hydrogen as a chain transfer agent or molecular weight regulators. Other chemicals which, to a lower extent, may act as chain transfer agents are also not desired but nevertheless be present as impurities or in other forms in the polymerization process. If desired, two or more polymerization stages working under different polymerization conditions, for example temperature and hydrogen concentration, UHMWPE produced with the present process can suitably be used in the production of sheets, plates, bars, rods, tubes and extruded profiles through compression molding or ram extrusion. In some cases also injection molding techniques can be used.

The following examples are given in order to further describe the present invention in a non- limiting manner.

CHARACTERIZATION

The properties are determined according to the following methods: Intrinsic viscosity measured in tetraline at 135°C.

Average Particle Size of the adduct and catalysts

Determined by a method based on the principle of the optical diffraction of monochromatic laser light with the "Malvern Master Sizer 2000" apparatus. The average size is given as P50.

Porosity and surface area with nitrogen: are determined according to the B.E.T. method (apparatus used SORPTOMATIC 1900 by Carlo Erba).

Porosity and surface area with mercury:

The measure is carried out using a "Porosimeter 2000 series" by Carlo Erba.

The porosity is determined by absorption of mercury under pressure. For this determination use is made of a calibrated dilatometer (diameter 3 mm) CD₃ (Carlo Erba) connected to a reservoir of mercury and to a high- vacuum pump (1 -10 -^"2 mbar). A weighed amount of sample is placed in the dilatometer. The apparatus is then placed under high vacuum (<0.1 mm Hg) and is maintained in these conditions for 20 minutes. The dilatometer is then connected to the mercury reservoir and the mercury is allowed to flow slowly into it until it reaches the level marked on the dilatometer at a height of 10 cm. The valve that connects the dilatometer to the vacuum pump is closed and then the mercury pressure is gradually increased with nitrogen up to 140 kg/cm . Under the effect of the pressure, the mercury enters the pores and the level goes down according to the porosity of the material.

The porosity (cm³/g), both total and that due to pores up to Ιμηι, the pore distribution curve, and the average pore size are directly calculated from the integral pore distribution curve which is function of the volume reduction of the mercury and applied pressure values (all these data are provided and elaborated by the porosimeter associated computer which is Bulk density: DIN-53194

Porosity and surface area with nitrogen: are determined according to the B.E.T. method

(apparatus used SORPTOMATIC 1900 by Carlo Erba).

Effective density: ASTM-D 1505

General procedure for the HDPE polymerization test

Into a 4.5 liters stainless steel autoclave, degassed under N₂ stream at 70°C, 1.6 liters of anhydrous hexane, 0.1 gr of catalyst component and the amount of triethylaluminum (TEAL) reported in table 1 were introduced. The whole was stirred, heated to 75 °C and thereafter 7 bar of ethylene were fed. The polymerization lasted 2 hours during which ethylene was fed to keep the pressure constant. At the end, the reactor was depressurized and the polymer recovered was dried under vacuum at 60°C.

EXAMPLE 1

Preparation of the spherical MgCh-EtOH adduct

A magnesium chloride and alcohol adduct containing about 3 mols of alcohol having spherical form and average size of about 9 μηι was prepared following the method described in example 3 of EP 1673157 using an molten adduct/mineral oil weight feeding ratio of 0.06.

The spherical support, underwent a thermal treatment, under N₂ stream, over a temperature range of 50-150°C until spherical particles having a residual ethanol content of about 35% (1.1 mole of ethanol for each MgCl₂ mole) were obtained.

Into a 2 1 glass reactor provided with stirrer, were introduced 1L of T1CI4, 70 g of the support prepared as described above and, at temperature of 0°C, 3,6 ml of 1 ,2- dimethoxypropane (1,2DMP) (Mg/DMP = 16 mol/mol). The whole mixture was heated and kept under stirring for 60 minutes at 100°C. After that, stirring was discontinued and the liquid siphoned off. Two washings with fresh hexane (1 liter) were performed at 60°C and then, other two more hexane washings were performed at room temperature. The spherical solid component was discharged and dried under vacuum at about 50°C.

The final solid composition is below reported:

Total titanium 6 % (by weight)

Mg 17.7 % (by weight)

1,2-DMP 2.7 % (by weight)

Its porosity measured according to the method reported in the description was 0.5 cm /g. The catalyst component was used in the polymerization of ethylene according to the conditions in Table 1 where also the results are shown.

EXAMPLE 2 Into a 2 1 glass reactor provided with stirrer, were introduced 0.86L of TiC , 60 g of the support prepared as described in example 1 , above and, at temperature of 0°C, 1.6g of 1,2- dimethoxypropane (1,2DMP) and 10.5g of hafnium tetrachloride. The whole mixture was heated and kept under stirring for 60 minutes at 100°C. After that, stirring was discontinued and the liquid siphoned off. Two washings with fresh isohexane (0.86L) were performed at 55°C and then, other two more hexane washings were performed at room temperature. The spherical solid component was discharged and dried under vacuum at about 50°C.

The composition of the solid was the following:

Ti 4.0 % (by weight)

Mg 15.2 % (by weight)

CI 59.3 % (by weight)

Hf 10.0 % (by weight)

1,2-DMP 0.7 % (by weight)

The catalyst component was used in the polymerization of ethylene according to the conditions in Table 1 where also the results are shown.

