Catalyst precursor and catalyst for the high-temperature (co)polymerization of alpha-olefin

Abstract

A precursor for the formation of catalysts for the (co)polymerization of -olefins, comprising titanium, magnesium, at least one metal selected from hafnium and zirconium, aluminum and chlorine, obtained with a process comprising treatment with a siloxane compound. Said solid precursor, used in combination with a suitable co-catalyst in high-temperature (co)polymerization processes of -olefins, shows an improved productivity, a high incorporation of co-monomers in the copolymerization of ethylene and an increased thermal stability with respect to the systems so far in use.

Claims

1. A catalyst precursor for the (co)polymerization of alpha-olefins, comprising titanium, magnesium, aluminum, chlorine and at least one metal M selected from hafnium and zirconium characterized in that it is obtained by means of a process comprising the following steps: (i) heating a mixture in a liquid hydrocarbon solvent comprising a magnesium chloride, a compound of titanium, a compound of said metal M, a carboxylic acid RCOOH, wherein R is an organic group having from 2 to 30 carbon atoms, in such quantities as to respect the following atomic or molar ratio ranges: M/Ti=0.2-5.0; Mg/Ti=3.0-20.0; RCOOH/(Mg+M)=1-8 at a temperature ranging from 50 to 200 C. for at least one minute and separating the possible solid residue remaining undissolved, to obtain a solution; (ii) adding to the solution obtained in step (i), an aluminum alkyl chloride having the following general formula (I):
AlR.sub.nCl.sub.(3n)(I) wherein: R is a linear or branched alkyl radical, containing from 1 to 20 carbon atoms, and n is a decimal number having values ranging from 0.5 to 2.5, in an amount sufficient for precipitating in the form of a solid compound at least 70% of the metals Mg, M and Ti present in said solution, and heating the mixture thus obtained to a temperature ranging from 40 to 130 C. for a time ranging from 5 to 240 minutes to obtain a solid precipitate comprising Mg, M, Al and Cl in atomic ratios with respect to the Ti within the following ranges: M/Ti=0.2-5.0; Mg/Ti=3.0-15.0; Al/Ti=0.1-4.0; Cl/Ti=15.0-60.0; (iii) separating the solid precipitate thus formed from the residual liquid solution; and (iv) putting said solid precipitate obtained in step (iii) in contact with a siloxane compound A, having from 2 to 40 carbon atoms and from 1 to 15 silicon atoms, comprising at least one siloxane group selected from the following formulae:
OSiC and SiOSi, in such an amount that the atomic ratio Si/Ti between the Si atoms in the siloxane compound A and the Ti atoms in the solid precipitate, is higher than or equal to 0.1 to obtain said catalyst precursor.

2. The catalyst precursor according to claim 1, characterized in that at the end of step (iv) it is obtained in the form of a suspension in a liquid hydrocarbon that optionally contains an excess amount of siloxane compound A.

3. The catalyst precursor according to claim 1, wherein titanium, magnesium, said metal M, aluminum and chlorine form at least 80% by weight of the catalyst.

4. The catalyst precursor according to claim 1, having a particle-size with a Gaussian distribution having a maximum ranging from 2 to 15 m, and granule dimensions which are such that 80% by weight of the same ranges from 1 to 30 m.

5. The catalyst precursor according to claim 1, wherein the titanium is in an amount of up to 10% by weight with respect to the total weight of the solid.

6. The catalyst precursor according to claim 1, wherein in step (i) of the preparation process, the molar ratio RCOOH/(Mg+M) ranges from 1.5 to 5.0.

7. The catalyst precursor according to claim 1, wherein in step (i) of the preparation process, the solid residue remaining undissolved is equal to or lower than 30% by weight with respect to the total weight of the metallic compounds of the mixture which are insoluble as such in the liquid hydrocarbon solvent at room temperature.

8. The catalyst precursor according to claim 1, wherein in step (i) of the preparation process, the heating of the mixture is effected in a closed container or under reflux conditions of the solvent.

9. The catalyst precursor according to claim 1, wherein in said preparation step (ii), the addition of the aluminum alkyl chloride having formula (I) to the solution obtained in step (i) is effected so that the temperature of the reaction mixture is not higher than 45 C.

