The macromolecule and its far-reaching consequences
Prof. Dr. Ullrich Scherf on the introduction of the term 'macromolecule' and the long way back to the closed substance cycle
In 1922, the chemist and later Nobel Prize winner Hermann Staudinger introduced the term "macromolecule" to science. Who was this man and what did he describe with this term?
Scherf: Hermann Staudinger, it is undisputed today, is the pioneer of the field of macromolecular chemistry. The initial resistance to his doctrines was immense and very pronounced 100 years ago. This was even partly understandable from the logic of science at the time. It had indeed been known for a long time that high molecular weights could be measured for various substances, some of which were also isolated from natural products, e.g. natural rubber, the rubbery substance from the milky sap (latex) of various rubber plants. But there were two different views. On one side were the proponents of the aggregate or colloid hypothesis, and on the other was the notion of the existence of macromolecules (polymers). Staudinger held the view that there are long filament molecules composed of periodic repeating units. It is important to know that this was not so easy to prove with the instruments at that time, because the analytical instruments of the natural sciences are by far not comparable with today's state of the art. Therefore, at that time it was already such a mixture of circumstantial evidence which he cited. It should be noted that Hermann Staudinger helped the macromolecule hypothesis to final victory after a few years, despite prominent resistance, among other things by measuring the viscosity of polymer solutions, by degradation experiments and by so-called polymer analogous conversions, which are chemical changes to already existing polymer chains. In the 1940s and 1950s, the first polymers such as nylon were produced fully synthetically, confirming his hypothesis across the board.
In the beginning, many scientific colleagues resisted this notion. Why?
Scherf: Perhaps the culture of dispute was even more pronounced 100 years ago than it is today. The existence of chemical compounds consisting, for example, of long chains of thousands of carbon atoms periodically built up from identical units (=repeating units) did not fit into the scientific worldview of many opponents of the macromolecule hypothesis. One argument was: many polymers are crystalline or more precisely semi-crystalline, and some crystallographers considered it fundamentally impossible for a molecule to be larger than its unit cell (a unit cell is the smallest unit of a crystal). It can be said that the opposition was not based on clear arguments, but when the opponents include prominent Nobel laureates, things can get tricky.
Macromolecular substances are divided into natural, semi-synthetic and synthetic substances. Can you give us a few examples?
Scherf: First of all, this is the typical classification for macromolecular substances. Polymer science is very technologically oriented, it is less about structures and mechanisms and more about technological aspects.
Naturally occurring polymers are those that are obtained from natural products without chemical modification, processed to shape them and used, i.e. cellulose (e.g. from cotton or wood), starch or natural rubber. Semi-synthetic polymers are chemically modified, altered natural products such as modified starch from the food industry, methyl cellulose as wallpaper paste, or vulcanized natural rubber, i.e. cross-linked with sulfur, as elastomers, i.e. elastically deformable plastics. Quite incidentally, such elastomers, i.e. rubbers, had already been developed and commercially produced since the mid-19th century and were used, for example, as tires for vehicles, of course without any knowledge of the macromolecular nature of the materials. Synthetic polymers include all polymers produced synthetically today as the main component of plastics. This is based on petrochemicals, i.e. crude oil and natural gas. There is a wide range of polymers, from bulk polymers such as polyethylene and polyvinyl chloride (PVC) to higher-grade polymers such as polyethylene terephthalate (from which our PET bottles are made) or polyurethanes (assembly foam in the construction industry) to specialty polymers. This is a real "polymer pyramid" with so-called bulk polymers (polyethylene, polypropylene, polystyrene, polyvinyl chloride PVC) as the base and high-value specialty polymers as the top.
One of the best-known macromolecular substances is plastic. Looking back, can we say that Staudinger's research laid the foundations for the rapid development of the chemical industry?
Scherf: Plastics are macromolecular substances that are provided with additives that serve to stabilize them, soften them, improve their properties, color them, and so on. These can be up to 40% additives. One must imagine that large parts of the chemical production today are located in the area of plastics. Today, plastics account for a huge share of chemical production. World production of plastics in 2020 amounted to 367 million tons, of which 55 million tons were produced in Europe alone. Naturally, plastics have been a striking technological achievement of the last 100 years because of their attractive property profiles (e.g. low weight combined with high strength or great elasticity, very good formability). They were also a driver for the development of the chemical industry, and this goes back to Staudinger, because his concept was fully confirmed in the 1940s.
