Acrylic: chemical background and pioneering patents
By Markus Weber, PhD. and Guido Deussing
In times like these, when fear of a viral infection is avoided, effective protection is particularly important where there is a high risk of infection, i.e. in the medical and food sectors. Here and there, a material has become the protector of choice over glass, namely the plastic polymethyl methacrylate (PMMA), better known as acrylic. K-online takes the current world news about the pandemic of a new corona virus as an opportunity to report on people and their achievements in connection with the development of acrylic.
Source: istock / Orientefootage
Polymethyl methacrylate (PMMA), which is better known as arcylic, 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 acrylic 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 trade 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 acrylic. 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 acrylic and many other plastics: what is known as polymerisation.
About polymers and monomers
A start can be made by pointing out that acrylic, which is known as “organic glass” because of its hydrocarbon content, is also called “acrylic glass”. Acrylic 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 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 acrylic 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 acrylic. 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 acrylic. 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!)
Do you remember? 90 years ago, on 27. October 1928, Röhm & Haas AG, Darmstadt, received a patent for an innovative new plastic with the awkward name polymethyl methacrylate (PMMA). German Imperial Patent (DRP) 656642 was the result of sixteen years of research in the acrylate chemistry field. Röhm & Haas, a medium-sized company with about 250 employees in 1918 (Ackermann 1967, 16 and 122), had won the race against such flagships of the chemical industry as BASF in Germany or Imperial Chemical Industries in Great Britain, but soon decided to try and co-operate with its rivals – on a contractual basis and for their mutual benefit. After the plastic patent had been granted, it still took a few more years of development work before PMMA began to cause a stir under the trade name “Plexiglas” (“Acrylic”). As a result, the annual sales of Röhm & Haas AG grew rapidly, from 466,000 Reichsmarks in 1934 to more than 23 million Reichsmarks in 1940 (Ackermann 1967, 84). Credit for this is due not only to the company founder Dr Otto Röhm (1876-1839), who obtained his doctorate at Tübingen University in 1901 with a thesis about acrylic acid polymerisation products, but also to the research chemists recruited by Röhm, above all Dr Walter Hermann Bauer (1893-1968). After the end of the First World War, Bauer had brought new ideas and fresh energy to the acrylic research operations at Röhm & Haas and made the decisive advances – initially in the synthesis of the monomers C4H6O2(acrylic acid methyl ester or methyl acrylate) and C5H8O2(methacrylic acid methyl ester or methyl methacrylate) and then in their polymerisation into marketable products in the form of safety glass and acrylic glass panelling.
Acrylic glass is a polymer of methacrylic acid methyl ester (C5H8O2). Although this textbook statement sounds banal, the process that chemists had to carry out to develop a marketable product that received the trade name “Plexiglas” in Germany in December 1933 was exacting and convoluted.
The company Röhm & Haas started the first laboratory tests for the purposes of plastics research as early as 1912 (Wittig 2007, 14). The initial aim was to synthesise acrylic acid methyl ester (C4H6O2) as the liquid, monomer counterpart to the solid, elastic polymers (polyacrylates) with which Otto Röhm had been familiarised by his doctoral advisor Hans Freiherr von Pechmann (1850-1902) and which now needed more intensive research. The acrylic acid (C3H4O2) needed as the raw material for this purpose had to be produced specially in the laboratory, because it could not be bought (Wittig 2007, 15) and was not available simply as a waste product, e.g. in coal tar manufacturing. It was, however, definite that coal chemistry was the place to start. This is because the acrylic acid molecule involves the combination of a carboxyl group (-COOH) with a hydrocarbon chain that has unsaturated double bonds (C=C). Synthesisation of acrylic acid therefore had to be approached gradually via other hydrocarbons that acted as preliminary and intermediate products:
“All the research was based on the fact that the acrylic acid molecule consists of three carbon atoms (C atoms) […]. The question for Röhm was how acrylic acid could be obtained best from existing substances. Two approaches seemed to him to be convincing: one either started with molecules that have two C atoms, to which a further C atom from a different substance group needed to be added via chemical reations. Or one looked for molecules that already had three C atoms but were combined with the wrong substance groups. The task was then to ‘convert’ them into the right substance’.” (Wittig 2007, 26)
On 21. June 1912, Röhm outlined by hand a number of proposals about how to synthesise acrylic acid ester, such as: “from: lactic salt + met. Na or Na alkoxide or Na amalgam or amalgam of a different metal e.g. zinc amalgam” (Wittig 2007, 15). In 1915, he made a list of more than twenty different synthesis approaches and submitted it to his chemists as a programme (Edschmid 1957, 51). By 1918, six chemists – who were, however, hampered in their efforts by the war – had finally investigated the following synthesis approaches in succession without achieving a breakthrough: Dr Andres / the lactic acid approach, Dr (Julius, editor’s note) Kohl / the propionic acid approach, Dr Göller / the acrolein approach, Dr Schwarzkopf, Messrs Dehn and M. (sic) / the acetylene, ethylene and malonic acid approach (Ackermann 1967, 14).
