Substantia. An International Journal of the History of Chemistry 1(2): 99-109, 2017 Firenze University Press www.fupress.com/substantia ISSN 2532-3997 (online) | DOI: 10.13128/substantia-30 Citation: S.C. Rasmussen (2017) The Early History of Polyaniline: Discovery and Origins. Substantia 1(2): 99-109. doi: 10.13128/substantia-30 Copyright: © 2017 S.C. Rasmussen. This is an open access, peer-reviewed article published by Firenze University Press (http://www.fupress.com/substan- tia) and distribuited under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All rel- evant data are within the paper and its Supporting Information files. Competing Interests: The author declared that no competing interests exist. Historical Article The Early History of Polyaniline: Discovery and Origins Seth C. Rasmussen Department of Chemistry and Biochemistry, North Dakota State University, NDSU Dept. 2735, P.O. Box 6050, Fargo, ND 58108-6050, USA E-mail: seth.rasmussen@ndsu.edu Abstract. Prior to the discovery of its conductive properties in the 1960s, polyaniline was studied and applied as a variety of colored materials and dyes. The history of the discovery and origins of polyaniline are presented beginning with the initial oxidation of aniline by F. Ferdinand Runge in 1834 and concluding with the first electrochemi- cal oxidation of aniline by Henry Letheby in 1862. In the process, the reports of ani- line oxidation products between 1834 and 1862 are evaluated and discussed in light of modern knowledge, highlighting the various historical contributions to the current field of conjugated polymers. Finally, an initial argument for polyaniline as the first synthetic organic polymer is presented. Keywords. Polyaniline, aniline black, emeraldine, conjugated polymers, oxidative polymerization. INTRODUCTION Modern society is largely a plastic-based culture in which organic plas- tics have become more ubiquitous than other common materials such as met- als, glass, or ceramics. As a result, some have postulated that there is sufficient justification to refer to the period beginning with the 20th century as the Age of Plastics.1 The bulk of commercial plastics are made up of various saturated organic polymers that are primarily electronic insulators. In contrast, conjugat- ed polymers (Figure 1) are a less common class of organic plastic materials that are native semiconducting materials. In addition, these materials are capable of enhanced electronic conductivity (in some cases quasimetallic) in either their oxidized (p-doped) or reduced (n-doped) state.2,3 Conjugated polymers are thus organic materials that combine the conductivity of traditional inorganic materi- als with many of the desirable properties of organic plastics, including mechani- cal flexibility and low production costs.2,3 This unique combination of proper- ties has led to considerable fundamental and technological interest in these materials over the last few decades, resulting in the current field of organic elec- tronics. In the process, a variety of modern technological applications have been developed, including sensors, electrochromic devices, organic photovoltaics (solar cells), field effect transistors, and organic light-emitting diodes (OLEDs).2,3 100 Seth C. Rasmussen Discussion of the history of these materials typical- ly begin in the mid-to-late 1970s with the collaborative work on conducting polyacetylene by Hideki Shirakawa, Alan G. MacDiarmid, and Alan J. Heeger,4-7 for which they were awarded the 2000 Nobel Prize in Chemistry. The common view that their work marks the historical origins of these materials is further supported by the language of the award which states that it is “ for the dis- covery and development of electrically conductive poly- mers”.8 More recently, however, a number of reports have begun to reveal a more accurate and complete account of the history of these materials, highlighting in particular the documented efforts that predate the commonly cited polyactylene work of the 1970s.9-16 The first report of the conductive nature of these materials was by Don Weiss in 1963,17-19 but the gen- eral study of these polymeric materials far predates even this pioneering work. Of the core parent polymers given in Figure 1, the oldest of these materials is gen- erally considered to be polyaniline, although exactly when this material was first studied is often a point of debate. Various authors have credited different figures with the origin and discovery of this material, originally known as aniline black.20-22 These figures have included F. Ferdinand Runge20-23 (1794-1867), Carl Fritzsche9,20 (1808-1871), John Lightfoot24,25 (1831-1872), and Henry Letheby11,26 (1816-1876). The current report will attempt to evaluate the work of these figures in light of current knowledge and thus present the first detailed account of the discovery, origins, and development of polyaniline up through the 1860s. CURRENT KNOWLEDGE OF POLYANILINE PRODUCTION AND CHARACTERISTICS In order to be able to evaluate the work of the