Nickel acetylene polymerization

Derivation Nickel-catalyzed polymerization of acetylene, a reaction discovered by Reppe in Germany about 1940. The mechanism has several possible pathways. 

The ability of nickel complexes, e.g., nickel carbonyl and its phosphine derivatives, to catalyze polymerization and other reactions of olefins and acetylenes has been studied extensively (46, 53), particularly by Reppe. 

One of the characteristic features of the metal-catalysed reaction of acetylene with hydrogen is that, in addition to ethylene and ethane, hydrocarbons containing more than two carbon atoms are frequently observed in appreciable yields. The hydropolymerisation of acetylene over nickel—pumice catalysts was investigated in some detail by Sheridan [169] who found that, between 200 and 250°C, extensive polymerisation to yield predominantly C4 - and C6 -polymers occurred, although small amounts of all polymers up to Cn, where n > 31, were also observed. It was also shown that the polymeric products were aliphatic hydrocarbons, although subsequent studies with nickel—alumina [176] revealed that, whilst the main products were aliphatic hydrocarbons, small amounts of cyclohexene, cyclohexane and aromatic hydrocarbons were also formed. The extent of polymerisation appears to be greater with the first row metals, iron, cobalt, nickel and copper, where up to 60% of the acetylene may polymerise, than with the second and third row noble Group VIII metals. With alumina-supported noble metals, the polymerisation prod-... 

One of the most interesting alternatives to the Shirakawa catalyst has been the systems disclosed by Luttinger 22-23) and later elaborated by Lieser et al. 24). The tris(2-cyanoethyl)phosphine complex of nickel chloride reacts with sodium boro-hydride to produce a catalyst system capable of polymerizing acetylene in solutions in either alcohol or, quite remarkably, water. A more efficient catalyst is obtained by replacing the nickel complex with cobalt nitrate. Interest in Luttinger polyacetylene seems to have waned in the last few years. 

Petrov and co-workers polymerized acetylene with simultaneous hydrogenation over nickel or nickel plus zinc chloride and obtained saturated and olefinic products. The ratio of products boiling in the gasoline range to heavier products depended upon the catalyst as well as the pressure. The liquid obtained from nickel consisted of a gasoline obtained in 50 % yield in the runs under atmospheric pressure, and in 70 % yield under 20 atm. pressure. The rest comprised diesel fuel hydrocarbons (292). The structure of the liquid hydrocarbons formed was unaffected by the presence of phosphoric acid or zinc in the catalyst. The gas contained up to 80% butenes, depending upon the conditions. This work of Petrov on the synthesis of hydrocarbons is apparently being continued at the present time. 

The tendency of acetylene to undergo thermal oligo- and polymerization to give (actually in very low yields ) linear polyenes or benzene is a reaction known for almost a century. Formally, the same process is implied in the formation of 137 from four acetylene molecules. Yet this deceptively simple cyclotetramer-ization scheme turned to be a viable reaction only after Reppe s discovery that a simple catalyst, nickel(n) cyanide, could serve as a highly efficient device to control the course of acetylene oligomerization. It is the ability of this catalyst to form a complex, 401, with four molecules of acetylene that ensured the required selectivity of cyclotetramer formation (Scheme 2.135). 

Acrylonitrile and related compounds displace all the carbonyl groups from nickel carbonyl to form [(RCH CHCN)2Ni], in which the nitrile bonds through the olefinic double bond 222, 418). The bis(acrylonitrile) complex catalyzes many reactions, including the conversion of acrylonitrile and acetylene to heptatrienenitrile and the polymerization of acetylene to cyclooctatetraene 418). Cobalt carbonyl gave a brown-red amorphous material with acrylonitrile, which had i cn absorptions typical of uncoordinated nitrile groups, but interestingly, the presence of C=N groups was also indicated 419). In acidic methanol, cobalt carbonyl converts a,j8-unsaturated nitriles to saturated aldehydes 459). 

