Addition metathesis polymerization ADMET

The third class of olefin methathesis in Scheme 21.1 is addition metathesis polymerization (ADMET). This reaction is an alternative method to stitch together olefins into polymers, in this case by a combination of dienes with extrusion of ethylene. Control of molecular weight by the ADMET process is less precise than that by ROMP, but this reaction has been used to make polymers with precise architectures, such as polymers that would be perfectly alternating ethylene-propylene copolymers. ... 

Dienes are cyclized by intramolecular metathesis. In particular, cyclic alkenes 43 and ethylene are formed by the ring-closing metathesis of the a,co-diene 46. This is the reverse reaction of ethenolysis. Alkene metathesis is reversible, and usually an equilibrium mixture of alkenes is formed. However, the metathesis of a,co-dienes 46 generates ethylene as one product, which can be removed easily from reaction mixtures to afford cyclic compounds 43 nearly quantitatively. This is a most useful reaction, because from not only five to eight membered rings, but also macrocycles can be prepared by RCM under high-dilution conditions. However, it should be noted that RCM is an intramolecular reaction and competitive with acyclic diene metathesis polymerization (ADMET), which is intermolecular to form the polymer 47. In addition, the polymer 47 may be formed by ROMP of the cyclic compounds 43. 

The same acidic chloroaluminate ionic liquids have been used as solvent for tungsten aryl oxide complexes for the metathesis of alkenes [24]. Slightly acidic chloroaluminates also dissolve the [Cl2W=NPh(PMe3)3] complex which catalyze ethene oligomerization without the addition of co-catalysts [25]. In a similar way, Ni-catalyzed 1-butene dimerization into linear octenes was carried out in acidic chloroaluminates buffered with small amount of weak bases [26]. Neutral chloroaluminates (l-ethyl-3-methylimidazolium chloride/AlCl3 = 1) were employed to immobilize ruthenium carbene complexes for biphasic ADMET (acyclic diene metathesis) polymerization of an acyclic diene ester [27]. 

Numerous friends and colleagues in the field of metathesis (the soldiers to whom we dedicate this book) have encouraged us to believe that a new book incorporating these recent developments would be both timely and welcome. We felt, however, that the book should still outline the historical development of the subject and not just be a supplement to the original book. This has necessarily meant some compression of earlier material and some restriction of discussion. The title has been expanded to include the words Metathesis Polymerization , which embraces not only ring-opening metathesis polymerization (ROMP), but also the metathesis condensation reactions of acyclic dienes (ADMET) and the addition reactions of acetylenes. The division of the material and the subjects of the chapters follow the same pattern as before. The literature has been covered up to mid-1996. 

In addition to cross-metathesis for making small molecules, the cross-metathesis of l,/ -dienes can lead to polymeric materials (Scheme 10). This process is known as acyclic diene metathesis polymerization, or ADMET polymerization. In theory, ADMET polymerization is competitive with ring-closing metathesis (RGM), and most of the successful examples of ADMET involve the use of dienes where the RGM process would produce an unfavorable ring size. Some examples of ADMET polymerization are depicted in Scheme 10, and include (i) amino acid-containing dienes (e.g., 75)," (ii) boronate-linked dienes 77," (iii) l,4-divinyl-2,5-bis(heptyloxy)-benzene 79," and (iv) phosphazine-containing dienes (e.g., 81)." " ... 

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. 

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). 

Further, it was demonstrated that the addition of benzoquinone to the polymerization mixture prevents the olefin isomerization. Therefore, second generation ruthenium metathesis catalysts can be used for the preparation of well defined polymers via an ADMET technique causing little isomerization (32). 

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. 

In recent years we have been able to delineate the key to success in ADMET chemistry, that being the selection of a catalyst free of Lewis acids. Acidic catalysts permit the intervention of vinyl-addition chemistry which precludes formation of high polymers through step polymerization. To prove the point, styrene was used as a model compound to explore possible mechanistic paths for reacting olefin systems. It was shown that if Lewis-acid containing catalyst systems are used, then vinyl-addition chemistry predominates, whereas the choice of a Lewis-acid free catalyst system (Shrock s tungsten catalyst is used in this example) results in complete domination of metathesis chemistry instead. These observations... 

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