researchers above, it is first necessary to briefly review our modern understanding of the chemistry and char- acteristics of polyaniline. Polyaniline is readily pro- duced by oxidative polymerization, via either chemical or electrochemical oxidation of aniline.21,22,26,27 Most often, such oxidations are carried out in acidic aque- ous environments with a pH below 3-4, which is most favorable for the production of the polymeric prod- uct.21,22 A significant excess of oxidant is required due to the fact that polymer product undergoes oxidation at a lower potential than the initial aniline, thus con- suming oxidant that cannot contribute to polymeriza- tion. As a consequence, the polymer product is also always isolated in its oxidized (p-doped) form. Neutral, non-doped samples are possible, but must be obtained via reduction following the oxidative polymerization process. Polyaniline can thus occur in a number of well-defined oxidation states, of which isolated poly- mer samples are viewed to exist as averages of these forms.22,27 These states and their corresponding names were initially proposed by Arthur G. Green (1864-1941) and Arthur E. Woodhead in 1910,28-31 and range from the fully reduced leucoemeraldine through the partially oxidized protoemeraldine, emeraldine and nigraniline, to the fully oxidized pernigraniline. The three primary forms are given in Figure 2. It should be pointed out that the terms emeraldine and nigraniline predate the Figure 1. Common parent conjugated organic polymers. Figure 2. Primary oxidation states of polyaniline. 101The Early History of Polyaniline: Discovery and Origins work of Green and Woodhead, with nigraniline origi- nally referring to true aniline black and emeraldine rep- resenting a green, reduced form of the material.23,32 Unlike the other parent conjugated polymers shown in Figure 1, the electrical conductivity of polyaniline is affected by both traditional doping via oxidation, as well as the extent of polymer protonation.21,22 This pro- tonation effect is often referred to as protonic acid dop- ing21,22,27 and the acid doping level can be tuned sim- ply by controlling the pH of the dopant acid solution. As such, each of the forms given in Figure 2 can exist as either its base or protonated salt. The most common forms of polyaniline, however, are either the emeraldine base or emeraldine salt (Figure 3). As detailed in Figures 2 and 3, the color of the poly- meric material is dependent on both its state of oxidation and protonation. Thus, the most common emeraldine salt is green to green-black, while the emeraldine base is vio- let-blue to blue-black. In both cases, the observed color is dependent on concentration. The state of the polymer also affects its relative solubility. Thus, while the emeral- dine base is soluble in organic solvents such as N-methyl- pyrrolidone, polyaniline protonated by common mineral acids is only soluble in aqueous acids.22,27 RUNGE, KYANOL, AND ITS OXIDATION The oldest of the figures to which the polymerization of aniline is first attributed is Friedlieb Ferdinand Runge (Figure 4), who was born in Billwärder, Germany (near Hamburg) on February 8, 1794.33-36 The son of a pas- tor,33-35 Ferdinand (he went by his middle name) was the third of seven children.35 His mother died in 1806 and his father later remarried, with an additional child from the second marriage.35 His family was quite poor and could not afford to send the children for further education beyond the local primary school.34,35 Thus, he was appren- ticed at age 15 to his uncle, studying pharmacy at the Rat- sapotheke in Lübeck.34-36 He then moved to Berlin in 1816 in order to study medicine, but transferred to Göttingen after two years, where he attended chemistry lectures by Friedrich Stromeyer (1776-1835).34,35 After only one semester, however, he transferred again to Jena. At Jena, Runge studied analytical chemistry under Johann Wolf- gang Döbereiner (1780-1849)34,35 and finished his Dr. med. with a dissertation on belladonna in 1819.33-36 Wanting to become a professor, Runge then returned to Berlin that same year to habilitate as Privatdozent.33-35 This required him to first acquire a Dr. phil. degree, which he completed in 1822 with a dissertation on indigo.33-35 He remained in Berlin where he lectured on phy- tochemistry and technical chemistry until October of Figure 3. Protonic equilibria of polyaniline’s emeraldine form. Figure 4. Friedlieb Ferdinand Runge (1794-1867). [Edgar Fahs Smith Memorial Collection. Kislak Center for Special Collections, Rare Books and Manuscripts. University of Pennsylvania]. 