Keywords
Ziegler–Natta catalyst - transition-metal catalyst - polymer
Karl Ziegler, a scientist from Germany, discovered that combining TiCl4 and Al(C2H5)3 produced a highly active catalyst that could polymerize ethylene in a stereoregular
manner at atmospheric pressure. Later, an Italian chemist named Giulio Natta expanded
upon Ziegler’s work by developing methods for using the catalyst with other olefins
like propylene. Natta also contributed to our understanding of the mechanism behind
the polymerization reaction, which led to the development of various forms of the
Ziegler catalyst. Over time, scientists have gained more control over stereospecific
polymerization thanks to these discoveries.[1]
[2]
[3]
[4]
The Ziegler–Natta catalyst is comprised of transition-metal chlorides, including titanium,
chromium, vanadium, and zirconium chlorides, that have a distinguished lineage, along
with organometallic complexes of triethylaluminium. The crystal structure of the titanium
chloride compound contains Ti atoms attached to five chlorine atoms on the surface,
with one empty orbital. When the compound reacts with Al(C2H5)3, the latter donates an Et group to Ti, causing one chlorine group to detach from
Ti.[5]
[6]
[7] This reaction activates the catalyst, as illustrated in Scheme [1], and initiates chain propagation and termination steps, also depicted in the same
diagram.
Keshav Taruneshwar Jha is a research Scholar and is pursuing his MPharm (Pharmaceutical Chemistry) from
ISF College of Pharmacy, Moga, Punjab and is carrying out research under the supervision
of Dr. Pooja A. Chawla.
Abhimannu Shome is a research Scholar and is pursuing his MPharm (Pharmaceutical Chemistry) from
ISF College of Pharmacy, Moga, Punjab and is carrying out research under the supervision
of Dr. Pooja A Chawla.
Pooja A Chawla is professor and head in the Department of Pharmaceutical Chemistry, ISF College
of Pharmacy Moga, Punjab, India. She has supervised more than 63 research scholars.
Scheme 1 The activation of Ziegler–Natta catalysts (ZNC)
These polymers are useful for manufacturing plastics, fibers, and films. Ziegler and
Natta’s work on this catalyst earned them the Nobel Prize in Chemistry in 1963.[8]
[9] The Ziegler–Natta catalysts have undergone several advancements, resulting in four
distinct generations of catalysts.
The first generation utilized diethyl aluminum and titanium chloride as co-catalysts.
In the second generation of catalysts, titanium chloride/AlEt2Cl was combined with an internal electron donor, such as ether or ester,[10]
[11] which enhanced the activity and stereospecificity of the catalysts. The third generation
of catalysts was introduced in 1968,[12] and it utilized a catalytic system made up of TiCl4 complexes supported by MgCl2. This method enabled the production of linear polyethylene and isotactic polypropylene.
The fourth generation[13]
[14] of catalysts utilized homogeneous catalysts for conducting olefin polymerizations.
Over the years, several noteworthy applications of Ziegler–Natta catalysts have been
developed.[8]
For example, these catalysts have been used to create high-density polyethylene, which
is used in products such as bottles and pipes. Additionally, they have been used to
create polypropylene, which is used in a wide range of products, including packaging
materials and automotive parts.[15]
[16] Overall, the Ziegler–Natta catalysts have played a significant role in the development
of modern polymer science and have contributed to the creation of numerous everyday
products.
Table 1 Applications of Zieger–Natta Catalysts
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(A) Yan et al. developed two aminosilane-based external donors, di(piperidyl) dimethoxysilane
(DPPDMS) and dipyrrolyldimethoxysilane (DPRDMS), for Ziegler–Natta catalysts used
in the homopolymerization of hexene-1 to produce elastomeric polyhexene-1.[17]
Procedure for the Synthesis of External Donors (ED)
DPPDMS
N-Heptane, piperidine, n-butyl lithium, and tetramethoxysilane are mixed in a five-neck round-bottom flask
at room temperature followed by transferring the resulting liquid to a three-neck
round-bottom flask and washing with n-heptane. After removing the solvent and unreacted chemicals, a colorless transparent
liquid is obtained via low-pressure distillation.
DPRDMS
The kinetic parameters used for the creation of DPPDMS were the same as those employed
in this experiment.
Polymerization of Hexene-1
1-Hexene, external donors, and triethylaluminium are combined in a round-bottom flask
filled with hexane and stirred while in an environment of nitrogen, water, and little
oxygen. After 10 min of 20–50 °C whirling, add tested catalyst. Following a 2 h reaction,
add 10% HCl, filter, dry, and weigh the liquid.
