A Ziegler–Natta catalyst, named after
Karl Ziegler
Karl Waldemar Ziegler (26 November 1898 – 12 August 1973) was a German chemist who won the Nobel Prize in Chemistry in 1963, with Giulio Natta, for work on polymers. The Nobel Committee recognized his "excellent work on organometallic compound ...
and
Giulio Natta, is a
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
used in the synthesis of
polymers
A polymer (; Greek '' poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
of 1-alkenes (
alpha-olefins). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility:
* Heterogeneous
supported catalysts based on titanium compounds are used in polymerization reactions in combination with cocatalysts,
organoaluminum compounds such as
triethylaluminium
Triethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al2( C2H5)6 (abbreviated as Al2Et6 or TEA), as it exists as a dimer. This colorless liquid is pyrophoric. It is an industrially ...
, Al(C
2H
5)
3. This class of catalyst dominates the industry.
* Homogeneous catalysts usually based on complexes of the
group 4 metals titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion i ...
,
zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...
or
hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri M ...
. They are usually used in combination with a different organoaluminum cocatalyst,
methylaluminoxane (or methylalumoxane, MAO). These catalysts traditionally contain
metallocenes but also feature
multidentate oxygen- and nitrogen-based
ligands
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
.
Ziegler–Natta catalysts are used to polymerize terminal
alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic ...
s (ethylene and alkenes with the
vinyl
Vinyl may refer to:
Chemistry
* Polyvinyl chloride (PVC), a particular vinyl polymer
* Vinyl cation, a type of carbocation
* Vinyl group, a broad class of organic molecules in chemistry
* Vinyl polymer, a group of polymers derived from vinyl ...
double bond):
:''n'' CH
2=CHR → −
2−CHR">H2−CHRsub>''n''−;
History
The 1963
Nobel Prize in Chemistry
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
was awarded to German
Karl Ziegler
Karl Waldemar Ziegler (26 November 1898 – 12 August 1973) was a German chemist who won the Nobel Prize in Chemistry in 1963, with Giulio Natta, for work on polymers. The Nobel Committee recognized his "excellent work on organometallic compound ...
, for his discovery of first titanium-based catalysts, and Italian
Giulio Natta, for using them to prepare stereoregular polymers from
propylene
Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petro ...
. Ziegler–Natta catalysts have been used in the commercial manufacture of various polyolefins since 1956. As of 2010, the total volume of plastics, elastomers, and rubbers produced from alkenes with these and related (especially Phillips) catalysts worldwide exceeds 100 million tonnes. Together, these polymers represent the largest-volume commodity plastics as well as the largest-volume commodity chemicals in the world.
In the early 1950s workers at
Phillips Petroleum
Phillips Petroleum Company was an American oil company incorporated in 1917 that expanded into petroleum refining, marketing and transportation, natural gas gathering and the chemicals sectors. It was Phillips Petroleum that first found oil in th ...
discovered that chromium catalysts are highly effective for the low-temperature polymerization of ethylene, which launched major industrial technologies culminating in the
Phillips catalyst. A few years later, Ziegler discovered that a combination of
titanium tetrachloride
Titanium tetrachloride is the inorganic compound with the formula . It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds ...
(TiCl
4) and
diethylaluminium chloride (Al(C
2H
5)
2Cl) gave comparable activities for the production of polyethylene. Natta used crystalline
α-TiCl3 in combination with
Al(C2H5)3 to produce first
isotactic
Tacticity (from el, τακτικός, taktikos, "relating to arrangement or order") is the relative stereochemistry of adjacent chiral centers within a macromolecule. The practical significance of tacticity rests on the effects on the physical p ...
polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.
Polypropylene
belongs to the group of polyolefins a ...
.
Usually Ziegler catalysts refer to
titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion i ...
-based systems for conversions of
ethylene
Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds).
Ethylene ...
and Ziegler–Natta catalysts refer to systems for conversions of
propylene
Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petro ...
. In the 1970s,
magnesium chloride was discovered to greatly enhance the activity of the titanium-based catalysts. These catalysts were so active that the residual titanium was no longer removed from the product. They enabled the commercialization of
linear low-density polyethylene
Linear low-density polyethylene (LLDPE) is a substantially linear polymer ( polyethylene), with significant numbers of short branches, commonly made by copolymerization of ethylene with longer-chain olefins. Linear low-density polyethylene differ ...
(LLDPE) resins and allowed the development of noncrystalline copolymers.
Also, in the 1960s,
BASF
BASF SE () is a German multinational chemical company and the largest chemical producer in the world. Its headquarters is located in Ludwigshafen, Germany.
The BASF Group comprises subsidiaries and joint ventures in more than 80 countries ...
developed a gas-phase, mechanically-stirred
polymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many f ...
process for making
polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.
Polypropylene
belongs to the group of polyolefins a ...
