Part 1: chemical background and pioneering patents
By Markus Weber und Guido Deussing
Deck chair made of acrylic or acrylic glass. Source: istock / fotokostic
Polymethyl methacrylate (PMMA), which is better known as Perspex, is a plastic that has become a modern classic. It was a difficult and lengthy process to develop it. Chemiefabrik Röhm & Haas OHG (now called Evonik Röhm GmbH, a subsidiary of Evonik Industries AG), which was established in Eßlingen / Germany in 1907 and was based in Darmstadt / Germany from 1909 onwards, acted boldly and tenaciously in tackling and completing this assignment. In 1901, the co-founder and head of the company, the chemist Dr Otto Karl Julius Röhm (1876-1939), devoted his dissertation to the polymerisation products of acrylic acid. It was to take until 1928 before PMMA was technically mature enough to be patented as a plastic. So Perspex has been in existence for 90 years now, although the German trade name (“Plexiglas”) was not registered as a trademark with the number 461639 until 4. December 1933. The new plastic caused a stir all over the world – in other countries with the names “Perspex” (Great Britain) or “Lucite” and subsequently “Acrylite” (USA). A success story that continues to this day, although it was overshadowed in the early stages by the dispute about who at Röhm & Haas deserved the credit for inventing it. k-online.de is telling the story of “organic glass” and the applications for it in a series that will be appearing at irregular intervals. It starts here with an outline of the chemical background and the pioneering patents.
It is thanks to the findings of organic chemistry that it has been possible to create a large family of completely new kinds of material that do not occur anywhere naturally – and are therefore called “Kunststoffe” (= “synthetic / artificial materials”) in German – from coal, oil and natural gas. It is a well-known fact that the term “organic” is reserved for chemical compounds that contain carbon. The carbon atom has a special position in the world of chemistry, because it can combine with others of its own kind to form chains or rings, that are in addition capable of bonding with other elements like hydrogen, oxygen or nitrogen. In this way, nature produces millions and millions of carbon compounds with many different molecular structures.
Compounds that consist of carbon (C) and hydrogen (H) are known as hydrocarbons. As would be expected, they are found in coal, oil and natural gas, as terpenes, carotenoids or rubber as well as in many plants. Hydrocarbons are the starting point for Perspex. The chemical formula for the latter is (C5H802)n, which indicates that the material is complex and makes it easy to understand that development of it from the laboratory to industrial production was a lengthy business that is being outlined below. Everything revolves here around the process that leads in the final analysis to Perspex and many other plastics: what is known as polymerisation.
About polymers and monomers
A start can be made by pointing out that Perspex, which is known as “organic glass” because of its hydrocarbon content, is also called “acrylic glass”. Perspex is based on methyl methacrylate (MMA), a colourless liquid that is originally a derivative of the “domestic raw materials wood, coal and limestone”, as it says in a Perspex information leaflet issued by the manufacturer Röhm & Haas in 1938. Under the influence of light and heat, methyl methacrylate molecules form long chains, which is encouraged further by the addition of initiators. This chain formation process is known as polymerisation; the macromolecule polymethyl methacrylate (PMMA) or Perspex is formed. A material that consists of macromolecules is therefore known as a polymer, with the Greek prefix “poly-“ indicating the large number of links in the chain. MMA, on the other hand, is a monomer, i.e. a substance that can polymerise, because its carbon atoms are unsaturated or, in other words, they are keen to form bonds. So “polymerisation” in a nutshell means “to create a bond between unsaturated molecules of the same kind” (Röhm 1901, 8), which “combine via carbon bonding” (ibid., 18). The length of the molecular chain created in this context decides not only whether the polymer is “low-molecular” or “high-molecular” but also the physical properties of the polymer, such as hardness, mouldability and ageing resistance.
