100 Years of Macromolecular Science

Abstract

The year 2020 is very special for polymer science. One hundred years ago, Staudinger, considered by many the father of polymer chemistry, published his famous manuscript “Über Polymerisation”, describing several polymerization reactions such as the polycondensation of dicrotonic esters, as well as the polymerization of formaldehyde and styrene.1 In 1922, he coined the term “macromolecules”, which we still use to this day to describe covalent polymers.2 However, back in those days, polymers, as described by Berzelius, had a completely different meaning;3 therefore, Staudinger's work was not very well received by the chemical community, to say the least.4 Indeed, Staudinger had to embark on a personal crusade in order to gain support for his concept of covalent macromolecules, as chemists at the time did not accept that molecules above 5 kDa could be made. Staudinger's pioneering work was eventually recognized when he received the 1953 Nobel Prize in chemistry “for his discoveries in the field of macromolecular chemistry”. Therefore, following our previous issues of Rosarium Philosophorum on Physical Organic Chemistry, Structural Chemistry, Organic Synthesis and Chemical Biology, this year, the Israel Journal of Chemistry opens with a new Rosarium Philosophorum on Macromolecular Chemistry, in which leaders in the field describe key historical contributions, their own contributions and personal views, as well as their perspectives on the future of polymer science. The result is a marvelous compilation that can serve as a guide for young (and less young) chemists looking to contribute to this important and impactful aspect of chemistry. It is also a short history lesson for those just curious to learn more about the beautiful chemistry started by Staudinger. We have collected several important points from the various contributions to this Rosarium: Luscombe, Russel et al. describe the view of the IUPAC Polymer Division concerning the role of polymers in society, starting from the time preceding Staudinger, but highlighting the important role Staudinger's vision played in defining macromolecules. These ideas allowed for the development of new materials without which our lives would be significantly more difficult. The history of polymers and macromolecules within the annals of IUPAC is also summarized. Referring to the role of polymers and polymer science in society, they mention both the positive aspects and the current challenges, especially those related to environmental impact. They also forecast some of the expected advances in polymer science for the next 30 years. Baris Kiskan and Yusuf Yagci take us back in history to Leo H. Baekeland and Bakelite, one of the most important polymeric materials ever invented. The invention of the first synthetic thermoset polymer paved the way for the development of phenolic resins. As a curiosity, their account includes the cover of Time magazine from 1924 featuring Baekeland, one of the few practicing chemists to have gained this distinction. Kiskan and Yagci explain the importance of the synthesis conditions to the properties of phenolic resins, including the ratio of phenol to formaldehyde, the catalyst system and the reaction temperature and pH. They detail the effects and types of curing methodologies and review some of the chemical modifications typically used to enhance material properties, such as long term thermo-oxidative stability, high modulus or high char yields. A particularly interesting resin is obtained when bisoxazolines are used as additional cross-linking agents, affording materials with properties suitable even for the aerospace industry. The account highlights the use of renewable sources as starting materials and the emergence of polybenzoxazines as competitors to the more established Novolak resins. The authors conclude that phenolic resins and their applications will continue to develop and expand during their second century of existence. Takuzo Aida and Bert E. W. Meijer describe the full circle achieved with the amazing results obtained in the field of supramolecular polymers. This is a very interesting story, given that during Staudinger's time, polymers, rubbers etc. were considered non-covalent aggregates of small molecules showing colloidal behavior. With the acceptance of Staudinger's concept of macromolecules, non-covalent interactions, which play an enormous role in the structure and function of biopolymers and materials, were somehow initially excluded from polymer science. The authors highlight the notion that polymers based on secondary interactions can achieve amazing properties, comparable to covalent macromolecules, as well as control over molecular structure. Aida and Meijer describe the evolution of the field, focusing on early work conducted in the 1980s and 1990s, and reaching the outstanding materials accessible nowadays. They describe the rich chemistry of supramolecular polymerization, and the relation between mechanism and molecular weight and dispersity, which now provide polymers with chemistries that are as well defined as covalent systems. They also describe the unique functions that can be obtained using the