Insertion polymerisation, alkyne

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... 

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. 

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. 

Alkynes. Homogeneous polymerisation is generally assumed to proceed through complex formation and subsequent insertion, though definitive proof of this pathway is rarely available. This mechanism has been illustrated by the isolation of appropriate intermediates such as (30) for... 

Ruthenium complex 81 reacted with alkynes yielding 82, which underwent migratory insertion of the NHC ligand into the Ru alkylidene to form 83 (Scheme 2.26). Trzeciak and co-workers demonstrated that [(NHC) RuCl2(Ti -p-cymene)] was able to polymerise phenylacetylene, and that the resulting product was terminated by an imidazolium species, probably via the same process. ... 

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