Poly lowest energy conformation

Farmer et al. evaluated the conformations and dynamics of poly(di- -hexylsilane). " - " The lowest energy conformer for a polymer containing eight silicon atoms was a helical arrangement with 30° torsional angles in the silane backbone. The authors also monitored different backbone and side chain torsions during dynamics simulations and concluded that the conformation present in the crystalline solid is controlled by intermolecular effects. 

Conformational calculations are carried out on poly(di-n-hexylsilanes). The most significant finding from the energy calculations is that the a -trans conformation is not the lowest energy structure for the symmetrically alkyl-substituted silane polymers. A helical structure is preferred for the isolated molecule. 

With poly(tetrafluoroethylene), on the other hand, the distance between fluorine atoms bonded to adjacent carbon atoms in the T conformation is 0.25 nm, which is smaller than the sum, 0.28 nm, of the van der Waals radii. Thus, the main chain atoms deviate from the ideal T conformation by means of a small change of from 0° to 16° in the torsion angle (see also Figure 4-7). The deviation of carbon chains from the T conformation increases with size of the substituents. For example, with isotactic poly(propylene), having the monomeric unit -(-CH2—CH(CH3), every second conformation in the lowest energy state is a G conformation, such that the whole chain adopts the. . . TGTGTG. . . conformation. This sequence of microconformations leads to a helix macroconformation. 

Values of t lower than 10 ( 10 -10 s) imply an activation entropy greater than zero, being associated with more complex mobility mechanisms for which the activation energy could be of the order of 160 - 210 kJ mof , as is the case for die P relaxation in polysaccharides [75,76] and in polymers where the conformational mobility of the main chain is severely restrict [77]. For the lowest members of the poIy( -alkyl methacrylates) series, activation energies of between 79 and 96 kJ mol were found for the p process that, as already mentioned, is accompanied by restricted rearrangements of the main chain. For longer poly(w-alkyl methacrylates) the relaxation process is more localized with a lower activation energy value (e.g. 21 kJ mof for PnLMA [73]). 

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