Acetylene polymerisation

Figure 5.14 Acetylene polymerisation II (adapted from Helens and Charnley 1996)...

The mechanism of polymerisation of alkynes with metathesis catalysts requires that the original triple bond of the acetylenic monomer becomes a single bond in the polymer [scheme (5) of Chapter 2], in contrast to the insertion mechanism of acetylene polymerisation with Ziegler-Natta catalysts, where the triple bond becomes a double bond [scheme (1)]. Ideas about the mechanism of metathesis polymerisation of cycloolefins suggested that isolable metal carbenes might promote the polymerisation of cycloolefins suggested that isolable metal carbenes might promote the polymerisation of alkynes, as indeed turned out to be true, as several metal carbenes were found [22-24] to cause alkyne polymerisation. 

An original method for acetylene polymerisation is that using bis(ylide)nick-el(II) catalysts with a highly active Ni-Ph bond [74,75] ... 

By contrast, the Fischer carbyne tnms-bromotetracarbonyl(phcnylmcthyli-dene)tungsten, Br(CO)4W = CPh [140-142], will polymerise monosubstituted acetylenes by a metathesis mechanism [143] but will not give transalkylidynation products. The Fischer carbyne and related carbynes are effective catalysts for the metathesis polymerisation of not only monosubstituted acetylenes, whose polymerisation can be brought about by a number of catalysts [108, 109, 118, 120-125, 128-134], but also of acetylenes polymerised previously by only a few or no catalysts, i.e. even disubstituted acetylenes [112-115, 126, 127, 144-148] and unsubstituted acetylene [19, 20, 42, 119]. 

It was proved that metal carbynes are sources of metal carbenes [e.g. scheme (9) in Chapter 6] promoting the polymerisation of acetylenic monomers. Therefore, related metal carbynes and carbenes appeared to catalyse the polymerisation of alkynes in the same way as regards the identity of the products, in particular as regards stereochemistry. For the terminal and internal alkynes, the Fischer carbyne acts much like the Casey and Fischer metal carbenes. The Fischer carbyne also promotes acetylene polymerisation, and it does this where the Fischer carbene fails and the Casey carbene is much less effective [22,143]. 

When obtained in a finely divided state by reduction of its oxides at 250° C. cobalt is pyrophoric. In this condition it decomposes acetylene in the cold, the metal becoming incandescent. A portion of the acetylene polymerises to benzene, whilst the remainder yields carbon and hydrogen.9... 

These processes have supplanted the condensation reaction of ethanol, carbon monoxide, and acetylene as the principal method of generating ethyl acrylate [140-88-5] (333). Acidic catalysts, particularly sulfuric acid (334—338), are generally effective in increasing the rates of the esterification reactions. Care is taken to avoid excessive polymerisation losses of both acryflc acid and the esters, which are accentuated by the presence of strong acid catalysts. A synthesis for acryflc esters from vinyl chloride (339) has also been examined. 

Since impurities can affect both the polymerisation reaction and the properties of the finished product (particularly electrical insulation properties and resistance to heat aging) they must be rigorously removed. In particular, carbon monoxide, acetylene, oxygen and moisture must be at a very low level. A number of patents require that the carbon monoxide content be less than 0.02%. 

The polymers were first described by Newkirk. Polymerisation may be brought about by subjecting acetylene-free vinyl fluoride to pressures to up to 1000 atm at 80°C in the presence of water and a trace of benzoyl peroxide. 

Reaction of adsorbed inhibitors In some cases, the adsorbed corrosion inhibitor may react, usually by electro-chemical reduction, to form a product which may also be inhibitive. Inhibition due to the added substance has been termed primary inhibition and that due to the reaction product secondary inhibition " . In such cases, the inhibitive efficiency may increase or decrease with time according to whether the secondary inhibition is more or less effective than the primary inhibition. Some examples of inhibitors which react to give secondary inhibition are the following. Sulphoxides can be reduced to sulphides, which are more efficient inhibitorsQuaternary phosphonium and arsonium compounds can be reduced to the corresponding phosphine or arsine compounds, with little change in inhibitive efficiency . Acetylene compounds can undergo reduction followed by polymerisation to form a multimolecular protective film . Thioureas can be reduced to produce HS ions, which may act as stimulators of... 

Reppe, W., Schlichting, O., Klager, K., Toepel, T. (1948) Cyclisierende Polymerisation von Acetylen. I. Uber Cyclooktatetraen. Justus Liebigs Annalen der Chemie, 560, 1-92. 

These values do not always refer to the same thing. The acetylene and propyne codes indicate the instability. The other compounds have codes which indicate the danger of polymerisation. Comparing the methylstyrene codes with the others raises doubts about the codes chosen. 

Figure 9.8 Relative reactivities (a) and selectivities (b) of palladium cluster size in the formation of C6H6, C4H8 and C4H6 in the polymerisation of acetylene. (Reproduced from Ref. 29).

