WO2016051315A1

WO2016051315A1 - Ziegler-natta catalyst system and use thereof

Info

Publication number: WO2016051315A1
Application number: PCT/IB2015/057327
Authority: WO
Inventors: Virendrakumar Gupta; Sunil DHAMANIYA; Amarjyoti KALITA; Hiren Manojkumar BHAJIWALA; Ramesh Joshi
Original assignee: Reliance Industries Limited
Priority date: 2014-09-29
Filing date: 2015-09-23
Publication date: 2016-04-07

Abstract

Ziegler-Natta catalyst system disclosed herein comprises at least one Ziegler-Natta pro-catalyst, at least one organo-aluminium co-catalyst and at least one first external electron donor containing at least one substituted sinalediyl diacetate compound of Formula-1 as, Formula-1 wherein, R1 and R2 are independently selected from the group consisting of C1-C6 alkyl groups, and aryl groups; and R3 and R4 are independently selected from C1-C6 alkyl groups and aryl groups. The Ziegler-Natta catalyst system can optionally comprise at least one second external electron donor. The process for olefin polymerization involves preparing the Ziegler-Natta catalyst system and polymerizing the olefin in the presence of the Ziegler-Natta catalyst system and hydrogen, as a chain termination agent. The polypropylene produced by this process is characterized in having polydispersity index ranging from 4.0 to 7.0 and melt flow index ranging from 4.0 to 12.1.

Description

ZIEGLER-NATTA CATALYST SYSTEM AND USE THEREOF

FIELD OF THE DISCLOSURE

The present disclosure relates to a Ziegler-Natta catalyst system. BACKGROUND

A Ziegler-Natta catalyst system is generally used in the synthesis of polymers from olefins. The Ziegler-Natta catalyst system generally comprises a pro-catalyst, composed of transition metal halides (i.e., halides of titanium, chromium, vanadium, zirconium), a co-catalyst composed of organic derivatives of nontransition metals, and at least one electron donor. The electron donor affects the activity of the catalyst and stereoregularity of the polymer formed during the polymerization process. The electron donor may be present internally in the catalyst system or added externally to the catalyst system. On many occasions, both internal and external electron donors may be present in the Ziegler-Natta catalyst system.

Silane based electron donors are commonly used in Ziegler-Natta catalyst systems for olefin polymerization.

US20100267911 suggests the use of substituted silanediyl-diacetate compounds as internal electron donors in Ziegler-Natta catalyst systems for propylene polymerization. The polypropylene obtained by the process suggested in US20100267911 has a maximum molecular weight distribution (measured as Polydispersity Index, PDI) of 5.1 and a maximum hydrogen response, which is measured as melt flow index (MFI), of 6.7g/10min. However, to obtain a polymer having effective processability and mechanical strength, a desired molecular weight distribution and melt flow index is required. Furthermore, the drawback associated with conventional Ziegler-Natta catalyst systems is that large amounts of electron donors are required to be used for the polymerization process, which renders the process costly. Another drawback is the requirement of a tedious and time consuming step of incorporating an internal electron donor during the preparation of the Ziegler-Natta pro- catalyst.

Accordingly, there is felt a need to provide a Ziegler-Natta catalyst system that can be used for the polymerization of olefins, in a process that is economical, simple and produces a polymer having a desirable molecular weight distribution and hydrogen response.

OBJECTS

Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:

It is an object of the present disclosure to provide a cost effective Ziegler-Natta catalyst system. It is an object of the present disclosure to provide a Ziegler-Natta catalyst system having improved hydrogen response.

It is another object of the present disclosure to provide a process for polymerization of olefin that provides a polymer having a desired molecular weight distribution.

It is yet another object of the present disclosure to provide a process for polymerization of olefin that provides a polymer having a desired melt flow index and isotacticity.

Other objects and advantages of the present disclosure will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present disclosure. SUMMARY

One aspect of the present disclosure provides a Ziegler-Natta catalyst system for olefin polymerization. The Ziegler-Natta catalyst system comprises at least one Ziegler-Natta pro- catalyst, at least one organo- aluminium co-catalyst and at least one substituted-silanediyl- diacetate compound of Formula- 1 as a first external electron donor,

Formula- 1

wherein,

Ri and R₂ are independently selected from the group consisting of Ci-C₆ alkyl groups, and aryl groups; and

R₃ and R4 are independently selected from Ci-C₆ alkyl groups and aryl groups.

