A world without polymers is without many technological advances that continue to spur the economy and improve the quality of human life. Polymers will continue to play an essential role in the 21st century. Indeed, there is a strong connection between innovations in polymer science and progress toward the 17 Sustainable Development Goals adopted by all United Nations Member States. For instance, polymer-based materials will play a vital role in new technologies that enable access to clean water in a manner that avoids a crippling trade-off with energy. Sustainable communities and infrastructure will benefit from developing polymers that allow materials to escape traditional property trade-offs—such as those between density and strength—the set limits on current design and construction. The realization of affordable and clean energy will require advancements in various technologies, including polymer solar cells, which remain a leading candidate for modular energy generation. While important, these examples barely represent the myriad ways polymers will feature prominently in future solutions to many of our most pressing challenges. Indeed, these solutions will be built on a foundation of fundamental advances in polymer science and soft matter.
This JACS Au virtual issue consists of nine recently published Articles and Perspectives which advance our fundamental understanding of polymers and soft matter. The subjects of these works encompass several disciplines, including polymer chemistry, colloids and interfacial science, composite materials, and spectroscopy. They have broad critical implications in sectors ranging from energy to sustainability to health. Collectively, the papers showcase the strong connection between rigor and relevance that underpins contemporary polymers and soft matter research.
Polymer properties, which define the use application of the material and usher in a wide variety of applications, are generally recognized to be determined by the constituent monomer in the case of homopolymers or monomers in the case of copolymers. Regarding the latter, how monomers are combined along the polymer chain profoundly impacts properties and use application. As the diversity of available chain structures increase, developing accurate structure-properties relationships become paramount. To advance our understanding of such relationships for bottlebrush polymers, Lawrence and co-workers synthesized topological uniform and fully discrete bottlebrush polymers by first preparing precise macromonomers, which were subsequently polymerized via ROMP using Grubbs third-generation catalyst (DOI: 10.1021/jacsau.2c00010). The work demonstrated a profound influence of structure on the glass transition temperature and packing efficiency of discrete and well-defined multiblock bottlebrush polymers.
Mechanical recycling is one way society will reduce the impact of polymers on the environment and lessen overall use. Yet, the challenge associated with mechanical recycling from mixed plastics, even for polymers with similar chemical structures, is incompatibility leading to phase separation and inhomogeneous materials. Traditional routes to improve polymer compatibility have relied on diblock copolymers or in situ interfacial reactions. In their perspective, C. J. Ellison, C. W. Macosko, and co-workers present the case, via several examples, that multiblock copolymers with either a linear or graft topology enable better blend compatibilization for mechanical recycling (DOI: 10.1021/jacsau.1c00500). They discuss emerging challenges, including deciphering the relative importance of trapped entanglements vs cocrystallization on the ability of multiblock copolymers to stabilize interfaces or an improved theoretical understanding of factors affecting transport and localization of multiblock copolymers to the interface, which would guide new designs. Overcoming these challenges, and others, holds promise that mechanical recycling may soon be viewed as an “upcycling” technology. The above papers show that control of monomer arrangement within a polymer chain is critical to form and function. A. DeStefano, R .A. Segalman, and E. C. Davidson take this notion further. Their perspective considers the opportunities available in new insights and material designs with the development of synthetic sequence-defined polymers (DOI: 10.1021/jacsau.1c00297). In particular, they discuss challenges related to synthesis and sequence quantification as well as fully understanding self-assembly and processing of sequence-defined polymers. In posing the thought question, “What to make when you can make anything?” the perspective makes the case that a predictive understanding of sequence–function relationships is needed and advances in computational approaches are paramount.
Beyond control of chain structure and monomer sequence, polymer-based composite materials, i.e., mixtures of two different polymers (or more), or polymers with nonpolymer additives, have garnered tremendous attention for developing materials with properties only realized by the constituent combinations. In many cases, polymer-based composites have enabled new technologies. One classic example is polymer solar cells for sustainable and portable energy. Recently, B. J. Kim, T-S. Kim, and co-workers developed a mixing strategy to simultaneously enhance the power conversion efficiency and stretchability of polymer solar cells by creating a ternary composite consisting of a polymer donor, small molecule acceptor, and a high molecular weight polymer acceptor (DOI: 10.1021/jacsau.1c00064). The latter component is critically important to the design—it facilitates an entangled network and tie molecules between near domains of acceptors, both important for electron hoping and stress dissipation. The addition of 20% wt of the high molecular weight polymer acceptor resulted in the most robust performance.
