CA1118747A - Ziegler catalytic system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Ziegler catalytic system, particularly suitable for the poly-merization of .alpha.-olefins at low pressure, formed by the combination of an organo-metallic compound and a compound of a heavy metal supported on a carrier, said carrier being a crystalline active complex obtained by reacting metallic magnesium, gaseous hydrogen chloride and one or more al-hols in the liquid or a gaseous state at a temperature of from 60° to 300°C, the said active complex having a content of free alcoholic hydroxyl groups of from 1 to 25%, by weight, a chlorine content of from 15 to 70% by weight, a magnesium, content of from 5 to 35% by weight, a porosi-ty of from 0.1 to 1 cc/g, a surface area of from 1 to 200 m2/g, and exhibiting x-ray diffraction bands at d=13.1 .ANG. and d-?.5 .ANG..

Description

111~3747 The present invention relates to Ziegler catalytic systems particularly suitabl~ for the homo- and co-polymeri-' zation, at low pressure~ of ~ -olefins.
i It is known that in similar processes the catalytic , system genera1ly consists of a Ziegler catalytic sy~tem formed by the cornbination of a transition metal compound and an organo-metallic compound.
I It is also known that t~e transition metal compound may be fixed to a support, such LS for example Al203, an alurnino-silicate, SiO2, MgO, MgC03 or Mg(OH)2. Processes of this type I j, have~ however, the con~iderable disadvantage of generally requiring a final washing of the polymers to purify the products obtained of harmful residue of the catalytio system, ¦ It is also known that halogenation (generally fluorination~
¦ of supports such as for example Al203, SiO2, MgO or SiO2. Al203 results in a considerable increase in the activity of the cayalytic system, ~uch that it becomes possible to avoid the costly operation o~ the final washing of the polymer9 as is ~cscribod. ~or example~ in the British Paten~ Specifiiration~s ~u Nos. i,314~7~4 and 1,315,770.
I These catalysts do not~ however9 allow a simple regulation ! . of the average molecular ~eight of the polymer. The polymers generally have a very high average molecular weight and a - Melt Index (measured by the ~STM D1238 method, with a weight of 2.16 Kg) very close to zero. Therefore they are often dif-ficult to process.
We have now found a support for the transition metal compound of the Zlegler catalytic system, which allows the I couplin~ of the advantages, and the elimination of the disad-vantages, of both types of catalysts which have been indicated above; there is thus obtained:
a) high catalytic activity in the polymerization at low pres~ures (~bout 5 ~g/cm ) and hence produc'i~ity ~uch that j it is possible to a~oid any ~ashing of the polymers obtained;
I b) easy contro~lability of the average mo]ecular weig~lt and the Melt Inde~ of the polymsrs, and also of their crystal-~ 8747 linity and density.
~ loreover, the process for obtaining the active complex which will be used as the support is Yery simple and erficient and results in a support which does not require those conven -` tional activation treatments which are usually carried out before contacting the support with the transition metal compound.
This support permits also, thanks to its accentuated ac tivity and to its excellent, particular state of subdivision, 1 ' 10 only the strictly stoichiometric quantity of the compound of I a heavy metal of group IV up to group VI to be used in the preparation of the catalytic complex, whereas an excess of I this compound is usually used in the known art.
I The invention provides a Ziagler catalytic system formed by the combinatioIL of an organo-metallic compound of a metal of Group I~ II or III of the Periodic System of the Elemen-ts -! according to Mendeleef and a compound of a heavy metal of Group IV, Y or YI of said Periodic System supported on a carrier~ ch~r~ctori~ed in ~h~t th~ ~rr;R~ i~ ~ ~r~c~lin~
i ~ ac~i~e cGmplex obtained by reacting metallic magneslum, gaseous hydrogen chloride and one or more aliphatic, cycloaliphatic, arvmatic or alk~laromatic alcohols in the llquid or gaseous state, at a temperature of from 600 to 300C, the said active complex having: `
(a) a content of free alcoholic hydroxyl groups of from 1 to