Recommendation for a new recruit from Jena
In order to make progress in the plastics research programme, Otto Röhm recruited a new chemist on 4. March 1918 as the successor to Dr Schwarzkopf: Dr Walter Bauer (1893-1968) (Ackermann 1967, 16). Ludwig Wolff (1857-1919), Associate Professor of Analytical Chemistry at Jena University, had advised the company to try Bauer in response to its inquiry of 12. February 1918 (Ackermann 1967, 14). Bauer came from a family of merchants and craftsmen based in Arnstadt/Thuringia. He started to study mathematics, chemistry and physics at Jena University in 1911 and then made chemistry his main subject at Munich University (Ackermann 1967, 238). He was very influenced there by the chemistry professors Adolf von Baeyer (1835-1917), Oskar Piloty (1866-1915) and Ludwig Vanino (1861-1944) (Ackermann 1967, 73). After he graduated in Munich in 1917, the 24-year-old Bauer obtained a doctorate in natural sciences “magna cum laude” the same year in Jena on a Liebig grant (Ackermann 1967, 238 and Wittig 2006).
At Röhm & Haas in Darmstadt, Bauer was “the only chemist deployed to carry out research in the acrylic field after he joined the company […]. No assistants were available” (Ackermann 1967, 16). “Otto Röhm compiled a work schedule for him, ‘in which it should be taken into consideration that carbon compounds can be produced by the most improbable removals and additions of atoms or groups of atoms’. The assignment in this context was to make systematic records of as many as possible of the reaction paths that did not seem to be plausible at first.” (Wittig 2007, 26) “Since compounds with two C atoms were easier to obtain on the market than compounds with three C atoms, Walter Bauer initially wanted to structure acrylic acid molecules by trying to add a third C atom to his basic molecules. Once he had failed to carry out any convincing acrylic acid synthesis in this way, he decided to use basic compounds with three C atoms, like propionic and malonic acid as his next step […].” (Wittig 2007, 27)
Bauer, on the other hand, reported that his focus on propionic acid was not a choice of his own but was rather attributable to the legacy he inherited and was required to work his way through:
“By way of instruction, Bauer was given three pages of handwritten rules about the production of acrylic acid methyl ester from propionic acid. […] An analysis of the acrylic acid ester made by his predecessor – which was supposed to be pure – revealed that the ester content was 80%. The rest consisted essentially of alcohol. No-one had noticed this error before. The propionic acid approach proved to be uneconomic and impractical. […] The lack of viable previous work forced Dr Bauer to start entirely from scratch. He began by writing down some 100 syntheses for acrylic acid methyl ester.” (Ackermann 1967, 16)
Trommsdorff 1976, 203 explains in this context: “In 1918, he worked on the propionic acid approach and then the malonic acid approach. In January 1919 he moved on the acetone approach […] and, subsequently, the ‘ethylene approach’.” The latter proved, finally, to be the ideal solution – synthesis of monomer acrylic acid methyl ester was achieved “on the basis of ethylene […] via ethylene cyanohydrin”. (Wittig 2007, 27)
A description of ethylene
Ethylene, which is known better nowadays as ethene, is a gaseous, highly flammable hydrocarbon (C2H4). It was originally obtained from fermentation spirit (basis: biomass) or carbide spirit (basis: limestone and coal) by removing water from the alcohol catalytically (Nagel 1971, 7 and Hölscher 1972, 15). The dye industry was first to make use of ethylene for the production of synthetic indigo, but gave this approach up again (Nagel 1971, 8). It was discovered very soon “that ethylene […] is suitable for numerous plastics” (Mark 1970, 63), since it is an unsaturated hydrocarbon that tends to polymerise. It is a well-known fact that the number of carbon-carbon bonds determines whether a molecule can form a polymer chain or not. “In hydrocarbons, the carbon atoms can link with each other by multiple bonding, with a reduction in the number of hydrogen atoms […]. The ethylene molecule has two bonds between the carbon atoms. One of them is easy to ‘break’, so that the molecule can be added to a chain.” (Mark 1970, 63) Incidentally, ethylene has been obtained primarily from crude oil or heavy naphtha (Mark 1970, 134) or natural gas or methane (Hölscher 1972, 15) instead of from coal tar since the middle of the 20thcentury.