Another technological-relevant field in which polymer mixing arises is colloids. One route to forming structured, i.e., Janus or patchy, colloids is via precipitation of polymer mixtures from a solution. Precipitation induces polymer–polymer phase separation and the formation of colloids, in which the structure is controlled by the competition between phase separation and kinetic arrest, e.g., vitrification. A better understanding of this complex process was revealed by combining continuum simulations, free-energy calculations, and experiments to demonstrate how specific pathways from precipitation to solidification influence the final colloid structure (DOI: 10.1021/jacsau.1c00110). In any polymer-based composite material, the presence of an interface on properties is important. In polymer nanocomposites, in which one phase (usually inorganic) has a dimension on the nanometer length scale, i.e., a nanofiller, the interfacial area is enhanced by many orders of magnitude. J. Huang, J. Zhou, and M. Liu broadly define the polymer component near the nanofiller that exhibits properties different from the bulk as the interphase. The concept suggests that a third phase, which may be dynamic, should be considered in the design construct of polymer nanocomposites. The case is made that the interphase may be exploited as an additional lever when fabricating multifunctional nanocomposites—the premise being that the interphase offers independent control of function. The ability to process polymer nanocomposites at scale with control of the interphase is a challenge that must be resolved (DOI: 10.1021/jacsau.1c00430).
Advances in polymer chemistry will underpin better materials that bring us closer to our global sustainability goals. Naturally sourced sugar-based polymers represent one promising alternative. Here, K. L. Wooley and co-workers explored the connection between monomer chemistry and polymer regiochemistry of polycarbonates prepared by organobase-catalyzed ring-opening polymerization (DOI: 10.1021/jacsau.1c00545). In particular, they discovered that polymerization of carbonate glucose monomers with different cyclic acetal protecting groups all produced regioirregular backbones. Transcarbonylation reactions, confirmed by small molecule studies, were responsible for scrambling the monomer connectivity. The findings aid in understanding how monomer groups influence chain structure, and make progress toward a path for designing functional monomers for producing sustainable and desirable polymers. Reducing the energy required to produce high-usage polyolefins is a worthy pursuit. In this regard, opportunities exist to understand better catalyst fragmentation which influences mass transfer, reactor fouling, and downstream polymer powder processing. X-ray computed nanotomography, with a 3D spatial resolution of 74 nm and capability of noninvasive reconstruction of a large number of prepolymerized Ziegler-type catalyst particles, allowed for the classification of three degrees of particle fragmentation: weak, moderate, and strong (DOI: 10.1021/jacsau.1c00130). In particular, F. Meirer, B. M. Weckhuysen, and co-workers showed that the shrinking core breakup model dominated the weakly fragmenting particles while moderate to strongly fragmenting particles were dominated by the continuous bisection breakup model. The method presented should apply to other polyolefin catalysts. Broader use of the technology will enable more significant insights into collective catalyst behavior. In pursuit of new materials, J. F. Hooper and co-workers demonstrated the successful synthesis of polymer–graphene and polymer–protein conjugates via the radial decarboxylation of activated ester to initiate cobalt-mediated radical polymerization (DOI: 10.1021/jacsau.1c00453). The work highlights the critical role of cobalt for not only molecular weight control but also for the facilitation of end-group control. The approach holds promise for the production of high-loading polymer-based composite materials.
As we continue to witness polymer science and soft matter breakthroughs, their use and importance in technology will only continue to grow. We present this virtual issue of JACS Au with the hope that you find insights and inspiration that will spur even more innovation in these amazing fields. At JACS Au, we are delighted to showcase our first selection of articles focusing on polymers and soft matter, representing a snapshot of what is available to all via open access.
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.