102 Seth C. Rasmussen 1823, after which he traveled Europe with the primary goal of visiting Paris.34,35 Upon his return, he joined the University of Breslau as Privatdozent in 1826. He was promoted to extraordinary professor of technical chem- istry in 1828,34,35 but this was not a permeant position and he found his conditions unfavorable for develop- ment of his practical ideas. He finally ended his academ- ic career in 1831.34,35 He moved to Berlin where he was offered a position in the chemical works of the Königli- che Seehandlungs Societät (Royal Sea Trade Society) at Oranienburg (small town near Berlin) in 183233-35 or 1833.36 It was here that he began the most fruitful period of his career. Among his accomplishments, he discov- ered a number of species from coal-tar, including pyr- role, quinoline, phenol, and aniline. He then became the technical director of the chemical plant in 1840.33 Runge constantly clashed with his superiors, however, and he was ultimately dismissed in 1851,36 with his last day of employment December 31, 1852.34,35 He was granted a small pension, however, providing that he resided locally where his advice could be sought if needed.35,36 He thus remained in Oranienburg where he died on March 25, 1867 at age 74.33-36 Shortly after the start of his new position at Oranienburg, Runge began the investigation of distil- lates of coal-tar, which he separated into acidic and basic fractions.37 From the basic fraction, he isolated a volatile oil with a scarcely noticeable, but peculiar, odor. When treated with acid, the oil formed colorless salts, and both the oil and its salts became aquamarine when treated with chlorine of lime. Because of this color response, he named this oil kyanol, from a combination of the Greek kuanós (“blue”) and the Latin oleum (“oil”).37 This oil was later shown to be identical to the product krystallin isolated from the dry distillation of indigo, which had been previously reported by Otto Unverdorben (1806- 1873) in 1826. These materials are now both known as aniline.33 Runge then found that treatment of aniline with other oxidizing agents resulted in the formation of vari- ous dark insoluble dyes. These efforts began with the application of a hydrochloric acid solution of gold oxide to a porcelain plate, followed by a drop of aqueous ani- line. Heating the combination to 100 °C then resulted in purple-colored spot, which became blue-gray when dried.38 He then went on to show that if one first coated a hot porcelain plate with a solution of copper oxide in hydrochloric acid, and allowed it to dry, an added drop of aniline nitrate solution heated to 100 °C then resulted in a dark green-black spot.38 Using copper oxide in nitric acid and aniline hydrochloride gave the same result. In order to confirm that it was the metal oxidant that was causing the color change, he then showed that heat- ing just aniline nitrate or aniline hydrochloride gave no decomposition or reaction. However, the addition of any copper salt then caused the formation of the black material.38 Lastly, he showed that the addition of aniline hydrochloride to a hot porcelain plate coated with potas- sium bichromate also produced a dark black spot. In the process, Runge noted that if sufficient amounts of the aniline nitrate or aniline hydrochloride salts could be produced, the colored products resulting from their treatment with metal species could provide a practical use.38 To illustrate this, he first treated cot- ton with lead chromate, after which he printed the fabric with aniline hydrochloride to give green patterns within twelve hours. If more concentrated solutions of the ani- line hydrochloride were used, black patterns developed rather than green. Both developed patterns remained unchanged by rinsing in water.20,38 Runge had a strong interest in dyeing and the col- oring of fabrics, as illustrated by his publication of a three-volume series entitled Farbenchemie. The first of these was published in 1834,39 the same year as his ini- tial aniline publications, and these volumes summa- rized the technical methods and state of the coloring field at the time.33 Due to this interest and the results of his studies from various coal tar species such as ani- line, Runge approached his superiors in 1836 to propose that the chemical works at Oranienburg could produce such synthetic dyes from coal tar, but such efforts were never approved.33,36 Runge moved his focus onto other studies and it was only much later that the merits of his discoveries with synthetic dyes were recognized. Fol- lowing Runge’s initial reports, further related studies on aniline did not appear until 1840 with the efforts of Carl Fritzsche. FRITZSCHE AND ANILINE FROM INDIGO Carl Julius Fritzsche was born on October 29, 1808 in Neustadt, Germany (then within the Kingdom of Saxony).40,41 His father was a physician and the town where Fritzsche grew up had no grammar school, so he was educated through private lessons until he was 14. He then moved to Dresden, where he was employed in the pharmacy of his uncle for five years.40 After that, he moved to Berlin where he became an assistant to Eil- hard Mitscherlich (1794-1863) in 1830.40,41 As Fritzsche took a doctorate in botany at the University of Berlin in 1833, Mitscherlich is credited with all of Fritzsche’s chemical training.40-42 His initial papers focused on botanical subjects, but with chemical character, and his 103The Early History of Polyaniline: Discovery and Origins following papers began to take on greater chemical con- tent.40 He then emigrated to Russia in 1834,40-42 where he became the manager of Struve’s mineral-water works in St. Petersburg.41 He became an adjunct member of the St. Petersburg Academy of Sciences in 1838, becom- ing an associate member in 1844, and a full member in 1852.40,42 His chemical work, most of which appeared initially in publications of the St. Petersburg Academy of Sciences,40 touched on uric acid derivatives, vari- ous oxides of nitrogen, and most critical of the discus- sion here, indigo and its derivatives. All of the efforts discussed below were carried out in a small laboratory next to his house, but he later shared the new, spacious chemical laboratory of the St. Petersburg Academy with Nikolay Zinin (1812-1880) beginning in 1866.40,42 Fritzsche enjoyed good health until he suffered a stroke in 1869.40-42 Although he recovered from it, he suffered from paralysis on one side, and his speech and memory suffered.40,42 Nevertheless, he continued working until his death on June 20, 1871.40,41 In 1840, Fritzsche published a paper detailing the isolation of a new species from the decomposition of indigo.43 Treating indigo with a hot, highly concentrat- ed solution of base gave a salt mass of reddish-brown color, from which was then distilled a brown oil. Further purification isolated a colorless oil, which he called ani- lin after the Spanish name of indigo, añil.41 This mate- rial was a base which formed light and highly crystal- line salts and was isolated as 18-20% of the original indigo.43 During the publication of this paper, the edi- tor of the Journal für praktische Chemie, Otto Erdmann (1804-1869), recognized the similarity of anilin with Unverdorben’s krystallin, and highlighted this in a note following Fritzsche’s paper.44 Two years later, Nikolay Zinin reduced nitrobenzene to give a species that he called benzidam.45 In a short note directly following Zinin’s paper, Fritzsche identified benzidam as his own anilin, stating that there can be no doubt about its iden- tity.46 The following year, August Hoffmann (1818-1892) then presented evidence that Unverdorben’s krystallin, Runge’s kyanol, Fritzsche’s anilin, and Zinin’s benzidam were all the same compound,47 which ultimately became known as phenylamine or aniline. Fritzsche found that aniline reacted with exposure to air, taking on first a yellow color and then turning into a brown mass. Continuing his 1840 study of the oil, he found that under ‘certain circumstances’ treat- ment with HNO3 caused the production of a blue or green material, which he thought might be indigo, but he was only able to generate it in small quantities.43 He then found that dissolving aniline salts in chromic acid (H2CrO4, typically as a H2SO4 solution) produced a dark green precipitate, which ultimately became black-blue in color.43 Combustion analysis of the precipitate revealed a significant amount of chromium. Lastly, he treated ani- line salts with potassium permanganate, which resulted in a brown precipitate containing manganese oxide. Fritzsche admitted that he had not been able to study these color-forming reactions in much detail and planned to return to these in later publications.43 Thus, while he did not follow up on the direct results reported in 1840, he did return to the treatment of aniline with oxidants in 1843, this time applying potassium chlo- rate.48 Mixing an alcohol solution of aniline salt with potassium chlorate in HCl resulted in a blue precipi- tate, which turned green upon washing with alcohol, and became dark green upon drying. Analysis of the composition of the solid revealed an empirical formula C24H20N4Cl2O, which is in near perfect agreement with the structure of the emeraldine salt given in Figure 3 (X = Cl-).48 Although this seems to be Fritzsche’s last report on the oxidation of aniline, the green aniline materials introduced by Runge and Fritzsche were later applied to commercial dyes by Frederick Crace-Calvert (1819-1873), Samuel Clift, and Charles Lowe. CALVERT, CLIFT, LOWE AND EMERALDINE Frederick Crace-Calvert was born near London on November, 14, 1819.49 In 1835, he moved to France where he studied chemistry at the University of Rouen under Gerardin. After two years, he then moved to Paris to continue his studies at the Sorbonne and the College de France. He then became the manager of the chemical works of Robiquet and Pelletici at the age of 21, but soon left to become an assistant to Michel Eugène Chevreul (1786-1889) in 1841.49,50 He returned to England in late 1846, where he was appointed as a honorary professor of chemistry at the Royal Institution, and later as lecturer on chemistry at the School of Medicine in Manchester. He became interested with the properties of carbolic acid and built works for its manufacture in 1865. Due to his various contributions, he became a Fellow of the Royal Society and of the Chemical Society, as well as other societies both at home and abroad. He was seized with a