Back to the pioneering years at Röhm & Haas, however, when the ethylene approach was tackled for the first time. Looking back, Bauer prided himself on his own intuition and the “courage to break with established tradition”; he “relied on his instinct to choose from the approximately one hundred possible syntheses for acrylic ester production a synthesis approach that proved successful immediately and has been exploited technically for decades” (Ackermann 1967, 115). Although Bauer could not claim to be the first to adopt the ethylene approach, everyone who had tried it beforehand had failed: “Dr Julius Kohl, who joined (Röhm & Haas, editor’s note) in January 1913, wrote a reaction plan for ethylene that went from ethylene cyanohydrin to acrylic acid ester […]. It was described in the laboratory files as the ‘ethylene approach’. The first stage went from ethylene […] to ethylene bromohydrin. This was possible in the laboratory without any difficulty.” (Trommsdorff 1976, 201) However, since the next stage, the “conversion of the halohydrin into cyanohydrin using alkali cyanide, which – according to the literature – was carried out in an alcoholic solution – […], only produced a yield of 29%”, the ethylene approach was in general abandoned (Trommsdorff 1976, 201). “The work […] was discontinued for 4 years” (Trommsdorff 1976, 203), until Bauer came on the scene: “After studying these records (made by Kohl, editor’s note) thoroughly, Walter Bauer succeeded in solving the yield problem, as a result of which this synthesis approach became viable.” (Wittig 2007, 27).
The seven-stage ethylene approach
Anyone who would like to understand in greater detail what Bauer’s achievement consisted of, has to go through the seven-stage ethylene approach (Wittig 2007, 27) intermediate product by intermediate product, until acrylic acid ester is reached. The article “From my life as a research scientist and inventor” that Bauer wrote in 1963 for the annual periodical published by the Deutscher Erfinder-Verband (“Creative achievement”) and was reprinted in Ackermann 1967, 237-243) makes this possible:
Stage 1: acetylene – ethylene bromide
Trommsdorff 1976, 203 explains: “Since ethylene was difficult to obtain at the time, acetylene (see box, editor’s note), which provided symmetrical ethylene bromide in light or in the presence of oxygen when combined with hydrogen bromide”, was used. Bauer himself reports: “In view of our raw material situation at the time, I chose acetylene as the source material, which is an unsaturated hydrocarbon that can, among other things, add two molecules of hydrogen bromide. This was done […] in such a way that both the bromine atoms engaged one and the same carbon atom and formed an asymmetrical reaction product according to Markovnikov’s rule. In order to synthesise acrylic ester, symmetrical addition of the hydrogen bromide was necessary, however. On the basis of what was known at this time, it therefore inevitably appeared to be pointless to start synthesis by this route. Common sense suggested that it was not worth trying. The inventor was, however, inspired by a bout of revolutionary optimism to check experimentally the accuracy of the results that had been produced regularly in the past. The outcome was that the reaction can be carried out with the required symmetrical results – contrary to Markovnikov’s rule – if certain catalysts like light and oxygen are present. This has been called the ‘peroxide effect’ in subsequent chemical literature.” (Ackermann 1967, 240)
Röhm & Haas AG had the production of ethylene bromide from acetylene patented (Ackermann 1967, 25); Bauer is named as the inventor both times in the patents DRP 368467 of 24. July 1919 (“Process for manufacturing hydrogen halide addition products from acetylene”) and DRP 394194 of 10. May 1921 (“Process for the synthesis of ethylene dibromide”).
