EP0662095A1

EP0662095A1 - Process for polymerizing alpha-olefin

Info

Publication number: EP0662095A1
Application number: EP93922327A
Authority: EP
Inventors: Edward Wilson Stanley
Original assignee: Union Carbide Chemicals and Plastics Technology LLC; Shell Oil Co
Current assignee: Union Carbide Chemicals and Plastics Technology LLC
Priority date: 1992-09-25
Filing date: 1993-09-23
Publication date: 1995-07-12
Also published as: CA2145538A1; WO1994007926A1; AU672637B2; AU5136493A; JPH08504446A

Abstract

A process for polymerizing one or more α-olefins of up to 10 carbon atoms which comprises contacting the one or more α-olefin under polymerization conditions with a catalyst system comprising: (a) a titanium halide-containing magnesium, containing procatalyst component wherein the component is obtained by contacting a magnesium compound of the formula MgR'R'', wherein R' and R'' are, independently, alkoxide group, aryloxide group or halogen, with a halogenated tetravalent titanium compound in the presence of a halohydrocarbon and an alkyl ester of a polycarboxylic acid electron donor, (b) an organoaluminum cocatalyst component, and (c) an organosilane selectivity control agent represented by general formula (I) wherein R?1, R2 and R3¿, are, independently, alkyl group of 1 to 12 carbon atoms; aryl group of 1 to 12 carbon atoms, alkaryl group of 1 to 12 carbon atoms, aralkyl of 1 to 12 carbon atoms or halogen; and R4 is hydrocarbyloxy of 1 to 2 carbon atoms. The process affords high catalyst productivity and produces polymer products that have broad molecular weight distribution while retaining low oligomer content properties.

Description

DESCRIPTION

PROCESS FOR POLYMERIZING ALPHA-OLEFIN

Technical Field

This invention relates to a process for producing highly stereospecific α-olefin polymers. More particularly, the invention relates to a process that utilizes a novel high activity stereoregular polymerization catalyst system to produce α-olefin polymers having improved properties. Background Art

The use of a solid, transition-metal based, olefin polymerization catalyst system including a magnesium- containing, titanium halide-based catalyst component to produce a polymer of an α-olefin such as ethylene, propylene, and butene-1, is well known in the art. Such polymerization catalyst systems are typically obtained by the combination of a titanium halide-based catalyst component, an organoaluminum compound and one or more electron donors. For convenience of reference, the solid titanium-containing catalyst component is referred to herein as "procatalyst", the organoaluminum compound, as "cocatalyst", and the electron donor compound, which is typically used separately or partially or totally complexed with the organoaluminum compound, as "selectivity control agent" (SCA) . It is also known to incorporate electron donor compounds into the procatalyst. The electron donor which is incorporated with the titanium-containing compounds serves a different purpose than the electron donor modifier referred to as the selectivity control agent. The compounds which are used as the electron donor may be the same or different compounds which are used as the selectivity control agent. The above-described stereoregular high activity catalysts are broadly conventional and are described in numerous patents and other references including Nestlerode et al, U. S. Patent 4,728,705, which is incorporated herein by reference.

Although a broad range of compounds are known generally as selectivity control agents, a particular lcatalyst component may have a specific compound or groups of compounds with which it is specially compatible. For any given procatalyst and/or cocatalyst, discovery of an appropriate type of selectivity control agent can lead to significant increases in catalyst efficiency, lower hydrogen demand as well as the improvement in polymer product properties.

Many classes of selectivity control agents have been disclosed for possible use in polymerization catalysts. One class of such selectivity control agents is the class of organo-silanes. For example, Jloppin et al, U.S. Patent 4,990,478, describes branched C_ - C _ alkyl-t- butoxydimethoxysilanes. Other aliphatic silanes are described in Hoppin et al, U.S. Patent 4,829,038. Although many methods are known for producing highly stereoregular α-olefin polymers, it is still desired to improve the activity of the catalyst and produce polymers or copolymers that exhibit improved properties such as broad molecular weight distribution^ and low xylene solubles. Further, it is desired to produce polymers or copolymers that exhibit a reduction in the amount of volatiles, e.g., smoke and/or oil, liberated during subsequent processing, e.g. extrusion. Disclosure of the Invention The invention relates to a process for the production of homopolymers or copolymers that have improved polymer properties.

