Coordination Polymerisation of Alkynes

Acetylene was first polymerised to polyacetylene (polyvinylene) according to scheme (1) by Ziegler, who patented a process employing new catalysts prepared from transition metal salts and metal alkyls [1]  

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]  

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. 

Polyacetylene has promise as a polymeric electroconducting material [26-31]. The simplest way to obtain this polymer is by polymerisation of acetylene, although alternative methods involving the metathesis polymerisation of cyclic polyenes are also effective. 

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

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. 

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 ... 

This review summarises the most interesting approaches in the polymerisation of alkynes and the ring opening metathesis polymerisation of cyclic olefins catalysed by seven-coordinate tungsten(II) and molybdenum(II) compounds. Special attention is given to the catalytically active intermediate compound formed in the reaction of the metal complex and the organic substrate (alkyne or cyclic olefin) and to the reaction mechanism. 

POLYMERISATION OF CYCLIC OLEFINS AND ALKYNES BY SEVEN-COORDINATE TUNGSTEN (II) AND MOLYBDENUM (II) COMPOUNDS... 

The binuclear compounds as well as the mononuclear seven-coordinate bis(nitrile) compounds are catalysts for the polymerisation of terminal alkynes such as phenylacetylene (PA) or ferf-butylacetylene (f-BA) and for the ROMP of norbornene (NBE) and norbornadiene (NBD) [20, 38-42]. The first step in all catalytic reactions is the coordination of the organic substrate to metal and the formation of a complex. Sometimes it was possible to isolate the products in crystalline form but frequently such adducts were observed only in situ by NMR investigations. 

Scheme 1. The possible intermediates formed in the polymerisation reaction of alkynes catalysed by seven-coordinate W and Mo compounds...

The seven-coordinate compounds react rapidly at room temperature with an excess of terminal alkyne, such as PA or r-BA, to give an unsaturated polymer in high yield. All binuclear compounds of tungsten(II) polymerise PA at comparable rates that are qualitatively very fast (Table 1). 

It is now apparent that seven-coordinate tungsten(II) and molybdenum(II) complexes of the type [(CO)4M( Li-Cl)3M(M Cl3)(CO)3] and [MCl(M Cl3)(CO)3(NCMe)2] (M = W, Mo M = Sn, Ge) can be synthesised and isolated in crystalline forms. These complexes readily react with alkenes and alkynes in mild conditions to give crystalline products. In the presence of an excess of terminal alkyne or cyclic olefins such compounds are catalysts of the polymerisation reaction. 

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. 

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