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One hundred years ago, in 1920, the German chemist Hermann Staudinger published his classic paper entitled "Über Polymerisation" ("On Polymerization"). In that paper, he proposed a chain structure for rubber, each chain-like molecule consisting of many identical chemical units (isoprene monomers). He also claimed that the unusual tensile strength and elasticity of rubber are a result of the great length of these chains or, as polymer scientists of today would say, of their high molecular weight, and the vast number of carbon-carbon double bonds between the subunits within each molecule.
Several years later, Staudinger termed such high molecular weight chain-like molecules "Makromolekül" ("macromolecule"). After a prolonged controversy, Staudinger's macromolecular theory was eventually accepted by the scientific community. This led to some of the most profound scientific developments of the 20th century, affecting many aspects of our lives.
Since the 19th century, researchers reported unusually high molecular weights for some natural products such as rubber, cellulose, resins, and proteins. Ordinary organic substances, including alcohol, petrol, or sugar, have a molecular weight in the range of 50 – 500. For the natural organic products in question, the range is from 50,000 to several million.
In some respects, these organic products behaved like inorganic colloids – they could reside in solutions as huge particles with very low diffusibility. At that time, it was challenging to imagine molecules with a considerable size capable of stable existence. Instead, many renowned organic chemists such as Wolfgang Ostwald, Hans Pringsheim, and Kurt Hess, suggested the concept of clusters or aggregates of much smaller molecules held together by strong intermolecular forces of aggregation.
Staudinger strongly opposed the analogy between inorganic colloids and organic high molecular weight substances. He argued that all physical and chemical properties of organic matter are defined by their intrinsic molecular structure and not from physical forces outside the molecules. This gave birth to a long-lasting colloid/macromolecule debate.
Rubber: From Small to Large Molecules
Between 1922 and 1930, Staudinger continued his experiments with chemically modified rubber and published his findings in 19 papers, where he thoroughly investigated the properties of catalytically-hydrogenated natural rubber. His results demonstrated that the loss of the double bond in saturated rubber did not affect the "colloidal properties" of the material. He firmly believed that this proved the presence of covalent bonds between the building blocks of the rubber molecules.
X-Rays Shed Light on Unusual Diffraction Patterns in Cellulose Fibers
In the early decades of the 20th century, novel physical methods, such as X-rays and electron diffractions, were adopted to study natural fibers (cellulose), rubber, and proteins. While Staudinger was publishing his findings on rubber modification, Reginald O. Herzog and W. Jancke studied the structure of cellulose fibers by X-ray diffraction. They observed unusual diffraction patterns consisting of diffused spots placed symmetrically in groups of four.
Subsequently, these diffractograms were interpreted by Michael Polanyi, who suggested that at least part of the cellulose material was crystalline and consisted of either long chains of glucose rings or dimers of rings. Despite that ambiguous conclusion, Polanyi's interpretation was a significant step towards the establishment of the macromolecular theory.
Between 1925 and 1926, Polanyi's colleagues J.R. Katz, E.A. Hauser, and H. Mark observed that the X-ray diffractogram of stretched rubber displayed a pattern that could be associated with partially crystallized material. This result was supporting the idea that flexible macromolecular chains, when exposed to mechanical deformation, can be ordered into thin, elongated, and well-oriented bundles with high internal order.
Around that time, the research into high molecular weight materials was picking up pace across the Atlantic. In 1926, O. L. Sponsler and W. H. Dore, working at the University of California, Los Angeles, presented a comprehensive structure of the cellulose molecule - a chain of glucose rings joined by covalent bonds.
In 1928, Wallace H. Carothers at the DuPont Company started ambitious and systematic research to synthesize new materials with specific structures through established organic reactions. As a result, he came up with a new class of fully synthetic polymers called polyamides, or "nylons," that could be melted and drawn into a remarkably strong fiber. More importantly, his results helped to establish, without further criticism, the idea of macromolecules as the building blocks of the organic matter.