10. The catalyst precursor according to claim 1, wherein at the end of the separation step (iii), the solid precipitate is subjected to washings with a hydrocarbon solvent until a molar concentration of aluminum in the hydrocarbon solvent lower than 1.5 mM, is reached.

11. The catalyst precursor according to claim 1, wherein said step (iv) is carried out at a temperature ranging from 10 to 120 C.

12. The catalyst precursor according to claim 1, wherein said siloxane compound A, in the preparation step (iv), comprises from 1 to 10 O atoms.

13. The catalyst precursor according to claim 1, wherein said siloxane compound A, in the preparation step (iv), is selected from those included in the following formulae (II) and (III):
SiR.sub.p(OR).sub.(4p)(II)
T.sub.1(Si(R.sup.5).sub.2O).sub.qT.sub.2(III) wherein each R is independently a linear, cyclic or branched alkyl group, having from 1 to 10 carbon atoms, possibly halogenated or an aryl group having from 6 to 10 carbon atoms, possibly halogenated, or an alkyl silyl group having the formula SiR.sub.3; each R is independently H, a halogen or an alkyl or aryl group included in the definition of the previous R; each R.sup.5 is independently H, a halogen, preferably chlorine, or an alkyl or aryl group included in the definition of the previous R, or an alkoxyl or aryloxy groups having the formula OR, wherein R is as previously defined, T.sub.1 can have any of the meanings of R.sup.5; T.sub.2 can have any of the meanings of R; p is an integer ranging from 0 to 3; q is an integer ranging from 2 to 15.

14. A catalyst for the (co)polymerization of alpha-olefins, comprising, in contact with each other, a cocatalyst consisting of a hydride or organometallic compound of a metal of groups 1, 2 or 13 of the periodic table, and a catalyst precursor according to claim 1.

15. The catalyst according to claim 14, wherein said cocatalyst is selected from aluminum trialkyls which contain from 1 to 10 carbon atoms in each alkyl radical.

16. The catalyst according to claim 15, wherein the atomic ratio between the aluminum (in the cocatalyst) and the titanium (in the precursor) is within the range of 1:1 to 500:1.

17. A process for the (co)polymerization of alpha-olefins, comprising polymerizing at least one alpha-olefin, either in continuous or batchwise, in one or more steps at low (0.1 -1.0 MPa), medium (1.0 -10 MPa) or high (10-150 MPa) pressure, at temperatures ranging from 20 to 300 C. in the presence of a catalyst according to claim 14.

18. The process according to claim 17, wherein at least one alpha-olefin is ethylene.

19. The (co)polymerization process according to claim 17, characterized in that it is carried out in an inert solvent solution, at temperatures ranging from 130 to 300 C. and at pressures ranging from 1 to 25 MPa.

20. The (co)polymerization process according to claim 19, wherein the polymerization temperature ranges from 160 to 260 C.

21. The process according to claim 17, wherein the process is carried out in an inert solvent.

Description

EXAMPLES

(1) Reagents and Materials

(2) The reagents and materials used in the following examples of the invention, and their possible pretreatment, are indicated in the following list; the supplier is indicated in brackets Titanium tetrachloride (Aldrich, 99.9%): distilled n-hexane Synthesis (Synthsol LP6 purity 95%) anhydrified on mixed bed columns with molecular sieves 4A/13 n-Decane Synthesis (Synthsol LP10, 95%): anhydrified on mixed bed columns with molecular sieves 4A/13 Triisobutylaluminium (TIBAL) (Chemtura pure): used as such Triethylaluminium (TEA) (Chemtura, pure): used as such Diethylaluminiumchloride (DEAC) (Chemtura, pure): used as such Diisobutylaluminiumchloride (DIBAC): 97%, Chemtura Isobutylaluminiumdichloride (IBADIC): 99%, Chemtura 1-hexene: 97%, Aldrich, distilled on calcium hydride 1-octene: 98%, Aldrich, distilled on calcium hydride Ethylene: Air Liquide grade 4.5, Purity99.9% Anhydrous Magnesium Chloride (Cezus-Areva): >99%, grade T.202, used as such Titanium Tetrabutylate (ACROS): purity >99%, used as such Hafnium Tetrachloride (ACROS): >95%, grade 101, used as such 2-ethylhexanoic acid: (BASF): anhydrified on molecular sieves 4A Diisopropyldimethoxysilane (Eurenor 5021, Chemtura), used as such.

