General ADMET Mechanism

Two of these are the cycloaddition of the methyhdene with ethylene (path E, non-productive), reaction of the methylidene with an internal olefin such that the alkyl substituent on the metallacyclobutane is in the j9-position (path H, non-productive). The other two pathways are the cycloaddition of the alkylidene with an internal olefin to give the trisubstituted metallacyclobutane (path G, frans-metath-esis, non-productive) and the reaction of the alkylidene with a terminal olefin to give the a,a -disubstituted metallacyclobutane (path F), which can be looked at as a chain transfer-type event, albeit not in the sense of a chain polymerization. In this case, the alkylidene is shifted from the end of one chain to the end of another chain. So, assuming that all pathways have somewhat similar rates, the elimination of ethylene will drive the reaction to high polymer. In the case of ADMET, these additional mechanistic pathways do not prevent the polymerization reaction, since these additional pathways are either degenerate or represent processes that do not affect the overall molecular weight distribution of the polymer. 

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At this point it is appropriate to discuss the mechanism for ADMET, because ADMET polymerization is more involved than its chain polymerization counterpart— ROMP. Figure 8.6 illustrates the accepted mechanistic pathway which leads to productive metathesis polymerization, as first described by Wagener et al.14a A general model reaction between an a,o>-diene with a metal alkylidene... 

This chapter will present some of the history of ADMET and olefin metathesis in general, although the emphasis will be on the mechanism and kinetics of ADMET polymerization. The general mechanism for olefin metathesis will be presented before any of the specific catalyst structures are introduced or discussed in order to provide the reader with a firm basis upon which to compare the various popularly used catalysts for ADMET polymerization. In addition, procedural information will be given at the end of the chapter to give the reader an idea of what is specifically involved in a typical ADMET polymerization. 

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

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. 

The ADMET reaction (see (7.3.1.7)) can similarly be explained by the general mechanism for metathesis. This is shown in Figure 7.7 where a dimer is formed. The same cycle when repeated with the dimer replacing the monomer would give the trimer. In other words, the polymer chain can grow by repeating this basic cycle. 

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