The first external electron donor in the Ziegler-Natta catalyst system of the present disclosure may be a substituted-silanediyl-diacetate compound selected from the group consisting of diethyl 2,2 -(dimethylsilanediyl)diacetate, diethyl 2,2 -(phenyl(methyl)silanediyl)diacetate and diethyl 2,2 -(diisopropylsilanediyl)diacetate.

The pro-catalyst used in the Ziegler-Natta catalyst system of the present disclosure can be a spheroidal magnesium alkoxide based Ziegler-Natta pro-catalyst comprising titanium. The organo-aluminium co-catalyst may be triethylaluminium. The ratio of the amount of the organo- aluminium co-catalyst and the amount of elemental titanium may range from 500: 1 to 10: 1. The molar ratio of the amount of the pro-catalyst and the amount of the first external electron donor may range from 1:5 to 1:20. The molar ratio of the amount of the organo- aluminium co-catalyst and the amount of the first external electron donor may range from 1: 1 to 50: 1.

The Ziegler-Natta catalyst system may further comprise at least one second external electron donor. The second external electron donor may be cyclohexylmethyldimethoxysilane. The ratio of the amount of the first external electron donor to the amount of the second external electron donor ranges from 5: 1 to 20: 1.

In another aspect of the present disclosure there is provided a process for preparing a Ziegler- Natta catalyst system. The process comprises mixing at least one organo- aluminium co-catalyst, at least one substituted silanediyl diacetate compound as a first external electron donor and optionally at least one second external electron donor to get a mixture; and adding at least one Ziegler-Natta pro-catalyst to said mixture to obtain the Ziegler-Natta catalyst system.

In yet another aspect of the present disclosure there is provided a process for polymerization of olefin using the Ziegler-Natta catalyst system of the present disclosure. The process comprises preparing a Ziegler-Natta catalyst system comprising at least one Ziegler-Natta pro-catalyst, at least one organo-aluminium co-catalyst and at least one substituted silanediyl diacetate compound as a first external electron donor and optionally at least one second external electron donor; and subjecting an olefin to polymerization in the presence of said Ziegler-Natta catalyst system and at least one chain transfer agent at a temperature ranging from 50°C to 100°C; to obtain a polyolefin. The chain transfer agent used for the polymerization process is hydrogen gas. The olefin used for the polymerization process may be propylene and the polyolefin can be polypropylene. The polymerization process is carried out under olefin pressure in the range from 5 Kg/cm 2 to 15 Kg/cm 2. The polymerization process is carried out for a time period ranging from 10 minutes to 120 minutes.

The polypropylene produced by using the Ziegler-Natta catalyst system of the present disclosure has a polydispersity index (PDI) ranging from 4.0 to 7.0 and melt flow index (MFI) ranging from 4.0 to 12.1. The polypropylene so obtained, having the desired PDI and melt flow index (MFI), exhibits a combination of requisite processability and mechanical strength.

DETAILED DESCRIPTION:

In accordance with one aspect of the present disclosure there is provided a Ziegler-Natta catalyst system. The Ziegler-Natta catalyst system comprises at least one Ziegler-Natta pro-catalyst, at least one organo-aluminium co-catalyst, and at least one substituted-silanediyl-diacetate compound of formula- 1 as a first external electron donor.

Formula- 1

wherein,

R₃ and R4 are independently selected from Ci-C₆ alkyl groups and aryl groups. In one embodiment of the present disclosure, the substituted-silanediyl-diacetate compound is diethyl 2,2 -(dimethylsilanediyl)diacetate. In another embodiment of the present disclosure, the substituted-silanediyl-diacetate compound is diethyl 2,2 -(phenyl(methyl)silanediyl)diacetate. In yet another embodiment of the present disclosure, the substituted-silanediyl-diacetate compound is diethyl 2,2 -(diisopropylsilanediyl)diacetate.