2~ by weight;
(b) a chlorine content of from 15 to 70~p' by weighti (c) a magnesium content of from 5 to 35~,' by weight;
(d) a porosity of from 0.1 to 1 cc/g; and (e) a surface area o r from 1 to 200 m /g.
~ lagnesium met~l may be used in the form of a powder, filings, turnings, etc. The gaseous hydrogen chloride may be either purc or mixed wit}l inert gases SUCll as H2, Nz~ He, etc.
The characteristics of the active complex are all original and typical. Thus~ for example, when using ethyl alcohol, there is obtailled ~n activc co~plex ha~ing the appearance of a ~ ite ------powder with a well-defined ~l-ain ~ize (of between 20 and 80 microns). ~nalysis of this product ~ith an X-ray diffrac-tometer re~realed the following characteristics:
1-) High crystallinity;
2) Characteristic dif~raction band~ at d - 13.1 A and d = 9.5 A~ These bands are not comparable with the diffrac-tion spectra ~ener~ted by the forms of anhydrous MgC12 known from the literature, nor from the spectra of the ethylate o~
Mg.
It therefore appears clear that~ under the reaction con-ditions of the present in~en~ion, there is obtained from the reaction Or metallic Mg~ gaseous HCl and an alcohol (in the I ~ascous or liquid state), and preferably in the presence of hydrogen~ a crystalline powder ha~ing its own well-de~ined lattice~ not known in the liter~ture.
It must be noted that a ~ide range of alcohols ~s usable for the resction. Good results haYe been obtained, fo~ example, as well as with ths ethyl alcohol cited above, also with methyl~
prop~l, a..~-l or ben~yl aicohol or ~ith p-nenols 2~ CreS015.
Ethyl alcohol or the branched aliphatic alcohols, in particular isopropyl alcohol~ are preferable.
~j It must be noted, moreover, that with a suitable choice of the alcohol, the obtaining of optimal values of the porosity ~ of the complexes produced may be fa~oured.
The reaction between the aforesaid reagents occurs rapidly and may be carricd out in the liquid phase or in the ~aseous phase (that is using the alcohol vapours).
I~ the reaction is carried out in the liquid phase, there may be added an inert diluent such as, for example, an anhydrous aliphatic~ cyclo aliphatic, aromatic, or alkylaromatic hydrocar-bon (less than 5 ppm of water) having a boilin~ point greater than or cqual to that of the alcohol used. Exa~ples of such hydrocarbons are hexane, heptane, cvclohexane, toluene, xylene etc. Hept~ne and cyclohexane are pre~erable.
It must be underlined that by choo3ing th~ reaction parameters suitably there may be obtained actlve complexes

-3-with the desircd characteristics, that ls to say the desired percentage by ~/eight of free a1coholic OM, of chlorine, of magnesium and tne desired porosity and surface area; and ha~ing X-ray diffraction spectra typical of crystalline sub-stances but different from one another.
Since both the specific activity of the final catalytic system texpressed as Xg of ~)lymer obtained / gram of transition metal/hour/atmosphere of ola.fin) and also the properties of the polymers produced, (such as average molecular weight, Mel-t Index, crystallinity, specific density~ etc), depend on the j~ total of these characteri~stics of the support it is clear that the reaction conditions for the prep~ration of the support willS from time to time~ be chosen in dependence on the charac-teristics which it is desired to-obtain in the final products.
The best values for the chemical and physical characteris-tics of the ncti~re complex are within the following ranges:
- fre~ alcDholic hydroxyl ~roup content of from 1 to 25~o by ~i~ht ~and preferably from 3 to 10% by weight);
- chlorine content of from 15~ to 70,% by wei~ht (and ~V prer~abiy frual 30-~ to ~ by weignt);
- ~agnesiu~ content of from 5~to 35% by ~eight (and pre~erably from 10~o to 25~ by weight);
- porosity of from 0.1 to 1 cc/g (and preferably from 0.3 to 0.7 cc/g);
- ~urface area of from 1 to 200 m /g (and preferably from 5 to 100 m /g).
~rom this it results that thè values usable for the main reaction parameters also lie within relatively wide ranges.
Th~ ratio ~etwaen equivalents of magnesium and moles of alcohol is generally fron~ 1:0~5 to 1:20, and preferably from 1:~ to 1:10~

If the reaction is carried out in the liquid phase, the quantity of hydrogen chloride used is preferably such a.s to satur~te the liquid reaction phase.
i If the reaction is carried out in the gas~ous phase 9 th~
hydrogen chloride/alcohol (a3 ~apour) molar ratio is generally !