More particularly, the present invention is a process for the production of polymers using a high activity olefin polymerization catalyst system which comprises (a) a titanium halide-containing procatalyst component obtained by halogenating a magnesium compound of the formula MgR'R", wherein R* and R" are alkoxide groups of 1 to 10 carbon atoms with a halogenated tetravalent titanium compound in the presence of a polycarboxylic acid ester electron donor, and a halohydrocarbon, (b) an organoaluminum cocatalyst component, and (c) an organosilane selectivity control agent having the general formula:

wherein R¹, R² and R³ are, independently, alkyl of 1 to 12 carbon atoms, aryl of 1 to 12 carbon atoms, alkaryl of 1 to 12 carbon atoms, aralkyl of 1 to 12 carbon atoms or halogens and R⁴ is a hydrocarbyloxy of 1 to 2 carbon atoms. The preferred selectivity control agents are t- butyldimethylmetho ysilane, (2-methyl-2- butyl)dimethylmethoxysilane, (3-ethyl-3-pentyl) - dimethylmethoxysilane and mixtures thereof. Best Mode for Carrying Out the Invent _m Although a variety of chemical compounds are useful for the production of the procatalyst, a typical procatalyst of the invention is prepared by halogenating a magnesium compound of the formula MgR'R", wherein R¹ is an alkoxide or aryloxide group and R" is an alkoxide, hydrocarbyl carbonate or aryloxide group or halogen, with a halogenated tetravalent titanium compound in the presence of a halohydrocarbon and an electron donor.

The magnesium compound employed in the preparation of the solid catalyst component contains alkoxide, aryloxide, hydrocarbyl carbonate or halogen. The alkoxide, v/hen present, contains from 1 to 10 carbon atoms. Alkoxides containing from 1 to 8 carbon atoms are preferable, with 2 to 4 carbon atoms being more preferable. The aryloxide, when present, contains from 6 to 10 carbon atoms, with 6 to 8 carbon atoms being preferred. When halogen is present, it is preferably present as bromine, fluorine, iodine or chlorine, with chlorine being more preferred. Preferred magnesium compounds are magnes: .n dialkoxides.

Suitable magnesium compounds are magnesium chloride, ethoxy magnesium bromide, isobutoxy magnesium chloride, phenoxy magnesium iodide, cumyloxy magnesium bromide, magnesium diethoxide, magnesium isopropoxide, magnesium ethyl carbonate and naphthoxy magnesium chloride. The preferred magnesium compound is magnesium diethoxide.

Halogenation of the magnesium compound with the halo-genated tetravalent titanium compound is effected by employing an excess of the titanium compound. At least 2 moles of the titanium compound should ordinarily be employed per mole of the magnesium compound. Preferably from 4 moles to 100 moles of the titanium compound are employed per mole of the magnesium compound, and most preferably from 4 moles to 20 moles of the titanium compound are employed per mole of the magnesium compound.

Halogenation of the magnesium compound with the halogenated tetravalent titanium compound is effected by contacting the compounds at an elevated temperature in the range from about 60°C to about 150°C, preferably from about 70°C to about 120°C. Usually the reaction is allowed to proceed over a period of 0.1 to 6 hours, preferably between 0.5 to 3.5 hours. The halogenated product is a solid material which is isolated from the liquid reaction medium by filtration, decantation or a suitable method.