Now methyl methacrylate is not a material that occurs naturally; on the contrary, it is a methyl ester of methacrylic acid, i.e. it is produced via combination of the acid with an alcohol (“esterification”). Although methacrylic acid itself is found in chamomile oil, the latter cannot be used to make Perspex. So methacrylic acid has to be produced synthetically, like acrylic acid – with which it is closely related chemically. Incidentally: esters polymerise less spontaneously (“of their own accord”) than unesterified acids, but they are more stable; chemists therefore prefer to use esters, so that they can control the polymerisation process more effectively.
Pioneers of acrylic research
Scientists focussed on acrylic acid – also known as propenoic acid – first, as long ago as the 19th century. It is a colourless chemical compound with a pungent odour similar to vinegar that is flammable, extremely caustic and liquid at room temperature. Acrylic acid can be characterised more specifically as a monocarboxylic acid. As a carboxylic acid, it is defined by the carboxyl group (-COOH), of which it has only one (monocarboxylic acid) here. A hydrocarbon chain, which has unsaturated double bonds (C=C) that form the basis for its tendency to polymerise, is connected to the carboxyl group of the acrylic acid as what is known as a substituent.
Methacrylic acid, which is also known as methylpropenoic acid or isobutyric acid, is also an unsaturated monocarboxylic acid that – in contrast to acrylic acid – has a methyl group (-CH3) or, as Frankland/Duppa 1865, 13 put it, “is derived from acrylic acid by replacing 1 at. of hydrogen in the same by 1 at. of methyl”. The people we are talking about here are the English chemist Edward Frankland (1825-1899), who obtained his doctorate in Marburg / Germany, and his assistant Baldwin Francis Duppa (1828-1873). The two of them were among the first research scientists to study the acrylic acid family in detail: “This family has definitely been neglected to a comparatively large extent because the acids concerned […] only had a few connections to other families of organic compounds, on the basis of which it would have been possible to develop a probable hypothesis about their inner architecture.” (ibid., 1) The two Englishmen tried “to convert the acids from the lactic acid family into acids from the acrylic family (ibid., 1) and reported about this is the “Chemistry and Pharmacy Annals”: “Conversion of the acids from the lactic acid family into acids from the acrylic acid family shows that there is a very simple relationship between the two families; what has in fact been demonstrated is that the removal of 1 at. of water from the basic part of an acid from the lactic acid family leads to the conversion of this acid into the corresponding one from the acrylic acid family” (ibid., 25). They succeeded in obtaining methacrylic acid by helping the ethyl ether of dimethoxalic acid (isobutyric acid) – which belongs to the lactic acid family – to react with phosphorus pentachloride. Frankland and Duppa determined that the “colourless, transparent, very runny liquid that has the very strong, nauseating odour of withered mushrooms” (ibid., 12) was “the ethyl compound of methacrylic acid” (ibid., 13). It “is easily broken down by a boiling alcoholic potassium solution, with the formation of alcohol and the potassium salt of the methacrylic acid […]. When distilled with excess diluted sulfuric acid, this potassium salt produces free methacrylic acid, that initially floats to the surface of the distillate as an oily layer but dissolves in the water that is produced with it at the same time.” (ibid., P. 13)
Rudolph Fittig (1835-1910), Professor of Chemistry at Tübingen University, and his student Ludwig Paul reported about the polymerisation of methacrylic acid into a solid, resinous substance in 1877. They had determined that “part (of the methacrylic acid, editor’s note) decomposed and left a white, resinous decomposition product behind in the distillation vessel after every new distillation operation, perhaps due to the presence of some phosphorus chloride or hydrochloric acid” (Fittig/Paul 1877, 55). A year before, Paul had already mentioned the “resinous decomposition products ( ) of methacrylic acid”, which “should probably be considered to be polymers of the same” in his dissertation (Paul, 1876, 27-28) and had added the following clarification: “That the hydrochloric acid alone does not have the polymerising effect but that other unexplained conditions lead to the above changes is already demonstrated by the fact that resinous products are formed in distillation even after the hydrochloric acid is eliminated […]. In addition to this, the same happens in […] operations where the substances involved do not come into contact with hydrochloric acid.” (ibid., 13)