The Golden Age of Polymers
The word "polymer" was introduced by Jöns Jacob Berzelius in the 1830s to describe molecules in which the same atomic groups were arranged repeatedly. He considered benzene (C6H6) to be a polymer of ethyne (or acetylene, C2H2). Over the years, the term covered larger and larger molecules, until eventually it was used to denote the long flexible macromolecules formed of multiple base units known as "monomers" (from Greek: monos - "one" and meros - "part"). The macromolecules themselves became "polymers" (poly is the Greek word for "many").
The 1930s and 1940s marked the golden age for the development of new synthetic polymers. Scientists in both academic and industrial laboratories were synthesizing new monomers from abundant and inexpensive raw materials. At the same time, the processes of polymerization (a chain-growth process, where the monomers attach sequentially to the active end of the growing polymer chain) and polycondensation (a step-growth process that combines two or more different monomers in an alternating structure) were refined for increased efficiency and yield.
New methods were developed for better characterization of the microstructure of the polymer macromolecules. This, in turn, allowed the engineering of polymers with specific physical and chemical properties by tailoring the structure and the arrangement of the polymer chains.
The availability of polyvinyl chloride (PVC), polyurethane (PU), nylon fibers and neoprene (the first synthetic rubber), polytetrafluoroethylene (PTFE or "Teflon"), and polystyrene (PS) revolutionized the manufacturing of human-made fibers, films, plastics, rubbers, coatings, and adhesives. These new synthetic materials had no resemblance to their raw materials (usually oil or natural gas). They were outperforming their natural counterparts, while at the same time being cheaper and more accessible.
The growing automotive industry and rubber demand during World War II stimulated the large-scale production of artificial rubber.
The booming success of synthetic fibers, synthetic rubber, and new functional materials taught the polymer industry a vital lesson: fundamental research can lead to products that replace natural materials.
The Scientific Recognition of Polymer Scientists
The combination of applied and fundamental research can sometimes lead to Nobel prizes.
Hermann Staudinger received the Nobel prize in chemistry in 1953 for his life-long work in the field of macromolecular chemistry.
In 1963, the Nobel prize in chemistry was awarded to Karl Ziegler and Giulio Natta for the development of a catalytic process that allowed scientists to conduct well-controlled polymerization at room temperature and atmospheric pressure. This paved the way for the mass-production of polyethylene and polypropylene, the two most widely-used commodity polymers.
Paul J. Flory received the Nobel prize in chemistry in 1974 for his fundamental contributions to polymer science. He was instrumental in developing the modern understanding of how polymer macromolecules behave. His research showed that the individual molecules in a polymer differ in terms of chain length and that the properties of synthetic polymers can be described only as "average" properties. These ideas were further developed into the concept of "molecular weight distribution," one of the fundamental properties of the polymer materials.
What are Bio-Polymers?
Bio-polymers are the basis of all life on our planet. Polypeptides (proteins) and polynucleotides (DNA and RNA) dominate the functional and informational machinery of life, while polysaccharides (sugars) are important in physical structure, energy storage, and intermolecular interactions.
In 1963, the Nobel prize in physiology and medicine was awarded to Francis H. C. Crick, James D. Watson, and Maurice H. W. Wilkins for their work in the 1950s on the structure of the nucleic acids, the most important of all informational bio-polymers.
Growing Interest in Organic Electronics and OLEDs
The Nobel prize in chemistry in 2000 was awarded to Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa for the discovery of the high electrical conductivity of doped polyethyne (or polyacetylene) in the middle of the 1970s. This sparked an ever-growing interest in the field of organic conductive polymers that can be tailored to achieve desired electronic and optical functionality. The use of organic compounds in microelectronics led to the development of organic light-emitting diodes (OLEDs) used in the modern display technology and high-performance lithium-polymer batteries.
Environmental Impact on Polymers
The use of polymers is still expanding and, with a global production of several hundred millions of tons annually, synthetic polymers have become a significant factor in the economy of all industrialized countries in the world. This has created the major environmental problem of how to deal with this non-biodegradable waste. Although recycling has become a major industry, it is limited by the available markets for reclaimed materials and is profitable only for limited classes of polymers.