fatal illness in 1873, and died in Manchester on October 24, 1873.49 Sometime prior to the summer of 1860, Crace- Calvert, Samuel Clift, and their assistant Charles Lowe developed green and blue dyes from the oxidation of ani- line for the coloring of cotton, for which they filed a joint patent on June 11, 1860.24,51,52 These methods involved the application of an aniline salt (either the hyrdochloride or 104 Seth C. Rasmussen tartate) and potassium chlorate to give a green color after 12 hours. This green color was given the name emeral- dine,24,50-55 which ultimately became the name adopted for the most common form of polyaniline (Figure 3). If the initially produced green dyed fabric was boiled in an alkaline or soap solution, the green color could be con- verted to a blue color, which was given the name azu- rine.24,51-54 Both the blue color and the method of its pro- duction from emeraldine is consistent with the formation of the emeraldine base as outlined in Figure 3. Crace- Calvert presented these results as part of an address before the Society of Arts, which was then published in the February 7th issue of the Journal of the Society of Arts in 1862.52 Later that same year samples printed by these techniques were exhibited in the chemical section at the London International Exhibition of 1862.55 The following year Crace-Calvert also included this in his book Lec- tures on Coal-Tar Colours, and on Recent Improvements and Progress in Dyeing and Calico Printing.53 Crace-Calvert, Clift, and Lowe induced the print- ers Wood and Wright to apply their methods to gener- ate green and blue colors in the end of 1860.53 Wood and Wright considered the resulting colors good enough to be commercialized and introduced improvements result- ing in a dark shade that could be considered black.25,53 The improvements were achieved by either adding iron salts or other oxidizing agents to the potassium chlorate, followed by treating the color produced on the fabric with either a weak solution of potassium bichromate or bleaching powder. It was also found that copper nitrate could be mixed with aniline hydrochlorate, without the addition of potassium chlorate, and the mixture could be printed on the fabric to gradually give a dark green or black. These two methods gave either a green or blue color, but one that was so dark that it could be viewed as black.53 About the same time, black dyes produced from the oxidation of aniline were introduced by John Light- foot and Heinrich Caro (1834-1910). LIGHTFOOT, CARO, AND ANILINE BLACK John Lightfoot, Jr. was the son of Thomas Lightfoot (1811-1866), but was named after his grandfather John Lightfoot, Sr. (1774-1820).25,50 His grandfather, father, and uncle were all colorists and connected with the Broad Oak Print Works of Accrington, about 20 miles north of Manchester.24,25,50 As part of his practical education, Lightfoot traveled to France and Alsace in 1854.50 He then continued the family tradition by joining Broad Oak in ca. 1855.25 At Broad Oak, Lightfoot played a leading role in the use of madder, archil (or orchil), and indigo colors. He also began experimenting with the semisyn- thetic colorant known as murexide, the behavior of ani- line colors, methods of fixing new dyes to wool and cot- ton, and imitations of madder and aniline colors.25 According to Lightfoot, it was November 1859 when he first observed that aniline hydrochloride in the pres- ence of potassium chlorate imparted a fast and brilliant black to cotton during printing.25,51 Initial efforts in the application of this aniline hydrochloride and potassium chlorate mixture produced little to no color after 24 hours, but he found that when the same formulation was printed with copper rollers, a green color was produced within 12 hours. Believing that the copper was a criti- cal factor, he then added a copper salt to the mix to ulti- mately give an intense black.24,51 The conclusion that the copper was a necessary factor is not consistent with the previous reports of either Runge38 or Fritzsche,48 both of which produced either green and black materials by treating aniline hydrochloride with chlorate salts alone. In reviewing the formulation given by Lightfoot,51 how- ever, it is clear that he used too little potassium chlorate to sufficiently oxidize the aniline salt and thus an addi- tional oxidant such as copper was required for him to generate the color in sufficient intensity. Of course, prior to the addition of the copper salts, Lightfoot’s process of printing cotton with the green or black aniline dyes was nearly identical to that originally reported by Runge.38 In fact, from a chemical standpoint, the methods only differed in the chlorate salt utilized. Although Lightfoot recognized that the high price of aniline in 1859 was a disadvantage, he continued with experiments in July 1860 and had produced a few samples by 1861.24,25 In 1862, he reported his discovery as a letter to the Editor in the December 6th issue of Chemical News and Journal of Industrial Science, along with