reversibility of non-covalent interactions, providing these polymers with unique abilities, which covalent polymers are now trying to mimic. These perspectives provide a rich and optimistic view of the future of this field. Virgil Percec describes his personal story and connection with polymer science, which started in the Herman Staudinger House in the University of Freiburg, as well as his interaction with Staudinger's former students, all influencing the beginning of his career and leading to his book that celebrated 60 years of Staudinger's Nobel Prize. Percec asks an interesting question regarding Ziegler's career and impact on the field of polymers. Ziegler had demonstrated some of the first examples of living anionic polymerization and could have continued researching that, but instead turned to the development of organometallic homogeneous catalysis. That work eventually led to the 1963 Nobel Prize in polymer science, only 10 years after Staudinger's Nobel Prize. Percec discusses in great detail the history of stereocontrolled polymerization and its importance, before moving on to a discussion of dendrimers and their self-assembly, as well as supramolecular polymerization. Yasuyuki Tezuka focuses on the molecular design aspect of polymers, combining the topics of polymer and topological chemistry. The author of the leading book on cyclic polymers describes his view on how to design chemistries to achieve control over the topology of macromolecules, starting from “simple” cyclic polymers to the recent advances in multicyclic macromolecules. The latter mimics some of the most complex biological macromolecules, such as DNA, which exhibits cyclic structures, knots and link topologies. He highlights the difficulties of controlling folding and bonding in disperse macromolecules, and the organic and supramolecular chemistry that has been developed to address such challenges. His personal views on the impact of polymer science on society are linked to the multidisciplinary approach required to advance the field. Finally, he describes the open challenges in topological polymer science, perhaps previewing some of the work to be expected in the field of topological macromolecules in the near future. Elizabeth R. Gillies focuses on the evolution of smart, bio-inspired polymers and the development of stimuli-responsive materials. Chemists are obviously fascinated by the self-repairing ability of living tissues and by the way small molecules can interact with proteins to induce signal cascades, which result in cell behavior. The most widely used stimuli to affect polymers are pH, temperature, light and, more recently, mechanical force. This insightful article describes how advanced synthetic methodologies may induce diverse functions of polymers, rendering them as chemical sensors, drug delivery vehicles and even as mechanical actuators. For this to become a reality, Gillies highlights some of the recent advances in macromolecular methodologies, such as controlled radical polymerization and ‘Click’ chemistry. Polymer science has greatly influenced society and, as predicted by Staudinger, long covalent chains have played a key role in the development of novel useful materials such as Nylon, Teflon, Kevlar and many others. Moreover, the improved synthetic possibilities and a better understanding of our environment have come full circle: in the past, polymers were made to replace functions of biopolymers, and today we are looking for biodegradable biopolymers to replace the sturdy plastic materials of our everyday life. The growing attention that ‘smart’ polymers are receiving bodes well for a future where polymers will be less contaminating and will be used for important tasks, such as drug delivery in cancer treatment and sensing. Christopher Barner-Kowollik et al. highlight the flourishing field of single-chain nanoparticles (SCNP). Perhaps related to the Lutz account, the advance of synthetic techniques that enable precise positioning of monomers along a polymer chain and the study of controlled polymer collapse, or folding, allows for the creation of polymers with protein-like structures and functions. Even though SCNPs have recorded impressive achievements, such as specific catalysis within cells, the authors point out the challenges that still need to be addressed, and which areas require further study and development in order to go beyond the state-of-the-art in the field, especially in areas such as catalytic nanoreactors and mimicry of quaternary protein structures. Stuart Rowan and Christopher Weder describe how multidisciplinary research can be used to develop functional, stimuli-responsive macromolecules. As relatively young leaders of the field, they remember that “stimuli responsive polymers” is still a very new term, originated at the beginning of this millennium, along with their own independent careers. They redefine the topic, explaining that they were personally inspired by stimuli-responsive small molecules and natural systems. They focus on the initial work in the field on the way from solid state to polymers. They also describe the use of metallopolymers, leading to unprecedented control over signal response as well as property change. They