Conformation and hindered rotation

In solution, the flexibility of the polymer chain allows the polymer to collapse into a compact, randomly coiled structure rather than to retain an extended form. A transition between these two forms can be observed depending on solvent quality and temperature and is best known as the rod-to-coil transition of biomacromolecules such as DNA. 

In the case of a conjugated polymer like polyacetylene, as shown in Figs. 1.8(a) and 1.8(b), the situation with respect to rotation about the nominally single bonds is very different it is controlled by the electronic structure of the backbone bonds rather than by steric hindrance. As will become clear later, when this type of polymer is discussed in relation to conduction, the partial 

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