Kahlbaum’s light polymer
In the literature of the time, a clear distinction was not always made between acrylic acid methyl ester (methyl acrylate, C4H6O2) and methacrylic acid methyl ester (methyl methacrylate, C5H8O2), which encouraged the use of them as synonyms. The German chemist Georg Wilhelm August Kahlbaum (1853-1905), who worked in Basel / Switzerland from 1876 onwards, initially in a private laboratory established with his own resources and then from 1892 onwards as a professor at Basel University, carried out research into polymer acrylic acid methyl esters. His publication about polymethyl acrylate appeared during the period before he started teaching:
“For the purposes of physical examination, I prepared acrylic acid methyl ester on various occasions […], which was stored in several securely closed containers; the ester was initially a runny liquid that boiled at 85° and had a penetrating odour that made the eyes water. After 6 months, it was unchanged, but a short time afterwards it turned into a jelly-like substance with bubbles in it that did not, however, lose any of its transparency. Similar transformations are not unusual with unsaturated compounds, particularly acrylic acid derivatives; (Bernhard, editor’s note) Tollens (1841-1918, editor’s note) […] demonstrated this with the allyl ester of this acid, but could not do the same with the methyl ester.” (Kahlbaum 1880, 2348)
A jelly-like substance with bubbles in it – on the basis of this information, “it was not possible to conclude that the polymer could be used […] as a technically viable and versatile moulded plastic” – to quote the accompanying text for the German Imperial Patent 656421, which was granted to Röhm & Haas AG in Darmstadt / Germany on 7. February 1928 and is entitled “Plastics moulded from polyacrylic acid, its compounds or combinations of the same”. A further quotation from the same document: “Kahlbaum had a prism ground from the transparent parts of this solid substance, with which he carried out optical measurements. He later succeeded in congealing the ester in a glass prism via exposure.”
Although what was involved with Kahlbaum’s light polymer was polyacrylic acid methyl ester and not of polymethacrylic acid methyl ester – or, to use modern terms: polymethacrylate (PMA) and not polymethyl methacrylate (PMMA) – it anticipates Perspex. Because “exclusively high-molecular products” (Ackermann 1967, 109) are formed in light polymerisation, which do not saponify, as a subsequent investigation revealed that Dr Walter Hermann Bauer (1893-1968) carried out, who was employed by Röhm & Haas as a chemist for 26 years and devoted his professional life to acrylate chemistry as head of the company’s research laboratory.
Initially, there were no reasons for the polymer pioneers at the end of the 19th century to concentrate on methacrylic acid of all things; the focus was on acrylic acid. Eduard Linnemann (1841-1886), Professor of General and Pharmaceutical Chemistry at Lemberg University, had announced in the “Chemistry and Pharmacy Annals” on 1. June 1872 with reference to its “spontaneous ( ) transformation”: “The product […] swells up gradually in water and alcohol, in order to dissolve as an acid similar to rubber.” (Linnemann 1872, 369) Tollens, the chemist who has already been mentioned, and his pupil W. A. Caspary reported in 1873 in the same magazine about the light polymerisation of acrylic acid allyl ether: “At normal temperature and particularly in sunlight, the ester changes into a clear, hard, transparent substance after some time.” (Caspary/Tollens 1873, 251) Ten years later, Felix Weger (1859-?), a doctoral student at Königsberg University, observed the “formation of this curious modification of acrylic acid esters […] on methyl, ethyl and propyl ester” (Röhm 1901, 11-12).
