Mechanical models Wiechert Model

The mechanical models discussed above are based on single relaxation (or retardation) time. Real polymer fibers have a spectrum or distribution of relaxation and retardation times due to the existence of different types of conformational changes. One convenient way to introduce a range of relaxation times into the problem is to constmct models consisting of a number of Maxwell and/or Kelvin-Voigt sub-models connected in parallel and/or series. Figure 16.24 shows a Maxwell-Wiechert model, which is constmcted by connecting an aibitraiy number of... 

Interestingly, the stereoselectivity of reactions of cyclohexanone vith iso-butyraldehyde and benzaldehyde vere first predicted by using density functional theory calculations on models based on Houk s calculated transition state of the Hajos-Parrish-Eder-Sauer-Wiechert reaction [125]. The transition states of inter- and intramolecular aldol reactions are almost super-imposable and readily explain the observed enantiofacial selectivity. Relative transition state energies vere then used to predict the diastereo- and enan-tioselectivity of the proline-catalyzed reactions of cyclohexanone vith iso-butyraldehyde and benzaldehyde. The predictions are compared vith the experimental results in Scheme 4.30. The good agreement clearly validates the theoretical studies, and provides support for the proposed mechanism. Additional density functional theory calculation also support a similar mechanism [126, 127]. 

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