:
87~7 from o.S:1 to 10:1, and preferably from 1:1 to 5:1.
The range of values of the reaction time~ usable in relat~vely wid0. These may vary from 15 to 250 minutes and preferably from 30 to 100 minutes.
The reaction temperature3 are very i~port~nt in that these i~fl1lence the basic characteristic~ of the active complex which i~ obtained, such as the percentage of free alcohol groups, the porosity and the surface area, in a marked manner. Also dependil1g on the type of alco~ol used, the tempe-rature may vary from 600 to 300C, and preferably from 80~ to 200C.
Reaction temperatures greater than 200C geller~lly resultin too low concentrations of the free alcohol groups in the active comple~es obtained, which then ha~e low c~talytic acti~ity.
`~ The reaction pressures do not however, constitute a criticai factor, even when the reactior. is carried out in the gaseous phase.
~t mu~t firal~y be in~ica~ed that~ w~le ~r the p~^pa ~tion in the gaseous phase, the reaction conditions are chosen so as to obtain the crystalline active complex directly in its ~inal state (solld and dry), in the preparation under "wet conditions" 7-the operating conditions chosen for solating the crystalline active complex (solid and dry) produced have a ~onsiderabla influence o~ its chemical and physical characteristics.
In particular, the time, th~ pressure and the temperature of evaporat;;on of the volatile components dete-;mine, in a sub-stantial manner~ the crystalline structure, the grain si~e, the porosity and the surface area of the crystalline active complex. Therefore, in the preparation in the liquid phas0, the removal from the active complex produced of the excess alcohol, and possible diluents, is conveni0ntly carried out by using an evaporation te~pe`rature of from 100 to 200 and prefer~bly from 130 to 180C, a pressure of from 1 atmosphere to 100 mm Hg and a time of from 1 to 10 hours, and preferably from 4 to 8 hours.

7~7 The catalyt~c systern is then prepQred by reaction of the support described above with a compound o~ a heavy m~tal of group IV to group Vl of the perio~ic system, and by sub~
sequent activation of the caltalytlc component thus formed by treatment with an organometa]Llic compound of a ~netal from ~roup I to group III of the periodic system.
The compounds of the heavy metals are preferably chosen from the halides, the oxyhalides~ the alkoxy~halides ~nd the alcoholates. The prcferred heav~ metals are titanium, vanadiwn and chromium.
Hence, for exa~llple, suitable compounds are TiC14, TiBr4, VC14, YOC13 9 VOBr~, CrO2C12, Ti(OC2~5)3Cl, Ti(O isoC4~9)2Cl~
Ti(GC2H5)4, Ti(O isoC4Hg)4, etc. The best results are obtained with TiC14.
The reaction between the support and the heavy metal compound is carried out under the usual conditions described - in the art. It must be noted, as already indicated, that the support allows strictl~- stoichiometric quantities of the heavy .~.stal co~po~n~ v~ respec~ to tne sa~ sup~o~t to be us~d ~inally, the organometallic compounds may be chosen rrom the metal-alkyls~ the halides or the hydrides of metal-alkyls~
or even the Grignard compounds. ~hese metals may be chosen, for e~ample, from Al, Zn, Mg, Na and Li~
Suitable compourlds are thus, for example~ Al(CH3)3 9 Al(C2H5~3, Al(isoC4H9~3~ Li(C4Hg)~ Al(C2H5)2Cls Al(C2H5)2Br~
Al(c2Hsj2H~ OC4Hg)2H~ A12(C2H5)3C13' (C2~ gBr~ etc-- The best results are generally obtained with the aluminium-allcyls and the halides or the hydrides of aluminum-alkyls9 and especially with aluminium-triethyl and aluminium-tri-isobutyl.
The catalytic system wbich is the sub~ect of the present lnvention i:3 applicable to the homo- or co-polyrnerization of the ~ -olefins, and especially of ethylene or propylene. The homo- or co--polymerization of the olefins may be carried ou$
by any of the usual methods, both in the gaseous phase and ln solution.
The regulation of the average rnolecular weight of the --6_ .~ .