. In that process, the particle bed in the reactor was either not fluidized or not fully fluidized. In 1968, the first gas-phase fluidized-bed polymerization process, the Unipol process, was commercialized by
Union Carbide
Union Carbide Corporation is an American chemical corporation wholly owned subsidiary (since February 6, 2001) by Dow Chemical Company. Union Carbide produces chemicals and polymers that undergo one or more further conversions by customers befo ...
to produce polyethylene. In the mid-1980s, the Unipol process was further extended to produce
polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.
Polypropylene
belongs to the group of polyolefins a ...
.
The features of the fluidized-bed process, including its simplicity and product quality, made it widely accepted all over the world. As of today, the fluidized-bed process is one of the two most widely used technologies for producing
polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.
Polypropylene
belongs to the group of polyolefins a ...
.
In the 1970s,
magnesium chloride-supported Ziegler–Natta catalysts were introduced. These catalysts exhibit activities so enhanced that costly steps could be omitted from the workup. These omitted processes included deashing (removal of residual catalyst) and removal of unwanted amorphous polymer.
Stereochemistry of poly-1-alkenes
Natta first used polymerization catalysts based on titanium chlorides to polymerize
propylene
Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petro ...
and other 1-alkenes. He discovered that these polymers are crystalline materials and ascribed their crystallinity to a special feature of the polymer structure called
stereoregularity.
The concept of stereoregularity in polymer chains is illustrated in the picture on the left with polypropylene. Stereoregular poly(1-alkene) can be
isotactic
Tacticity (from el, τακτικός, taktikos, "relating to arrangement or order") is the relative stereochemistry of adjacent chiral centers within a macromolecule. The practical significance of tacticity rests on the effects on the physical p ...
or
syndiotactic
Tacticity (from el, τακτικός, taktikos, "relating to arrangement or order") is the relative stereochemistry of adjacent chiral centers within a macromolecule. The practical significance of tacticity rests on the effects on the physical p ...
depending on the relative orientation of the
alkyl
In organic chemistry, an alkyl group is an alkane missing one hydrogen.
The term ''alkyl'' is intentionally unspecific to include many possible substitutions.
An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...
groups in polymer chains consisting of units −
2−CHR">H2−CHR��, like the CH
3 groups in the figure. In the isotactic polymers, all stereogenic centers CHR share the same configuration. The stereogenic centers in syndiotactic polymers alternate their relative configuration. A polymer that lacks any regular arrangement in the position of its alkyl substituents (R) is called atactic. Both isotactic and syndiotactic polypropylene are crystalline, whereas atactic polypropylene, which can also be prepared with special Ziegler–Natta catalysts, is amorphous. The stereoregularity of the polymer is determined by the catalyst used to prepare it.
Classes
Heterogeneous catalysts
The first and dominant class of titanium-based catalysts (and some
vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pass ...
-based catalysts) for alkene polymerization can be roughly subdivided into two subclasses:
* catalysts suitable for homopolymerization of ethylene and for ethylene/1-alkene
copolymer
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are ...
ization reactions leading to copolymers with a low 1-alkene content, 2–4 mol% (
LLDPE
Linear low-density polyethylene (LLDPE) is a substantially linear polymer ( polyethylene), with significant numbers of short branches, commonly made by copolymerization of ethylene with longer-chain olefins. Linear low-density polyethylene differ ...
resins), and
* catalysts suitable for the synthesis of isotactic 1-alkenes.
The overlap between these two subclasses is relatively small because the requirements to the respective catalysts differ widely.
Commercial catalysts are supported by being bound to a solid with a high surface area. Both
TiCl4 and
TiCl3 give active catalysts.
The support in the majority of the catalysts is
MgCl2. A third component of most catalysts is a carrier, a material that determines the size and the shape of catalyst particles. The preferred carrier is
microporous
A microporous material is a material containing pores with diameters less than 2 nm. Examples of microporous materials include zeolites and metal-organic frameworks.
Porous materials are classified into several kinds by their size. The reco ...
spheres of
amorphous silica with a diameter of 30–40 mm. During the catalyst synthesis, both the titanium compounds and MgCl
2 are packed into the silica pores. All these catalysts are activated with organoaluminum compounds such as
Al(C2H5)3.
All modern supported Ziegler–Natta catalysts designed for polymerization of propylene and higher 1-alkenes are prepared with
TiCl4 as the active ingredient and
MgCl2 as a support. Another component of all such catalysts is an organic modifier, usually an
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...
of an
aromatic diacid or a
diether. The modifiers react both with inorganic ingredients of the solid catalysts as well as with organoaluminum cocatalysts.
These catalysts polymerize propylene and other 1-alkenes to highly crystalline isotactic polymers.
Homogeneous catalysts
A second class of Ziegler–Natta catalysts are soluble in the reaction medium. Traditionally such homogeneous catalysts were derived from
metallocenes, but the structures of active catalysts have been significantly broadened to include nitrogen-based ligands.
Metallocene catalysts
These catalysts are metallocenes together with a cocatalyst, typically MAO, −
3)">−Al(CH3)sub>''n''−. The idealized metallocene catalysts have the composition
Cp2MCl
2 (M = Ti,
Zr,
Hf) such as
titanocene dichloride. Typically, the organic ligands are derivatives of
cyclopentadienyl Cyclopentadienyl can refer to
* Cyclopentadienyl anion, or cyclopentadienide,
** Cyclopentadienyl ligand
* Cyclopentadienyl radical, •
* Cyclopentadienyl cation,
See also
* Pentadienyl
{{Chemistry index ...
