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Cis-transoid form

Figure 8 (a) The cis-transoid form (left of the defect) is more stable than the trwis-cisoid form (right), (b) Spin defect trapped at one end of the chain. [Pg.673]

The process results in a cis-transoid structure. The formation of trans-polyacetylene is suggested to take place through isomerization of the new segment formed by cis insertion before it can crystallize.412... [Pg.769]

The reaction of benzylamine with 2,3-acetonitrile solvent was examined in the presence of one molar equivalent of siivernitrate no acid catalyst was added. The reaction took a different course Instead of forming SI, 54, or 55, the dioxane ring remained unopened and a 9,l0-diaza-l,4,5,8-tetraoxaperhydroanthracene denvative (61a) was obtained (42% yield). This reaction was also observed with pyridylmethylamine, in the absence of either silver nitrate or add catalyst, to yield 61b (67% yield). The structures 61a,b are in agreement with their spectral data. An x-ray crystal structure determination established the structure of 61a. Both 61 and the related 49 exhibit cis-transoid-cis ring stereochemistry. [Pg.119]

A Polyacetylene cis-transoid structures is as a result formed. Formation a trans-structure at heats speaks thermal isomerization, previous crys-... [Pg.88]

Polymerization of phenylacetylene in compressed (liquid or supercritical) CO2 has been studied using a Rh catalyst, [(nbd)Rh(acac)l2 (167). Higher polymerization rate is obtained in CO2 than in conventional organic solvents such as THF and hexane. Polymerization in the presence of a phosphine ligand, p-[F(CF2)6(CH2)2]-CeH4 3P, predominantly produces cis-transoidal polymers, while, without the ligand, both cis-transoidal and cis-cisoidal polymers are formed. [Pg.17]

As shown in Figure 21.2, four steric (geometric) structures are theoretically possible for polyacetylenes, that is, cis-cisoid, cis-transoid, trans-cisoid, and trans-transoid, because the rotation of the single bond between two main chain double bonds in the main chain is more or less restricted. Polyacetylene can be obtained in the membrane form by use of a mixed catalyst composed of Ti(0-n-Bu)4 and EtsAl, the so-called Shirakawa catalyst (1) both the cis- and trans-isomers are known, which are thought to have cis-transoidal and trans-transoidal structures, respectively (Table 21.1). Phenylacetylene can be polymerized with a Ziegler-type catalyst, Fe(acac)3/Et3Al (2) (acac = acet-ylacetonate), Rh catalysts (7), and metathesis catalysts (3-5) that contain Mo and W as the central metals, to provide cis-cisoidal, cis-transoidal, cis-rich, or trans-rich polymers, respectively. [Pg.555]

Polyacetylenes are the most typical and basic r-conjugated polymers, and can ideally take four geometrical structures (trans-transoid, trans-cisoid, cis-transoid, cis-cisoid). At present, not only early transition metals, but also many late transition metals are used as catalysts for the polymerization of substituted acetylenes. However, the effective catalysts are restricted to some extent, and Ta, Nd, Mo, and W of transition metal groups 5 and 6, and Fe and Rh of transition metal groups 8 and 9 are mainly used. The polymerization mechanism of Ta, Nd, W, and Mo based catalysts is a metathesis mechanism, and that of Ti, Fe, and Rh based catalysts is an insertion mechanism. Most of the substituted polyacetylenes prepared with W and Mo catalysts provide trans-rich and cis-rich geometries respectively. Polymers formed with Fe and Rh catalysts selectively possess stereoregular cis main chains. [Pg.569]

Polyacetylene the simplest example of this class of poly-conjugated systems was synthesized in 1958 by Natta et al.(77) as the trans form by means of typical Ziegler catalysts. Cis and trans configurations of polyacetylene were reported first by Watson, McMordie and Lands (78) in 1961. Shirakawa and Ikeda (79 81) synthesized an all-cis and an all-transpolyacetylene and pointed out the cis-transoid(IV)and trans-transoid(V)structure of these polymers. [Pg.29]

For example, the 10-membered 6-pelargonolactam in solution at 25 °C consists mainly of the tram isomer with only 5-10% of the cis isomer, while in crystals, it exists as a mixture of cis and tram isomers. Conversely, in solutions of the 11-membered lactam, only the tram isomer is present. Interestingly, in protonated lactams, the amide configuration is sometimes different from that of the neutral lactams, as shown by the analysis of the hydrochlorides. Drastic is the effect of protonation on ri-caprylolactam, which changes its configuration from the nonplanar transoid form to a nearly planar cisoid form. [Pg.335]

Reactions a and b in Scheme 8 represent different ways of coordination of butadiene on the nickel atom to form the transoid complex 27a or the cisoid complex 27b. The hydride addition reaction resulted in the formation of either the syn-7r-crotyl intermediate (28a), which eventually forms the trans isomer, or the anti-7r-crotyl intermediate (28b), which will lead to the formation of the cis isomer. Because 28a is thermodynamically more favorable than 28b according to Tolman (40) (equilibrium anti/syn ratio = 1 19), isomerization of the latter to the former can take place (reaction c). Thus, the trans/cis ratio of 1,4-hexadiene formed is determined by (i) the ratio of 28a to 28b and (ii) the extent of isomerization c before addition of ethylene to 28b, i.e., reaction d. The isomerization reaction can affect the trans/cis ratio only when the insertion reaction d is slower than the isomerization reaction c. [Pg.304]


See other pages where Cis-transoid form is mentioned: [Pg.378]    [Pg.133]    [Pg.6]    [Pg.804]    [Pg.153]    [Pg.198]    [Pg.199]    [Pg.378]    [Pg.133]    [Pg.6]    [Pg.804]    [Pg.153]    [Pg.198]    [Pg.199]    [Pg.463]    [Pg.463]    [Pg.56]    [Pg.62]    [Pg.76]    [Pg.382]    [Pg.382]    [Pg.69]    [Pg.123]    [Pg.261]    [Pg.263]    [Pg.330]    [Pg.383]    [Pg.112]    [Pg.88]    [Pg.183]    [Pg.481]    [Pg.15]    [Pg.23]    [Pg.555]    [Pg.20]    [Pg.66]    [Pg.20]    [Pg.231]    [Pg.198]    [Pg.709]    [Pg.335]    [Pg.673]    [Pg.124]    [Pg.556]    [Pg.410]    [Pg.289]    [Pg.306]   
See also in sourсe #XX -- [ Pg.6 ]




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