Grazing angle XAS techniques (XANES) can be applied to ultrathin film systems [316]. Selected NEX-AFS, XPS and FUR spectroscopy results were obtained for plasma-polymerised films with different monomers (styrene, acetylene, ethylene and butadiene) [317]. 

The most commonly used form of polyacetylene is produced by the Shirakawa method, which involves the direct polymerisation of acetylene gas onto a substrate at... 

Feast, W. J. et al J. Chem. Soc., Chem. Comm., 1985, 202-203 The second stage of an improved synthesis of poly(acetylene), which involves disproportionation of a soluble polymer by heating a thin film at 75°C to give 1,2-bis(trifluoromethyl)benzene and poly(acetylene), must not be done in bulk because the reaction then becomes explosive. The earlier synthesis by direct polymerisation of acetylene was considerably more dangerous... 

See other acetylenic compounds, aldehydes, glass incidents, polymerisation INCIDENTS... 

This tetramer of acetylene decomposes violently on distillation at 156°C, but not at reduced pressure [1]. It polymerises on standing to a solid detonable by shock [2], Explosions occurred dining attempted analytical combustion [3],... 

Formation of complexes with excess methyl propiolate or dimethyl acetylenedicar-boxylate must not be allowed to proceed at above +20°C, or violently explosive polymerisation of the acetylene esters will occur. 

Potentially very explosive, it may be handled and transferred by low temperature distillation. It should be stored at —25°C to prevent decomposition and formation of explosive polymers [1]. The critical pressure for explosion is 0.04 bar, but presence of 15-40% of diluents (acetylene, ammonia, carbon dioxide or nitrogen) will raise the critical pressure to 0.92 bar [2], Further data on attenuation by inert diluents of the explosive decomposition of the diyne are available [3], During investigation of the cause of a violent explosion in a plant for separation of higher acetylenes, the most important finding was to keep the concentration of 1,3-butadiyne below 12% in its mixtures. Methanol is a practical diluent [4], The use of butane (at 70 mol%) or other diluents to prevent explosion of 1,3-butadiyne when heated under pressure has been claimed [5], It polymerises rapidly above 0°C. 

Aldol Also called the Four-step process. A process for converting acetylene to synthetic rubber, used on a large scale in Germany during World War II. A four-step synthesis converted the acetylene to butadiene, and this was then polymerised by the Buna process. The four steps were ... 

If two identical molecules combine chemically a dimer is obtained. Acetylene for instance, is dimerised to vinylacetylene. If smaller molecules of a substance unite then a large molecule, a polymer, of high Molecular weight is obtained. The individual small molecule from which a polymer is formed is called a monomer. The chemical process for the formation, of a polymer is called polymerisation. This is exemplified in the following equation. 

Compare with this the polymerisation of acetylene to benzene.)... 

Catalytic forms of copper, mercury and silver acetylides, supported on alumina, carbon or silica and used for polymerisation of alkanes, are relatively stable [3], In contact with acetylene, silver and mercury salts will also give explosive acetylides, the mercury derivatives being complex [4], Many of the metal acetylides react violently with oxidants. Impact sensitivities of the dry copper derivatives of acetylene, buten-3-yne and l,3-hexadien-5-yne were determined as 2.4, 2.4 and 4.0 kg m, respectively. The copper derivative of a polyacetylene mixture generated by low-temperature polymerisation of acetylene detonated under 1.2 kg m impact. Sensitivities were much lower for the moist compounds [5], Explosive copper and silver derivatives give non-explosive complexes with trimethyl-, tributyl- or triphenyl-phosphine [6], Formation of silver acetylide on silver-containing solders needs higher acetylene and ammonia concentrations than for formation of copper acetylide. Acetylides are always formed on brass and copper or on silver-containing solders in an atmosphere of acetylene derived from calcium carbide (and which contains traces of phosphine). Silver acetylide is a more efficient explosion initiator than copper acetylide [7],... 

Initially alkynes were polymerised by trial and error with the use of Ziegler type recipes and the mechanism for these reactions may well be an insertion type mechanism. Undefined metathesis catalysts of ETM complexes were known to give poly-acetylene in their reaction with alkynes (acetylene) [45] and metallacycles were proposed as intermediates. Since the introduction of well-defined catalysts far better results have been obtained. The mechanism for this reaction is shown in Figure 16.24 [46], The conductive polymers obtained are soluble materials that can be treated and deposited as solutions on a surface. 

Polyacetylene, (CH),(, is a simple, conjugated polymer which may have either a trans or a cis configuration (Fig. 9.1). Free-standing films of polyacetylene can be easily obtained by catalytic polymerisation of gaseous acetylene, the most common procedure being the Shirikawa... 

The reactions of acetylene, including its polymerisation, that occur in the presence of cupric chloride solution have been studied 670) in the (additional) presence of biguanide dihydrochloride. This solution has the unique property of promoting the formation of acetic acid from acetylene. 

---