~ 118~7 polymer which it i~ desired to obtain can be achieved~ a~
well as by means of sultable choice of the support, prepared according to the invention, also by means of the additlon of`
one or more of the usual chain teminating ag~nts, such as hydrogen, alcohols9 C029 ~inc-alkyls and cadmium~alkyls.
The pvlymer obtained, and especially the polyethylene produced, are substantially 1~inear and typically have a Melt Index of from 0.1 to 20, a crystallinity of from 50~o to 84~
and a dsnsity of from 0.95 to 0.97. Since the Melt Indcx n~ay Yary within an extremely wide range of values, these products are suitable practically for any type ofprocessing, such as~
for example extrusion, blow-moulding etc.
Se~eral e~amples will be gi~ren hereinafter purely for illustratiYe purposes. These must not be considered as a limitation of the inYention7 In the examples the Melt Index was;deter~nined by the ASTM D1238 method, with a weight of 2.t6 K~.
Exa~ple 1 10 gr~ms oF .~i~ turn-~gs are tre~ted n 3 gla~s -eac'vr1 Iurnished with an a~itator and dehydrated under hot conditions ` with anhydrous ~2~ with 100 cc~. of anhydrous ethyl alcohol `~ ~nd with gaseous HCl (~eed rate of 20 l/h) at reflu~: tempera-ture for 60 minutes.
Evapo~ation is then carried out for 4 hours at 130C and at atmospheric pressure, thus reco~ering 47.5 grams of a ~hite powder with a grain size of from 30 to 80 microns.
X-ray analysis of this powder reveals a spectrum typical of a highly crystalline substance, with two characteristic bands o~ considerable intensity at d = 9.5 A and d = 13.1 A.
The porosity i~ 0.57 cc/g~ the content of C2H50H groups is 25C,~ by weight and the surface area is 47 m /g.

10 g of this powder are treated with 100 cc of anhydrous n-heptane containing 1 cc of ~iC14 for 4 hour~ and at reflux temperature.
A~ter 4 hours -the mixture is e~aporated to dryness and a catalytic component containing 3C~o by weight ol Ti and 54.7~o ~ .. _ . . ~ . .. _ .. , . _ _ _ _ .. , . .. . _ _ _ .. _ . . , . _ . _ . _ . . .. ~ _ . ... .. . . _ , . .
_ _ _ .. .... .. .. _ _ . . ~