The pro-catalyst used in the Ziegler-Natta catalyst system of the present disclosure can be a magnesium alkoxide based Ziegler-Natta pro-catalyst comprising titanium halide. However, other transition metal halides (i.e., halides of titanium, chromium, vanadium, zirconium) may also be used.

In one embodiment of the present disclosure, the pro-catalyst used is a spheroidal magnesium alkoxide based Ziegler-Natta pro-catalyst, comprising titanium tetrachloride.

The organo-aluminium co-catalyst includes but is not limited to triethylaluminium, tridecylaluminium, tri-n-butylaluminium, tri-isopropylaluminium, tri-isoprenylaluminium, tri- isobutylaluminium, ethyl aluminium sesquichloride, diethylaluminium chloride, di-isobutyl aluminium chloride, triphenylaluminium, tri-n-octylaluminium and tri-n-decylaluminium.

In one embodiment of the present disclosure, the organo-aluminium co-catalyst is triethylaluminium.

Inventors of the present disclosure further found that to produce a polymer of the desired PDI and MFI, the components of the Ziegler-Natta catalyst system are to be used in a specific proportion/ratio.

In the Ziegler-Natta catalyst system of the present disclosure, the ratio of the amount of organo- aluminium co-catalyst and the amount of elemental titanium ranges from 500: 1 to 10: 1. In one embodiment of the present disclosure the ratio is 250: 1. The ratio of 250: 1 was found to be most suitable for polymerization.

The ratio of the amount of the pro-catalyst and the amount of the first external electron donor ranges from 1:5 to 1:20.

The molar ratio of the amount of organo- aluminium co-catalyst and the amount of the first external electron donor ranges from 1: 1 to 50: 1.

The Ziegler-Natta catalyst system of the present disclosure may further comprise at least one second external electron donor. The second external electron donor can be selected from the group consisting of cyclohexylmethyldimethoxysilane, dicyclopentyldimethoxysilane, diethyldiethoxysilane and diisobutyldimethoxysilane.

In one embodiment of the present disclosure, the second external electron donor is cyclohexylmethyldimethoxysilane.

The ratio of the amount of the first external electron donor and the amount of the second external electron donor ranges from 5: 1 to 20: 1.

In accordance with yet another aspect of the present disclosure there is provided a method for preparing the Ziegler-Natta catalyst system.

Initially, a Ziegler-Natta pro-catalyst is prepared. Any of the known methods may be employed for preparing the Ziegler-Natta pro-catalyst.

In one embodiment, the Ziegler-Natta pro-catalyst is prepared by the known procedure involving a multistep process of repeatedly reacting spheroidal magnesium alkoxide with a mixture of titanium tetrachloride and chlorobenzene mixture (1: 1). In the next step, at least one organo-aluminium co-catalyst, at least one first external electron donor containing at least one substituted silanediyl diacetate compound, and optionally at least one second external electron donor are mixed to get a mixture. To the mixture is added at least one Ziegler-Natta pro-catalyst to obtain the Ziegler-Natta catalyst system.

In accordance with still another aspect of the present disclosure there is provided a process for olefin polymerization. First, a Ziegler-Natta catalyst system is prepared from at least one Ziegler-Natta pro-catalyst, at least one organo-aluminium co-catalyst, at least one substituted silanediyl diacetate compound as a first external electron donor and optionally at least one second external electron donor. An olefin is subjected to polymerization in the presence of said Ziegler-Natta catalyst system and at least one chain transfer agent at a temperature ranging from 50°C to 100°C; to obtain a polyolefin.

The chain transfer agent used for said polymerization process is hydrogen gas.

The olefin used in said polymerization process can be at least one selected from the group consisting of propylene, ethylene, 1-butene, 1-hexene and 1-octene. In one embodiment of the present disclosure, the olefin used in the polymerization process is propylene and the polyolefin is polypropylene.

The polymerization process is carried out under olefin pressure ranging from 5 Kg/cm to 15 Kg/cm².

The polymerization process is carried out for a time period ranging from 10 minutes to 120 minutes. The polypropylene obtained by the polymerization process of the present disclosure is characterized by the following properties.

