Ziegler–Natta catalyst

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.

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.

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 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, −[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.

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 (O₂), nitrogen (N₂), phosphorus (P), and sulfur (S). 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. 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 Cp₂ZrCl₂ represents a typical precatalyst. It is unreactive toward alkenes. The dihalide reacts with MAO and is transformed into a metallocenium ion Cp₂CH₃, 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.

Orbital interactions of initial coordination

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.

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_nTi–CH₂−CHR–polymer + CH₂=CHR → L_nTi–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_nTi–CH₂−CHR–polymer + H₂ → L_nTi−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
• Polypropylene
• Copolymers of ethylene and 1-alkenes
• Polybutene-1
• Polymethylpentene
• Polycycloolefins
• Polybutadiene
• Polyisoprene
• Amorphous poly-alpha-olefins (APAO)
• Polyacetylene