“The glycol obtained subsequently in the synthesis process (by saponification/hydrolysis of the ethylene bromide, editor’s note) was converted into glycol chlorohydrin (C2H5OCl, editor’s note) with hydrogen chloride. The very aggressive hydrogen chloride, which formed hydrochloric acid in the presence of moisture – something that was feared because of its corrosive effect on equipment – was produced in quartz equipment at the time […]. Disregarding all prejudices, metal burners and metal chambers were used, which produced excellent results under the conditions experienced here” (Ackermann 1967, 240; see also Trommsdorff 1976, 203).
“Kohl failed with the next stage […], the production of ethylene halohydrin with alkali cyanide, because he only achieved a yield of 29% in alcohol. Bauer succeeded in carrying out the reaction in water with a high yield.” (Trommsdorff 1976, 203-204) Bauer himself wrote: “Ethylene cyanohydrin, which is sensitive to water as a nitrile, is produced in the next reaction stage […]. The chemist avoided the presence of water in the production of such substances. However, the reaction also goes very unsatisfactorily when it is carried out at high pressure and temperature. Excellent yield levels were already obtained here easily at room temperature, when glycol chlorohydrin and an aqueous (sic!) sodium cyanide solution were combined to react together […]. This was another example of the problems posed by established thinking. And experiments are regularly the way to produce accurate information.” (Ackermann 1967, 240)
“Starting with ethylene cyanohydrin, three more reaction stages (with the processes saponification, dehydration and esterification) would have been necessary in order, finally, to obtain acrylic ester. Production of the acrylic acid that is obtained in the penultimate stage was difficult because of its easy polymerisability […]. This was the most problematic stage in the entire synthesis procedure due to the yield losses that were to be expected because of premature polymerisation. This difficulty was overcome by carrying out the three reaction stages at the same time in a ‘one-pot process’ – for the first time in chemistry history.” (Ackermann 1967, 240) This one-pot process, as it is known, was described in 1918 and involves the “single-stage conversion of the nitrile group (= ethylene cyanohydrin, editor’s note) into the ester group with sulphuric acid and alcohol” (Trommsdorff 1976, 204): “The transformation of ethylene cyanohydrin to acrylic ester involves 3 reactions, i.e. conversion of the nitrile group into the carboxyl group, esterification of the carboxyl group with alcohol and formation of the double bond via dehydration.” (ibid.) Conversion of ethylene cyanohydrin with the help of sulphuric acid and alcohol (methanol) produces acrylic acid methyl ester directly, i.e. only one single stage is needed instead of three. Bauer stresses: “Professor (Eduard, editor’s note) Vongerichten (1852-1930, editor’s note) from Jena University emphasised to the young doctor (= Walter Bauer, editor’s note) during his farewell visit to the university that it was essential for every intermediate reaction product to be manufactured as purely as possible when making organic products, because this was the only way to obtain excellent final yields. And as many as three stages were now being manufactured in a single operation without making the intermediate products individually, with the obtainment of a yield amounting to 85% of the theoretical maximum even so. Doubts about the accuracy of existing thinking prove as a result to be justified again and again. Research scientists frequently need to act in a way that appears irrational.” (Ackermann 1967, 240)
Incidentally: Röhm & Haas only filed a patent application for the “one-pot process” ten years later “for secrecy reasons” (Wittig 2007, 27) (DRP 571123 of 19. June 1928: “Process for the transformation of β-oxynitriles or their derivatives into unsaturated esters”); Bauer was named as the inventor.
