We are happy to present this first collection of Perspectives on the “Grand Challenges in Polymer Science”. These six Perspectives from world-leading experts in various subfields of polymer science and engineering present their views on the important problems that researchers in the polymer community could tackle to find sustainable long-term solutions. Some of these technical questions are new as the field evolves, some are yet to be answered due to limitations in existing─synthetic, characterization, computation/theory─methods, while others have been answered partially or continue to be debated by researchers with opposing scientific observations. The Perspectives in this virtual special issue highlight the past, present, and future for each of these complex problems in polymer science. As we started writing this Editorial on ACS Polymers Au’s “Grand Challenges in Polymer Science”, it dawned on us that this issue comes exactly five years after Tim Lodge, then Editor-in-Chief of Macromolecules, penned his Editorial “Celebrating 50 years of Macromolecules”. (1) In his retrospective and forward-looking Editorial, he presented his thoughts on the “top ten technological and intellectual achievements” in polymer science over the past five decades as well as the “top ten current challenges” in polymer science that remained to be solved. Table 1 below lists these top ten current challenges from his Editorial. List taken from Table 3 of the Macromolecules Editorial by Tim Lodge in 2017, DOI: 10.1021/acs.macromol.7b02507. Unsurprisingly, many of the problems presented in the Perspectives in this ACS Polymers Auvirtual special issue fall under one or more of these ‘top ten challenges’ (Table 1). Additionally, these Perspectives capture recent advances in the past five years since that Editorial by Lodge, and present the current critical needs for new experimental and computational approaches to find answers to the many unsolved complex questions in polymer science. In this Editorial, we briefly summarize the topic(s) covered in each of these six Perspectives, along with quotes from the lead author and/or statements from their Perspective. We also share our personal views on why we think these are important problems and highlight with bold-italicized phrases the Grand Challenges in Polymer Science that we think each Perspective connects to. We start with the Perspective by Glenn Fredrickson and co-workers [DOI: 10.1021/acspolymersau.2c00026] titled “Ionic Compatibilization of Polymers” (Figure 1). Fredrickson says, “The compatibilization of immiscible polymers has been a central theme since the genesis of polymer science and technology. Underappreciated, however, is the use of ionic bonds or ionic correlations. Vast categories of dissimilar polymers can be homogenized using this strategy, modifying not only mechanical, optical, and electronic properties, but processability.” We believe that with the growing number of new processing techniques involved in high-throughput manufacturing of multicomponent polymer materials, where incompatible polymer chemistries are blended/layered, compatibilization of those interfaces between immiscible polymer chemistries will continue to be an important problem to address. This Perspective presents some solutions to improve compatibilization within multicomponent blends of ion-containing polymers and ways to use that knowledge to engineer new and improved polymer-based electronics. Figure 1. Table of Contents Image from “Ionic Compatibilization of Polymers” [DOI: 10.1021/acspolymersau.2c00026]. Fredrickson et al. also discuss how ionic compatibilization can be useful in plastic waste upcycling, which we believe is a significant grand challenge in polymer science. Polyolefin waste, which is composed primarily of isotactic polypropylene (iPP) and various grades of polyethylenes (PE), constitutes a large fraction of the plastics manufactured globally. Isolating the individual iPP and PE components in the waste stream is a challenge. When molten, mixtures of iPP and PE phase separate into macroscopic iPP and PE-rich domains; in solid-state, the interfaces between these dissimilar domains exhibit poor entanglement leading to brittle materials. Fredrickson et al. present alternative approaches involving the ionic compatibilization of these interfaces and discuss the cost implications of such procedures. They conclude with this statement: “Evidently, the level of ionic modification required for strong interfaces and materials, the chemistries used to install such (ionic) functionality, and the optimal selection of acid, base, or salt units are subjects that will all require significant fundamental research”. motivating future research in this direction. The Perspective by Rachel Segalman and co-workers [DOI: 10.1021/acspolymersau.2c00024] titled “Decoupling Ion Transport and Matrix Dynamics to Make High Performance Solid Polymer Electrolytes” (Figure 2) tackles the subject of ion transport through solid polymer electrolytes (SPEs). Nanostructured SPEs provide a promising alternative route to homogeneous liquid electrolytes because SPEs can achieve high ionic conductivity and selectivity. Design of effective SPEs requires a fundamental understanding of ion motion in polymer materials, which depends on a complex “interplay of intrachain transport, interchain hopping, and co-diffusion of segments and ions.” As a