ADMET with classical metathesis catalysts

Most recently it has been demonstrated that classical metathesis catalyst systems such as those shown above are capable of inducing ADMET condensation chemistry [34]. These classical systems, 15 and 16, are precursors to actual metal carbenes, and they must be activated with the presence of an alkylating agent such as tetrabutyltin or tributyltin hydride. The ADMET condensation chemistry proceeds at a reasonable rate and high molecular weight polymers can be obtained. 

Several early attempts at ADMET polymerization were made with classical olefin metathesis catalysts [57-59]. The first successful attempt was the ADMET polymerizations of 1,9-decadiene and 1,5-hexadiene with the WClg/EtAlf l,. catalyst mixture [60]. As mentioned in the introduction, the active catalytic entities in these reactions are ill-defined and not spectroscopically identifiable. Ethylene was trapped from the reaction mixture and identified. In addition to the expected ADMET polymers, intractable materials were observed, which were presumed to be the result of vinyl polymerization of the diene to produce crosslinked polymer. Addition to double bonds is a common side reaction promoted by classical olefin metathesis catalysts. Indeed, reaction of styrene with this catalyst mixture and even wifh WCl, alone led to polystyrene. Years later, classical catalysts were revisited in fhe context of producing tin-containing ADMET polymers wifh tungsten phenoxide catalysts [61], Alkyl tin reagents have long been known to act as co-catalysts in classical metathesis catalyst mixtures, and in this case the tin-containing monomer acted as monomer and cocatalyst [62]. Monomers with less than three methylene spacers between the olefin and tin atoms did not polymerize (Scheme 6.14). 

This hypothesis was supported by the reaction of WCl5/EtAlCl2 with styrene, which yielded polystyrene, presumably by cationic polymerization, rather than stilbene, the expected product of metathesis. The realization that vinyl addition competes with olefin metathesis in reactions using these classical catalyst systems was the key. This work serendipitously coincided with Schrock and coworkers report of their Lewis acid-free metathesis catalyst. These two advances, taken together, allowed ADMET to be realized as a method of forming a high polymer. 

ADMET depolymerization has been explored in a variety of ways [174]. In its simplest form, an unsaturated polymer subjected to metathesis conditions in the presence of ethylene will undergo a retro-ADMET reaction, resulting from CM with ethylene. This concept had been explored with classical catalyst systems [le]. Schrock s catalyst was used to depolymerize PBD, poly-tr s-isoprene, polynor-bornene, andKraton (a butadiene/styrene diblock copolymer) [175] in the presence of ethylene (ethenolysis), and a mixture of products comprising the monomer and some oligomers were produced. 

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