some sample swatches.56 At the end of Lightfoot’s letter, the editor added a note that the color was not a pure black, and as with the previous cases of either Runge38 or Wood and Wright,53 was most likely so dark a blue or green as to appear black. However, Lightfoot’s black was considered superior to that of Wood and Wright.25 Patents for this process were filed in early 1863,51 with the US patent granted in May of the same year.57 In this patent, Light- foot describes the dyeing of fabric with mixtures of ani- line hydrochloride and potassium chlorate, followed by the addition of copper chloride.24,55,57 The patent, how- ever, also covered the use of other copper salts or even salts of other metals.57 The rights were then sold to Jakob J. Muller-Pack of Basle.25 A significant limitation in Lightfoot’s printing pro- cess was that it caused severe corrosion to the rollers of the printing machines, which also led to streaking on 105The Early History of Polyaniline: Discovery and Origins the cloth.24,25,55 This limitation was solved by Charles Lauth, who registered a patent in 1964 that specified the use of insoluble copper sulfide. The insoluble salt was inactive at the time of printing and later converted to soluble copper sulfate by action of the chlorate salt dur- ing development of the aniline black, which dramati- cally reduced the damage to the rollers of the printing machines.24,55 Lauth’s process was sold to Muller-Pack, who combined it with Lightfoot’s process in an effort to dominate the European and US markets.25 During this same time period, a second black ani- line dye was produced by Heinrich Caro (Figure 5) in 1860.24,25 Born on February 13, 1834 in Posen, Caro attended the Realm Gymnasium in Köln from 1842- 1852.58 From 1852-1855 he attended the Königliche Gewerbeinstitut (Royal Trade Institute), which trained students for industry, while also attending lectures at Berlin’s Friedrich-Wilhelms-Universität (now the Hum- boldt University of Berlin).58,59 In April 1855, Caro took a modest appointment in the Troost factory in Mülheim, where he mainly performed analytical work outside the printing shop.58 In March 1857, Caro was then sent on a study trip to England in March of 1857, where he visited a large number of printing and dyeing factories, in par- ticular that of John Dale and Roberts. Upon his return to Germany, he then carried out his military service in 1857-1858.58 In November, 1859, Caro moved to England to try his luck there. Although his initial efforts were in vain, he was finally able to obtain a job with John Dale, co-owner of the Cornbrook Chemical Works of Roberts, Dale & Co. in Manchester.58-60 He later returned to Germany in 1866, and on November 1, 1868 he became the coordinating technical director for the Badische Aniline and Soda Fab- rik (BASF).24,58-60 There he oversaw development of new colors including artificial alizarin, eosin, methylene blue, and azo dyes, as well as the initial stages of the indigo synthesis, and contributed towards the structural elucida- tion of the triphenylmethane dyes.59 He was appointed to the company’s board of directors in 1884 and continued to serve on the board until 1890.60 Caro died after a short illness on September 11, 1910 in Dresden.58 In 1960, while working for Roberts, Dale & Co., Caro developed a process for making aniline purple by the oxidation of aniline with copper salts. After alco- holic extraction of the desired purple dye, a black resi- due remained which provided an excellent fast black dye for printing on cotton. Roberts, Dale & Co. then com- mercialized this dye for sale to printers in 1862.24,54,59 As with Lightfoot’s original formulation, this residue played havoc with the printing machinery, but it could be suc- cessfully printed with wooden hand blocks.24 The efforts of Roberts, Dale & Co. established the advantages of aniline black over other black colorants of the time and it has been proposed to have been the motivating fac- tor for Lightfoot to patent his process in 1863.24 As dis- cussed above, the color found more widespread applica- tion in British printworks following the introduction of insoluble copper sulfide by Charles Lauth in 1864.59 By 1871, these black dyes from aniline became known as aniline black.51 Aniline black later became the first real general term for polyaniline. Following the introduction of aniline black dyes, the next major innovation came from the physician and chemist Henry Letheby. LETHEBY AND THE ELECTROLYSIS OF ANILINE SULFATE Henry Letheby (Figure 6) was born in Plymouth, England in 1817,61-63 and received his early education Figure 5. Heinrich Caro (1834-1910). [Edgar Fahs Smith Memorial Collection. Kislak Center for Special Collections, Rare Books and Manuscripts. University of Pennsylvania]. 