Plastic Can be Made Electrically Conductive - Research and Development on Polyacetylene -
Hideki Shirakawa
This paper was presented at Aula Magna, Stockholm University on December 8, 2000, as a Nobel Prize Lecture, which describes my previous work that I had carried out just before we reached to the discovery of chemical doping of polyacetylene films.
Polyacetylene synthesized by polymerization of acetylene was usually a black powder that was insoluble in any solvent and infusible.
Soon after I joined in Ikeda's research group, we succeeded in synthesizing polyacetylene directly in a form of thin film by a fortuitous error in 1967. After a series of experiments to reproduce the error, we noticed that we used the concentration of the Ziegler-Natta catalyst nearly thousand times as large as that usually used. Although the film showed a beautiful metallic luster, the conductivity was as low as those of semiconductor or insulator.
Halogen molecules such as chlorine and bromine react readily with polyacetylene to give halogenated polymer. To investigate the halogenation process, in situ measurements were planned to obtain the infrared spectra during the chlorination process. Contrary to the expectation, the infrared spectrophotometer recorded no spectrum but only a 100 % absorption line in the full range of 4000 to 400 cm-1 immediately after addition of a trace amount of chlorine to the film. The polyacetylene film changed into an opaque material due to something very strong absorptions.
Later, we knew that the very strong absorptions are so-called "doping induced infrared band" composed of three bands at 1397, 1288, and 888 cm-1, which have extremely strong absorption coefficients compared with those of the as-prepared polymer. At present, the carbocation formed in long conjugated polyenes is widely known as a positively charged soliton that act as a charge carrier for the electrical conduction. At that time, to my regret, I did not recognize that this carbocation could be a charge carrier and thus polyacetylene could be the first conducting polymer.
To open an era of conducting polymers, we had to wait until we intentionally carried out the doping experiment with use of bromine on Tuesday 23rd, 1976 at University of Pennsylvania, and successively with iodine.