Download Free Radical Copolimerization Dispersions Glassy State by T.S. Chow, L.B. Kandyrin, S. Kuchanov, V.N. Kuleznev PDF

By T.S. Chow, L.B. Kandyrin, S. Kuchanov, V.N. Kuleznev

This quantity features a sequence of essays which discover fresh advancements within the polymer sciences.

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E. the energy difference is magnified in the transition state of the reaction. Bond-formation towards the exo-face of dienophilic units such as those in 38 may also receive a degree of stereoelectronic assistance [106-108] as a result of antiperiplanar cy ~ (cy)* hyperconjugative donation from the strained c r l - 9 a and ~ 4 - 4 a carbon-carbon single bonds (Scheme 9) to the cy-orbitals under construction on the exo-face (see Fig. 9). The corresponding situation for endo-face approach is less favorable, the endoxide ( C - O ) bonds are not able to assume such an antiperiplanar relationship.

The desired syn-isomer 38 is chromatographically less mobile than 39 in which the two endoxide bridges assume the anti-configuration. Whilst both adducts, 38 and 39, are indistinguishable by 1H and 13C NMR spectroscopies, X-ray structural analyses of single crystals of both adducts has allowed unambiguous assignments of their stereochemistries to be made (Figure 10) [91]. 39 o Fig. 10. The X-ray crystal structures of the syn- and anti-bisdienophiles, 38 and 39, respectively, clearly show the relative configurations between the two endoxide bridges The bicyclic nature and relative configurations of the two endoxide bridges in 38 imposes a curvature upon the molecule which is compatible with the subsequent macropolycyclization to give a belt-type structure.

This compound is reported to undergo mono and bis addition [76, 80, 81, 113] in cycloadditions with moderately reactive dienophiles at very different rates. Indeed, mono-addition reportedly occurs approximately 100-300 times faster than bis-addition. This rate difference is believed to occur primarily as a result of the introduction, during mono-addition, of an endocyclic double bond on the opposite sides of the ring from the remaining exocyclic s-cis-butadiene unit. The outcome of this differential reactivity is that a discrete, stepwise, repetitive Diels-Alder protocol can be devized in which use is made of alternatively mild and forcing conditions to control the oligomerization process.

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