Cossee–Arlman polymerization mechanism for Ziegler-Natta catalyst: (a) traditional Ziegler-Natta catalyst and (b) modified Ziegler-Natta catalyst.
Alkene polymerization
Key points: Heterogeneous Ziegler-Natta catalysts are used in alkene polymerization; the Cossee-Arlman mechanism describes their function; low molar mass homogeneous catalysts also catalyse the alkene polymerization reaction; considerable control over polymer tacticity is possible with judicious ligand design.
Polyalkenes, which are among the most common and useful class of synthetic polymers, are most often prepared by use of organometallic catalysts, either in solution or supported -on a solid surface. The development of alkene polymerization catalysts in the second half of the twentieth century, producing polymers such as polypropene and polystyrene, ushered in a revolution in packaging materials, fabrics, and constructional materials. In the 1950s J.P. Hogan and R.L. Banks discovered that chromium oxides supported silica, a so-called Philips catalyst, polymerized alkenes to polyenes. Also in the 1950s Ziegler, working in Germany, developed a catalyst for ethene polymerization based on catalyst formed from TiCl4 and Al(C2H5)3, and soon thereafter G. Natta in Italy used this of catalyst for the stereospecific polymerization of propene (see below). Both the Ziegler-Natta catalysts and the chromium-based polymerization catalysts are widely used today.
The full details of the mechanism of Ziegler—Natta catalysts are still uncertain, but the Cossee—Arlman mechanism is regarded as highly plausible. The catalyst is prepared from TiCl4 and Al(C2H5)3, which react to give polymeric TiCl3 mixed with AlCl3 in the form of a fine powder. The alkylaluminium alkylates a Ti atom on the surface of the solid and an ethene molecule coordinates to the neighbouring vacant site. In the propagation steps for the polymerization, the coordinated alkene undergoes a migratory insertion reaction. This migration opens up another neighbouring vacancy, and so the reaction continue and the polymer chain can grow. The release of the polymer from the metal a occurs by 13-hydrogen elimination, and the chain is terminated. Some catalyst remains the polymer, but the process is so efficient that the amount is negligible.
The proposed mechanism of alkene polymerization on a Philips catalyst involves the coordination of one or more alkene molecules to a surface Cr(II) site followed by rearrangement to metallocycloalkanes on a formally Cr(IV) site. Unlike Ziegler—Natta catalysts, solid-phase catalyst does not need an alkylating agent to initiate the polymerization reaction; instead, this species is thought to be generated by the metallocycloalkane directly or by formation of an ethenylhydride by cleavage of a C—H bond at the chromium site.
Homogeneous catalysts related to the Philips and Ziegler—Natta catalysts provide additional insight into the course of the reaction and are of considerable industrial significance in their own right, being used commercially for the synthesis of specialized polymers. Most examples are from Group 4 (Ti, Zr, Hf) and are based on a bis(cyclopentadienyl) metal tern: the tilted ring complex [Zr(n5-Cp)2(CH3)L]+ is a good example. These Group 4 metallocene complexes catalyse alkene polymerization by successive insertion steps that involve prior coordination of the alkene to the electrophilic metal centre. Catalysts of this type are used in the presence of the so-called methyl aluminoxane (MAO), a poorly defined compound of approximate formula (MeA1O)n, which, among other functions, serve to methylate a starting chloride complex.
Additional complications arise with alkenes other than ethene. We shall discuss terminal alkenes such as propene and styrene, as these are relatively simple. The first plication to consider arises because the two ends of the alkene molecule are diffe principle, it is possible for the polymer to form with the different ends head-to-head head-to-tail (14), or randomly. Studies on catalysts such as (12) show that the chain migrates preferentially to the more highly substituted C atom of the alkene, giving a polymer chain that contains only head-to-tail orientations.
Alkene polymerization
Key points: Heterogeneous Ziegler-Natta catalysts are used in alkene polymerization; the Cossee-Arlman mechanism describes their function; low molar mass homogeneous catalysts also catalyse the alkene polymerization reaction; considerable control over polymer tacticity is possible with judicious ligand design.
Polyalkenes, which are among the most common and useful class of synthetic polymers, are most often prepared by use of organometallic catalysts, either in solution or supported -on a solid surface. The development of alkene polymerization catalysts in the second half of the twentieth century, producing polymers such as polypropene and polystyrene, ushered in a revolution in packaging materials, fabrics, and constructional materials. In the 1950s J.P. Hogan and R.L. Banks discovered that chromium oxides supported silica, a so-called Philips catalyst, polymerized alkenes to polyenes. Also in the 1950s Ziegler, working in Germany, developed a catalyst for ethene polymerization based on catalyst formed from TiCl4 and Al(C2H5)3, and soon thereafter G. Natta in Italy used this of catalyst for the stereospecific polymerization of propene (see below). Both the Ziegler-Natta catalysts and the chromium-based polymerization catalysts are widely used today.
The full details of the mechanism of Ziegler—Natta catalysts are still uncertain, but the Cossee—Arlman mechanism is regarded as highly plausible. The catalyst is prepared from TiCl4 and Al(C2H5)3, which react to give polymeric TiCl3 mixed with AlCl3 in the form of a fine powder. The alkylaluminium alkylates a Ti atom on the surface of the solid and an ethene molecule coordinates to the neighbouring vacant site. In the propagation steps for the polymerization, the coordinated alkene undergoes a migratory insertion reaction. This migration opens up another neighbouring vacancy, and so the reaction continue and the polymer chain can grow. The release of the polymer from the metal a occurs by 13-hydrogen elimination, and the chain is terminated. Some catalyst remains the polymer, but the process is so efficient that the amount is negligible.
The proposed mechanism of alkene polymerization on a Philips catalyst involves the coordination of one or more alkene molecules to a surface Cr(II) site followed by rearrangement to metallocycloalkanes on a formally Cr(IV) site. Unlike Ziegler—Natta catalysts, solid-phase catalyst does not need an alkylating agent to initiate the polymerization reaction; instead, this species is thought to be generated by the metallocycloalkane directly or by formation of an ethenylhydride by cleavage of a C—H bond at the chromium site.
Homogeneous catalysts related to the Philips and Ziegler—Natta catalysts provide additional insight into the course of the reaction and are of considerable industrial significance in their own right, being used commercially for the synthesis of specialized polymers. Most examples are from Group 4 (Ti, Zr, Hf) and are based on a bis(cyclopentadienyl) metal tern: the tilted ring complex [Zr(n5-Cp)2(CH3)L]+ is a good example. These Group 4 metallocene complexes catalyse alkene polymerization by successive insertion steps that involve prior coordination of the alkene to the electrophilic metal centre. Catalysts of this type are used in the presence of the so-called methyl aluminoxane (MAO), a poorly defined compound of approximate formula (MeA1O)n, which, among other functions, serve to methylate a starting chloride complex.
Additional complications arise with alkenes other than ethene. We shall discuss terminal alkenes such as propene and styrene, as these are relatively simple. The first plication to consider arises because the two ends of the alkene molecule are diffe principle, it is possible for the polymer to form with the different ends head-to-head head-to-tail (14), or randomly. Studies on catalysts such as (12) show that the chain migrates preferentially to the more highly substituted C atom of the alkene, giving a polymer chain that contains only head-to-tail orientations.
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