The halogenated tetravalent titanium compound employed to halogenate the magnesium compound contains at least two halogen atoms, and preferably contains four halogen atoms. The halogen atoms are chlorine atoms, bromine atoms, iodine atoms or fluorine atoms. The halogenated tetravalent titanium compounds has up to two alkoxy and/or aryloxy groups. Examples of suitably halogenated tetravalent titanium compounds include diethoxytitanium dibromide, isopropoxytitanium triiodide, dihexoxytitanium dichloride, phenoxytitaniu trichloride, titanium tetrachloride and titanium tetrabromide. The preferred halogenated tetravalent titanium compound is titanium tetrachloride.

Halogenation of the magnesium compound with the halogenated tetravalent titanium compound, as noted, is con- ducted in the presence of a halohydrocarbon and an electron donor. If desired, an inert hydrocarbon diluent or solvent may also be present, although this is not necessary. The halohydrocarbon employed is an aromatic or aliphatic, including cyclic and all cyclic compounds. Preferably the halohydrocarbon contains 1 or 2 halogen atoms, although more may be present if desired. It is preferred that the halogen, independently, is chlorine, bromine or fluorine. Suitable aromatic halohydrocarbons include chlorobenzene, bromobenzene, dichlorobenzene, dichlorodibromobenzene, o-chlorotoluene, chlorotoluene, dichlorotoluene, chloronaphthalene. Chlorobenzene, o- chlorotoluene and dichlorobenzene are the preferred halohydrocarbons, with chlorobenzene and o-chlorotoluene being more preferred.

The aliphatic halohydrocarbons which can be employed suitably of 1 to 12 carbon atoms. Preferably such halohydrocarbons of 1 to 9 carbon atoms and at least 2 halogen atoms. Most preferably the halogen is present as chlorine. Suitable aliphatic halohydrocarbons include dibromomethane, trichloromethane, 1,2-dichloroethane, trichloroethane, dichlorofluoroethane, hexachloroethane, trichloropropane, chlorobutane, dichlorobutane, chloropentane, trichloro-fluorooctane, tetrachloroisooctane, dibromodi-fluorodecane. The preferred aliphatic halohydrocarbons are carbon tetrachloride and trichloroethane. Aromatic halohydrocarbons ire preferred, particularly those of 6 to 12 carbon atoms, and especially those of 6 to 10 carbon atoms.

Typical electron donors that are incorporated within the procatalyst include esters, particularly aromatic esters, ethers, particularly aromatic ethers, ketones, phenols, amines, amides, imines, nitriles, phosphines, phosphites, stibines, arsines, phosphoramides and alcoholates. Alkyl esters of aromatic polycarboxylic acids are frequently incorporated into electron donors. Illustrative of such electron donors are methyl benzoate, ethyl benzoate, diethyl phthalate, diisoamyl phthalate, ethyl p-ethoxybenzoate, methyl p-ethoxybenzoate, diisobutyl phthalate, dimethyl napthalene-dicarboxylate, diisobutyl maleate, diisopropyl terephthalate, and diisoamyl phthalate. Diisobutyl phthalate is the preferred alkyl ester of aromatic carboxylic acid. After the solid halogenated product has been separated from the liquid reaction medium, it is treated one or more times with additional halogenated tetravalent titanium compound in order to remove residual alkoxy and/or aryloxy groups and maximize catalyst activity. Preferably, the halogenated product is treated multiple times with separate portions of the halo-genated tetravalent titanium compound. Better results are obtained if the halogenated product is treated twice with separate portions of the halogenated tetravalent titanium compound. As in the initial halogenation, at least 2 moles of the titanium compound should ordinarily be employed per mole of the magnesium compound, and preferably from 4 moles to 100 moles of the titanium compound are employed per mole of the magnesium compound. Most preferably from 4 moles to 20 moles of the titanium compound per mole of the magnesium compound.

Optionally, the solid halogenated product is treated at least once with one or more acid chlorides after washing the solid halogenated product at least once with additional amounts of the halogenated tetravalent titanium compound. Suitable acid chlorides include benzoyl chloride and phthaloyl chloride. The preferred acid chloride is phthaloyl chloride.