Otto Röhm’s doctorate
The polymerisation process had not been understood to a large extent yet, so it was not possible to control it. Following the success achieved with rubber, celluloid and galalith, more and more chemists were nevertheless expecting to achieve a great deal by continuing to implement the approach that had been adopted and by carrying out additional research activities. Hans Freiherr von Pechmann (1850-1902), Chemistry Professor in Tübingen, for example, suspected that polyacrylic acid methyl ester had the potential to become technically significant. With the help of alcohol-free sodium alcoholate, Pechmann had succeeded in “polymerising the esters of unsaturated acids into higher-molecular compounds via carbon bonding” (Röhm 1901, 8). He subsequently asked his 24-year-old student Otto Röhm to carry out a doctorate in this field, in order “to determine how acrylic acid ester behaves when treated with sodium alcoholate” (ibid., 9). The study revealed that not only acrylic acid methyl ester but also acrylic acid ethyl ester polymerises, while “methyl ester turns solid more easily than ethyl ester” (ibid., 12). Röhm therefore concentrated afterwards on the methyl ester and its solid modification, produced by the combination of light and heat (ibid., 25). He characterised it as a “colourless, transparent, very elastic substance, that only smelled slightly like the liquid ester” (ibid., 12), the physical properties of which were somewhere “between tough, flexible glass and stiff rubber” (Trommsdorff 1976, 193). On the basis of his subsequent analysis twelve years after Röhm’s death, Bauer concluded that this “solid ( ), brittle ( ) product, which has not been described yet ( )” was a low- to medium-molecular ionic polymer, that should not be confused with Kahlbaum’s high-molecular light polymer, something that had happened to the young Röhm, according to Bauer (Ackermann 1967, 109): “The experimenter who […] is familiar with the slowness of light polymerisation is struck by the unusual speed at which the polymerisation product is formed […] here. What appears to be involved is ion chain polymerisation.” (ibid., 110)
When the dissertation was submitted to the examination committee at Tübingen University in August 1901, Pechmann was no longer Röhm’s doctoral advisor. Due to anxiety depression – the medical diagnosis at the time was “severe melancholy” – the scholar had left the academic community, after which Professor Carl Bülow (1857-1933) took over the chemical/pharmaceutical institute, including Pechmann’s doctoral students. In November 1901, Röhm received his doctorate “magna cum laude”. Whereas Pechmann had been able to offer him the prospect of a position as assistant and thus a university career while he was still in good health (Edschmid 1957, 16), this option was eliminated once and for all on 19. April 1902, when the professor’s nerves failed him and he took his own life. Röhm put his dissertation aside and was only to come back to the results of it ten years later – as a businessman rather than as a scientist. In the meantime, he and the banking professional Otto Haas (1872-1960), who – like he – came from Swabia, had established Chemiefabrik Röhm & Haas OGH in Eßlingen / Germany in 1907 and had relocated the company to Darmstadt / Hesse in 1909. Here Röhm produced and marketed an enzymatic leather staining agent he had developed, for which there was steadily growing demand from all over the world and which put the company on a sound economic basis. This enabled Röhm to resume acrylic chemistry research activities in the company’s own laboratory in 1911.
The path to “ester rubber”
The focus was now on the company’s objective of developing a plastic with properties that made it marketable and made it seem capable of replacing other materials either when required or completely, because it was superior to them. A vision for which the term “ester rubber” (Edschmid 1957, 51) was coined, encouraged by the success of Russian and German chemists who had created synthetic rubber. In 1899, Ivan Lavrentyevich Kondakov (1857-1931) synthesised dimethylbutadiene and obtained the first completely synthetic rubber from it in 1901. Dr Fritz Hofmann (1866-1956) continued this work at Elberfelder Farbenfabriken and produced what was called methyl rubber from dimethylbutadiene in 1909 under the influence of heat and pressure. It proved to be more stable than Kondakov’s autopolymer (Röker 2007, 202). Before this, Hofmann had already synthesised isoprene, the natural equivalent of which occurs in plants that contain rubber, and polymerised it into synthetic rubber.
