Olefin poly-, metathesis

Poly (acetylenes) [16], There are several catalysts available for polymerization of substituted acetylenes. Whereas Ziegler-Natta catalysts are quite effective for polymerization of acetylene itself and simple alkylacetylenes, they are not active towards other substituted acetylenes, e.g. phenylacetylenes. Olefin-metathesis catalysts (Masuda, 1985 Masuda and Higashimura, 1984, 1986) and Rh(i) catalysts (Furlani et al., 1986 Tabata, 1987) are often employed. In our experience, however, many persistent radicals and typical nitrogen-containing functional groups serve as good poisons for these catalysts. Therefore, radical centres have to be introduced after construction of the polymer skeletons. Fortunately, the polymers obtained with these catalysts are often soluble in one or other organic solvent. For example, methyl p-ethynylbenzoate can be polymerized to a brick-coloured amorph- See the Appendix on p. 245 of suffixes to structural formula numbers. 

Ruthenium(II)-NHC systems ean be used for atom transfer radical polymerization (ATRP). Generally, similar results as for the analogous phosphine complexes are obtained. For the ATRP of styrene and methyl methacrylate (MMA) [(NHC)2peBr2] was found to rival copper(I)-based systems and to yield poly (MMA) with low polydispersities. Polymerizations based on olefin metathesis that are catalyzed by ruthenium-NHC complexes are discussed separately vide supra). 

Polymers of monocyclic olefins (cyclopentene, cyclooctene) produced by ring-opening metathesis are linear elastomers. Their properties are somewhat similar to those of poly eri-1,4-hn i.idiene). Polymers of dicy-clopentadiene produced with the same catalysts are heavily cross-linked resins displaying high toughness and tensile strength as well as excellent impact strength at low temperatures. 

Fig. 4 Renewable platform chemicals used in olefin metathesis (a) plant oils and fatty acids, (b) terpenes and terpenoids, (c) phenylpropanoids, (d) natural rubber (cw-1,4-poly isoprene), (e) carbohydrates, (f) amino acids and peptides, and (g) furans...

Olefin metathesis does not generate stereogenic centers, however, the reaction may be employed in the desymmetrization of prochiral (poly)olefins of the kinetic resolution of racemates. In the example depicted in Scheme 17, a trialkene is desymmetrized, and the preference for the cyclization reaction with one of the two symmetry-equivalent C = C double bonds leads to the enantioselective formation of the reaction product, a chiral dihydrofuran. The following principal conclusions can be drawn from this study ... 

Scheme 25 Olefin metathesis complexes bound to (poly)divinylbenzene monoliths...

The ethynyl groups of ethynylbenzenes undergo polymerization by the action of Group 6, 7, and 9 transition metal catalysts, often used for olefin metathesis, to give the corresponding polyenes which are frequently called poly(phenylacetylene)s (43) [15, 47]. 

Olefin metathesis represents a comparably young area of chemistry. Originally strongly related to Ziegler—Natta chemistry, its basic principles started to be elucidated as late as in the early 1960s. In 1960 Truett, probably inspired by some previous patent disclosures, reported for the first time on the poly-... 

Ivin, K. J. Mol, J. C. Olefin Metathesis and Metathesis Poly merization. Academic Press San Diego, 1997. 

Scheme 12.16 Olefin ring-expansion metathesis of cy-clooctene using a cyclic Grubbs catalyst to produce cyclic poly(cyclooctenamer) [56].

Poly- and oligo-(p-phenylenevinylene) (PPV OPV) as well as their substituted derivatives belong to the conjugated systems with valuable opto-electronical properties [1] as a consequence, the preparation of highly regular PV-products is in the focus of interest of many research groups, worldwide. In this contribution an overview will be given about olefin metathesis as a tool for direct synthesis of PPV, OPV and of their substituted derivatives. Furthermore, results of the product characterization and the product properties will be presented exemplarily. 

When cyclic olefins are subjected to olefin metathesis, breaking of the double bonds proceeds with opening of the unsaturated cyclic unit, and poly(alkenylenes), that is, macromolecular products that are also known as polyalkenamers, can be obtained (Scheme 20.3). Common examples of cyclic... 

Metathesis polymers obtained from unsubstituted monocyclic olefins, such as cyclopentene or cyclooctene, can contain two types of repeating units. These are units in which the carbon-carbon double bond has the cis or trans configuration.Sequences of cis- and trans-enchained poly(l-octenylene) are shown in Figure 20.1. 

The catalyst (mesitylene)W(CO)3/EtAlCl2/exo-2,3-epoxynorbornane produces predominantly trans-isotactic poly(iso-5,5-Me2NB) (85 15 trans cis) at a monomer concentration of 0.4 mol/L. The NMR spectrum of the polymer obtained from enantiomerically enriched monomer displays two prominent signals in the region of S 128-138 ppm that correspond to the trans-HT and trans-TH olefin carbon nuclei. At higher monomer concentrations, that is, 2.7 mol/L, the proportion of cis units increases to 40% (it is noted that the cis/trans and m/r dyad ratios of metathesis polymers are often measured by the intensity of the relevant NMR signals, and small errors in these ratios can... 

---