The reaction conditions employed to treat the solid halogenated product with the titanium compound are the same as those employed during the initial halogenation of the magnesium compound.

After the solid halogenated product has been treated one or more times with additional halogenated tetravalent titanium compound, it is separated from the liquid reaction medium, washed at least once with an inert hydrocarbon of up to 10 carbon atoms to remove unreacted titanium compounds, and dried. Exemplary of the inert hydrocarbons that are suitable for the invention are isopentane, isooctane, hexane, heptane and cyclohexane.

The final washed product has a titanium content of from 0.5 percent by weight to 6.0 percent by weight, preferably from 2.0 percent by weight to 4.0 percent by weight. The atomic ratio of titanium to magnesium in the final product is between 0.01:1 and 0.2:1, preferably between 0.02:1 and 0.1:1.

The cocatalyst is an organoaluminum compound which is typically an alkylaluminum compound. Suitable alkylaluminum compounds include trialkylaluminum compounds, such as triethyl-aluminum or triisobutylaluminum; including dialkylaluminum halides such as diethylaluminum chloride and dipropylaluminum chloride; and dialkylaluminum alkoxides such as diethylaluminum ethoxide. Trialkylaluminum compounds are preferred, with triethylaluminum being the preferred trialkylaluminum compound.

The organo. lane selectivity control agents in the catalyst system contain at least one silicon-oxygen-carbon linkage. Suitable organosilane compounds includes compounds having the following general formula:

R¹ R³

Si

R²^ ^R⁴ wherein R¹, R² and R³ are, independently, alkyl of 1 to 12 carbon atoms, aryl group of 1 to 12 carbon atoms, alkaryl group of 1 to 12 carbon atoms, aralkyl group of from 1 to 12 carbon atoms or halogen; and R⁴ is a hydrocarbyloxy group of 1 to 2 carbon atoms. It is preferred that R¹, R², and R³ are alkyl groups and R⁴ is a alkoxy group. It is further preferred that R⁴ is a methoxy group. Examples of suitable organosilane selectivity control agents include t- butyldimethyl-methoxysilane, (2-methyl-2- butyl) dimethylmethoxysilane, (3-ethyl-3-pentyl) - dimethylmethoxysilane and mixtures thereof. The preferred organosilane selectivity control agent is t- butyldi ethylmethoxysilane. The invention also contemplates the use of mixtures of two or more selectivity control agents. The selectivity control agent is provided in a quantity such that the molar ratio of the selectivity control agent to the titanium present in the procatalyst is from about 2 to about 60. Molar ratios from about 8 to about 45 are preferred, with molar ratios from about 10 to about 35 being more preferred.

The high activity stereoregular polymerization cata-lyst is employed in a chemical reaction to effect polymerization by contacting at least one α-olefin under polymerization condi-tions. In accordance with the invention, the procatalyst component, organoaluminum cocatalyst, and selectivity control agent can be introduced into the polymerization reactor separately or, if desired, two or all of the components may be partially or completely mixed with each other before they are introduced into the reactor. In any event, the organoaluminum cocatalyst is employed in sufficient quantity to provide from 1 mole to about 150 moles of aluminum per mole of titanium in the procatalyst. It is preferred that the cocatalyst is present in sufficient quantities to provide from 10 moles to about 100 moles of aluminum per mole of titanium in the procatalyst.

The particular type of polymerization process utilized is not critical to the operation of the present invention and the polymerization processes now regarded as conventional are suitable in the process of the invention. The polymerization is conducted under polymerization conditions as a liquid phase or a gas-phase process employing a fluidized catalyst bed.