106 Seth C. Rasmussen there.61 He continued his chemical studies in the labo- ratory of the Royal Cornwall Polytechnic Society in Fal- mouth,61,62 where he eventually became assistant and did some lecturing.61 He then moved to London in 1837 to attend the Aldersgate Medical School, where he became assistant to Jonathan Pereira (1804-1853), professor of chemistry at the London Hospital.61,62 Letheby became a licentiate of the Society of Apothecaries in 1837, and then received his M.B. (Bachelor of Medicine) from Lon- don University in 1842,62,63 after which he succeeded Pereira as the chair of chemistry and toxicology at the London Hospital in 1846.61,62 He was elected as London’s medical officer of health in 1855,61-64 while also serving as the city’s public analyst.62-64 In addition, he served as the city’s chief gas examiner 62-64 and as a consult- ing chemist to the Great Central Gas Company61,64 In 1858, he received a M.A. and Ph.D. from an unknown German university.62 He was a fellow of both the Lin- nean Society of London and the Chemical Society of London.62,63 Because of failing health, Letheby resigned his city posts in February 1874.62 After a brief illness, he died at the age of 60 on March 28, 1876, at his home in London,61-64 and was buried in Highgate cemetery on March 30th.62 After the investigation of two cases of fatal poison- ings by nitrobenzenes, Letheby began investigations into chemical tests for the presence of aniline, as he had found that nitrobenzene was reduced to aniline after ingestion. Thus, he reported the study of acidic solutions of aniline with various oxidizing agents to produce blue- to-purple colors in 1862.65 Continuing his investigations, Letheby then oxidized sulfuric acid solutions of aniline via a Pt electrode at the positive pole of a small Grove’s cell (an early high current battery) to generate a deep blue to bluish-green pigment that adhered to the elec- trode as a fine powder.65 As with many of the previous reports of chemical oxidation, the exact color observed depended on the concentration of the aniline solution used. He then prepared the material on a larger scale, using greater quantities of aniline and two larger Grove’s cells connected together for intensity.65 Using this setup, a thick layer of dirty bluish-green pigment quickly cov- ered the large platinum sheet (4 × 6 in) acting as the positive electrode. The pigment was then removed from the electrode, washed with water and dried to give a bluish-black powder. The powder was insoluble in water, alcohol, ether, or ammonia, and was only soluble in sul- furic acid. Immersion in ammonia did cause the powder to acquire a brilliant blue color, but the resulting blue pigment still did not dissolve. When dissolved in concentrated sulfuric acid, the solution formed was either blue, green, or violet depend- ing on the degree of concentration. Diluting the result- ing acid solution with water then resulted in the precipi- tation of a dirty emerald green powder, which could be made blue with treatment of concentrated ammonia or blue to purple with the addition of concentrated sulfu- ric acid. The blue pigment generated by any of the means above could also be partly decolorized by various reduc- ing agents, and in this condition, it became soluble.65 ANALYSIS AND DISCUSSION Although the various studies presented above included little to no structural or compositional data, all of the reaction conditions and resulting proper- Figure 6. Henry Letheby (1816-1876) [by W. & D. Downey albu- men carte-de-visite, 1860s © National Portrait Gallery, London]. 107The Early History of Polyaniline: Discovery and Origins ties reported are completely consistent with the mod- ern knowledge of the oxidative polymerization of ani- line, as well as the chemistry and characteristics of the resulting conjugated polymer, polyaniline. In addition, it is clear that all four of the historical figures that have been previously credited with the origin and discovery of polyaniline did indeed play critical roles in the early history of this material. However, if we are to assign pri- ority to any of these figures, the credit must be properly given to Runge. Not only was he the second to discover aniline itself after Unverdorben, but he was the first to report its oxidative polymerization and the first to docu- ment the successful application of these products as both green and black dyes for printing on cotton.38 Although Runge’s contributions were not usually recognized dur- ing the time periods discussed above, Heinrich Caro paid tribute to Runge during the celebration of his own 70th birthday in 1904, crediting Runge with various dis- coveries including the first formation of emeraldine by copper salts.33 While Caro does not go so far as to credit Runge with the discovery of aniline black, author Emilio Noelting (1851-1922) begins his 1889 book Scientific and Industrial History of Aniline Black with the statement that the history of aniline black can be traced to the “very old researches of Runge”.20 The proper recognition of Runge should not dimin- ish the contributions of the remaining three gentlemen, however, all of which left their mark on the history of polyaniline. Although Fritzsche did little to advance the oxidative polymerization of aniline