(a) melt flow index ranging from 4.0 to 12.1.

(b) polydispersity index ranging from 4.0 to 7.0.

High hydrogen response of the Ziegler-Natta catalyst system of the present disclosure is reflected by the high melt flow index of the polymer obtained by the polymerization process.

The present disclosure is further described in light of the following examples which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure.

The substituted-silanediyl diacetate compounds used as the first external electron donor for the preparation of the Ziegler-Natta catalyst system in the exemplary embodiments of the present disclosure have the following structures.

Sr. Name Code Structure

1 diethyl 2,2 -(dimethylsilanediyl)diacetate DSD1

2 diethyl 2,T- DSD2

(phenyl(methyl)silanediyl)diacetate

&S02 3 diethyl 2,T- DSD3

(diisopropylsilanediyl)diacetate

Example 1:

Preparation of the Ziegler-Natta procatalyst

0.01 Kg Magnesium ethoxide was transferred to a 1 : 1 v/v mixture of TiCL_t and chlorobenzene (230 ml) and heated at 97°C for 60 minutes. Heating was stopped and the reaction mixture was allowed to settle and cool. The supernatant was decanted and residue was taken to the second step. In the second step a 1: 1 v/v mixture of TiC and chlorobenzene (230 mL) was added to the residue and heated at 97°C for 30 minutes. Heating was stopped and the reaction mixture was allowed to cool and the supernatant was decanted. This step was repeated one more time. The residue thus obtained was washed 4-5 times with 100 ml n-hexane to remove excess titanation material. The solid was dried to get the Ziegler-Natta procatalyst.

Example 2

Process for olefin polymerization using the Ziegler-Natta catalyst system comprising substituted-silanediyl diacetate compounds as first external electron donors

In 100 ml dried n-hexane 1.04g triethylaluminium (9.128 mmol) was added, followed by addition of 0.07 g procatalyst prepared by the procedure described in Example- 1. After 3 minutes, 0.104g (0.4487 mmol) DSDl was added and stirred to obtain the Ziegler-Natta catalyst system. The polymerization was carried out in a reactor equipped with an over-head stirrer. The reactor was back-filled four times with nitrogen and then charged with the 2.5 L n-hexane. The reaction mixture was stirred vigorously at 30°C under 3Kg/cm of propylene for 15 minutes. The reaction vessel was depressurized and 0.07g Ziegler-Natta catalyst system was added to the reactor. 240ml hydrogen was then charged to the reactor at 3 kg/cm . The reactor was charged with propylene and the content was stirred at 70°C under propylene pressure of 6 Kg/cm for 120 min. The propylene supply was cut off and reactor was cooled at 30°C. The solvent was removed and dry polymer resin was collected.

Similar polymerization processes were also carried out by using a Ziegler-Natta catalyst system comprising DSD2 and DSD3, by the procedure mentioned above.

Example 3:

Polypropylene synthesis experiments were performed by using the Ziegler-Natta catalyst system of the present disclosure having the molar ratio of triethylaluminmm (TEAl) and Ti (TEAl/Ti) of 250 and the molar ratio of triethylaluminmm and electron donor (TEAl/ED) of 20. For comparing the results, an experiment with PEEB (p-ethoxy ethyl benzoate) as a first external electron donor was also conducted under the same conditions. The results are shown in Table 1.

Table 1: Catalyst activity and characterization data of polypropylene resin synthesized using Ziegler-Natta catalyst systems comprising substituted silanediyl diacetate compounds.

(mol/mol); BD: Bulk density; XS: Xylene soluble; MFI: Melt flow index.

In the polymerization process using the Ziegler-Natta catalyst system with substituted-silanediyl- diacetate compounds, the percentage xylene soluble i.e. the atactic part of polymer, was found to be in range of 3.7 to 4.8 at TEAl/ED ratio of 20, whereas with PEEB the atactic part of polymer was found to be 3.7 at TEAl/ED ratio of 5. Thus, the the Ziegler-Natta catalyst system of the present disclosure, with substituted silanediyl diacetate compounds, shows comparable isotacticity even when a lesser amount of the electron donor is used.

