Tungsten catalysts polymers

While molybdenum and tungsten catalysts are associated with metathesis polymerization (see Organic Synthesis Using Metal-mediated Metathesis Reactions), chromium catalysts are widely used in the production of polymers. The best-known system is the so-called Phillips catalyst, Cp2Cr (see... 

The third route involves metathesis polymerization of cyclooctatetraene with tungsten catalysts, yielding polyacetylene as an insoluble film along with oligomers (iOi). By first polymerizing cyclooctene and then adding cyclooctatetraene, a soluble, red block copolymer was obtained. On the basis of the visible absorption spectrum, at least two or three cyclooctatetraene units were concluded to have been added to the polymer chain forming a short polyacetylene block. No conductivity data were reported for this copolymer. 

The third group also includes unsaturated boranes, which have the advantage of (i) the stability of borane groups towards tungsten catalysts (ii) the stability of borane monomers and polymers in hydrocarbon solvents and (iii) the versatility of borane groups, which can be transformed to a variety of functionalities under mild reaction conditions (Chung 1992a). 

However, the polymer prepared using the tungsten catalyst has a main chain cis double bond content higher, reaching 70%. In the H-NMR spectrum we can see well resolved signals due to the olefinic protons of poly-NBD with a cis configuration at 5.56... 

Ethers were the first functionalized dienes to be polymerized successfully by ADMET [74]. It was found, however, that at least three methylene spacers between the oxygen atom and the olefin were required for successful polymerization with Schrock s catalysts. The resultant polymers were cross-linkable by both chemical and photochemical means [75]. Both [Mo]2 and [W]l were capable of producing the polymer, but polymerization with the molybdenum catalyst proceeded at a rate roughly 10 times faster than that with the tungsten catalyst [76]. ADMET polymerization of diallyl ether was attempted with both [Mo]2 and [Ru]l, resulting in a low molecular weight polymer. The major reaction product for both catalysts is 2,5-dihydrofuran, the result of RCM (Figure 13.6). Divinyl ether was not metathesis-active with any of Schrock s catalysts. 

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... 

Dicyclopentadiene is also polymerized with tungsten-based catalysts. Because the polymerization reaction produces heavily cross-Unked resins, the polymers are manufactured in a reaction injection mol ding (RIM) process, in which all catalyst components and resin modifiers are slurried in two batches of the monomer. The first batch contains the catalyst (a mixture of WCl and WOCl, nonylphenol, acetylacetone, additives, and fillers the second batch contains the co-catalyst (a combination of an alkyl aluminum compound and a Lewis base such as ether), antioxidants, and elastomeric fillers (qv) for better moldabihty (50). Mixing two Uquids in a mold results in a rapid polymerization reaction. Its rate is controlled by the ratio between the co-catalyst and the Lewis base. Depending on the catalyst composition, solidification time of the reaction mixture can vary from two seconds to an hour. Similar catalyst systems are used for polymerization of norbomene and for norbomene copolymerization with ethyhdenenorbomene. 

Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl2 [758], Me4Sn/Al203 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. 

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