Rotational rearrangements

One can conclude that the presence of crystallites prevents the amorphous chains from deforming exclusively due to rotational rearrangements and the deformation is accompanied by volume changes. For undrawn PE and PP with degrees of crystallinity of 60-70%, the Poisson s ratio is approximately 0.35 at room temperature. This value is typical for solids and it corresponds to the increase of the volume during unidirectional stretching. 

It has often been remarked from experimental data that the internal viscosity effect appears to be approximately proportional to the solvent viscosity [102,103]. This suggests that the molecular mechanism of the rotational rearrangements may be considered as an example of Kramer s... 

As a concluding remark on this subject, we point out that the present approach rests upon the simplifying assumption that the chain rotational rearrangements may be reduced to a single, average mechanism displaying the characteristic time Tq. Reality is likely to be more complex, and the existence of a spectrum of such characteristic times should be accounted for in a more sophistieated theoretical approach. 

Since relaxation Involves rotational rearrangements, there are energy barriers associated with the process, t Is related to the energy (H) by the equation,... 

In the presence of an external electric field, polar chains reorient themselves in the direction of the field (Fig. 12.5). The orientational motions of polar macromolecules give rise to dielectric absorption. The rotational rearrangement of macromolecules in a solution takes a finite time, increasing the polarization P of the system in the direction of the field, where P is the sum of the constituent dipoles per unit... 

No molecule is completely rigid and fixed. Molecules vibrate, parts of a molecule may rotate internally, weak bonds break and re-fonn. Nuclear magnetic resonance spectroscopy (NMR) is particularly well suited to observe an important class of these motions and rearrangements. An example is tire restricted rotation about bonds, which can cause dramatic effects in the NMR spectrum (figure B2.4.1). 

Synthesis of (A) started with the combination of 2,4,6-trimethylphenol and allyl bromide to give the or/Ao-allyl dienone. Acid-catalyzed rearrangement and oxidative bydroboration yielded the dienone with a propanol group in porlactone ring were irons in the product as expected (see p. 275). Treatment with aqueous potassium hydroxide gave the epoxy acid, which formed a crystalline salt with (R)-l-(or-naphthyl)ethylamine. This was recrystallized to constant rotation. 

Furthermore, the catalytic allylation of malonate with optically active (S)-( )-3-acetoxy-l-phenyl-1-butene (4) yields the (S)-( )-malonates 7 and 8 in a ratio of 92 8. Thus overall retention is observed in the catalytic reaction[23]. The intermediate complex 6 is formed by inversion. Then in the catalytic reaction of (5 )-(Z)-3-acetoxy-l-phenyl-l-butene (9) with malonate, the oxidative addition generates the complex 10, which has the sterically disfavored anti form. Then the n-a ir rearrangement (rotation) of the complex 10 moves the Pd from front to the rear side to give the favored syn complex 6, which has the same configuration as that from the (5 )-( )-acetate 4. Finally the (S)-( )-mal-onates 7 and 8 are obtained in a ratio of 90 10. Thus the reaction of (Z)-acetate 9 proceeds by inversion, n-a-ir rearrangement and inversion of configuration accompanied by Z to isomerization[24]. 

Beckmann rearrangement, 4, 292 phototransposition, 4, 204 synthesis, 4, 223 Wittig reaetion, 4, 294 Wolff-Kishner reduetion, 4, 291 Pyrrole, 1-aeyl-barrier to rotation, 4, 193 IR speetra, 4, 21, 181 rearrangement, 4, 41 synthesis, 4, 82 thermal rearrangement, 4, 202 Pyrrole, 2-aeyl-aeidity, 4, 297 cleavage, 4, 289 conformation, 4, 33... 

The di-TT-methane rearrangement is a stereospecific reaction. There are several elements of stereochemistry to be considered. It is known that the double bond that remains uncyclized retains the E or Z configuration present in the starting material. This result excludes any intermediate with a freely rotating terminal radical. The concerted... 

In the presence of catalytic amounts of Pd(0), silicon-substituted vinyloxiranes can rearrange into the corresponding ot-silyl- 3,y-unsaturated aldehydes (Scheme 9.34) [151]. Treatment of 80 with Pd(OAc)2 and P(OPh)3 results in the formation of 7t-allylpalladium complex 81. Bond rotation to give 82, followed by migration of the silyl moiety, affords aldehyde 83, which is trapped in situ to provide the Felkin-Anh product 84. The reaction proceeds with retention of configuration and the ee of the starting material is retained in the product. The size of the silicon substituents is critical for the outcome of the reaction, as is the choice of ligands on palladium. 

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