High hydrogen response of the Ziegler-Natta catalyst system of the present disclosure is reflected by high melt flow index of the polymer obtained by the polymerization process. By using the Ziegler-Natta catalyst system of the present disclosure, an MFI as high as 12.1 was obtained when 240 mL of hydrogen gas was used as a chain termination agent. Under similar conditions using PEEB, a low MFI of 1 was obtained.

Further, it is evident from the data that as the bulkiness of the substituent increases the percentage xylene soluble fraction decreases. This indicates that the steric factor plays an important role in governing the activity as well as isospecific activity of the polymer. Higher isotacticity of the polymer is confirmed by the C-NMR data. The results are shown in Table 2.

Table 2: C-NMR data of polypropylene resin synthesized using Ziegler-Natta catalyst systems comprising substituted silanediyl diacetate compounds.

The results of C-NMR stereo sequence distribution of the polypropylene samples show a relative amount of pentads (mmmm) and triad (mm) in the range of 83-86% and 90-92% respectively for different substituted silanediyl diacetate compounds.

The comparative data of the hydrogen response of polymerization process, and molecular weight and molecular weight distribution is tabulated in Table 3.

Table 3: Characterization data of polypropylene resin synthesized using Ziegler-Natta catalyst systems comprising substituted silanediyl diacetate compounds.

Key: ED: External electron donor; TEA1/ED: Triethylaluminium and electron donor ratio (mol/mol); Mn: number average molecular weight; Mw: weight average molecular weight; MFI: Melt flow index, PDI: polydispersity index The Ziegler-Natta catalyst system of the present disclosure with substituted silanediyl diacetate compounds shows a far better hydrogen response, measured as melt flow index (MFI) (4.0-12.1 g/lOmin), as compared to the PEEB (1.0 g/lOmin). These results show that the Ziegler-Natta catalyst system of the present disclosure requires less amount of hydrogen for the synthesis of isotactic polypropylene.

Further, it is observed that DSD3 has high hydrogen response, which is corroborated by its low molecular weight data and this observation can be attributed to the increased chain transfer propagation rate during the polymerization.

The polydispersity index (PDI) in the range of 5.7 to 6.7 demonstrates that the polypropylene synthesized by the catalyst system of the present disclosure has moderate molecular weight distribution, which is required for obtaining the desired processability as well as mechanical properties of polypropylene.

Data from Table 1 shows that the catalyst system of the present disclosure has less activity as compared to PEEB, but that can be compensated by carrying out the polymerization at high monomer pressure. The catalyst system of the present disclosure provides polymer resins with high bulk densities (0.40- 0.42 g/cc).

Furthermore, the mechanical properties of the polypropylene were determined by the melt rheological measurements. These measurements demonstrate that polypropylene synthesized by using DSD3 has relatively less storage modulus as compared to DSD1 and DSD2 which is evident from the low molecular weight data for DSD3 system obtained by gel permeation chromatography (GPC) measurements. The particle size distribution (PSD) analysis demonstrates that with an increase in the steric hindrance/bulkiness of the substitution in electron donor system, the polymer particle size increases. The polymer particles have different PSD patterns, for example, a highly broad PSD for DSD1 and DSD3 donor systems, in contrast to narrow PSD for the DSD2 donor system. Moreover, scanning electron microscope (SEM) analysis reveals that the DSD2 based Ziegler- Natta catalyst system where the substitution on Silicon is a planer aromatic ring, produces globular homogeneous sized polymer particles whereas the Ziegler-Natta catalyst systems based on DSD1 and DSD3 provide particles with uneven shape. It indicates that the reaction kinetics during the polymerization process depends on the nature of substitution present on Silicon in the first external electron donor.

Example 4

Process for polymerization using the Ziegler-Natta catalyst system comprising substituted- silanediyl diacetate compounds as a first external electron donor along-with cyclohexylmethyldimethoxysilane as a second external electron donor

To 100 ml dried n-hexane was added 1.04g (9.128 mmol) triethylaluminium, followed by addition of 0.07 g procatalyst obtained by the procedure described in Example 1. After 3 minutes was added 0.102 g (0.4487 mmol) of mixed donor system of DSD1 (0.0919 g) and cyclohexylmethyldimethoxysilane (0.0102 g) (9: 1 w/w) followed by mixing to provide the Ziegler-Natta catalyst system.

