Ziegler–Natta catalyst

A Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, is a catalyst used in the synthesis of polymers 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, Al(C₂H₅)₃. This class of catalyst dominates the industry.
• Homogeneous catalysts usually based on complexes of the group 4 metals titanium, zirconium or hafnium. 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.

Ziegler–Natta catalysts are used to polymerize terminal alkenes (ethylene and alkenes with the vinyl double bond):

:n CH₂=CHR → −[CH₂−CHR]_n−;

History

The 1963 Nobel Prize in Chemistry was awarded to German Karl Ziegler, for his discovery of first titanium-based catalysts, and Italian Giulio Natta, for using them to prepare stereoregular polymers from propylene. 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 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 (TiCl₄) and diethylaluminium chloride (Al(C₂H₅)₂Cl) gave comparable activities for the production of polyethylene. Natta used crystalline α-TiCl₃ in combination with Al(C₂H₅)₃ to produce first isotactic polypropylene. Usually Ziegler catalysts refer to titanium-based systems for conversions of ethylene and Ziegler–Natta catalysts refer to systems for conversions of propylene.

Also, in the 1960s, BASF developed a gas-phase, mechanically-stirred polymerization process for making polypropylene. 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 to produce polyethylene. In the mid-1980s, the Unipol process was further extended to produce polypropylene.

In the 1970s, magnesium chloride (MgCl₂) was discovered to greatly enhance the activity of the titanium-based catalysts. These catalysts were so active that the removal of unwanted amorphous polymer and residual titanium from the product (so-called deashing) was no longer necessary, enabling the commercialization of linear low-density polyethylene (LLDPE) resins and allowed the development of fully amorphous copolymers.

The fluidized-bed process remains one of the two most widely used processes for producing polypropylene.

Stereochemistry of poly-1-alkenes

Natta first used polymerization catalysts based on titanium chlorides to polymerize propylene 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.

thumb|upright=1.8|left|Short segments of polypropylene, showing examples of isotactic (above) and syndiotactic (below) [[tacticity.]]

The concept of stereoregularity in polymer chains is illustrated in the picture on the left with polypropylene. Stereoregular poly(1-alkene) can be isotactic or syndiotactic depending on the relative orientation of the alkyl groups in polymer chains consisting of units −[CH₂−CHR]−, like the CH₃ 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.

Ultimately, the stereoregularity of the polymer is determined by the catalyst used to prepare it. Most polymers produced with a Ziegler-Natta catalyst are enantioselective site-controlled, meaning the stereochemistry of each added monomer is dependent primarily on the stereochemistry of the catalyst (barring stereo errors) rather than the stereochemistry of the previous monomer. Thus, the chirality of the catalyst greatly influences tacticity. For example, achiral bis(2-phenylindenyl)zirconium dichloride produces isotactic polymers whereas its chiral variant produces syndiotactic polymers. The attachment of bulky ligands to the metal center results in steric hindrance, designed to restrict the orientation from which the incoming alkene can attack the metal center.

Unique behaviors of the catalyst can also strongly impact tacticity. For example, the open site of VCl₄/Al(C₂H₅)₂Cl alternates between the axial and equatorial site with every monomer addition. As a result, each successive propylene monomer will insert into the vanadium-carbon bond from opposite sides. The alternating active site geometry thus produces a polymer chain with alternating enantiomeric configurations, producing syndiotactic polypropylene. See #Mechanism of Ziegler–Natta polymerization for the detailed mechanism.

Heterogeneous catalysts

The first and dominant class of titanium-based catalysts (and some vanadium-based catalysts) for alkene polymerization can be roughly subdivided into two subclasses, both heterogeneous catalysts:

• catalysts suitable for homopolymerization of ethylene and for ethylene/1-alkene copolymerization reactions leading to copolymers with a low 1-alkene content, 2–4 mol% (LLDPE 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 spheres of amorphous silica with a diameter of 30–40 mm. During the catalyst synthesis, both the titanium compounds and MgCl₂ are packed into the silica pores. All these catalysts are activated with organoaluminum compounds such as Al(C₂H₅)₃.]]

Metallocene catalysts

These catalysts are metallocenes together with a cocatalyst, typically MAO, −[O−Al(CH₃)]_n−. The idealized metallocene catalysts have the composition Cp₂MCl₂ (M = Ti, Zr, Hf) such as titanocene dichloride. Typically, the organic ligands are derivatives of cyclopentadienyl. In some complexes, the two cyclopentadiene (Cp) rings are linked with bridges, like −CH₂−CH₂− or >SiPh₂. 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.

Metallocenes are referred to as single-site catalysts, where the active site across each metallocene complex is structurally identical. The surface of a heterogeneous catalyst is not uniform, so there are many different active sites with differing electronic environments. As the structure of the active site directly affects the stereoregularity and tacticity, heterogeneous catalysts may produce polymers with a broader range of tacticities. Metallocenes do not suffer from this weakness, and so often result in polymers with greater stereoregularity and narrower molecular weight distributions. The chirality of the metallocene is a major determinant in whether the polymer is isotactic or syndiotactic. 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.

Orbital interactions of initial coordination

thumb|The electrons in the filled pi-orbital (white) of the alkene are donated to the empty d-orbital (grey) of the titanium.

thumb|The electrons in one of the filled d-orbitals (white) of titanium are accepted by the antibonding pi orbital (grey) of the alkene.

Upon coordination of the alkene to the catalyst, the alkene donates electrons from its pi-bonding orbital into the metal center's empty d orbital while also accepting electrons from the metal's filled d orbitals into its empty pi anti-bonding orbital. The synergy of these interactions stabilize the overall bond between the alkene and metal center and thus facilitates initial coordination.