37~7 by ~ei~ht of Cl2 i3 Obtailled.
95 mg of this catalytic component are disperded in 2 litres of anhydrou~ n-heptane containing 1 cc of aluminium-triethyl and the whole is poured into a steel~ 4 litre auto-j clave furnished wit~ an agitator. Polymerl~ation Or ethylene is then carried out at a temperature of 90C and at a total pressure of 5 Kg/cm (4 Kg/cm of ethylene and 1 Kg/cm of hydrogen) for a period of 30 ~inutes.
700 g of polyme~ ~re obtained. The o~tput is 3680 g of polyethylene/g catalyst/h/at~osphere of ethylene. The specificactivity of the catalyst is 122.6 Kg of polyethylene/~ of ', Ti/h/atmosphere of ethyleDe.
The polymer ob~ained has the following characteristics:
_ Melt Index o.67 I - Crystallinity 745 \- Density C.96t1.
1 ,~xample 2 In this Example the preparation o~ the catalytic support i~ ca.r~ed out b~ Id-y pro_ess", ra~her +han by a llw~
proces~".
10 g of ~Ig powde~ are loaded into a tubular glass reactor (~acketed a~d furnished with an agitator) ha~ing a porous septum at the bottom. The reactor is first heat dried with anhydrous N2. The temperature is maintained at 1400C. There is then introduced from the bottom, thro~gh tlle porous plate, a gaseous }ICl/ethyl alcohol mixtu~e in a molar ratio of 1:1 and with a feed rate of HCl of 20 l/h.
~ fter 4 hours there is obtained a white powder having 8 grain size of from 20 to 80 micronsO X-ray analysis shows the same chalracteristic b~nds as those Or the support of Example 1. l'he product has a surface area of 50 m /g, a porosity of o.6 cc/g and a content of C2H50H groups of 21%
by weight. '~
10 g oi~ this powder are treated with TiCl4 as desc~ibed in Exampie 1. There is obtained a catalytic component contai-ning 3.6~o by weight of Ti and 57,15~ by weight of Cl2o i 1~ 37 ~7 54 mg of this catalytic component ara disper~ed in 2 litres of anhydrous n-heptane containillg 1 cc of aluminium-triethyl~ The polymerization of ethylenc is carried out as in Example 1. 501 g of polymer are obtain~d. The output is 4630 g of polyethylene/g catalyst/hour/atmosphere of ethylene, ; and the specific activity of the catalyst is 128.6 Xg of ; polyethylene/g of Ti/hour/atniosphere of ethylene.
~he polymer obtained has the following character~stics:
, - Melt Index 0.7 t - 10 - C~ystallinity 74 ; - Density o.g6~ .
Example 3 This control example is given to show that by operating under conditio~s such as to obtain an active complex almost I free ~rom alcoholic OH, the activity of the final catalyst obt~ined is greatly reduced.
10 g of Mg turnin~s are treated with ethyl alcohol and HCl ~s in Example 1. Evaporation is then carried out at 250~C
I-Or 5 ~ours and ~t at~ospheric pressure.
A white powder is obtained havings a grain size of from 20 to 80 microns. X-ray analysis of the powder shows a spectrum colnpletely different from that of the support Or Example 1 9 the strong typical bands at d = 9.5 A and d = 13.1 A being a~sent. The porosity is 0.3 cc/g, the content of C2H50H gro~lps is less than 1C,' by weight and the surface area is 38 m /g.
10 g of this po~dar are treated with TiCl4 as described in Example 1~ thus obtaining a catalytic component containing o.6~ by weilght o~ Ti and 77~ by weight of Cl2.
60 mg of this catalytic component are disper~ed in 2 litres o~ anhydrous n-heptane containing 1 cc of aluminium-tri¢thyl. The polymeri~ation of ethylene is carried out as in Example 1, but with a reaction time of 1 hour.
20 g o:f polymer are obtained. The output is 83 g of polyethylene/g catalyst/hour~at~osphere Or ethylene, and the specific activity of the catalyst i9 13.8 Kg of polyethylene/g of Ti/h/atmosphere of ethylene.