(3) Elemental Analysis

(4) a) Determination of Mg, Al, Hf and Ti.

(5) For the determination of the quantity by weight of the metals Mg, Al, Hf and Ti, in the precipitates and solid precursors of the present invention, an aliquot weighed exactly, operating in a dry-box under a flow of nitrogen, of about 30-50 mg of sample, was placed in a platinum crucible of about 30 ml, together with a mixture 0.25 ml of H.sub.2SO.sub.4 at 96% and 1 ml of HNO.sub.3 at 70%. The crucible was then heated on a plate increasing the temperature until the appearance of white sulphuric fumes (about 200 C.). The mixture thus obtained was cooled to room temperature, 1 ml of HNO.sub.3 at 70% was added and the mixture was then brought again to the appearance of fumes. After repeating the sequence a further two times, a limpid, almost colourless solution was obtained. 1 ml of HNO.sub.3 and about 15 ml of water were then added without heat, heating to 80 C. for about 30 minutes. The sample thus prepared was diluted with water having a MilliQ purity up to a weight of about 50 g, weighed exactly, to obtain a solution on which instrumental analytic determination was effected using a Perkin Elmer OPTIMA3200RL ICP-OES (optical detection plasma) spectrometer, for comparison with solutions at a known concentration. For this purpose, for each analyte, a calibration line within the range of 0-10 ppm was prepared, measuring solutions with a known titre obtained by dilution by weight of certified solutions.

(6) The solution of the sample prepared as described above was diluted again by weight so as to obtain concentrations close to those used as reference, before effecting spectrophotometric analysis. All the samples were prepared in duplicate. The results were considered acceptable if the single data of the tests in duplicate did not differ by more than 2% relative with respect to their average value.

(7) b) Chlorine Determination

(8) Approximately 30-50 mg of sample were weighed exactly in 100 ml glasses in a dry-box under a stream of nitrogen. 2 g of Na.sub.2CO.sub.3 were added and 50 ml of MilliQ water were added, outside the dry-box. The mixture was brought to boiling point on a plate under magnetic stirring for about 30 minutes. It was left to cool, diluted H.sub.2SO.sub.4 1/5 was added until the reaction became acid and the mixture was titrated with AgNO.sub.3 0.1N with a potentiometer titrimeter.

(9) Granulometric Analysis

(10) The determination of the average particle size and distribution of the catalytic solids was effected with the optical method using a MALVERN MASTERSIZER2000 instrument.

(11) Characterization of the polymers and copolymers The Melt Flow Index, MFI of the polymers was measured according to the standard ASTM D-1238E, with a weight of 2.16 kg. The so-called shear sensitivity (SS) was also determined with the same equipment, as a ratio between MFI with a weight of 21.6 kg and MFI with a weight of 2.16 kg.

(12) The density of the polymeric products obtained was measured by means of a gradient column, according to the method ASTM D1505-68.

(13) The average molecular weights of the olefinic polymers Mn and Mw and the relative distribution MWD, were determined by means of Gel-Permeation Chromatography (GPC), using a WATERS 150-CV chromatograph with a Waters differential refractometer as detector, eluting with 1,2,4-trichlorobenzene (stabilized with Santonox) at 135 C. A set of -Styragel HT columns (Waters) were used of which three with a pore dimension of 10.sup.3, 10.sup.4, 10.sup.5 respectively, and two with a pore dimension of 10.sup.6 , establishing a flow-rate of the eluent of 1 ml/min. The data were acquired and processed by means of Maxima 820 software version 3.30 (Millipore); the calculation of the number average molecular weights (Mn) and weight average molecular weights (Mw) was effected by means of universal calibration, selecting standards of polystyrene with molecular weight within the range of 6,500,000-2,000, for the calibration.

Example 1

(14) The following products are charged in order into a 1,000 ml Buchi glass reactor containing 200 ml of n-decane: 3.20 g (33.6 mmoles) of MgCl.sub.2, 0.96 g (2.82 mmoles) of Ti(n-OBu).sub.4 and 1.18 g (3.68 mmoles) of HfCl.sub.4.

(15) 16.12 g (111.8 mmoles, 17.8 ml) of 2-ethylhexanoic acid (EEA), equal to a molar ratio EEA/(Mg+ Hf)=3, are subsequently added, at room temperature, slowly and under stirring.

