Classical Metathesis Catalysts

These materials can be considered linear copolymers of ethylene and propylene or precisely methyl-branched polyethylene. In addition, copolymerizations of the methyl-containing monomers with 1,9-decadierie yield polymers with lower propylene content [50]. These materials are of great interest to the polyolefin community, especially in the physical understanding of the effects of branching on physical properties. Polyethylenes with a variety of main chain functionality have also been synthesized and analyzed [51-54]. 

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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 examples of sequential isomerization/ring-closing metathesis for the preparation of heterocycles have also been performed by using two successive catalytic reactions catalyzed by two different ruthenium catalysts, but the second catalyst was introduced after completion of the first catalytic reaction. The isomerization was usually catalyzed by RuHCl(CO)(PPh3)3 [48], or RuCl2(= CHPh)(PCy3)(bis(mesityl)imidazolylidene) in the presence of trimethylsilyl vinyl ether [49], whereas a classical metathesis catalyst was subsequently introduced for the cyclization [48,49]. 

These 1987 resnlts concluded that classical metathesis catalyst systems were not sufficient and that Lewis acid cocatalyst-free systems were necessary if successM ADMET condensation polymerization were to become a reality. The key to snccessM ADMET polymerization was demonstrated " nsing the Lewis acid-free tungsten alkylidene metathesis catalyst (5a), the structure of which had been reported by Schrock et just one year earlier. When this... 

It has also been reported that a classical metathesis catalyst, RCH=Ru(PCy3)2 (CO)Cl, catalyzes the reaction between ethylene and alkynes to give mainly hydrovinylation products [90]. 

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). 

Scheme 6.14 Polymerization of organostannanes with classical metathesis catalysts.

Note that TiCl4/AIEt3 is acting here as a classical metathesis catalyst, in contrast to its behavior as a Ziegler-Natta catalyst with acetylene. 

At the beginning of the sixties, laboratory studies showed that diisobutene (2,4,4-trimethyl pent-2-ene) could be cleaved with ethylene over a classical metathesis catalyst to produce neohexene and isobutene [13 -15]. 

Many classical metathesis catalysts, like WCySnlL, and also many well defined Schrock type carbene complexes catalyse the ADMET reaction of dienes, Figure 5. 

Components of classic metathesis catalysts, i.e. group 5 and 6 transition-metal chlorides and organometallic cocatalysts, are more or less sensitive to moisture and air, and so should be handled in a dry, inert gas atmosphere. Although a few Schrock carbenes are commercially available, they are very sensitive to moisture and air, and so must always be handled under a strictly dried inert gas. Rh catalysts are relatively stable to air in the solid state but decompose readily in solution. 

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