result, understanding ion transport is very much coupled with understanding polymer dynamics and segmental relaxation time (i.e., glassy dynamics in polymers). However, “balancing the low glass transition temperature as well as other requirements of an electrolyte presents a major challenge, particularly since mechanical robustness is generally a major justification for choosing a polymer over a liquid electrolyte.” This Perspective by Segalman and co-workers presents alternative strategies one could use to decouple the polymer matrix dynamics and ion transport. We think identifying approaches to independently control multiple coupled phenomena (e.g., ion transport and glass transition) for creation of new and improved polymeric materials for batteries and (ion-selective) membranes are substantial grand challenges for the polymer community to address. Figure 2. Table of Contents image from “Decoupling Ion Transport and Matrix Dynamics to Make High Performance Solid Polymer Electrolytes” [DOI: 10.1021/acspolymersau.2c00024]. The Perspective written by Tim Lodge [DOI: 10.1021/acspolymersau.2c00033] is titled “Dynamics and Equilibration Mechanisms in Block Copolymer Particles” (Figure 3). Lodge writes, “Self-assembly of block copolymers into interesting and useful nanostructures, in both solution and bulk, is a vibrant research area. Nanoparticles prepared by block copolymer self-assembly exhibit structures that are often not at equilibrium. A quantitative understanding of the mechanisms by which equilibrium is approached is still lacking, and presents a rich challenge for theory, simulation, and experiment.” This Perspective on the classic block copolymer self-assembly problem highlights newer unsolved/partially solved issues related to understanding and tailoring non-equilibrium effects of processing to achieve desired nanostructures in polymers. We think this Perspective also nicely highlights the power of structural characterization techniques like time-resolved small-angle neutron scattering, liquid-phase TEM, and the combination of simulations and experiments to better understand time evolution of macromolecular assembled structures near and far from equilibrium. Figure 3. Table of Contents Image from “Dynamics and Equilibration Mechanisms in Block Copolymer Particles” [DOI: 10.1021/acspolymersau.2c00033]. Understanding nonequilibrium effects in polymers is not only an experimental challenge but also a quest in molecular simulations. We quote Friederike Schmid: “The (next) big challenge in polymer science is to gain a comprehensive and predictive understanding of the behavior of polymeric systems over all relevant scales and up to very late times, close to and far from equilibrium.″ We think this computational challenge can be partly addressed through bridging of scales–molecular to continuum. Schmid’s Perspective titled “Understanding and Modeling Polymers: The Challenge of Multiple Scales” [DOI: 10.1021/acspolymersau.2c00049] (Figure 4) describes popular models that are used to study polymers at different length scales and the strategies (e.g., static and dynamic coarse-graining methods and multiresolution approaches) to bridge across different scales. Figure 4. Table of Contents Image from “Understanding and Modeling Polymers: The Challenge of Multiple Scales” [DOI: 10.1021/acspolymersau.2c00049]. Highlighting the power of molecular simulations, Lodge said in his 2017 Editorial that “hardware and current algorithms have elevated simulations to the level of an essential family of techniques, both for fundamental understanding and for prediction.” [DOI: 10.1021/acs.macromol.7b02507]. We agree with that sentiment completely. We also believe that, in the past 5–10 years, researchers in the polymer community have begun to see the tremendous value not only of physics-based (e.g., molecular simulations) methods but also of data-driven computational methods (e.g., machine learning) for predicting new polymer chemical designs, elucidating new polymer physics, in objective interpretation of characterization data, and establishing design-nanostructure–property relationships. Debra Audus and Tyler Martin’s Perspective points toward key questions and methodological needs within the growing subfield of data-driven polymer science [DOI: 10.1021/acspolymersau.2c00053] (Figure 5). Quoting Audus and Martin, “Polymer research driven by machine learning has made tremendous progress in the past five years, but barriers remain. The movement toward Open Science offers many opportunities to advance the field and address key challenges in polymer machine learning, such as data scarcity, and data quality. A culture shift where we become more collaborative and more open in data and code, while building capabilities in autonomous science, use of domain knowledge, and uncertainty quantification could enable unprecedented progress.” Figure 5. Table of Contents Image from “Emerging Trends in Machine Learning: A Polymer Perspective” [DOI: 10.1021/acspolymersau.2c00053]. Based on the two Perspectives from Schmid, and Audus and Martin, and personal views (including biases of yours truly, Jayaraman), we feel that the combination of physics-based and data-driven computational methods (e.g., linking theory and machine learning or molecular simulations and machine learning) as well as the combination of computations and experiments for automation in polymer science (e.g., machine