ADMET catalytic cycle

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

Scheme 6.8 Generic catalytic cycle for productive ADMET.

The tungsten complex 1 was used to synthesize a variety of ADMET polymers including silicon and ether-containing materials [31, 33]. The double bonds of the resultant polymers are predominantly of the trans configuration (75-95%, typically over 90%) [31]. The initial alkylidene is lost in the first turnover of the reaction to afford the alkylidene complex that enters the catalytic cycle (Scheme 6.15). 

Nonetheless, ADMET is a versatile technique that allows the incorporation of a wide variety of functional groups into the resultant polymers. Scheme 1.9 shows the catalytic cycle of ADMET, controlled by the metathesis catalyst, which can be either ruthenium- [76, 77] or molybdenum-based [78, 79]. While the kinetics are controlled by the catalyst (there is no reaction in its absence), it still follows the kinetic picture described in Section 1.3.2. This is because the catalyst is removed from the chain end after each successful alkene metathesis reaction (i.e., coupling) and the olefin with which it subsequently reacts is statistically random. 

By the general mechanism of metathesis reactions, the structures of polymers obtained from ROMP and ADMET could also be fully explained. In ROMP reactions when the metal alkylidene species remain attached to the polymer chain, the polymerization process is called living polymerization (see Section 6.3). Such a situation is possible if the extent of side reactions (e.g., cross-metathesis) is minimal. The catalytic cycle for the living ROMP reaction starting with a metal alkyl is shown in Figure 7.6. 

Figure 7.7 Generalized catalytic cycle for metal alkylidene-based ADMET reaction.

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