The ligand L, which can be a triphenylphosphine molecule, hinders the fourth molecule of the acetylene from coordinating and, by preventing the formation of metallic nickel, makes the process catalytic. In fact, the same Schrauzer (163) obtained the polymerization of acetylene to cyclo-octatetraene by a stoichiometric reaction with bisacrylonitrilenickel without any phosphine. He interpreted the reaction course as the formation of the intermediate, Ni(C2H2)4, which then gives metallic nickel and the tetramer ... 

As early as 1948, Reppe et al. reported the discovery of the cyclic polymerization of acetylene to cyclooctatetraene (eq. (29)) using nickel catalysts [84]. This discovery represented a true landmark in transition metal catalysis. 

Bis(ylide)nickel catalysts are of high chemical variability and show superior performance in the activation of unsaturated substrates such as acetylene. The normalized polymerization activity in dimethyl sulfoxide (DMSO) of 500 mol polymerized acetylene per mol nickel (h aim) by far exceeds that of structurally related phosphane catalysts by a similar order of magnitude as observed in ethylene polymerizations (see Sections 1.2 and 1.3.1). To our knowledge this activity even exceeds that of all other nickel catalysts reported so far (Fig. 1.4). 

The high polar group tolerance of ylide nickel catalysts enables the polymerization of acetylene in polymer solutions not only of low polarity but also of medium and high polarity. These options provide synthetic access to a wide range of novel matrix polyacetylenes (MATPAC). Examples of polymers that may be used as matrix... 

There is a notable tendency to form oligomers when acetylenic substances interact with compounds of metals, and this tendency is also shown by butadiene 117) (see Section IV, B,d). This is particularly so with the carbonyls of iron and cobalt, and the oligomerization reactions are favored with nickel 121) and with palladium compounds 113, 122, 123). This phenomenon may be related to the hydropolymerization of acetylenes on metal surfaces, and it may be that such polymerization processes would be better described in terms of ir-complexes. 

Fourth, the complexes of acetylenes and diolefins react chiefly in polymerization, this tendency being most marked with the complexes of nickel and palladium. Platinum complexes are not generally active in polymerization. In the catalytic hydropolymerization of acetylene, nickel displays the greatest activity, palladium takes second place and platinum third place. The remaining noble metals are less active than platinum. Thus, again, a correlation between the two fields is observed to hold. 

Several important homogeneous catalytic reactions (e.g. hydroformylations) have been accomplished in water by use of water-soluble catalysts in some instances water can act as a solvent and as a reactant for hydroformylation. In addition, formation of aluminoxanes by partial hydrolysis of alkylaluminum halides results in very high activity bimetallic Al/Ti or Al/Zr metallocene catalysts for ethene polymerization which would be otherwise inactive. Polymerization of aryl diiodides and acetylene gas has recently been achieved in water with palladium catalysts. Finally, nickel-containing enzymes, such as carbon monoxide dehydrogenase (CODH) and acetyl-CoA synthase, operate in water with reaction mechanisms comparable with those of the WGSR or of the Monsanto methanol-to-acetic-acid process. ... 

A number of metal complexes catalyses specific alkyne polymerizations, giving rise to four-, six- or eight-membered carbocyclic rings. The first work in this area was the nickel-catalysed formation of cyclooctatetraene (40) from acetylene by the group of Reppe ", but since then formation of cyclic systems from acetylenes has been found to be also catalysed by molybdenum, cobalt, iridium and tantalum. ... 

This was the appearance of publications by W. Reppe and co-workers in followed by Badische Anilin und Soda Fabrik patents/ They showed that various triphenylphosphine complexes of nickel, especially [Ni(CO)2(PPh5)2](Ph = QHs), were more effective than other nickel complex catalysts for the polymerization of olefinic and acetylenic substances and that others, especially [NiBr2(PPh3)2], catalyzed the formation of acrylic acid esters from alcohols (ROH), acetylene, and carbon monoxide ... 

Work in this area has focused on nonphosphine supports. Polymerization of ethylene was reported with TiCU on a homopolymer or copolymer of vinylphenylphosphine AIR3 was added to activate the catalyst. With a nickel salt on a phosphinated polystyrene, addition of NaBH4 resulted in a catalyst active in the oligomerization and polymerization of acetylenic monomers (see Table 7). 

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