Alkyne Polymerisation

Three reviews covering the use of transition metal complexes in carbonylation reactions using synthesis gas have appeared.338 

Part of a large overview is of interest, as is a theoretical study.277 Addition of Me2SiCHsCH2 allows polymer molecular weight control for the system l-chl xrooct-l-yne in the 

Many applications of catalytic carbonylation chemistry to the synthesis of orgaiuc molecules are covered in three major reviews.2 3 

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An interesting case is the coordination polymerisation of acetylene and higher alkynes. It may proceed by a mechanism quite similar to the metathesis polymerisation of cycloalkenes involving metal carbene and metallacycle (metallacyclobutene) species [45], The initiation and propagation steps in alkyne polymerisation (leading to a polymer of cis structure) in the presence of a catalyst with a diphenylcarbene initiating ligand are as follows ... 

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. 

In an alkyne polymerisation system with Ziegler-Natta catalysts, chain transfer and termination reactions, similar to those postulated for olefin polymerisation, have been suggested to take place [25]. A possible chain transfer reaction is the formation of the Ti H species from the propagating chain end by /h hydride elimination ... 

There are implications for both alkyne polymerisation and alkene metathesis in the above observations. The sequence ( M2MU) comprises the transformation of one p-carbene to another vi an alkyne insertion followed by rearrangement. If one envisages a molecule of alkyne in the role played by CO in the (Hlj) conversion then the sequence can be taken a step further through insertion of alkyne into the new p-carbyne. Successive insertions and rearrangements of this type then provide a mechanism... 

Dimetallocycles have been discovered which exhibit high reactivity with respect to carbon-carbon bond-making and -breaking processes. They allow the synthesis of a variety of simple but important hydrocarbon ligands bridging a dinuclear metal centre. y-Carbene complexes are readily available by several routes and their reactions have implications for both alkyne polymerisation and alkene metathesis. A substantial chemistry of organic species co-ordinated at dinuclear metal centres is in prospect, with significance for metal surface chemistry and catalysis. 

In 1976 Fischer had tested the ROMP reactions of cycloalkanes with carbyne tungsten(O) complexes as catalysts (6). With addition of Lewis acids as cocatalysts the Fischer type carbyne complexes were active in cycloalkane metathesis polymerisation. Fischer type carbyne complexes are also active catalysts for alkyne polymerisations, as found by Katz in 1984 (7). The catalytic reactions of Schrock type carbyne tungsten(VI) or molybdenum(VI) complexes were focussed on alkyne metatheses reactions (8). 

Many alkene metathesis catalysts are active catalysts for alkyne polymerisation. We tested the polymerisation of 1-alkynes with all 3 Schrock type carbyne complexes. Clo(dme)WCtBu, Np3WCtBu and (tBuO)3WCtBu are active catalysts for 1-alkyne polymerisation. In addition Clo(dme)WCtBu) catalyse the metathesis of internal alkynes and (tBuO)3WCtBu gives alkyne metathesis as shown by Schrock (18). 

Schrock and Fischer type carbyne tungsten or molybdenum complexes are very interesting catalysts for alkene metathesis or alkyne polymerisation reactions. Within the first reaction steps they form carbene complexes and on these carbene complexes further metathesis or polymerisation occur. 

Coordinatively unsaturated species can achieve saturation through partial bonding to the hydrogen or carbon atoms of organic ligands. Metal-hydride-metal and metal-hydride-carbon interactions in transition-metal complexes play an important role in catalytic reactions like carbon monoxide reduction, olehn metathesis, alkyne polymerisation and methylene transfer. 

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. 

Coordination polymerisation via re complexes comprises polymerisation and copolymerisation processes with transition metal-based catalysts of unsaturated hydrocarbon monomers such as olefins [11-19], vinylaromatic monomers such as styrene [13, 20, 21], conjugated dienes [22-29], cycloolefins [30-39] and alkynes [39-45]. The coordination polymerisation of olefins concerns mostly ethylene, propylene and higher a-olefins [46], although polymerisation of cumulated diolefins (allenes) [47, 48], isomerisation 2, co-polymerisation of a-olefins [49], isomerisation 1,2-polymerisation of /i-olcfins [50, 51] and cyclopolymerisation of non-conjugated a, eo-diolefins [52, 53] are also included among coordination polymerisations involving re complex formation. 

Several monosubstituted acetylenes (terminal alkynes) with a small (not presenting steric hindrances) substituent, R C=CH (where R=primary or secondary alkyl group), have also been polymerised using Ziegler Natta catalysts [11-15]. 

Acetylenic monomers also appeared to undergo polymerisation with conventional olefin metathesis catalysts. This relates to monosubstituted highly branched alkylacetylenes and arylacetylenes as well as disubstituted acetylenes (internal alkynes) [16-18], It has been demonstrated that acetylene itself may also be polymerised using olefin metathesis catalysts [19,20]. The polymerisation of alkynes [scheme (2)] involves a metathesis reaction [scheme (5) of Chapter 2] analogously to that of cycloolefins [21] ... 

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]. 

Name and characterise the main types of coordination polymerisation of alkynes and give representative catalysts for each type. 

Until recently, intermolecular enyne metathesis received scant attention. Competing CM homodimerisation of the alkene, alkyne metathesis and polymerisation were issues of concern which hampered the development of the enyne CM reaction. The first report of a selective ruthenium-catalysed enyne CM reaction came from our laboratories [106]. Reaction of various terminal alkynes 61 with terminal olefins 62 gave 1,3-substituted diene products 63 in good-to-excellent yields (Scheme 18). It is interesting that in these and all enyne CM reactions subsequently reported, terminal alkynes are more reactive than internal analogues, and 1,2-substituted diene products are never formed thus, in terms of reactivity and selectivity enyne CM is the antithesis of enyne RCM. The mechanism of enyne CM is not well understood. It would appear that initial attack is at the alkyne however, one report has demonstrated initial attack at the alkene (substrate-dependent) is also possible, see Ref. [107]. 

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