Trommsdorff 1976, 204 pays tribute to Bauer’s “smooth preparative synthesis” of acrylic acid ester using the ethylene approach as efficient enough “to produce the monomer quantities required for development work during the initial phase”; this approach to synthesis was “not”, however, “economic yet because of its multi-stage process (see also Wittig 2007, 27). This was to change gradually: Bauer himself made “numerous simplifications, including the switch to acetone (C2H2) as the basic material instead of acetylene, as early as 1920” (Wittig 2007, 27).
In 1922, Röhm & Haas AG combined its experimental operations, which were still small, in what was called the organic synthetic department (OSA) (Wittig 2007, 27). But although the internal acrylate research was focussed specifically on the development of plastics, it took a while before they materialised: the company had limited personnel and financial resources, while economic slumps in the 1920s also had a debilitating impact. In addition to this, the money-maker – “Oropon”, the leather tanning product developed by Otto Röhm – started to falter: its price dropped 50 per cent between 1925 and 1933 (Wittig 2007, 33). This was due to the Great Depression in 1929 and increasing competitive pressure after patent protection expired in 1930. The first marketable products were not therefore polymers but by-products of acrylic ester synthesis; fire extinguishers were, for example, filled with ethylene bromide (Wittig 2007, 29). From 1927 onwards, Röhm & Haas was, on the other hand, able to obtain “a direct upstream product of the key substance ethylene cyanohydrin” from I.G. Farbenindustrie, so that expensive in-house production was no longer needed to some extent (Wittig 2007, 29). “Once ethylene oxide and prussic acid had become technical products, it was possible to obtain ethylene cyanohydrin in a single step […], which was then only followed by the one-pot process. Production of acrylic ester from ethylene cyanohydrin remained the only manufacturing process used technically anywhere in the world for many years.” (Trommsdorff, 204-205)
The road to safety glass
But what did the company do with its laboriously synthesised monomer, acrylic acid ethyl ester, to polymerise it? And what was to be done with the polymers, the polyacrylates? “Insulation (agents, editor’s note) for the electrical industry were the first application to be considered for the comparatively flexible plastics. A mature polymerisation process was not yet needed for this purpose. On the contrary: the OSA chemists used residue from uncontrolled polymerisation” (Wittig 2007, 29). Further applications were coatings or finishing agents for the textile industry, lacquers or bonding agents for the dye industry and acrylic adhesives (Wittig 2007, 29, 30 and 32). The first major success was attributable to a coincidence:
“In order obtain panels from the residues (of uncontrolled polymerisation, editor’s note), the chemist Dr Adolf Gerlach pressed them between two panes of glass under the influence of heat and wrote afterwards: ‘I discovered in this context that the panes of glass could not be separated any more after cooling and adhered firmly to the polyacrylic layer. Even when they were smashed, the splinters could not be separated from the polymer any more. Although the glass broke when attempts were made to hammer a nail through the panes, shards did not come off. An innovative new kind of laminate glass had been created, that did not turn yellow as it aged, in contrast to the celluloid available until then. OSA had its first plastic product.” (Wittig 2007, 29; see also Trommsdorff 1976, 214-215 and Hölscher 1972, 69)
It was entered in the trademark register with the name “Luglas” on 26. October 1929. Gerlach shared the relevant patent with his laboratory manager Bauer (DRP 676672 of 5. April 1932: “Process for the production of safety glass”). One of the paragraphs of the patent was as follows:
“The safety glass can be produced by standard methods, e.g. in such a way that finished polymer films are applied to or between the panes of glass and are bonded firmly to them, for instance by applying pressure and heat. Polymerisation of the monomer compounds or further polymerisation of pre-polymerised products can be carried out on or between the panes of glass too. In this context, the monomer or pre-polymerised products can be applied to or between the panes of glass with or without additives, if necessary in solutions or solution mixtures as well. Finished polymers can also be applied to the panes in solution and the solvent can be eliminated by evaporation. Layers that guarantee better shatterproof properties and demonstrate higher bond strength levels can be obtained by including solvents and by removing the solvent to a to all intents and purposes comprehensive extent.”