Small-Molecule Release from Long Polymer Chains
Industrial synthesis of long polymer chains is a somewhat random process and often leaves some unreacted monomers in the final product that can be harmful to the environment. Formaldehyde, styrene (from polystyrene used in food containers, for example), vinyl chloride, and bisphenol-A (from polycarbonates) are known carcinogens. Other polymers contain modifiers (plasticizers, flame retardants, UV protectors, etc.) to alter their properties. Many of the monomers and the modifier molecules are small enough to be able to diffuse through the polymer matrix and eventually to be released into any liquid or air in contact with the polymer.
Micro-Plastic in the Environment and the Prospects of Biodegradation of Synthetic Polymers
A growing concern is that the non-biodegradable polymers are disposed of in landfills or are floating in the oceans. These are subjected to weathering and partial photodecomposition, catalyzed by the exposure to sunlight, resulting in small plastic fragments with sizes down to a few microns, which are released in the groundwater or the marine habitats. Many of these materials are less dense than water, and once they enter aquatic systems, they tend to remain there indefinitely. Aquatic animals can also swallow larger fragments of plastic waste by mistake.
Hope for the future is that considerable efforts are devoted to finding ways for biodegradation of synthetic organic polymers. The process of biodegradation involves microorganisms (such as bacteria, fungi and yeasts) that can break down the organic macromolecules into small molecular weight fragments, which can be further degraded aerobically or anaerobically, to carbon dioxide (CO2), methane (CH4) and water.
Of particular interest is the biodegradation of synthetic oil-derived polymers such as polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP) and polystyrene (PS). These amount for up to 60% of the global commodity polymer production (and waste).
In 2016, Shosuke Yoshida and his colleagues at the Kyoto Institute of Technology reported a newly isolated microorganism that can aerobically degrade PET to small molecular weight products. The bacterium uses two separate hydrolytic enzymes to break down the PET polymer chains in a two-step process. The bacterium uses the resulting products, ethylene glycol and terephthalic acid (one of the PET monomers), as an energy source with CO2 and water as end products.
Non-hydrolyzable polymers (PE, PP and PP) are significantly harder to break down compared with polymers containing hydrolyzable bonds (such as PET). However, in 2018, scientists from Stanford University discovered that the microorganisms residing in the gut of the mealworm beetle larvae are capable of depolymerization of PS and PE to small molecular weight intermediates that can be used as a primary energy source for the larvae metabolism.
These remarkable discoveries offer the possibility to create genetically engineered microbial enzymes with an enhanced aerobic or anaerobic activity towards synthetic polymers. This would allow much faster aerobic biodegradation of organic waste. More importantly, the development of enzymatic systems for anaerobic degradation of these materials would lead to the production of methane gas, which can be harvested as a renewable fuel.
The video below looks at the future of polymer science and the impacts it has on society:
Video Credit: Advanced Science News/YouTube.com
References and Further Reading
Holger Frey and Tobias Johann, Celebrating 100 years of "polymer science": Hermann Staudinger's 1920 manifesto. Polym. Chem., 2020, 11, 8. Available at: DOI: 10.1039/C9PY90161B
Pioneers in Polymer Science, Raymond B. Seymour (Ed.), Kluwer Academic Publishers, 1989. Available at: DOI 10.1007/978-94-009-2407-9
Dorel Feldman (2008) Polymer History, Designed Monomers, and Polymers, 11:1, 1-15. Available at: DOI: 10.1163/156855508X292383
Yasu Furukawa, Polymer Science: From Organic Chemistry to an Interdisciplinary Science, in Chemical Sciences in the 20th Century: Bridging Boundaries, C. Reinhardt (Ed.), WILEY‐VCH Verlag GmbH, 2001. Available at: https://doi.org/10.1002/9783527612734.ch12
S. Yoshida et al., A bacterium that degrades and assimilates poly(ethylene terephthalate). Science 2016, 351, 1196-1199. Available at: DOI 10.1126/science.aad6359.
A. M. Brandon et al., Biodegradation of Polyethylene and Plastic Mixtures in Mealworms (Larvae of Tenebrio molitor) and Effects on the Gut Microbiome. Environ. Sci. Technol. 2018, 52, 11, 6526-6533. Available at: DOI 10.1021/acs.est.8b02301.