The polymerization conducted in the liquid phase employs as reaction diluent an added inert liquid diluent or alternatively a liquid diluent which comprises the olefin, such as propylene or 1-butene, undergoing polymerization. If a copolymer is prepared wherein ethylene is one of the monomers, ethylene is introduced by conventional means. Typical polymerization conditions include a reaction temperature from about 25°C to about 125°C, with temperatures from about 35°C to about 90°C being preferred and a pressure sufficient to maintain the reaction mixture in a liquid phase. Such pressures are from about 150 psi to about 1200 psi, with pressures from about 250 psi to about 900 psi are preferred. The liquid phase reaction is operated in a batchwise manner or as a continuous or semi-continuous process. Subsequent to reaction, the polymer product is recovered by conventional procedures. The precise controls of the polymerization conditions and reaction parameters of the liquid phase process are within the skill of the art.

As an alternate embodiment of the invention, the polymerization may be conducted in a gas phase process in the presence of a fluidized catalyst bed. One such gas phase process olymerization process is described in Goeke et al, U.S. Patent 4,379,759, incorporated herein by reference. The gas phase process typically involves charging to reactor an amount of preformed polymer particles, gaseous monomer and separately charge a lesser amount of each catalyst component. Gaseous monomer, such as propylene, is passed through the bed of solid particles at a high rate under conditions of temperature and pressure sufficient to initiate and maintain polymerization. Unreacted olefin is separated and recovered and polymerized olefin particles are separated at a rate substantially equivalent to its production. The process is conducted in a batchwise manner or a continuous or semi- continuous process with constant or intermittent addition of the catalyst components and/or α-olefin to the polymerization reactor. Typical polymerization temperatures for a gas phase process are from about 30°C to about 120°C and typical pressures are up to about 1000 psi, with pressures from about 100 to about 500 psi being preferred.

In both the liquid phase and thi gas-phase poly¬ merization processes, molecular hydrogen is added to the reaction mixture as a chain transfer agent to regulate the molecular weight of the polymeric product.. Hydrogen is typically employed for this purpose in a manner well known to persons skilled in the art. The precise control of reaction conditions, the rate of addition of feed component and molecular hydrogen is broadly within the skill of the art.

The present invention is useful in the polymerization of α-olefins of up to 10 carbon atoms, including mixtures thereof. It is preferred that α-olefins of 3 carbon atoms to 8 carbon atoms, such as propylene, butene-1 and pentene-1 and hexane-1, are polymerized. If α- olefins are to be copolymerized, the preferred α-olefins include ethylene.

The polymers produced according to this invention are predominantly isotactic. Polymer yields are high relative to the amount of catalyst employed. The process of the invention produces homopolymer and copolymers including both random and impact copolymers, that have a relatively broad molecular weight distribution while maintaining a relatively low oligomers content (determined by the weight fraction of C₂₁ oligomer) of less than 180 ppm. The production of polymers having an oligomers content of less than 130 ppm is preferred, with an oligomers content of less than 115 ppm being more preferred. <

Other features, advantages and embodiments of the invention disclosed herein will be readily apparent to those exercising ordinary skill after reading the foregoing disclosure. In this regard, while specific embodiments of the invention have been described in detail, variations and modifications of these embodiments can be effected without departing from the spirit and scope of the invention as described and claimed. The invention described herein is illustrated, but not limited by the following Illustrative Embodiments and the

Comparative Example. The following terms are used throughout the Illustrative Embodiments and Comparative Example:

SCA (selectivity control agent) PEEB (ethyl p-ethoxybenzoate)

TBDMMS (t-butyldimethylmethoxysilane) NPTMS (n-propyltrimethoxysilahe) DIBDMS (diisobutyldimethoxysilane) DIBDES (diisobutyldiethoxysilane) ILLUSTRATIVE EMBODIMENT I

(a) Preparation of Procatalyst Component

The procatalyst was prepared by adding magnesium diethoxide (2.17 g, 19 mmol) to 55 ml of a 50/50 (vol/vol) mixture of TiCl₄/chlorobenzene. After adding diisobutyl phthalate (0.66 ml, 2.50 mmol), the mixture was heated in an oil bath and stirred at 110°C for 60 minutes. The mixture was filtered hot and slurried in 55 ml of a 50/50 (vol/vol) mixture of TiCl₄/chlorobenzene. Phthaloyl chloride (0.13 ml, 0.90 mmol) was added to the slurry at room temperature. The resulting slurry was stirred at 110°C for 60 minutes, filtered, and slurried again in a fresh 50/50 mixture of TiCl₄/chlorobenzene. After stirring at 110°C for 30 minutes, the mixture was filtered and allowed to cool to room temperature. The procatalyst slurry was washed 6 times with 125 ml portions of isooctane and then dried for 120 minutes, at 25°C, under nitrogen.