anticipate the future of this important class of polymers, mainly regarding the challenges of incorporating them into our day-to-day materials, as well as the development of novel polymers capable of multi-responses to multiple signals in parallel. Francesca Lorandi and Krzysztof Matyjaszewski summarize the history and importance of the controlled radical-polymerizations, describing the challenges of catalysts for ATRP. Polymer chemistry, even before Staudinger, is a chemistry that depends significantly on the development of efficient, environmentally friendly and cheap catalysts. ATRP provides unprecedented control over polymer molecular weight and dispersity. With the diminishing quantities of very simple and cheap metal salts, one may ask why do we need more active ATRP catalysts? The authors try to answer this question focusing on the complex relation between catalyst quantity and polymerization kinetics, which affect material properties. They also focus on the newest advances in the fields, such as the use of light, reducing agents, electrons and mechanical forces to control the equilibrium between propagating and resting states, and describe the challenges that can be met by better ATRP catalysts. Sam Stupp et al. focus on supramolecular and hybrid bonding polymers, combining the advantages of covalent and non-covalent interactions in the formation, functionalization and function of polymeric molecules and materials. They describe the beginning of the field of polymers and the connection between supramolecular polymers and the “old view” of polymers before Staudinger. Then, they focus on sequential covalent and non-covalent polymerization, which led to new materials with unique properties, structures and functions. Finally, they highlight the relations between this new field of polymer science and Staudinger's concepts, mentioning a 1990 conversation between Stupp and Staudinger's wife about the difficulties Staudinger confronted in his time and his views on the importance of macromolecules. Ziwen Jiang and Sankaran (Thai) Thayumanavan delve into disulfide-containing macromolecules for therapeutics. They discuss the role and chemistry of disulfide bonds in polymers, with a special focus on applications in therapeutic delivery systems. The reversibility of the disulfide bond is beneficial for systems where careful control of structure and drug delivery are needed. If the disulfide bonds are part of the polymer backbone, their disruption would result in polymer degradation. Alternatively, if they are part of the side-chains, they offer opportunities for convenient modifications of the polymer by thiol-disulfide exchange reactions, as well as intramolecular cross-linking. Moreover, the disulfide function allows for conjugation to biological macromolecules, such as proteins and nucleic acids, thus expanding the capability of these special polymers to function in vivo. As discussed in other articles in this Rosarium, polymers can be designed to interact with biological systems. Michael Silverstein demonstrates how the recent expansion of modern polymerization methods can be combined with new techniques for the generation of porous polymers. These important materials can find applications in water purification, gas separation and energy storage. Relating to the great insight of Staudinger, he nicely details the history of foams and pore-containing polymers. He discusses the techniques for making “holes”, traversing the various polymer size scales and then going on to briefly summarize polymerization techniques. Silverstein concludes that “The key to producing porous polymers can lie in achieving a balance in the often simultaneous processes of generating the pores and locking-in the porous structure.” He also emphasizes the societal and environmental aspects of porous polymers, highlighting the major applications of porous polymers and their expected future. Jean-Francois Lutz asks the provocative question, “Can life emerge from synthetic polymers?” As humans have been making polymers for a century, is it conceivable to imagine that-in an analogy to life-supporting polymers such as proteins, nucleic acids and polysaccharides-perhaps life could also emerge from synthetic polymers? He explains what is needed for a polymer to support life, and briefly details how “synthetic” life could be made, either by manipulating existing biopolymers or by more challenging non-biological approaches. This intriguing article by Lutz poses thought-provoking questions such as are we close to making a new life form with polymers? Should we create and disseminate synthetic life forms to other planets? We invite you to dive into this Rosarium Philosophorum to discover various aspects and ideas presented by this select group of “philosophers”. We are living, undoubtedly, in the “Polymer Age”, and therefore we hope this issue may inspire and invite fellow chemists to become a part of this field. We thank all the authors for contributing to this important compilation, as well as the “undisclosed philosophers” who had the even harder job of reviewing these personal views of the field of macromolecular science. Enjoy your reading, Gabi Lemcoff, Charles Diesendruck and Ehud Keinan