Acetylene Ziegler-Natta polymerization

Of greater relevance catalytically is that the combined use of l3C enrichment and 13C nutation NMR spectroscopy can distinguish between proposed rival mechanisms for the Ziegler-Natta catalyzed polymerization of acetylene. In the four-center insertion mechanism the enriched acetylene (HC =C H) is incorporated as shown in Scheme 6. It is to be noted that the, 3C—13C bond label is here incorporated into a carbon-carbon double bond, the length of which is significantly smaller than that of a carbon-carbon single bond, which is how the enriched acetylene would be incorporated in the two-center mechanism shown in Scheme 7. The results of nutation experiments leave little doubt that the Ziegler-Natta polymerization of acetylene proceeds by a four-center mechanism. 

When monomer coordination to the active centre is prevented, polymerization cannot occur. Coordination is a reversible reaction strongly solvating agents deactivate centres in a ratio very close to 1 1 (acetylenes, allene, ketene, tetrahydrofuran, ROH, H20, COS, CO, C02, R3N) but weaker donors must be present in some excess in order to cause total inhibition. It appears that isotactic and atactic centres of Ziegler-Natta polymerizations, particularly centres with TiCl3 and Et2AlCl, exhibit a different ability to coordinate donors (different acidity). 

At the same time, in his lab in Tokyo, Hideki Shirakawa was applying Ziegler-Natta polymerization to the simplest conceivable monomer - acetylene, C2H2. By a fortuitous error he discovered the effect of using 1000 x the usual concentration of a particularly soluble organometalhc catalyst. When he bubbled acetylene gas through the solution, a shiny, silvery film was deposited on the walls of the tube. The morphology of these films... 

J. Steinmetz, H.J. Lee, S. Kwon, D.S. Lee, C. Goze-Bac, E. Aboul-Hamad, H. Kim, Y.W. Park, J.Y. Choi, Routes to the synthesis of carbon nanotube-polyacetylene composites by Ziegler-Natta polymerization of acetylene inside carbon nanotubes, Curr. Appl. Phys., 7, 39 1 (2007). 

Various routes to polyacetylene and derivatives. A. Direct Ziegler-Natta polymerization of acetylene as developed by Shirakawa. B. Feast s retro Diels-Alder, precursor polymer route. C. Ring-opening metathesis pol)rmerization (ROMP) of a substituted cyclooctatetraene as developed by Grubbs. 

Butadiyne does not have a susceptibility to Ziegler-Natta polymerization comparable to acetylene. 

Besides their application at the industrial level (e.g. in the Alfol process or in Ziegler—Natta polymerization catalysts), organoaluminums are the basis of various useful organic reactions. In fact, hydroalumination of acetylenes (a Cp2ZrCl2 catalyzed process when Me3Al is the metallation reagent) is particularly useful for the stereospecific synthesis of prostaglandin precursors. 

Poly (acetylenes) The most common chemical synthesis of poly (acetylene), as first effected by Shirakawa, involves use of a procedure common in the polymer world, viz. stereospecific Ziegler-Natta polymerization. In a typical procedure, a toluene slurry of AlEtj and Ti(OBu)4 (4 1) is used to coat a reaction vessel (e.g. a Schlenk tube). Acetylene gas is then admitted at pressures ranging from 2 cm to 76 cm Hg. A well-formed poly(acetylene) film starts growing in a few seconds, with polymerization generally carried out to an hour. The polymer films are washed in solvent of polymerization, and dried, when they can be peeled off to yield free standing films, with thicknesses up to 0.5 cm. 

Polyacetylenes. The first report of the synthesis of a strong, flexible, free-standing film of the simplest conjugated polymer, polyacetylene [26571-64-2] (CH), was made in 1974 (16). The process, known as the Shirakawa technique, involves polymerization of acetylene on a thin-film coating of a heterogeneous Ziegler-Natta initiator system in a glass reactor, as shown in equation 1. 

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

Poly (acetylenes) [16], There are several catalysts available for polymerization of substituted acetylenes. Whereas Ziegler-Natta catalysts are quite effective for polymerization of acetylene itself and simple alkylacetylenes, they are not active towards other substituted acetylenes, e.g. phenylacetylenes. Olefin-metathesis catalysts (Masuda, 1985 Masuda and Higashimura, 1984, 1986) and Rh(i) catalysts (Furlani et al., 1986 Tabata, 1987) are often employed. In our experience, however, many persistent radicals and typical nitrogen-containing functional groups serve as good poisons for these catalysts. Therefore, radical centres have to be introduced after construction of the polymer skeletons. Fortunately, the polymers obtained with these catalysts are often soluble in one or other organic solvent. For example, methyl p-ethynylbenzoate can be polymerized to a brick-coloured amorph- See the Appendix on p. 245 of suffixes to structural formula numbers. 

Acetylene is polymerized to polyacetylene [IUPAC poly(ethene-l,2-diyl)] by Ziegler-Natta initiators such as titanium tetraisobutoxide with triethylaluminum [Ito et al., 1974 Shelburne and Baker, 1987 Shirakawa, 2001 Theophilou and Naarman, 1989]. Polymerization at... 

When heated, polyvinyl chloride (PVC) and polyvinyl alcohol (PVA) lose HC1 and H20, respectively, to produce dark-colored conductive polyacetylene. Superior polymers of acetylene can be made by the polymerization of acetylene with Ziegler-Natta catalysts. The conductivity of polyacetylene is increased by the addition of dopants, such as arsenic pentafluoride or sodium naphthenide. 

Ziegler-Natta catalysts are not active at all in polymerization of disubstituted acetylenes.415 Mo- and W-based systems (for alkynes with small substituents) and Nb- and Ta-based catalysts (for alkynes with bulky groups), in turn, are very effective catalysts used to convert disubstituted acetylenes into polymers with very high molecular weight.414 415 A polymerization mechanism similar to that of metathesis polymerization of cycloalkenes are supported by most experimental observations.414 423 424... 

The 2000 Nobel Prize in Chemistry was awarded for work on poly acetylenes. Acetylene can be polymerized using a Ziegler-Natta catalyst. The cis or trans stereochemistry of the products can be controlled by careful selection and preparation of the catalyst. The resulting polyacetylene is an electrical semiconductor with a metallic appearance. cw-Polyacetylene has a copper color, and frawi-polyacetylene is silver. 

For substitution of monodentate 77-hydrocarbon ligands (ethylene, acetylene) a priori both mechanisms are possible. In this case an ability to change the coordination number in the transition state will be decisive. It is probable that square-planar complexes react by an associative mechanism with an increase in coordination number in the transition state. For the octahedral complexes, intermediates with lower coordination number are preferable (D-type mechanism). There is as yet no evidence for a transition state involving a-bonded ethylene or acetylene. However, both molecules are capable of inserting into transition metal-carbon u-bonds 10). It is quite probable that such an insertion mechanism operates in the Ziegler-Natta ethylene polymerization 11). 

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