~ L87~7 The polynler obtained has the following charac-teristics:
; - ~lelt Indcx 0.08 ; - Crystallinity 50~o E~-~mple 4 10 g of ~ig powder are tr~eated as in Example 2, but operati~lg at a temperature of 180C for 4 hours.
A white powder is obtainled, 90~o of which has a grain size of from 20 to 80 microns. X-ray analysis shows the s~me charac teristic bands as in ~x~ple 2. The product has a surface area of 70 m /~, a porosity of o.63 cc/g and a content of C2H50H
groups of 14~ao by weight.
10 g of this powdsr ara treat~d ~ith TiC14 as in Example 2, thus obtaining a catalytic component containing 1.6% by weight of Ti and 67.15~,~ by ~-eight of C12.
-78 mg of this catalytic component are dispersed in 2 litrès of anhydrous n-heptane containing 1 cc of aluminiu~-triethyl. The p~lymeri~ation of e$hylene is carried out ~s i Example 1.
,~CJU g Uî ~i~ly:n~r a~ :~ octainad- -lnc output lS i y,::, g OI
pol~e~hylene/g catalyst/ho~r/atmosphere of ethylene and the specific acti~ity of the catalyst is 120.2 Kg of polyethylene/
g of Ti/hour!atmosphere of ethylene.
The polymer obtained hasthe fo~lo~ing characteristics:
- Melt Inde~ 0.2 - Crystallinity 65~,~
o - Density 0.955.
Example 5 The preparation of the active complex is carried out as in Example 1, i 30 10 g of this product are treated with 100 cc of anhydrous n-hept~ne containing 1 cc of TiC14 for 4 hours at ambient tempcrature~ Th~ mixture is then dried for one hour at 1400C, thus obtaining a c~talytic co~lponent containing 2~% by weight of Ti and 50 ~ 7~o by w~ight of C~2, 50 mg of this catalytic component are dispersed in 2 litres of anhydrous n heptane containing 1 cc of alu;ninium-37d~7 ., triethyl an~ the polymeri~ation of ethylene is carried out as in E~an~ple 1.
290 g of polymer are obt;ained. The output is 2900 g of polyethylene/g catalyst/hour/atmosphere of ethylene and the specific activity of the catalyst ls 145.0 ~g of polye-thylene/g of Ti/hour/at~osphere of ethylene.
The poly~er obtained has~ the follo~/ing characteristics:
- Melt Index 0.71 - ~rystallinity 74~o Q ~ 10 - Density 0.961.
Example 6 ~his test is carried out as in Example 1, except that the polymerization is carried out at the same tot~l pressure but with different partial pressures of the ethylene (2 Kg/
c~ ) and of the hydrogen (3 Xg/cm ).
330 g of polymer are obtained. The output is 3473 g of _ polyethylene/g catalyst/hour/atmosphsre of ethylene and the specific activity of the catalyst is 115.7 K~ of polyethylenc/
g of Ti/hour/atmosphere of ethYleIleO
The polymer obtained has a Melt Indcx of 14.
Exam~le 7 -Two runs are carried out as in E~ample 1 except that~
rather than using 1 cc of aluminium-triethyl, there are used respective7y:
n case A: 1 cc of monochlorodiethylaluminium;
In ca~e B: t cc of aluminium-triisobutyl.
The re~sults thus obtained are respectively:
In caso A:
- 0~1tput of 2460 g of polyethylene/g catalyst/hour/atmosphere of ethylene;
- specific activity of 82.0 Kg of polyethylene/g of Ti/}lour/
atmosphere of ethylene.
In case B:
- output Qf 1~3~o g of polyethylene/g catalyst/hour/atmosphere of ethylene;
- specific activity of 144.0 Kg of polyethylene/g o~ Ti/hour/

- ~ ~187~

i atmosphere of ethylene.
Exam ~e 8 Three runs are carried out as in Example 1 except that 9 rather than using 1 cc Or TiC14 in 100 cc of anhydro~s n-heptane, there are used r~spectively:
In case A: 100 cc of a soluti.on containing 25C~o by w~ight of Ti~12(0C4Hg)2 in anhydrous n-heptane;
I~ case B: 100 cc of a solution containing 25~o by weight of YOC13 in anhydrou~ n-heptane;
' In case C: 100 cc of a solution containing 25~o by weight of The three catalytic components obtainsd have respectively:
In case A: 2f~ by weight cf Ti;
_n case B: 2.85/~o by wei~t of V;
case C: 108~ ~y weight of Cr..
~The three final catalysts obtained by activation with -, aluminium-triethyl carried out in each case as in Example 1 yield the ~ollo~ing results:
In c~s~ A~
- out~ut of t53S g ol polyethylerle~g catalyst/hour/atmosphere of ethylene;
- specific acti~ity of 76.7 Kg o~ polyethylene/g of Ti/h/
atmosphere of ethylene;
In case B:
- output of 1824 g of polyethylene/g catalyst/hour/atmosphere of ethylene; - .
- specific activity of 64.o Xg of polyethylene/g of V/hour/
atmosphere of ethylene;
In case C:
- output of 1260 g of polyethylene/g catalyst[hour/atmosphere of ethylene;
- specific 'activity of 70.0 Kg o~ polyethylene/g of Cr/h/
atmosphere o f ethylene.