(16) The suspension thus obtained is heated to 90 C. and maintained at this temperature for 30 minutes in a closed vessel. A slightly opalescent light yellow-green-coloured solution is obtained, which is then filtered under heat on a filtering septum, leaving a residue of 0.67 g (7.03 mmoles) of MgCl.sub.2 equal to 20.9% of the initial MgCl.sub.2. A solution of 28.8 g (185.8 mmoles) of isobutylaluminium dichloride (IBADIC) in 92 ml of n-decane is added dropwise to the limpid solution thus obtained, cooled to room temperature. During the addition of IBADIC, the temperature of the reaction mixture does not exceed 40 C. The ratio between the chlorine atoms in the IBADIC and alkoxide and carboxylic groups initially introduced is equal to 3. The reaction mixture thus formed is heated to 80 C. and kept at this temperature for 2 hours.

(17) The pink-brown-coloured solid precipitate obtained is separated from the mother liquor by decanting, subsequently washed with three aliquots, each of 200 ml, of n-hexane, and dried under vacuum at room temperature.

(18) 4.35 grams of solid precipitate (PREP 1) are obtained, having the following composition (weight % with respect to the total weight of the solid): titanium: 2.95% by weight magnesium: 12.09% by weight hafnium: 13.70% by weight aluminium: 2.85% by weight chlorine: 62.82% by weight zirconium: 0.2% by weight organic fraction: 5.39% by weight (containing about 80% by weight of residues of 2-ethylhexanoic acid).

(19) The preparation yield of Ti with respect to the Ti initially introduced is 95% by weight. The other metals in the solid precipitate thus obtained are present in the following molar proportions with respect to the Ti:

(20) Ti.sub.1Mg.sub.8.08Hf.sub.1.25Al.sub.1.71Cl.sub.28.77Zr.sub.0.04

(21) The small quantity of Zr found analytically derives from impurities contained in the compound HfCl.sub.4 used as reagent.

(22) 1.015 grams of the solid precipitate are suspended in 50 ml of anhydrous n-decane in a graded tailed test-tube, under a nitrogen flow. 1 ml of a solution at 0.31 M of diisopropyldimethoxysilane in n-decane (55 mg of siloxane compound, equal to 0.31 mmoles) are added to the suspension thus formed, with a Si/Ti atomic ratio equal to 0.5. The mixture is kept under stirring for 30 minutes at room temperature. The titre in titanium is determined on a 5 ml aliquot of the dried suspension, using the elemental analysis method described above. The suspension of the catalyst precursor thus obtained (according to the present invention), having a concentration of Ti equal to 0.59 g/l, is used as such in the subsequent polymerization tests.

Example 2

(23) 50 ml of anhydrous n-decane and 1.565 grams of the solid precipitate obtained as described in the previous Example 1, are charged into a tailed test-tube, under a nitrogen flow. 0.77 ml of a solution at 0.31 M of diisopropyldimethoxysilane in n-decane (equal to 42.3 mg, 0.24 mmoles of siloxane compound), are added to the suspension thus formed, with a Si/Ti atomic ratio equal to 0.25. The mixture is kept under stirring for 30 minutes at room temperature. The suspension of the catalyst precursor thus obtained (according to the invention), having a concentration of Ti equal to 0.91 g/l, is used as such in the subsequent polymerization tests.

Example 3

(24) The following products are charged in order into a 10 l Borosilicate glass reactor containing 2 l of n-decane: 32 g (336 mmoles) of MgCl.sub.2, 9.6 g (9.7 ml, 28.2 mmoles) of Ti(n-OBu).sub.4 and 12.91 g (40.3 mmoles) of HfCl.sub.4.

(25) 162.79 g (1129 mmoles, 179.7 ml) of 2-ethylhexanoic acid (EEA), equal to a molar ratio EEA/(Mg+Hf)=3, are subsequently added, at room temperature, slowly and under stirring.

(26) The suspension thus obtained is heated to 90 C. and maintained at this temperature for 30 minutes in a closed vessel. A slightly opalescent light yellow-green-coloured solution is obtained, which is then filtered under heat on a filtering septum, leaving a residue of 6.9 g (72.5 mmoles) of MgCl.sub.2 equal to 21.6% of the initial MgCl.sub.2. A solution of 288.7 g (1.862 mmoles) of isobutylaluminium dichloride in 950 ml of n-decane is added dropwise and under vigorous stirring, to the limpid solution thus obtained, cooled to room temperature. The addition phase of IBADIC lasts about 2 hours and the temperature is maintained at values ranging from 30 to 40 C. The ratio between the chlorine atoms in the isobutylaluminium dichloride and the alkoxide and carboxylic groups initially introduced is equal to 3.