Plastic as a macromolecular synthetic material could be recycled in a closed-loop process. But this is not happening. Why are we treading water on the subject of environmental protection when it comes to plastics?
Scherf: This brings us to the current problems of plastics utilization. The material cycles are still largely open today, and hundreds of millions of tons of plastics are fed into the cycles every year. In the worst case, the plastics end up uncontrolled in the environment. A prominent example is the plastic-polluted oceans. In the second worst case, the plastics are landfilled, with the risk that mostly low-molecular additives and so-called residual monomers are washed out and released into the environment. These additives are often not harmless, as evidenced, for example, by the current discussion about plasticizers in PVC. In some cases, the EU has now taken regulatory action, even after a broader public has been sensitized. In Brussels, this always happened and still happens in the interplay and struggle of lobby groups for influence, and the lobby of plastics manufacturers and plastics processors is traditionally strongly represented in Europe. In Germany, many plastics from waste are currently recycled for energy, i.e. burned in waste incineration plants. However, this also produces fine dusts and ashes that have to be disposed of properly and at great expense as hazardous waste. Closing the material cycles by recycling plastic waste, e.g. by pyrolysis processes, is generally possible, and the scientific basis has been established. However, huge investments and probably also regulatory interventions are necessary to initiate and realize the change of direction.
What can we do?
Scherf: With our current problems on plastics utilization, a quote from Albert Einstein comes to mind: "You can never solve problems with the same way of thinking that created them." Simply modifying and optimizing established polymer manufacturing processes will not produce the needed turnaround. Today, the 'pure` manufacturing processes are often already optimized to such an extent that they are highly energy-efficient and conserve resources. In polyethylene production, for example, ethylene gas and very small quantities of highly efficient catalysts are fed into continuously operating gas phase reactors on the one hand, and finished polyethylene granules are removed on the other, without the use of solvents or other auxiliary materials. In this way, hardly any waste is generated during production. At present, the very low production and sales price of bulk polymers such as polyethylene or polypropylene, as the main component of many plastics used today, is hindering essential efforts to make further progress in closing the material cycles. An important and probably necessary step in this direction would be to hold polymer and plastic producers responsible for the general recycling of plastic waste, and if necessary to force them to do so. In this context, it seems necessary to me to reduce plastics production to an ecologically acceptable level, i.e. to drastically reduce the production of new plastics in particular. At the same time, all plastic products fed into the market in the future must be strictly oriented and aligned with the yardstick of the possibility and feasibility of their material recycling. As already mentioned, the manufacturing processes for macromolecular materials already meet the high standards for sustainable processes to a considerable extent. The problem is to close the material cycles again after the end of the useful life of the plastic products.
Mr. Scherf, you head the Macromolecular Chemistry working group at Bergische Universität and are working with cooperation partners on an ultrafast optical switch. What is the role of macromolecular chemistry in this project?
Scherf: Today, science has to work together on an interdisciplinary basis, that's quite clear. My research group is working on special polymers with very specific electronic and optical properties, far away from the plastics problem just outlined, right at the top of the "polymer pyramid." One of the lines of development here are optical components based on the principle of strong light-matter coupling. Attempts are being made to generate very high data transmission rates with such optical components, data transmission rates that are still far above those commonly used today. The research field is called quantum optics. Here, the focus is on the very high-speed transmission of data. One application vision for this would be the efficient networking of quantum computers. In 2019, a collaboration between IBM Research Zurich and the University of Southhampton could present the first ultrafast optical transistor, which operates at room temperature. The Scherf working group at Bergische Universität supplied the required active polymer materials for this. With these optical components, current no longer flows; instead, the new medium for data transmission is now light.
Uwe Blass (conversation of Jan. 24, 2022)
Prof. Dr. Ullrich Scherf heads the Macromolecular Chemistry Department in the Faculty of Mathematics and Natural Sciences at Bergische Universität. In 2010, he became managing director of the Interdisciplinary Center "Institute for Polymer Technology" there.