-t2-

Claims (20)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A ziegler catalytic system formed by the combination of an organo-metallic compound of a metal of Group I, II or III of the Periodic System of the Elements according to Mendeleef and a compound of a heavy metal of Group IV, V or VI of said Periodic System supported on a carrier, characterized in that the carrier is a crystalline active complex obtained by reacting metallic magnesium, gaseous hydrogen chloride and one or more aliphatic, cycloaliphatic, aromatic or alkylaromatic alcohols in the liquid or gaseous state, at a temperature of from 60° to 300°C, the said active complex having:
(a) a content of free alcoholic hydroxyl groups of from 1 to 25% by weight;
(b) a chlorine content of from 15 to 70% by weight;
(c) a magnesium content of from 5 to 35% by weight;
(d) a porosity of from 0.1 to 1 cc/g;
(e) a surface area of from 1 to 200 m2/g; and (f) x-ray diffraction bands at d = 13.1 °A and d = 9.5 °A.

2. A catalytic system according to claim 1, characterized in that the carrier has a content of free alcoholic hydroxyl groups of from 3 to 10%
by weight, a chlorine content of from 30 to 60% by weight, a magnesium content of from 10 to 25% by weight, a porosity of from 0.3 to 0.7 cc/g and a surface area of from 5 to 100 m2/g.

3. A catalytic system according to claim 1, characterized in that said active complex is prepared by using a ratio of from 1:0.5 to 1:20 between the equivalents of magesium and the moles of alcohol.

4. A catalytic system according to claim 3, characterized in that said ratio is from 1:1 to 1:10.

5. A catalytic system according to claim 1, characterized in that the active complex is prepared in the gaseous phase by maintaining a molar ratio of from 0.5:1 to 10:1 between hydrogen chloride and gaseous alcohol.

6. A catalytic system according to claim 5, characterized in that said molar ratio is from 1:1 to 5:1.

7. A catalytic system according to claim 1, characterized in that said active complex is prepared in the liquid phase with a quantity of hydrogen chloride such as to saturate said liquid phase.

8. A catalytic system according to claim 7, characterized in that said liquid phase contains an inert liquid diluent chosen from aliphatic, cycloaliphatic, aromatic and alkyl-aromatic hydrocarbons.

9. A catalytic system according to claim 7, characterized in that said liquid phase contains an inert liquid diluent chosen from heptane and cyclohexane.

10. A catalytic system according to claim 7, characterized in that the active complex is separated from the liquid phase by evaporating the latter at a temperature of from 100° to 200°C, at a pressure of from 1 Atmosphere to 100 mm Hg and for period of from 1 to 10 hours.

11. A catalytic system according to claim 1, characterized in that said alcohols are chosen from methyl, ethyl, propyl, amyl and benzyl alcohols, phenols and cresols.

12. A catalytic system according to claim 1, characterized in that said alcohols are chosen from ethyl and isopropyl alcohols.

13. A catalytic system according to claim 1, characterized in that said gaseous hydrogen chloride is used in the form of a mixture with one or more inert gases.

14. A catalytic system according to claim 13, characterized in that hydrogen is used as an inert gas.

15. A catalytic system according to claim 1, characterized in that said compound of a heavy metal is a compound of tita-nium, vanadium or chromium.

16. A catalytic system according to claim 1, characterized in that said compound of a heavy metal is a halide, an oxy-halide, an alkoxyhalide or an alcoholate.

17. A catalytic system according to claim 1, characterized in that said organometallic compound is an aluminium-alkyl, a halide of an aluminium-alkyl or a hydride of an aluminium-alkyl.

18. A catalytic system according to claim 1, characte-rized in that said organo metallic compound is aluminium-triethyl or aluminium-triisobutyl.

19. A catalytic system according to claim 1, charac-terized in that said compound of a heavy metal is titanium tetrachloride.

20. A process for the homo- or co-polymerization of .alpha.-olefins, characterized in that said homo- or co-poly-merization is carried out according to the low pressure Ziegler method in the presence of a catalytic system as claimed in claim 1.