(27) The reaction mixture thus formed is heated to 80 C. and kept at this temperature for 2 hours. The solid precipitate in suspension thus obtained is separated from the mother liquor by decanting. The suspension is left to rest for about 30 minutes and the overlying clear liquor is separated. A volume of 3 liters of fresh n-decane is then added, stirring the mixture until a homogeneously dispersed suspension is re-formed. A second decanting is then effected after a rest time of about 60 minutes. The precipitate is subsequently filtered, washed with three aliquots, each of 2 l, of n-hexane, and dried under vacuum at room temperature. The concentration of IBADIC in the mother liquor at the third washing was lower than 0.5 mmoles/liter.

(28) 42.65 grams of solid precipitate (PREP-2) are obtained, having the following composition (weight % with respect to the total weight of the solid): titanium: 3.04% by weight magnesium: 12.9% by weight hafnium: 15.70% by weight aluminium: 2.88% by weight chlorine: 61.18% by weight zirconium: 0.3% by weight organic fraction: 4.00% by weight (containing about 80% by weight of residues of 2-ethylhexanoic acid).

(29) The synthesis yield of Ti with respect to the Ti initially introduced is 96% by weight. The other metals in the solid precipitate thus obtained are present in the following molar proportions with respect to the Ti:

(30) Ti.sub.1Mg.sub.8.36Hf.sub.1.39Al.sub.1.68 Cl.sub.27.19Zr.sub.0.05

(31) The small quantity of Zr found analytically derives from impurities contained in the compound HfCl.sub.4 used as reagent.

(32) The granulometric analysis effected on a portion of the solid precipitate showed an average dimension of the granules of 5.1 m, and a distribution which is such that 80% by weight of the granules has a dimension ranging from 2.5 to 13.1

(33) 1.355 grams of said solid residue PREP-2 in 50 ml of anhydrous n-decane are charged into a graded tailed test-tube, under a nitrogen flow. 1.39 ml of a solution at 0.31 M of diisopropyldimethoxysilane in n-decane (equal to 76 mg, 0.43 mmoles, of siloxane compound) are added to the suspension thus formed, with a Si/Ti atomic ratio equal to 0.5. The mixture is left under stirring for 30 minutes at room temperature.

(34) The suspension of the catalyst precursor (according to the present invention) thus obtained, having a concentration of Ti equal to 0.8 g/l, is used as such in the subsequent polymerization tests.

Example 4

(35) Solid catalyst precursor obtained by means of two consecutive treatments with siloxane (according to the invention).

(36) 50 ml of anhydrous n-decane and 1.184 grams of the solid precipitate PREP-1, obtained as described in the previous example 1, are charged into a tailed test-tube, under a nitrogen flow. 1.20 ml of a solution at 0.31 M of diisopropyldimethoxysilane in n-decane (for 66 mg of siloxane compound, equal to 0.37 mmoles) are added to the suspension thus formed, with a Si/Ti atomic ratio equal to 0.5. The mixture is left under stirring for about 30 minutes, and a further 1.20 ml of the solution of diisopropyldimethoxysilane in n-decane (for a further 66 mg of siloxane) are added again, with a final overall ratio between silicon atoms and titanium atoms equal to 1.0. The reaction mixture is kept under stirring for a further 30 minutes at room temperature. The suspension of the catalyst precursor (according to the invention) thus obtained contains 0.66 g/l of Ti and is used as such in the subsequent polymerization tests.

Example 5

(37) The preparation with two consecutive treatments of siloxane compound A according to the previous Example 4, is repeated, but using 1.05 grams of the solid precipitate PREP-2, obtained as described in the previous example 3. The suspension of the catalyst precursor (according to the invention) thus obtained contains 0.61 g/l of Ti and is used as such in the subsequent polymerization tests.

