Although organic polymers have replaced metals in many structural contexts, thus far they have failed to replace metals when the latter are used as electrical conductors or semiconductors. The impetus for such replacement includes, among others, lower cost, lower weight of materials, and increased processing variability for polymers as compared with metals. For example, polymers readily can be cast as films, foils, and fibers by standard, time-tested procedures. Polymers can be formed into a limitless variety of shapes and dimensions by standard processing procedures, thereby adding to the potential benefit of electrically conducting polymers.
One potential use for electrically conducting polymers is as electrodes or components of batteries, where their low weight and vast scope of design are attractive. Electrically conducting polymers also could find use in construction of solar cells. Where such polymers are photoconducting they would undoubtedly find applications in the electrophotographic industry.
Among the classes of polymers which hold promise, at least conceptually, as electrical conductors, semiconductors and as photoconductors, are aromatic azopolymers. These polymers are characterized by an extended conjugated system having the azo linkage, --N.dbd.N--, as a distinguishing feature in the repeating unit. The invention herein is a method of preparing such polymers simply and in relatively good yield. More specifically, our invention is a method of synthesizing aromatic azopolymers from aromatic diamines using sodium perborate.
Aromatic diamines have been oxidatively coupled to produce mainly dimers using oxygen and cuprous chloride in pyridine. I. L. Kotlyarevski, M. P. Terpugova, and E. K. Andrievskaya, Chem. Abst. 62, 6571f (1964). Both the low degree of polymerization and the apparent low product yield militate against this preparative method. Several years later Bach and Black (J. Polym. Sci., Part C, 22, 799 (1969)) improved upon the earlier work by using a pyridine and N,N-dimethylacetamide mixed solvent system. Although the degree of polymerization was substantially improved, polymer yield remained uncertain.
Aromatic monoamines are known to be oxidatively coupled to azo compounds by hydrogen peroxide and other peroxides, such as persulfate. Although sodium perborate seems to be best of the peroxides, (S. M. Mehta and M. V. Vakilwala, J. Am. Chem. Soc., 74, 563 (1952); P. Santurri, F. Robbins, and R. Stubbings, Org. Syn. Coll. Vol. V, 341 (1973)) even here product yields of 20-45% are the norm, with reported yields only infrequently approaching 60%.
Since polymer syntheses require a high yield reaction with few, if any, side reactions, it was expected that oxidation of aromatic diamines with perborate would be an unsatisfactory route to aromatic azopolymers. To our surprise and gratification we have found that aromatic azopolymers can be formed in quite good yield, even up to about 97%, by perborate oxidation of aromatic diamines. This unusual result thus affords a relatively facile access to a class of polymers whose electrical conduction and photoconduction merit continued interest.