The polymerization was carried out in a reactor equipped with an over-head magnetic stirrer. The reactor was back-filled four times with nitrogen and charged with the 2.5 L n-hexane. The reaction mixture was vigorously stirred at 30°C under 3 Kg/cm of propylene for 15 minutes. The reaction vessel was depressurized and the 0.07 g of Ziegler-Natta catalyst system in 2 ml of slurry in decane was injected in reactor. 240ml hydrogen was then charged to the reactor at 3 kg/cm . The reactor was charged with propylene and the content was stirred at 70 °C under propylene pressure of 6 Kg/cm for 120 min. The propylene supply was cut off and reactor was cooled at 30 °C. The solvent was removed and dry polymer resin was collected. Similar polymerization processes in the presence of a Ziegler-Natta catalyst system comprising DSD-2 and DSD-3 were carried out by following the procedure mentioned above.

Example 5:

A Ziegler-Natta catalyst system with mixed external electron donors, containing substituted silanediyl diacetate compounds as a first external electron donor and cyclohexylmethyldimethoxysilane (CHMDMS) as a second external electron donor was employed for the preparation of the polypropylene in the ratio of TEA1/Ti=250 and TEA1/ED=20. The results are shown in Table 4.

Table 4: Catalyst activity and characterization data of polypropylene resin synthesized using Ziegler-Natta catalyst system comprising mixed external electron donors.

Key: ED: External electron donor; TEA1/ED: Triethylaluminium and electron donor ratio (mol/mol); BD: Bulk density; APS: average particle size; XS: Xylene soluble; MFI: Melt flow index, CHMDMS=Cyclohexylmethyldimethoxysilane The Ziegler-Natta catalyst system was prepared using substituted silanediyl diacetate compounds and cyclohexyl methyl dimethoxy silane (CHMDMS) in the molar ratio of 9: 1 (wt %). Polymerization using the Ziegler-Natta catalyst system containing mixed electron donor system produced polypropylene with enhanced activity as compared to substituted silanediyl diacetate compounds alone.

A comparative data of hydrogen response of the polymerization process, and molecular weight and molecular weight distribution is tabulated in Table 5.

The polydispersity index data indicates that the polypropylene synthesized by mixed donor systems has a moderate molecular weight distribution (4.9-5.7) that optimizes processability as well as mechanical properties of polypropylene along with a good hydrogen response.

Table 5: Characterization data of polypropylene resin synthesized using Ziegler-Natta catalyst systems comprising mixed external electron donors.

Key: ED: External electron donor; TEA1/ED: Triethylaluminium and electron donor ratio (mol/mol); Mn: number average molecular weight; Mw: weight average molecular weight; MFI: Melt flow index, PDI: polydispersity index . The C-NMR data of stereo-sequence distribution of the polypropylene samples show that the polypropylene has high tacticity. The results are shown in Table 6.

Table 6: C-NMR data of polypropylene resin synthesized using Ziegler-Natta catalyst systems comprising mixed external electron donors.

methylcyclohexyldimethoxysilane

ECONOMICAL SIGNIFICANCE AND TECHNICAL ADVANCEMENT:

The process of the present disclosure employs comparatively less amount of electron donor since the electron donor is used externally making this process cost-effective.

The process of the present disclosure involves a single step addition of electron donor to the catalyst system which makes this process simple as compared to the multistep addition when the electron donor is used internally.

The process of the present disclosure has improved hydrogen response.

The process of the present disclosure produces comparatively less atactic polymer. Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The use of the expression "at least" or "at least one" suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.

Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.

While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

Claims

CLAIMS:

1. A Ziegler-Natta catalyst system comprising:

(a) at least one pro-catalyst;

(b) at least one organo-aluminium co-catalyst; and

(c) at least one substituted-silanediyl-diacetate compound of formula- 1 as a first external electron donor,

Formula- 1

wherein,

R₃ and R4 are independently selected from Ci-C₆ alkyl groups, and aryl groups.

