The best known member within this family of linear polymers, polyacetylene
(PA)[18,19], has been the subject of numerous
structural studies. One of the earliest reported structural references to
polyacetylene is in 1958. With the advent of the Shirakawa
synthesis high-quality fibrillar PA films (s-PA) first
became available. In general these films (and PA samples prepared using
alternate processes) are found to be predominately
crystalline in nature. Even the early studies of these films identified a rich
interplay in the structural ordering. When s-PA is synthesized at relatively
low temperatures the PA polymer chains are found to exist primarily in a
cis conformation, as shown in Fig. 3, within an orthorhombic Pnma unit
Of the two possible cis forms, cis-transoid or trans-cisoid,
structural studies indicate a preference for the cis-transoid isomer.
Thermal annealing at ca. 150C or doping induces a
solid state transformation to the highly conducting trans conformational
form with associated changes in the crystal structure.
Scattering studies of trans-PA have given rise to numerous debates concerning its three-dimensional (3D) ground state crystal structure. All experimental studies are consistent with the 2D p2gg equatorial packing of the PA chains. With respect to the 3D unit cell three different space groups, P21/a, P21/n[,30,31] and Pnam, have all been proposed. The difficulty in reaching a definitive answer can be attributed to the smallness of the monoclinic distortion, , incomplete thermal cis-trans conversion of test samples, limitations of the PA structural ordering within the films, and the subtle differences among the proposed structures.
In particular there has been considerable controversy, both experimental and
concerning the relative phase relationship between the two chains which
contribute to the PA unit cell and in the degree of bond alternation between
the C-C single and double bond lengths. From a theoretical standpoint PA is a
very attractive model compound because the small number of CH units (four)
which comprise the unit cell enables comprehensive total energy calculations.
The P21/a structure has the two chains in an in-phase relationship while the
P21/n structure yields an out-of-phase arrangement as shown in Fig. 4. An
early analysis of the non-equatorial scattering intensities by Fincher et
al. found strong evidence for P21/n symmetry with
double and single bond lengths of 1.44Å and 1.36Å respectively. While
other claims have been made in the interim period, the most recent studies
available continue to support this model with bond
lengths close to those just stated.