Example 3

Preparation of the solid component

The synthesis of the precursor was performed as described in Example 1 of

USP4,220,554. The so obtained support had the following composition:

Mg, 20.2 wt.%

CI, 29.8 wt.%

EtO groups 41.5 wt.%

In a 500 cm³ four-necked round flask equipped with a mechanical stirrer and purged with nitrogen, 220 cm of T1CI4 were charged. The temperature was set at 0°C and 15.3 g (127 mmoles. of Mg) of the solid support were slowly fed. Then, 1,2-dimethoxypropane (1,2DMP) was added into the reactor in such an amount to get Mg/1.2DMP = 16 mol/mol. The temperature was raised to 100 °C and the mixture was stirred for 2 hours. Then, the stirring was discontinued, the solid product was allowed to settle and the supernatant liquid was siphoned off. The solid was washed twice with anhydrous heptane (2 x 100 cm³) at 40°C and twice at 25°C, recovered, dried under vacuum and analyzed.

The composition of the solid was the following: Ti 4.4% (by weight)

Mg 18.0% (by weight)

CI 60.0% (by weight)

1,2-DMP 4.3% (by weight)

EtOH 7.0% (by weight)

Example 4

Preparation of the solid component

The synthesis of the precursor was performed as described in Example 1 of

USP4,220,554. The so obtained support has the following composition:

Mg, 20.2 wt.%

CI, 29.8 wt.%

EtO groups 41.5 wt.%

Into a 2 1 glass reactor provided with stirrer, were introduced 1.2 1 of titanium tetrachloride and 60g of the support prepared as described above and, at temperature of 0°C, 42g of hafnium tetrachloride. The whole mixture was heated and kept under stirring for 60 minutes at a 100°C. After that stirring was discontinued and the liquid siphoned off.

After 2 washing steps with anhydrous isohexane at 55°C and other 3 washes at room temperature more, the solid component was recovered (120g).

After drying under vacuum at about 50°C, the solid showed the following characteristics:

Ti 4.8% (by weight)

Mg 9.0% (by weight)

CI 51.8 % (by weight)

Hf 18.7 % (by weight)

Solvent 3.7% (by weight)

The catalyst component was used in the polymerization of ethylene according to the conditions and giving the results shown in Table 1.

Comparison example 1. A catalyst prepared according to the procedure described in Example 1 of EP523657 was used under the same polymerization conditions. The data are reported in Table 1.

Comparison example 2

A catalyst prepared according to the procedure same procedure described in example 3 but omitting the use of the electron donor of formula (I) was used under the same polymerization conditions. The data are reported in Table 1.

TABLE 1

Claims

1. A process for the preparation of an ethylene polymer having intrinsic viscosity measured in tetrahydro naphthalene at 135°C higher than 10 dl/g, comprising (co)polymerizing ethylene in the substantial absence of hydrogen as a chain transfer agent and in the presence of a catalyst system comprising the product obtained by contacting:

(a) a solid component having average particle size 5-20 μηι comprising Ti, Mg, halogen, and an electron donor compound selected from the group consisting of 1 ,2- diethers, mono or diesters of 1,2-dihydroxy compounds, and monoethers-monoesters of 1,2-dihydroxy compounds with

(b) an aluminum alkyl compound.

2. Process according to claim 1 in which the electron donor compound is selected from those belonging to formula (I) below

RRiC(OR₄)-CR₂R3(OR₅) (I) in which R, Ri, R₂ and R₃ are, independently, hydrogen or C1-C20 hydrocarbon groups which can also be condensed to form a cycle, R4 and R5 are C1-C20 alkyl groups, or ReCO- groups where R6 is a C1-C20 alkyl or aryl group, or they can be joined with R and R₃ respectively to form a cycle; said R to R6 groups possibly containing heteroatoms selected from O, Si, halogens, S, N and P.

3. The process according to claim 2 in which in the electron donor compound of formula (I), R, R4 and R5 are methyl.

4. The process according to claim 3 in which Ri to R₃ are hydrogen.

5. The process according to claim 2 in which R4 and R5 are linear, branched or cyclic C1-C5 alkyl groups.

6. The process according to claim 2 in which the ReCO- groups are acetyl and benzyl.

7. The process according to claim 2 in which the average particle size of component (a) is from 6 to 15 μηι.

8. The process according to claim 2 in which the solid catalyst component (a) is in a substantially spherical form.

9. The process according to claim 2 in which the electron donor compound of formula (I) is present in an amount ranging from 0.1 to 20%wt based on the total weight of component (a).

10. The process according to claim 1 in which the average particle size of component (a) is from 6 to 15 μηι.

1 1. The process according to claim 1 in which in the solid catalyst component (a) at least 70% of the titanium atoms are in the +4 valence state.

12. The process according to claim 1 in which in the solid catalyst component (a) has porosity (P_F) higher than 0.3 cm³/g.

13. The process according to claim 1 in which in the solid catalyst component (a) has a surface area determined by BET method lower than 100 m²/g.

14. The process according to claim 1 in which the alkyl-Al compound (b) is selected from the trialkyl aluminum compounds.