Gerlach and Bauer had acted as joint inventors for the first time before this, in 1928; the DRP 655570 in question (“Process for manufacturing conversion products of acrylic acid or its derivatives”) did not, however, yet involve safety glass. What it involved instead was the production of lacquers and adhesives – by means of polymerisation of the acrylic acid and/or its esters by using agents that release oxygen, solvents and thinners and by applying pressure and heat. Further safety glass patents held by Röhm & Haas are in Bauer’s name alone (DRP 674712 of 30. December 1934 “Laminated glass” and DRP 708842 of 5. April 1932 “Safety glass”) as well as in the names of Bauer and Dr Paul Weisert (DRP 716323 of 19. February 1939: “Process for the production of multilayer safety glass”).
The initial application areas for Luglas were safety goggles and gas masks. “In August 1929, Röhm writes about this to Haas: ‘that the production of splinterproof special glass has been possible now to an extent of 10,000 glasses per month’.” (Trommsdorff 1976, 215) “The production of car windscreens became very significant, because the number of severe cut injuries sustained in car accidents was reduced with LUGLAS.” (Wittig 2007, 29-30) “The first deliveries probably went to Auto-Union” (Trommsdorff 1976, 216), although there were customers in the Netherlands, Switzerland, Austria, Italy and Sweden too. However, “the plastics operations continued to make a loss at the purely economic level. The revenues from the sale of […] LUGLAS were minimal, so that Otto Haas invested some 100,000 Reichsmarks in the plastic research operations in 1930.” (Wittig 2007, 32-33) “In 1933, a licensing agreement was concluded with ‘Sicherheitsglas GmbH in Kunzendorf (in Lower Lusatia, now known as Kunice/Poland, editor’s note), which satisfied Röhm’s desire for expansion of the business. This company was supplied with polymer solutions and films and sold its multilayer glass for road vehicles and aircraft under the name ‘Sigla’.” (Trommsdorff 1976, 216)
Polymerisation in solvents started in 1928 (Wittig 2007, 31), although “the organic synthetic department was still carrying out basic research about polymerisation. The actual chemical process during polymerisation was unknown.” (Wittig 2007, 30) Polymerisation of thinned monomers was chosen as the focus primarily for safety reasons, because the chain reaction of the esters used was quite often – literally – explosive. I.G. Farbenindustrie then unexpectedly received the patent for what is known as emulsion polymerisation (“Process for manufacturing polymerisation products”, DRP 654989 of 18. February 1930):
“It has been determined that polymerisation of acrylic acid derivatives and their homologues can be carried out advantageously in such a way that the above-mentioned substances are emulsified and then polymerised in water, either alone or in blends […]; the quantity of polymerised components exceeds the quantity of the non-polymerisable components substantially. In this context, latex-like liquids are generally obtained, which produce high-quality polymers of high purity on coagulation and are easy to remove from the reaction vessels.” […] The acrylic acid derivatives are surprisingly resistant to the saponification impact of the water.”
Dr Hans Fikentscher (1896-1983) from I.G. Ludwigshafen (Badische Anilin- und Sodafabrik AG, BASF) and Dr Claus Heuck (1900-1991) from I.G. Leverkusen (Farbenfabriken, formerly Friedrich Bayer & Co. AG) were registered as the inventors. Fikentscher had started to investigate acrylic acid and its compounds in 1929. His research focussed not on the production of organic glass but on artificial silk (Hölscher 1972, 69). The first artificial silk products were produced at the I.G. laboratory in Ludwigshafen in 1930 from copolymers of acrylic acid nitrile and/or vinylidene chloride (Hölscher 1972, 123). The approach used in synthesis was as follows: “Acrylonitrile was produced from ethylene cyanohydrin, which the indigo and, later, inorganic department manufactured by processing ethylene oxide with prussic acid, via catalytic dehydration, while the free acrylic acid was formed by saponification or its esters were obtained via simultaneous processing with alcohol.” (Nagel 1971, 17; see also Hölscher 1972, 125) “Peroxides acted as catalysts for polymerisation” (Hölscher 1972, 70).
Acrylic contract with I.G. Farben
Since emulsion polymerisation “was essential for most of the acrylate products, Röhm & Haas had to reach agreement with I.G. Farbenindustrie” (Wittig 2007, 32). In 1932, what was known as the acrylic agreement was therefore concluded in 1932. This was an “amicable arrangement” with the aim of “specifying each side’s areas of interest” (Hölscher 1972, 69) and contained essentially the following agreements:
The I.G. Farben management granted Röhm & Haas the right, to share use of the I.G. emulsion patent.