Example 6

Copolymerization of Ethylene and 1-hexene to LLDPE in an Adiabatic Batch Reactor

(38) A vacuum-nitrogen flushing is exerted for at least three times and for an overall duration of about 2 hours in a 5-liter steel autoclave, of the Brignole type, equipped with a pressurized burette for the addition of the catalyst, a propeller stirrer and a heating thermoresistance connected to a thermostat for the temperature control. The following products are introduced in order into the autoclave: 2 l of anhydrous n-decane, 60 ml (40 g, 0.476 moles) 1-hexene, 1.0 ml of a solution at 1M of TIBAL (1.0 mmoles) in n-decane as cocatalyst. The temperature inside the reactor is brought to 209.4 C. and the reactor is pressurized with ethylene until a stable pressure of 14.5 MPa is reached at equilibrium. Under these conditions, a total of 46.3 g of ethylene are charged into the autoclave, subdivided into gaseous phase and ethylene dissolved in n-decane, as can be calculated on the basis of the known physico-chemical parameters of the system.

(39) 2.55 ml of the suspension of solid catalyst precursor obtained according to the previous Example 1, corresponding to 1.5 mg (0.0313 mmoles) of titanium (atomic ratio Al.sub.TIBAL/Ti=32), are collected and introduced into the pressurized burette together with about 5 ml of decane. The precursor suspension is charged into the autoclave, applying a slight overpressure of ethylene.

(40) The heating of the thermoresistance is interrupted and a temperature increase is observed due to the exothermicity of the polymerization reaction under pseudoadiabatic conditions, i.e. without removing the heat produced by the polymerization with cooling means. The entity of the temperature variation (T) can be directly correlated to the quantity of ethylene polymerized and is proportional to the catalytic activity obtained. The polymerization is continued for 5 minutes, without feeding further ethylene, and the reaction is then interrupted by the introduction of about 20 ml of ethanol into the autoclave. It is observed that the temperature reaches a peak of 217.8 C., with a T equal to 8.4 C., then dropping to 217.6 C. when the reaction is interrupted, with a pressure reduced to 4.8 MPa. On the basis of these data, a production of 45 g of ethylene copolymer (LLDPE polyethylene) can be calculated, equal to a conversion of 97% of the ethylene initially introduced. Under the above-mentioned adiabatic process conditions, an activity of 30 kg of polyethylene per gram of Ti, is obtained.

(41) The mixture is left to cool and the contents of the reactor are then discharged into about 3 liters of ethanol. The polymer is separated by filtration, washed with acetone and dried in an oven under vacuum (about 100 Pa) at 90 C. for about 12 hours. The dried polymer is analyzed to determine the molecular weights, the Melt Flow Index and density, according to the methods previously indicated.

(42) The polyethylene thus obtained has the following properties: melt flow index (2.16 Kg)=0.34 dg/min; shear sensitivity (SS)=32.5; density=0.919 g/ml; Mn=43,000 D; Mw=186,100 D; Mw/Mn=4.33

(43) The results are schematically summarized in Table 1 below.

Examples 7 to 10 and Comparative Example 11

(44) Some copolymerization tests were carried out under similar conditions to those previously specified in Example 5, but using each time the catalysts prepared according to the previous examples 1 to 4 and varying the quantity of 1-hexene. A polymerization test was also effected for comparative purposes with respect to Example 6, using, as solid catalyst component, the solid precipitate PREP-1, obtained as intermediate in the preparation of the catalyst precursor according to Example 1.

(45) The reaction conditions and results of each test are indicated in Table 1. The characterizations of the copolymers obtained are indicated in Table 2.

Examples 12 And 13 and Comparative Examples 14 and 15

(46) Two copolymerization tests were carried out with analogous procedures to those specified in the previous examples 6 and 7 and comparative example 11, but using 1-octene as comonomer instead of 1-hexene. The reaction conditions and results obtained are indicated in Table 1. The characterizations of the copolymers are indicated in Table 2.

Examples 16 And 17 and Comparative Example 18

(47) Some copolymerization tests were carried out with 1-hexene with analogous procedures to those specified in the previous examples, but using the catalyst precursors obtained on an industrial scale according to the previous examples 3 and 5. The reaction conditions and results obtained are indicated in Table 1. The characterizations of the copolymers are indicated in Table 2.