(b) Polymerization of Propylene

Various catalysts were produced using several organosilanes as the selectivity control agent, some of which are within the scope of the invention (TBDMMS) and others that are .not within the scope of the invention (NPTMS, DIBDES and DIBDMS) . Propylene (2700cc) and molecular hydrogen were introduced into a 1 gallon autoclave. The temperature of the propylene and molecular hydrocarbon was raised to 67°C. An organosilane selectivity control agent, triethylaluminum, and the procatalyst --lurry produced above were premixed for about 20 minutes and then the mixture was introduced into the autoclave. The amount of silane utilized in the polymerization also varied. The amount of triethylaluminum (0.56 mmoles) and the amount of the procatalyst slurry (sufficient quantity of procatalyst to provide 0.008 mmoles of titanium to the autoclave) remained constant. The autoclave was then heated to about 67°C and the polymerization was continued at 67°C for one hour. The polypropylene product was recovered from . the resulting mixture by conventional methods and the weight of the product was used to calculate the reaction yield in millions of grams of polymer product per gram (MMg/g) of titanium in the procatalyst. The term "Q" was calculated as the quotient of the weight average molecular weight (M_^,) and the number average molecular weight (M , determined by gel permeation chromatography. The term "M_z" as defined in "Encyclopedia of Polymer Science and Engineering, 2nd Edition", Vol. 10, pp. 1-19 (1987) incorporated herein by reference, is the z- average molecular weight. The term "R" was calculated as the quotient of M_z and I ,,. "Melt Flow" is determined according to ASTM D-1238-73, condition L. "Xylene Solubles" were determined in accordance with U.S. Food and Drug Administration Regulations, 21 CFR 177.1520. The results of a series of polymerizations are shown in TABLE I.

TABLE I

¹ Comparison

² mmoles of hydrogen added to the liquid phase reactor system ³XS = Xylene solubes by % weight

To further illustrate the advantages obtained using the catalyst system of the invention, viscosity ratio values were taken for polymers having a melt flow of about 3 dg/min using the smooth curves (viscosity ratio vs. melt flow) . "Viscosity Ratio" was determined by cone and plate rheometry (dynamic viscosity measurements) as a ratio of the viscosity of the product at a frequency of 0.1 Hz divided by the viscosity of the product at a frequency of 1.0 Hz. As the viscosity ratio of polymer product increases, the molecular weight distribution increases. The values are shown in TABLE II.

Table II

SCA Viscosity Ratio at 3 dg/min

TBDMMS 1.75 NPTMS¹ 1.56 DIBDMS¹ 1.56

'For comparison

It is seen from TABLE II that the catalyst systems of the invention direct a higher viscosity ratio and therefore a broader molecular weight distribution than a conventional catalyst systems using NPTMS as the selectivity control agent. ILLUSTRATIVE EMBODIMENT II Injection Molding of Polypropylene Product

Some of the polypropylene products, produced according to Illustrative Embodiment I, were recovered by conventional means. Each recovered .product was mixed and pelletized with the following additives package: 1000 ppm of Irganox® 1010 hindered phenolic primary antioxidant, 1000 ppm of Irgafos® 168 phosphite secondary antioxidant and 500 ppm of Calcium Stearate as an acid acceptor. The pelletized polymer products were injection molded in an Arburg Injection Molder. The final "melt temperature" (Tmt, °C) is obtained from a differential scanning calorimetry curve for each polymer product produced. A higher melt temperature correlates to higher isotacticity of the polymer product.

