Ziegler–Natta catalyst
A Ziegler–Natta catalyst, named after
- Heterogeneous organoaluminum compounds such as triethylaluminium, Al(C2H5)3. This class of catalyst dominates the industry.[1]
- Homogeneous catalysts usually based on complexes of the ligands.[2]
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
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.[4]
The fluidized-bed process remains one of the two most widely used processes for producing polypropylene.[5]
Stereochemistry of poly-1-alkenes
Natta first used polymerization catalysts based on titanium chlorides to polymerize
The concept of stereoregularity in polymer chains is illustrated in the picture on the left with polypropylene. Stereoregular poly(1-alkene) can be
Classes
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:
- catalysts suitable for homopolymerization of ethylene and for ethylene/1-alkene LLDPEresins), 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
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.[7] These catalysts polymerize propylene and other 1-alkenes to highly crystalline isotactic polymers.[6][7]
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.[6][7][9][10]
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
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.[3][11] 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:
- LnTi–CH2−CHR–polymer + CH2=CHR → LnTi–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:
- LnTi–CH2−CHR–polymer + H2 → LnTi−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
References
- ISBN 0471238961.
- ISBN 9780470504437.
- ^ a b Natta, G.; Danusso, F., eds. (1967). Stereoregular Polymers and Stereospecific Polymerizations. Pergamon Press.
- ISBN 9780470504437.
- ISBN 978-0-615-66694-5.
- ^ a b c Hill, A. F. (2002). Organotransition Metal Chemistry. New York: Wiley-InterScience. pp. 136–139.
- ^ a b c d e Kissin, Y. V. (2008). "Chapter 4". Alkene Polymerization Reactions with Transition Metal Catalysts. Amsterdam: Elsevier.
- PMID 26151395.
- ISBN 9780198558132.
- PMID 11749264.
- ^ Elschenbroich, C.; Salzer, A. (1992). Organometallics: a Concise Introduction. New York: VCH Verlag. pp. 423–425.
Further reading
- Kissin, Y. V. (2008). Alkene Polymerization Reactions with Transition Metal Catalysts. Amsterdam: Elsevier.
- Corradini, P.; Guerra, G.; Cavallo, L. (2004). "Do New Century Catalysts Unravel the Mechanism of Stereocontrol of Old Ziegler–Natta Catalysts?". PMID 15096060.
- Takahashi, T. (2001). "Titanium(IV) Chloride-Triethylaluminum". Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons.
- Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. (1999). "The Search for New-Generation Olefin Polymerization Catalysts: Life beyond Metallocenes". PMID 29711786.