. In some complexes, the two
cyclopentadiene
Cyclopentadiene is an organic compound with the formula C5H6.LeRoy H. Scharpen and Victor W. Laurie (1965): "Structure of cyclopentadiene". ''The Journal of Chemical Physics'', volume 43, issue 8, pages 2765-2766. It is often abbreviated CpH beca ...
(Cp) rings are linked with bridges, like −CH
2−CH
2− or >SiPh
2. Depending on the type of their cyclopentadienyl ligands, for example by using an
''ansa''-bridge, metallocene catalysts can produce either isotactic or syndiotactic polymers of propylene and other 1-alkenes.
Non-metallocene catalysts
Ziegler–Natta catalysts of the third class, non-metallocene catalysts, use a variety of complexes of various metals, ranging from scandium to lanthanoid and actinoid metals, and a large variety of ligands containing
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
,
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
,
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...
, and
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...
. The complexes are activated using MAO, as is done for metallocene catalysts.
Most Ziegler–Natta catalysts and all the alkylaluminium cocatalysts are unstable in air, and the alkylaluminium compounds are
pyrophoric
A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...
. The catalysts, therefore, are always prepared and handled under an inert atmosphere.
Mechanism of Ziegler–Natta polymerization
The structure of active centers in Ziegler–Natta catalysts is well established only for metallocene catalysts. An idealized and simplified metallocene complex Cp
2ZrCl
2 represents a typical precatalyst. It is unreactive toward alkenes. The dihalide reacts with MAO and is transformed into a metallocenium ion Cp
2CH
3, which is ion-paired to some derivative(s) of MAO. A polymer molecule grows by numerous insertion reactions of C=C bonds of 1-alkene molecules into the Zr–C bond in the ion:
Many thousands of alkene insertion reactions occur at each active center resulting in the formation of long polymer chains attached to the center. The
Cossee–Arlman mechanism describes the growth of stereospecific polymers.
This mechanism states that the polymer grows through alkene coordination at a vacant site at the titanium atom, which is followed by insertion of the C=C bond into the Ti−C bond at the active center.
Termination processes
On occasion, the polymer chain is disengaged from the active centers in the chain termination reaction. Several pathways exist for termination:
:Cp2−(CH2−CHR)''n''−CH3 + CH2=CHR → Cp2−CH2−CH2R + CH2=CR–polymer
Another type of chain termination reaction called a β-hydride elimination reaction also occurs periodically:
:Cp2−(CH2−CHR)n−CH3 → Cp2−H + CH2=CR–polymer
Polymerization reactions of alkenes with solid titanium-based catalysts occur at special titanium centers located on the exterior of the catalyst crystallites. Some titanium atoms in these crystallites react with organoaluminum cocatalysts with the formation of Ti–C bonds. The polymerization reaction of alkenes occurs similarly to the reactions in metallocene catalysts:
:L''n''Ti–CH2−CHR–polymer + CH2=CHR → L''n''Ti–CH2-CHR–CH2−CHR–polymer
The two chain termination reactions occur quite rarely in Ziegler–Natta catalysis and the formed polymers have a too high molecular weight to be of commercial use. To reduce the molecular weight, hydrogen is added to the polymerization reaction:
:L''n''Ti–CH2−CHR–polymer + H2 → L''n''Ti−H + CH3−CHR–polymer
Another termination process involves the action of protic (acidic) reagents, which can be intentionally added or adventitious.
Commercial polymers prepared with Ziegler–Natta catalysts
* Polyethylene
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including b ...
* Polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.
Polypropylene
belongs to the group of polyolefins a ...
* Copolymers of ethylene and 1-alkenes
* Polybutene-1
Polybutylene (polybutene-1, poly(1-butene), PB-1) is a polyolefin or saturated polymer with the chemical formula (CH2CH(Et))n. Not be confused with polybutene, PB-1 is mainly used in piping.
Production
Polybutylene is produced by polymerisation ...
* Polymethylpentene
* Polycycloolefins
* Polybutadiene
Polybutadiene utadiene rubber BRis a synthetic rubber. Polybutadiene rubber is a polymer formed from the polymerization of the monomer 1,3-butadiene. Polybutadiene has a high resistance to wear and is used especially in the manufacture of ti ...
* Polyisoprene
Polyisoprene is strictly speaking a collective name for polymers that are produced by polymerization of isoprene. In practice polyisoprene is commonly used to refer to synthetic ''cis''-1,4-polyisoprene, made by the industrial polymerisation of i ...
* Amorphous poly-alpha-olefins ( APAO)
* Polyacetylene
References
Further reading
*
*
*
*
{{DEFAULTSORT:Ziegler-Natta catalyst
Coordination complexes
Catalysts
Polymer chemistry
Industrial processes
1953 in science
1953 in Germany