The "Oligomers Content" was determined by the overnight extraction of a polypropylene sample in a chloroform solution containing hexadecane (n-C₁₆) as an internal standard. An aliquot of the extract is shaken in methanol and filtered to remove, trace amounts of atactic material. The filtered liquid is ttien injected onto a capillary column which uses a flame ionization gas chromatograph. Relative amounts of the extracted components are calculated based on the weight of polymer extracted using the internal standard quantitation against the C₂₁ oligomer groups. The oligomers content is an indicator of the amount of volatileε, e.g. smoke and/or oil that will be liberated by the polymer during extrusion. For instance, a lower oligomers content for a polymer product translates into lower smoke generation during further processing (e.g. extrusion) of the polymer product for film and textile applications. The results of the various analysis of polymer products are shown in Table IV. Comparative Example

(a) Preparation of Procatalyst Component

The procatalyst was prepared by adding magnesium diethoxide (50 mmol) to 150 ml of a 50/50 (vol/vol) mixture of chlorobenzene/TiCl₄. After adding ethyl benzoate (16.7 mmol) , the mixture was heated in an oil bath and stirred at 110°C for approximately 30 minutes. The resulting slurry was filtered and slurried twice with 150 ml of a fresh 50/50 (vol/vol) . Benzoyl chloride (0.4 ml) was added to the final slurry. After stirring at 110°C for approximately 30 minutes, the mixture was filtered. The slurry was washed six times with 150 ml portions of isopentane and then dried for 90 minutes, at 30°C, under nitrogen. (b) Polymerization

Using the above-described procatalyst (section a) , propylene was polymerized as described in Illustrative Embodiment II, section (b) , except the selectivity control agent was PEEB. The resulting polypropylene product was mixed, pelletized and injection molded as described in Illustrative Embodiment III. The results are furnished in TABLE III.

'Final melt temperature (°C) was obtained by differential scanning calorimetry (DSC) according to ASTM D-3417-83. For comparison

Comparative catalyst system

Claims

CLAIMS 1. A process for polymerizing one or more α- olefins of up to 10 carbon atoms which comprises contacting the one or more α-olefins under polymerization conditions with a catalyst system comprising:

(a) a titanium halide-containing procatalyst component containing magnesium, titanium, halide and a polycarboxylic acid ester, said procatalyst component being obtained by halogenating a magnesium compound of the formula MgR'R", wherein R' and R" are alkoxide groups of 1 to 10 carbon atoms, with a halogenated tetravalent titanium compound, in the presence of a halohydrocarbon and an alkyl ester of a polycarboxylic acid electron donor;

(b) an organoaluminum cocatalyst component; and (c) an organosilane selectivity control agent having the formula:

wherein R¹, R² and R³ are, independently, alkyl group of 1 to 12 carbon atoms, aryl group of 1 to 12 carbon atoms, alkaryl groups of 1 to 12 carbon atoms, aralkyl group of 1 to 12 carbon atoms or halogen; and R⁴ is a hydrocarbyloxy group of 1 to 2 carbon atoms.

2. The process of claim 1, wherein said organosilane selectivity control agent is present in a quantity such that the molar ratio of the selectivity control agent to titanium present in the procatalyst component is from about 1 to about 70.

3. The process of claim 2, wherein R¹, R², and R³ are alkyl groups and R⁴ is an alkoxy group.

4. The process of claim 3, wherein R⁴ is a methoxy group.

5. The process of claim 4, wherein the organosilane selectivity control agent is t-butyldimethyl- ethoxysilane, (2-methyl-2-butyl)dimethylmethoxysilane, (3- ethyl-3-pentyl)dimethyl-methoxysilane or mixtures thereof.

6. The process of claim 5, wherein the organosilane selectivity control agent is t-butyldimethyl¬ methoxysilane.

7. The process of claim 6, wherein t-butyl- dimethylmethoxysilane is present in a quantity such that the molar ratio of t-butyldimethylmethoxysilane to the titanium present in the procatalyst component is from about 2 to about 60.

