Acyclic well-defined metathesis catalysts

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... 

Catalyst and monomer development in acyclic diene metathesis remains a subject of interest, the goal being to obtain macromolecules with well-defined backbone structures and architectures by easily accessible and less expensive means. By the application of an appropriate design of monomers and a careful choice of catalysts, a variety of non-functionalised and functionalised dienes have been polymerised via metathesis condensation to high molecular weight polymers. 

Explain why well-defined metal alkylidienes, but not conventional two- or three-component metathesis catalysts, are preferably used for acyclic diene metathesis polycondensation. 

Ring closing and cross metathesis allow the rapid synthesis of simple cyclic and acyclic systems. The metathesis activity that is now possible using well-defined catalysts allows for the rapid generation of complexity from simple starting materials by relay processes and combinations of metathesis steps. Many of these reactions have been recognized only recently, are now beginning to be used in complex synthetic transformations. A few of these types of reactions will be outlined here to demonstrate the power of these multistep, relay processes. In these processes, an initial metathesis step leads to a new carbene that results in further transformations of the substrate. 

Heterogeneous catalysts for the metathesis of acyclic unsaturated esters are mainly based on rhenium. The rhenium oxide catalysts (Table 7.2) not only have the advantage that they are active at room temperature, but they are also highly selective, and can be easily recovered (Mol 1991). Mo03/Si02, that has been photoreduced in a CO atmosphere and subsequently treated with cyclopropane, has an even higher activity (Berezin 1991, Mol 1994b). Several well-defined metal... 

Well defined high valent Schrock type carbene complexes of Mo or W with imido ligands are nowadays favoured as very active and selective catalysts for Ring opening Metathesis Polymerization (ROMP) and Acyclic Diene Metathesis Condensation (ADMET). Low valent Fischer type carbene complexes are less active and often need cocatalysts to enhance activities [1]. 

Acyclic Diene Metathesis (ADMET) Polymerization represents a versatile route towards the synthesis of linear well-defined unsaturated polymers [1,2]. The reaction developed by Wagener et al. occurs in the presence of highly active tungsten and molybdenum metathesis catalysts such as Schrock alkylidenes, e.g. 

The design of versatile, highly active, and well-defined catalysts remains one of the main objectives of the research in olefin metathesis [1-3]. This is especially true for the application of metathesis to acyclic or cyclic olefins bearing functional groups which probably constitutes one of the most promising uses of this reaction. In fact, metathesis offers many interesting possibilities for the synthesis of valuable organic products or polymers that are often difficult to obtain by other methods [4-6]. 

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