I.G. Farben took over all the monomer production, including the one-pot process, in return for the payment of royalties (Hölscher 1972, 74), as well as parts of the polyacrylate production on the basis of the Röhm & Haas patents. The only product excluded from this arrangement was Luglas (Wittig 2007, 32).
The acrylic agreement with I.G. Farben gave the application research about polyacrylates at Röhm & Haas further momentum: “Polyacrylic acid esters […] are used in the lacquer industry as well as for adhesives. Emulsions of polyacrylic acid esters, produced by emulsion polymerisation of monomer esters, are used in the artificial leather and leather industry for the production of outside surface layers […]. They are also manufactured as granular powder […] and are used mainly to produce cable sheathing by the injection moulding process.” (Meyer and Mark 1950, 238; see also Trommsdorff 1976, 220-221) An exceptionally heterogeneous product range, for which solely the brand name “Plexigum” was used, which led to considerable confusion among staff and customers” (Wittig 2007, 31)
Plexigum was „the first Röhm & Haas brand for purely plastic resins and solutions” (Wittig 2007, 31) and was entered in the brand register on 25. June 1927, i.e. a good two years earlier than Luglas. The prefix “Plexi-“ did, however, already have label character before Plexiglas even existed. The origin of the word is unknown. There is an anecdote, according to which Otto Röhm was so perplexed by the rubbery stretchability of the first acrylate films that he came up with the word “Plexigum” for them as a modified form of “Gummiplex” (Wittig 2007, 31). Etymologists, in turn, attribute “Plexi-“ to the Latin word “plectere” (in English: “braid”, “twine”, hold together”).
Acetylene: a brief summary
Acetylene, which is also known as ethyne, is a colourless gas with the molecular formula C2H2and is therefore the simplest of the homologous series of acetylene hydrocarbons (alkynes). A characteristic feature of these is that they have at least one triple carbon bond (R–C≡C–R) in the molecule. Due to this triple bond between two carbon atoms, acetylene is, on the one hand, the most explosive hydrocarbon, while it is, on the other hand, “very valuable as a component of acrylic fibres and synthetic rubber” (Mark 1970, 62).
Acetylene was synthesised for the first time in 1836 by the Irish chemist Edmund Davy (1785-1857). It was obtained initially from calcium carbide (Trommsdorff 1976, 206), i.e. from coal and limestone, and later from crude oil or natural gas and/or methane (Hölscher 1972, 15) and oxygen, involving dehydrogenation (Nagel 1971, 35 and 64). For decades, into the 20thcentury, the only use that was found for it was to burn it, to use it either as lighting gas or as welding gas (flame temperature together with oxygen: > 3,000 degrees). Until it was discovered that it could be polymerised and was a suitable – and inexpensibly manufactured – basic material for plastics and synthetic fibres.
Acetylene was processed into, for example, carboxylic acids like acetic acid (Nagel 1971, 37) or acrylic acid. The acrylic acid of relevance for poly(meth)acrylate was manufactured from acetylene, carbon monoxide and water (Hölscher 1972, 17); “if alcohols were used instead of water, then good quantities of the appropriate esters already formed under very mild reaction conditions” (Nagel 1971, 58). This process, a carbonylation reaction (Nagel 1971, 58), is called Reppe synthesis, which is named after Dr Walter Reppe (1892-1969), the longstanding head of the BASF research operations, who focussed on acetylene chemistry from 1928 onwards (Nagel 1971, 38-39). “Due to wartime and post-war events, this work did not lead to the technical production of ‘direct acrylic acid ester’ (ethyl and n-butyl acrylate) in quantities of several hundred tonnes a month until the end of 1952. Economic considerations made it seem necessary to abandon the old cyanohydrin process and the process for manufacturing acrylic acid esters from acrylonitrile, which had been used in the meantime, completely towards the mid-1950s.” (Hölscher 1972, 74)
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