Acyclic diene metathesis mechanism

Metathetical polycondensation of acyclic dienes has not been successful with conventional catalysts used for the ring-opening metathesis polymerisation of cycloolefins, which is due to the fact that Lewis acids are usually present, and produce deleterious side reactions [13,16,17]. Only Lewis acid-free, well-defined catalysts have been successfully applied for acyclic diene metathesis polycondensation the key success has been to choose catalysts that obviate other pathways not involving the metathesis mechanism [18-20]. It was Wagener et al. [16,21] who first were able to convert an acyclic a, co-diene (1,9-decadiene), by using an acid-free metal alkylidene catalyst, to a high molecular weight... 

Figure 8.1 Schematic presentation of the mechanism for acyclic diene metathesis condensation polymerisation...

Figure 23 General reaction and mechanism for acyclic diene metathesis (ADMET) polymerization.

Dennis W. Smith Jr. joined The Dow Chemical Company Central Research Laboratory as Sr. Research Chemist in 1993, working primarily on the synthesis and characterization of high performance thermosets for thin film microelectronics applications. He then j oined Clemson in 1998 and was promoted to Professor of Chemistry in 2006, and in 2010, he joined the University of Texas at Dallas as Robert A. Welch Professor of Chemistry and was elected as Fellow of the American Chemical Society. His research interests include synthesis, mechanism, structure-property relationships, and applications of polymeric materials and composites. Smith received his BS from Missouri State University and his PhD from the University of Florida under the guidance of Prof. Ken Wagener on the scope and mechanism of acyclic diene metathesis (ADMET) polymerization. 

Acyclic diene metathesis (ADMET) [75] is the process by which a transition metal catalyst leads to a stepwise condensation polymerization of diene monomers, characterized by loss of gaseous ethylene and the production of linear polyolefins containing regular unsaturations along the polymer backbone (Scheme 1.8). In fact, many of the polymeric structures accessible by ADMET can be made by alternate mechanisms (e.g., 1,4-polybutadiene made by ADMET polymerization of 1,6-hexadiene is more commonly made by the anionic polymerization of 1,4-butadiene). 

It is interesting to note that if the sticky olefin hypothesis were correct, the conventional mechanism would also account for both these consequences as well as for cyclic olefins and terminal olefins yielding essentially no cross products. This can be seen, for example, in Scheme 3, where the reaction of cyclic and acyclic olefins is considered. Here, if the end of the olefin to which R is attached is more easily displaced from the metal than the end to which R is attached, reaction 2 will be faster than reaction 1. This will preferentially form conventional product—diene capped by the same groups, R and R, as the starting olefin. The other product, B, upon rapid metathesis yields A, and the cycle is then repeated. The effect would be that if one of the paths in Scheme 3 were preferred, only the conventional product would form. 

A chain mechanism for olefin metathesis explains product-time distributions in reactions between cyclo-octene and acyclic olefins. Even at the start of the reaction between cyclo-octene, trans-hut-2-ene, and trans-oct-4-ene in the presence of the catalyst, a significant amount of tetradeca-2,10-diene was found. " Tetradeca-1,9-diene was the principal product of metathesis reactions between cyclo-octene and hex-l-ene in the presence of tungsten catalysts. Ethylene and dec-5-ene formed by self-metathesis of the hex-l-ene also underwent cross-metathesis with the cyclo-octene to give deca-1,9-diene and octadeca-5,13-diene further reactions gave higher members of these homologous series. ... 

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