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Heterotactic associations

Several experiments gave evidence against homotactic or heterotactic association of diene and dienophile. Vapor-pressure measurement for cyclopentadiene (CPD) (Figure 7.2), in pure water and in 10% (w/w) n-PrOH/water, show that Henry s law is obeyed (vapor pressure varies linearly with solute concentration) until [CPD] is 0.03 M in pure water and 0.06 M in ri-PrOH/water. In kinetic measurement the concentration of CPD was always below 0.002 M, indicating that association is highly unlikely. Similar results have been obtained with methyl vinyl ketone, ethyl vinyl ketone, and naphthoquinone. [Pg.162]

Mixtures of diastereomers of 2,4,6-trlphenylheptane are epimerized. The mole fractions olisotactic, heterotactic, and syndiotactic isomers at equilibrium at 343 K are 0.217, 0.499, and 0.284, respectively. There results are interpreted according to the theory of stereochemical equilibrium. The theory of equilibria between isomers and the associated theory of the conformer populations for each isomer provide a mutually consistent interpretation of the two kinds of results, the same arbitrary parameters being used for both. Stereochemical equilibria and conformer population calculated for PS for the same parameters differ considerably from those for the oligomers. [Pg.170]

Lactic acid reacts with diacid or diol to form telechelic polylactic acid, then through further a linking reaction it forms high-molecular-weight lactic acid copolymers [33, 44, 45]. Polymerization of a racemic mixture of L- and D-lactides usually leads to the synthesis of poly-DL-lactide (PDLLA)which is amorphous. Use of stereospecific catalysts can result in heterotactic PLA which is found to show crystallinity [46, 47]. The degree of crystallinity and many associated properties are greatly controlled by the ratio of D to l enantiomers in the polymer [48]. [Pg.10]

For a given monomer, the stereocontrol depends primarily on the polarity of the medium. If polymerization is carried out in highly polar solvents, those that encourage dissociation, the result will be polymers with relatively high proportions of syndiotactic triads (see Table 18-6). However, the proportion of heterotactic triads is likewise very significant. Changing to apolar solvents, the proportion of isotactic triads Xa is found to increase, while the proportion of heterotactic triads remains approximately constant. It is obviously impossible to achieve holotactic polymers by ionic polymerizations with free ions. The influence of the polarity of the solvent shows that the increase in the proportion of isotactic triads probably corresponds to the increase in the concentration of contact ion pairs or ion associates. [Pg.657]


See other pages where Heterotactic associations is mentioned: [Pg.164]    [Pg.164]    [Pg.428]    [Pg.135]    [Pg.153]    [Pg.428]    [Pg.25]    [Pg.9]   


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