Higher plasticity, lower elasticity, water sensitivity and stickiness are what distinguished the polymer acrylic acid methyl ester obtained by Kahlbaum by light polymerisation from synthetic rubber. Although natural rubber had similar shortcomings, it had become elastic enough and thus technically viable by means of vulcanisation (treatment with elementary sulfur). Röhm therefore had the idea of vulcanising polyacrylic acid methyl ester too. On 31. January 1912, he received German Imperial Patent 262707 for the “Process for manufacturing a product with the properties of vulcanised rubber”. The text of the patent, which identified him as inventor, says:
“It was determined that the acrylic acid ester transformed into an elastic substance by polymerisation can also be turned into a technically viable rubber substitute by the familiar methods of vulcanisation. This success could not be anticipated in view of the chemical diversity of the acrylic acid esters, on the one hand, and of the isoprene, on the other hand. The process in question has the advantage of greater economy over the process for the production of rubber from isoprene etc. Because the product from which isoprene is obtained is expensive turpentine oil, whereas far less expensive glycerine and/or lactic acid and similar compounds are the source of acrylic acid ester. The substance produced by vulcanisation of acrylic acid esters can be put to the same uses as the material obtained from natural rubber.”
In order to carry out vulcanisation of polymer acrylic acid ester, a special trial rolling mill was installed at Röhm & Haas in Darmstadt (Ackermann 1967, 16). It did not prove to be a success, however, because the patent was not technically viable: “In contrast to rubber, polymer acrylic esters cannot be vulcanised. Understandably, because polymer acrylic esters are saturated compounds, i.e. they do not contain any unsaturated bonds, to which sulfur can be attached.” (ibid., 13) Looking back, Walter Bauer’s conclusion about Röhm, his former boss, was negative:
“Outwardly, polyacrylic ester is similar to natural and synthetic rubber. It was therefore obvious to assume that it must be possible to process polyacrylic ester into a technically viable vulcanisation product with the help of sulfur. What is involved here is therefore an inspiration of the human spirit that appears to be sound at first glance. However, since polyacrylic ester has a different molecular structure, it cannot […] be vulcanised with sulfur, as the experiment shows. This means that the patent lacked technical feasibility. It should not have been granted by the Patent Office. With no mention of the fact that it is not viable, this patent is regularly referred to in literature etc., with the impression being given that it represents an inventive achievement.” (ibid., 239)
To overcome this impasse, Röhm concentrated once again on acrylic acid ester synthesis trials, developed more than twenty different synthesis approaches in 1915 and submitted them to his chemists as a programme (Edschmid 1957, 51). It was, after all, completely different to obtain ester in a laboratory than it was to supply it for industrial purposes; in the latter case, the aim was to produce a polymerisation substrate of convincing quality and quantity in the most economic possible way and, in addition, to develop a polymerisation process for a really marketable product. The First World War inevitably interrupted the research activities in the acrylic field, so that they did not get back into full swing again until after 1918. Even so, it was to take until 1928 before Chemiefabrik Röhm & Haas was able to obtain the “Plastic” patent (German Imperial Patent 656642) about polymethyl methacrylate. (To be continued!)
Katharina Ackermann (Ed.): Dr Walter H. Bauer and his 67 German Imperial Patents. Achievements and experiences of a German research chemist, compiled on the basis of documents. Jugenheim/Bergstrasse: published by the editor in 1967, 264 pages
W. Caspary: About acrylic acid and acrylic acid ether. Diss. Göttingen 1873, 30 pages
W. Caspary and B[ernhard] Tollens: Conversion of β-bibromopropionic acid into acrylic acid. In: Chemistry and Pharmacy Annals 167 (1873), P. 240-257
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Klaus-D. Röker: The first attempts to use synthetic rubber. In: Society of German Chemists – History of Chemistry Study Group (publisher): Announcements 19 (2007), P. 199-216
Ernst Trommsdorff: Dr Otto Röhm. Chemist and entrepreneur. Düsseldorf and Vienna: Econ 1976, 294 pages
Felix Weger: Analysis of saturated and unsaturated esters and some associated compounds. In: Justus Liebig’s Chemistry Annals 221 (1883), P. 61-107