(48) Table Legend

(49) Table 1 indicates, for each example, whose number is specified in the first column on the left, in order: the preparation example of the solid catalyst precursor; the quantity of Ti contained in the solid precursor introduced into the reactor, in the column Ti; the quantity in grams of 1-hexene or 1-octene introduced into the reactor, in the column alpha-olefin; the quantity of ethylene charged into the reactor, in the column ethylene; the initial reaction temperature in the column T.sub.in; the increase in temperature observed during the polymerization, in the column T; the quantity of polymer obtained, in the column Yield; the percentage conversion, expressed as a ratio between the grams of polyethylene produced with respect to the total grams of ethylene present in the polymerization reactor, in the column Conversion; the activity of the catalyst expressed in kilograms of polymer per gram of Ti in the column Activity.

(50) Table 2 indicates, for each example, whose number is specified in the first column on the left, in order: the Melt Flow Index M.F.I. with a weight of 2.16 kg in the column MFI; the shear sensitivity SS, the density of the polymer obtained, the number average molecular weights M.sub.n and weight average molecular weights M.sub.w and the dispersion index M.sub.w/M.sub.n. Finally, the column of the activities obtained in each example, already indicated in Table 1, is repeated.

(51) From a comparison of the results in Tables 1 and 2 the increase in the average molecular weight M.sub.W together with the improved (lower) melt flow index, is evident, whereas the overall activity of the catalyst is substantially the same as that of the solid precipitate not treated with the siloxane compound. These results are an evident indication of a greater thermal stability of the active polymerization sites in the catalyst according to the present invention.

(52) TABLE-US-00001 TABLE 1 Ethylene/1-hexene and ethylene/1-octene copolymerization; TIBAL co-catalyst; P.sub.ethylene about 1.5 MPa, time = 5 minutes; Al.sub.TIBAL/Ti = 32 Precursor alpha-olefin Polymer. Ti Quantity Ethylene T.sub.in T Yield PE Activity Ex. Ex. Nr (mg) Type (g) calc. (g) ( C.) ( C.) (g) (kg.sub.PE/g.sub.Ti) 6 1 1.5 1-hexene 40 46.3 209.4 8.4 45.0 30.00 7 1 1.5 1- hexene 100 46.3 209.4 7.7 45.4 30.26 8 2 1.5 1- hexene 40 49.1 209.7 10.4 48.5 32.33 9 2 1.5 1 hexene 100 50.3 209.5 8.8 49.1 32.73 10 4 1.5 1- hexene 40 48.1 209.9 9.0 47.4 31.58 11(Comp) PREP-1 1.5 1- hexene 40 50.4 208.7 8.9 49.9 33.3 12 1 1.5 1-octene 40 49.8 211.0 10.4 49.0 32.66 13 1 1.5 1-octene 100 50.1 210.1 11.4 49.9 33.3 14(Comp) PREP-1 1.5 1-octene 40 52.4 210.4 11.5 52.2 34.78 15(Comp) PREP-1 1.5 1-octene 100 57.1 210.2 10.5 56.3 37.53 16 3 1.75 1- hexene 50 44.7 205.2 8.2 42.99 24.57 17 5 1.75 1- hexene 50 47.8 207.5 8.6 45.99 26.28 18(Comp) PREP-2 1.75 1- hexene 40 46.9 200.1 9.3 46.0 26.3

(53) TABLE-US-00002 TABLE 2 Characteristics of the copolymers Activity Den- Example (kg.sub.PE/g.sub.Ti) MFI SS sity M.sub.n M.sub.w M.sub.w/M.sub.n 6 30.00 0.34 32.53 0.919 43000 186100 4.33 7 30.26 0.54 31.08 0.906 39900 157800 3.95 8 32.33 0.37 33.51 0.919 44100 171000 3.88 9 32.73 0.78 31.67 0.903 41300 146400 3.54 10 31.58 0.22 30.43 0.920 39800 183700 4.62 11 (Comp) 33.3 0.84 32.17 0.914 39700 136100 3.43 12 32.66 0.39 31.6 0.913 42600 180000 4.23 13 33.3 0.84 35.8 0.900 40300 138600 3.44 14 (Comp) 34.78 0.59 32.05 0.919 36700 155000 4.22 15 (Comp) 37.53 0.95 33.76 0.905 37900 133000 3.51 16 24.57 0.65 33.5 0.924 33089 144600 4.37 17 26.28 0.70 30.9 0.925 35037 143300 4.09 18 (Comp) 26.3 0.73 32.2 0.926 31570 130700 4.14