Polypyrrole and Polythiophene Systems

Some of the interest in these polymers follows from their chemical stabili-ties i 1 7,118) ppy solubilities and low-temperature fusibilities of the 

FIGURE 5.17. Chain structures and molecular geometries of (a) polypyrrole and (b) polythiophene. Bond lengths are in A. 

A contributory factor at least in the morphological complexity of these polymer systems is the tendency of their chains to form helical conformations. The application of various electron microscopy methods to PPy, PTh, and their substituted products has revealed the presence of helical and sometimes superhelical structures, as well as (in one case at least) nearly parallel, straight-chain conforma- 

FIGURE 5.18. Unit cell of pristine polythiophene (also assumed for polypyrrole). (Adapted from Ref. 125 by permission of the publishers.) 

In a theoretical approach Cui and Kertesz conducted a series of semiem-pirical quantum chemical calculations on the conformational potential energy surfaces of single chains of PPy and PTh and their substituted products. Their results predicted distinct energy minima at a planar and helical chain conformation 0.08 eV higher, separated by a barrier of 0.2 eV. 

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