Twisted Chains

Polymer science is underdeveloped in terms of descriptions of the structure and properties of stiff-chain polymers. The conducting polymers fall mostly within this blind spot. They also present a number of novel possibilities such as the conversion from a flexible-chain precursor to a rigid-chain polymer, and the conversion between doped and undoped states in the soluble polythiophenes. Likewise, solid-state physics has yet really to tackle the transport of electrons in, and between, disordered, twisted chains. For each of the disciplines involved, the explosion of interest in conducting polymers has brouht a host of new question and new ideas. The process is far from over. 

The use of this model requires a detailed conformational analysis of flexible molecules to determine whether a given twisted chain of atoms shows a net preference for right- or left-helicity under the influence of chiral centers. Thus, R-2,2,3-trimethyl pentane (3) should exist predominantly in the conformation shown, with the twisted chain of four carbon atoms forming a right-handed helix. The substance is, in fact, dextrorotatory. 13>... 

It becomes important, thus, to consider in some detail the geometric attributes of twisted chains of atoms regarded as irregular helices, this being one of the significant ways in which the chirality of asymmetric atoms finds structural expression and becomes manifest. The present... 

In the preceding section we showed that a relatively short twisted chain of atoms can be treated as a one-turn (or, in some cases, two-turn) helix with an arbitrary axis parallel to the end-to-end line. In this section we turn attention to helices having many turns in which an actual axis can be discerned. It is necessary at this point to make some distinctions among several different types of helix that may be of significance in organic chemistry. 

Brewster, J. H. On the Helicity of Variously Twisted Chains of Atoms. 47, 29-71 (1974). Brocas, J. Some Formal Properties of the Kinetics of Pentacoordinate Stereoisomerizations. 

Increasing the degree of cross-finking leads to the distortion of the chelate nodes resulting from the twisted chain strain in the formation of a second and further coordination sites [14d],... 

Figure 4. (a) ORTEP diagram (at 50% probability) of IV showing the atom-labeling scheme, (b) Polyhedral representation of the twisted chain, (c) Three-dimensional hydrogen-bonded network of IV. 

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