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(B) A new Ziegler–Natta catalyst for polymerization of ethylene and ethylene/1-hexene
was produced by Wang and colleagues.[18] The novel catalyst comprises of (SiO2/MgO/MgCl2)·TiClx
Preparation of Catalyst
Dehydrated silica was heated in a fluidized-bed quartz reactor for 2 h, mixed with
magnesium acetate solution, and evaporated. The resulting SiO2/MgO samples were heated and stored. TiCl4 was then added, refluxed, rinsed, and dried to produce a stored fluid powder.
Polymerization of Ethylene and 1-Hexene
During polymerization, a pre-measured catalyst was placed in a 250 mL three-neck round-bottom
flask in an oil bath at 70 °C. Ethylene monomer was added to the reactor until the
pressure reached 0.12 MPa. Co-catalysts (TEA or TIBA) were added along with n-hexane and n-heptane, and the solution was saturated with ethylene pressure. Polymerization was
carried out for 1 h at 70 °C.
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(C) Tavakoli and team synthesized a bisupported ethoxide-type Ziegler–Natta catalyst
composed of TiCl4/MCM-41/MgCl2 for in situ polymerization of ethylene.[19]
Preparation of Catalyst
ZME, ZM, and ZE are three types of Ziegler–Natta catalysts. ZME is made by adding
magnesium ethoxide with toluene and hexane to preheated MCM-41, followed by addition
of TiCl4 and DIBP as an electron donor. After heating and washing with hexane, the ZME catalyst
residue is obtained. Two more single-supported catalysts, ZM and ZE, were produced
to assess productivity levels.
In situ Polymerization of Ethylene
Polymerization using ethylene and TEA as co-catalyst carried out in a three-neck round-bottom
flask on a burner followed by purging of nitrogen gas to ensure inert conditions.
Polymerization was initiated by injecting a small amount of catalyst into the reactor
containing hexane and co-catalyst at ambient temperature and pressure. The polymerization
was stopped by treating with acidified ethanol. The polymer was collected, filtered,
and dried at 60 °C under vacuum.[20]
[21]
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(D) Meng et al. modified Ziegler–Natta catalysts for ethylene–hexene (E–H) copolymerization
using siloxane compounds and internal donors like esters, phosphates, and diethers.[22]
Preparation of Catalyst
Catalyst made in a 250 mL four-neck round-bottom flask with anhydrous MgCl2, n-hexane and isobutyl alcohol at room temperature. Internal electron donors 1 and 2
were added, followed by TiCl4 at 70 °C for 2 h. Filtered and washed with n-hexene.
Preparation of Copolymer
Copolymer was synthesized at 45 °C in a 250 mL three-neck flask with ethylene, n-hexane, 1-hexene, and co-catalyst TEA or TIBA. The reaction was stopped by adding
acidified ethanol. The polymer was filtered, washed with ethanol, and dried at 60
°C under vacuum.
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(E) Recently in 2022, Abazari and coworkers reported a high-performance three-metallic
Ziegler–Natta catalyst for ethylene polymerization was synthesized using titanium
tetrachloride as the active center, magnesium ethoxide as the support, and ethylaluminum
sesquichloride (EASC) as a chlorinating agent.[23]
Preparation of Catalyst
A mixture of diesel oil and magnesium ethoxide was stirred for 8 h at room temperature
and then for 10 h at 80 °C. Next, a solution of titanium tetrachloride was added and
heated at 110 °C for 1 h, followed by the addition of ethylaluminum sesquichloride
and holding at 110 °C for 2 h. The resulting solid catalyst was washed with n-hexane and dried.
Preparation of Copolymer
Ethylene was polymerized in a slurry using n-hexane as the medium at a pressure of 8 bar. The reactor had a thermocouple, a temperature-control
unit, and a mechanical stirrer. The reagents, including TEA, catalyst, and ethylene,
were added sequentially at 60 °C, and the reaction was run for 2 h. The reaction was
stopped by venting the reactor. Polyethylene samples were then washed, filtered, and
dried. The [Al]/[Ti] ratio for the experiment was 714 mol/mol.
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Conclusion
To conclude, the spotlight article provides an overview of the significance and versatility
of Ziegler–Natta catalysts in contemporary polymer science (Table [1]). The paper explores the fundamental principles of Ziegler–Natta catalysis, including
the activation, insertion, chain propagation, and termination steps involved in the
polymerization of olefins. Furthermore, the paper emphasizes the crucial role of Ziegler–Natta
catalysts in controlling the stereoregularity of polymers, enabling the synthesis
of tailor-made materials with specific properties. With their wide-ranging applications
in the production of polyethylene and polypropylene, Ziegler–Natta catalysts continue
to drive advancements in materials science, offering opportunities for innovative
research and development in various industries.