beyond what was previously reported by Runge, he was the third to dis- cover aniline and more importantly gave it the name by which we recognize it today.43 Needless to say, there would not be the current term polyaniline without the contributions of Fritzsche! Also of significant impor- tance is the fact that Fritzsche was the first to analyze the elemental composition of the polymerized product to reveal an empirical formula in near perfect agree- ment with the structure of the modern emeraldine salt of polyaniline.48 Of course, the work of both Runge and Fritzsche may have been lost as academic curiosities if it had not been for the efforts of Crace-Calvert, Light- foot, and Caro to develop these materials into commer- cial products. In addition, it is their application of this material as a black dye that resulted in the longstand- ing name aniline black, the primary common designa- tion for this material until the ultimate use of the mod- ern polyaniline. It is hard to quantify how much impact their work had on maintaining interest in polyaniline until its more critical electronic properties were ulti- mately discovered in the 1960s.9,10,16 Lastly, one cannot overlook the contributions of Letheby. Although he most certainly was not the first to report the oxidation of ani- line to polyaniline, as is often claimed, he was the first to demonstrate its electrochemical oxidation and the very first to report on this method for the production of conjugated materials.65 The electrochemical generation of conjugated polymers was the most common route to conductive films of these materials prior to the 1990s, and this still is the most common method utilized for the production of polyaniline. In closing, I would like to postulate a final ques- tion for further consideration. If the work discussed above places the first report of the polymerization of aniline to polyaniline with the work of Runge in 1834, is polyaniline then the earliest known synthetic organic polymer? I am not the first to propose this possibility,26 but I still feel that this is worth bringing to light once more and to provide at least initial support for such a claim. Although the term polymer was first coined by Berzelius in 1832, it did not have the same mean- ing as its modern usage and did not originally refer specifically to macromolecules.66,67 In addition, none of the studies above ever used the term to refer to the oxidation products of aniline and the modern concept of macromolecules was not accepted until the 1920s, after which it was decades before Staudinger, Caroth- ers, Flor y, and others could convince the scientific community that these unusual molecules were real.9 The earliest man-made polymer is generally consid- ered to be polystyrene, whose polymerization was first obser ved in 183968 in which freshly distilled stora x resin produced an oil (styrene, but then called styrol) that converted into a rubberlike substance in the pres- ence of air, light, and heat.66,69 Of course, from the dis- cussion above, Runge’s conversion of aniline to green- black materials predates the obser ved formation of polystyrene by five years. In addition, unlike the spon- taneous polymerization of styrene, aniline was polym- erized via the purposeful addition of an oxidizing agent and its polymerization was carried out under a number of different conditions. A complicating factor in this argument is that what made polystyrene inter- esting was its obvious changes in physical properties, while the obvious changes in the polymerization of aniline was the resulting color produced, rather than any particular physical change. As such, in that time period of the early 19th century there were no related properties to connect the two materials and polyani- line was not recognized as what we would call an oli- gomer or polymer until 1910.28-31 Still it is clear that polyaniline represents not only an important begin- ning in the path to conducting organic materials, but to organic polymers in general. 108 Seth C. Rasmussen ACKNOWLEDGEMENTS I would like to thank Tony Travis for sharing some of his publications on Lightfoot, Caro, and anline black, as well as for a number of helpful discussions. Thanks also to Dave Lewis for the helpful discussion on the rela- tionship between Fritzsche and Zinin. I would also like to thank the Department of Chemistry and Biochemis- try of North Dakota State University (NDSU) for con- tinued support of my historical research, as well as the NDSU Interlibrary Loan Department, who went out of their way to track down many elusive and somewhat obscure sources. REFERENCES 1. 100+ Years of Plastics. Leo Baekeland and Beyond (Eds.: E. T. Strom, S. C. Rasmussen), ACS Symposi- um Series 1080, American Chemical Society, Wash- ington, D.C., 2011, pp. 1-3. 2. Handbook of Conducting Polymers, 3rd ed. (Eds.: T. A. Skotheim, J. R. Reynolds), CRC Press, Boca Raton, FL, 2007. 3. Handbook of Thiophene-based Materials (Eds.: I. F. Perepichka, D. F. 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