Polymerisation of Allenes

The polymerisation of allene (1,2-propadiene), the simplest cumulene, is of interest not least because polyallene can be considered to be a versatile intermediate polymer blank , if formed with the chemical structure resulting from polymerisation by the scheme 

Such polyallene could in theory be converted to either isotactic or syndiotactic polypropylene using asymmetric hydrogenation catalysts [438], 

Allene has been polymerised to high molecular weight linear polymers by various Ziegler-Natta catalysts based on compounds of such transition metals as Ti, V, Cr, Mn, Fe, Co and Ni as precursors and alkylaluminium compounds as activators [439-441], Crystalline and amorphous polymers have been produced, in various proportions, with each of the catalysts used. The crystalline polymers consist predominantly of 1,2-linked (head-to-tail) monomeric units formed by insertion polymerisation as in scheme (68), but some regioirregular-ities resulting from the 2,1-insertion, leading to head-to-head and tail-to-tail arrangements, may be present in the polymer chain  

Amorphous allene polymers also contain 1,1-linked monomeric units and cis-1,3-linked monomeric units [241]  

A nickel-based Ziegler-Natta catalyst, Ni(Acac)2—Al(/-Bu)3, yields a regular allene polymer consisting almost exclusively of head-to-tail linked (1,2-inserted) monomeric units [441], 

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Various monocomponent catalysts based on nickel compounds, such as Ni(All)2, Ni(All)Cl, Ni(All)OCOCF3 and Ni(Cod)2, have been used for the polymerisation of allene crystalline polymers of rather high molecular weight were yielded, but they were characterised by lower structural regularity as a result of diads made up of 1,2- and 2,1-linked monomeric units [438,441, 442]. Complexes of other transition metals, especially m-Rh(CO)2ClPPh3, Co(A11)3, Co2(CO)8 (in hydrocarbon solutions) and Pd complexes (in acetic acid solutions) are also active in allene polymerisation [241]. 

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