2. The Ziegler-Natta catalyst system as claimed in claim 1, wherein the first external electron donor is at least one selected from the group consisting of diethyl 2,2"- (dimethylsilanediyl)diacetate, diethyl 2,2 -(phenyl(methyl)silanediyl)diacetate and diethyl 2,2 -(diisopropylsilanediyl)diacetate.

3. The Ziegler-Natta catalyst system as claimed in claim 1, wherein the pro-catalyst is a spheroidal magnesium alkoxide based Ziegler-Natta pro-catalyst comprising titanium.

4. The Ziegler-Natta catalyst system as claimed in claim 1, wherein the organo-aluminium co-catalyst is at least one selected from the group consisting of triethylaluminium, tridecylaluminium, tri-n-butylaluminium, tri-isopropylaluminium, tri- isoprenylaluminium, tri-isobutylaluminium, ethyl aluminium sesquichloride, diethylaluminium chloride, di-isobutyl aluminium chloride, triphenylaluminium, tri-n- octylaluminium and tri-n-decylaluminium.

5. The Ziegler-Natta catalyst system as claimed in claim 1, wherein the organo-aluminium co-catalyst is triethylaluminium.

6. The Ziegler-Natta catalyst system as claimed in claim 3, wherein the ratio of the amount of the organo-aluminium co-catalyst and the amount of elemental titanium ranges from 500: 1 to 10: 1.

7. The Ziegler-Natta catalyst system as claimed in claiml, wherein the molar ratio of the amount of the pro-catalyst and the amount of the first external electron donor ranges from 1 :5 to 1:20.

8. The Ziegler-Natta catalyst system as claimed in claim 1, wherein the molar ratio of the amount of the organo-aluminium co-catalyst and the amount of the first external electron donor ranges from 1 : 1 to 50: 1.

9. The Ziegler-Natta catalyst system as claimed in claim 1, wherein said system optionally comprises at least one second external electron donor selected from the group consisting of cyclohexylmethyldimethoxysilane, dicyclopentyldimethoxysilane, diethyldiethoxysilane and diisobutyldimethoxysilane.

10. The Ziegler-Natta catalyst system as claimed in claim 9, wherein the second external electron donor is cyclohexylmethyldimethoxysilane.

11. The Ziegler-Natta catalyst system as claimed in claim 9, wherein the ratio of the amount of the first external electron donor and the amount of the second external electron donor ranges from 5: 1 to 20: 1.

12. A method of preparing a Ziegler-Natta catalyst system, said method comprising the following steps:

(1) mixing at least one organo-aluminium co-catalyst, at least one substituted silanediyl diacetate compound as a first external electron donor and optionally at least one second electron donor to get a mixture; and (2) adding at least one Ziegler-Natta pro-catalyst to said mixture to obtain the Ziegler- Natta catalyst system.

13. A process for olefin polymerization, said process comprising:

(a) preparing a Ziegler-Natta catalyst system comprising at least one Ziegler-Natta pro- catalyst, at least one organo-aluminium co-catalyst, at least one substituted silanediyl diacetate compound as a first external electron donor and optionally at least one second electron donor; and

(b) subjecting olefin to polymerization in the presence of the Ziegler-Natta catalyst system and at least one chain transfer agent at a temperature ranging from 50°C to 100°C; to obtain polyolefin.

14. The process as claimed in claim 13, wherein the olefin is selected from the group consisting of propylene, ethylene, 1-butene, 1-hexene, and 1-octene.

15. The process as claimed in claim 13, wherein the chain transfer agent is hydrogen gas.

16. The process as claimed in claim 13, wherein the polymerization is carried out under olefin pressure in the range from 5 Kg/cm 2 to 15 Kg/cm 2.

17. The process as claimed in claim 13, wherein the polymerization is carried out for a time period ranging from 10 minutes to 120 minutes.

18. The process as claimed in claim 14, wherein the olefin is propylene and the polyolefin is polypropylene.

19. Polypropylene obtained by the process as claimed in claim 18, wherein said polypropylene is characterized by:

(a) melt flow index ranging from 4.0 to 12.1; and

(b) polydispersity index ranging from 4.0 to 7.0.