8. The process of claim 7, wherein the halogenated tetravalent titanium compound is titanium tetrachloride...

9. -_a_!,The process of claim 8, wherein the magnesium compound is magnesium ethoxide.

10. The process of claim 9, wherein the poly- carboxylic acid ester electron donor is diisobutyl phthalate.

11. The process of claim 10, wherein the α-olefins are propylene and ethylene.

12. The process of claim 11, wherein the α-olefin is propylene.

13. In a process for polymerizing at least one α- olefin of up to 10 carbon atoms which comprises contacting at least one α-olefin t ler polymerization conditions with a catalyst system compι sing:

(a) a titanium halide containing procatalyst obtained by halogenating magnesium compound of the formula

MgR'R", wherein R' and R" are, independently, alkoxide group or aryloxide group with a halo¬ genated tetravalent titanium compound, in the presence of a halohydrocarbon and an alkyl ester of a polycarboxylic acid,

(b) an organoaluminum cocatalyst, and

(c) an organosilane selectivity control agent, the improvement in the process wherein the organosilane selectivity control agent has the formula:

wherein R¹, R² and R³ are, independently, alkyl group of 1 to 12 carbon atoms; aryl group of 1 to 12 carbon atoms, alkaryl group of 1 to 12 carbon atoms, aralkyl group of 1 to 12 carbon atoms or halogen; and R⁴ is hydrocarbyloxy group of 1 to 2 carbon atoms.

14. The process of claim 13, wherein R¹, R² and R³ are alkyl groups and R⁴ is an alkoxy group.

15. The process of claim 14 wherein R⁴ is a methoxy group.

16. The process of claim 15, wherein the organo¬ silane selectivity control agent is selected from the group consisting of t-butyldimethylmethoxysilane, (2-methyl-2- buty)dimethyl- ethoxysilane, (3-methyl-3-pentyl)dimethyl¬ methoxysilane and mixtures thereof.

17. An olefin polymerization catalyst system comprising:

(a) a titanium halide-containing procatalyst component obtained by halogenating a magnesium compound with a halogenated tetravalent titanium compound, in the presence of an alkyl ester of a polycarboxylic acid compound and a halogenated hydrocarbon,

(b) an organoaluminum cocatalyst component, and

(c) an organosilane selectivity control agent having the formula R¹ R³

Si R²^ ^R⁴ wherein R¹, R² and R³, are, independently, alkyl group of 1 to 12 carbon atoms; aryl group of 1 to 12 carbon atoms, alkaryl group of 1 to 12 carbon atoms, aralkyl group of 1 to 12 carbon atoms or halogen; and R⁴ is a hydrocarbyloxy group of 1 to 2 carbon atoms.

18. The olefin polymerization catalyst system according to claim 17, wherein the molar ratio of the selectivity control agent to the titanium present in the procatalyst is from about 1 to about 70.

19. The olefin polymerization catalyst system according to claim 18, wherein the selectivity control agent is t-butyldimethylmethoxysilane, (3-methyl-3-pentyl)- dimethylmethoxysilane, (2-methyl-2-butyl)dimethylethoxy- silane and mixtures thereof.

20. The olefin polymerization catalyst system according to claim 19, wherein the selectivity control agent is t-butyldimethylmethoxysilane.

21. The olefin polymerization catalyst system according to claim 20, wherein the magnesium compound is a magnesium dialkoxide, the halogenated tetravalent titanium compound contains at least two halogen atoms, and the organoaluminum cocatalyst component is a trialkylaluminum compound.

22. The olefin polymerization catalyst system according to claim 21, wherein the magnesium compound is magnesium diethoxide, the alkyl ester of the polycarboxylic acid compound is diisobutyl phthalate, and the halohydrocarbon is chlorobenzene or o-chlorotoluene.