learning-driven synthesis of new polymers, machine learning enabled fast and automated characterization) are meaningful grand challenges for the polymer community. With the growing influence of artificial intelligence (AI) in chemical and materials-focused industries, it is vital to find ways to integrate physics, chemistry, and machine learning in our pursuit for finding answers to complex new questions as the polymer field continues to evolve. How could there be a virtual special issue on the grand challenges in polymer science without a Perspective focused on polymerization? All subfields of polymer science (polymer physics, polymer theory, simulations, machine learning, polymer processing, polymer engineering) rely on robust and controllable polymerization methods. The Perspective from Nikos Hadjichristidis‬‬‬‬‬ [DOI: 10.1021/acspolymersau.2c00058] (Figure 6) titled “Quo Vadis Carbanionic Polymerization?” presents “significant achievements, current status, where it is going (Quo Vadis) and what the future holds for” living anionic polymerization of vinyl monomers. We quote Hadjichristidis: “Carbanionic polymerization is, without any doubt, the best method to design and produce well-defined polymers with different macromolecular architectures. It paved the way for other controlled/living polymerizations and industries when precise structures were needed. On the other hand, controlled/living polymerizations paved the way for simplicity. The next big challenge is to find a way to simplify carbanionic polymerization so that it is accessible to more laboratories and industrial products.” Along with the advances in anionic polymerization, great strides have also been made over the past decades in refining other polymerization strategies, such as radical and carbocation-based processes as well as catalytic polymerizations, toward the preparation of precisely defined polymers. The quest for scalable and sustainable polymer synthesis methods that enable precise control over monomer sequence, molecular weight, and architecture, however, continues to be a grand challenge. Figure 6. Table of Contents Image from “Quo Vadis Carbanionic Polymerization?” [DOI: 10.1021/acspolymersau.2c00058]. Table 2 lists all of the grand challenges we have identified from the Perspectives in this virtual special issue. We note that the problems highlighted in these six Perspectives only constitute a partial list of the grand challenges we face as polymer researchers. There are additional topics that are “Grand Challenges in Polymer Science”, including (but not limited to) new and improved polymers for personalized medicine and sustainable, cheap, and environment-friendly new commodity plastics. We hope to have leading experts in these subfields share their perspectives on these important topics in the near future. We also hope to have Perspectives and Editorials published in ACS Polymers Au on how we as a community can do better in (a) tackling mental health-related challenges that many researchers at all career stages face and (b) improving diversity, equity, inclusion, and respect (DEIR) in our workplaces. Many consider these to be important social issues but, unfortunately, treat them as issues separate from education and research. In contrast, we believe that these are closely coupled, because the health and welfare of our current and next generation of researchers are closely linked to them realizing their scientific potential and fostering their creativity to develop innovative sustainable solutions for the many Grand Challenges in Polymer Science. The 2022 Nobel Laureate Carolyn Bertozzi, Editor-in-Chief of ACS Central Science, has shared in her speeches, after receiving the 2022 Nobel Prize, how diversity, equity, inclusion, and respect toward different cultures and backgrounds in her lab fostered true creativity and innovation. So, we wrap up this Editorial with an inspirational quote from her: “Diversity powers creativity and advances science.” (2) We thank you, the readers, for reading this Editorial, and we hope you are as inspired as we were reading the Perspectives in this first Grand Challenges in Polymer Sciencevirtual special issue. We thank the authors for their hard work and thoughtfulness as they prepared these Perspectives. Finally, as always, we are grateful to the reviewers for their time and effort in constructively critiquing these Perspectives during peer review. This article references 2 other publications. This article has not yet been cited by other publications. Figure 1. Table of Contents Image from “Ionic Compatibilization of Polymers” [DOI: 10.1021/acspolymersau.2c00026]. Figure 2. Table of Contents image from “Decoupling Ion Transport and Matrix Dynamics to Make High Performance Solid Polymer Electrolytes” [DOI: 10.1021/acspolymersau.2c00024]. Figure 3. Table of Contents Image from “Dynamics and Equilibration Mechanisms in Block Copolymer Particles” [DOI: 10.1021/acspolymersau.2c00033]. Figure 4. Table of Contents Image from “Understanding and Modeling Polymers: The Challenge of Multiple Scales” [DOI: 10.1021/acspolymersau.2c00049]. Figure 5. Table of Contents Image from “Emerging Trends in Machine Learning: A Polymer Perspective” [DOI: 10.1021/acspolymersau.2c00053]. Figure 6. Table of Contents Image from “Quo Vadis Carbanionic Polymerization?” [DOI: 10.1021/acspolymersau.2c00058]. This article references 2 other publications.