The classical division of polymeric materials into thermoplastics and thermosets based on covalent network structure often implies that these categories are distinct and irreconcilable. Yet, the past ...two decades have seen extensive development of materials that bridge this gap through incorporation of dynamic crosslinks, enabling them to behave as both robust networks and moldable plastics. Although their potential utility is significant, the growth of covalent adaptable networks (CANs) has obscured the line between “thermoplastic” and “thermoset” and erected a conceptual barrier to the growing number of new researchers entering this discipline. This Perspective aims to both outline the fundamental theory of CANs and provide a critical assessment of their current status. We emphasize throughout that the unique properties of CANs emerge from the network chemistry, and particularly highlight the role that the crosslink exchange mechanism (i.e., dissociative exchange or associative exchange) plays in the resultant material properties under processing conditions. Predominant focus will be on thermally induced dynamic behavior, as the majority of presently employed exchange chemistries rely on thermal stimulus, and it is simple to apply to bulk materials. Lastly, this Perspective aims to identify current issues and address possible solutions for better fundamental understanding within this field.
The chemical structure and function of biomacromolecules has evolved to fill many essential roles in biological systems. More specifically, proteins, peptides, nucleic acids and polysaccharides serve ...as vital structural components, and mediate chemical transformations and energy/information storage processes required to sustain life. In many cases, the properties and applications of biological macromolecules can be further expanded by attaching synthetic macromolecules. The modification of biomacromolecules by attaching a polymer that changes its properties in response to environmental variations, thus affecting the properties of the biomacromolecule, has led to the emergence of a new family of polymeric biomaterials. Here, we summarize techniques for conjugating responsive polymers to biomacromolecules and highlight applications of these bioconjugates reported so far. In doing so, we aim to show how advances in synthetic tools could lead to rapid expansion in the variety and uses of responsive bioconjugates.
Reversible deactivation radical polymerization (RDRP) has revolutionized modern polymer chemistry over the past two decades, thus laying the groundwork for the synthesis of complex macromolecules and ...enabling the preparation of previously inaccessible materials. Reversible addition-fragmentation chain transfer (RAFT) polymerization has emerged as one of the most promising techniques because of its functional group tolerance, applicability to a wide range of vinyl monomers, and its nondemanding experimental conditions. However, despite the promise and clearly demonstrated utility of RAFT, limitations of the method sometimes still exist, including the occasional need for extended polymerization times, limited access to high molecular weight polymers, low “livingness” due to unavoidable radical termination events, etc. This Perspective focuses on recent advances that have been specifically designed to address many of these perceived limitations to reinforce the promise of RAFT for the synthesis of complex and well-defined polymers under facile conditions.
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The field of stimuli-responsive, or smart polymers, has commanded significant interest over the past decade. However, most examples of stimuli-responsive polymers have relied on ...macroscopic changes that arise via simple alterations in chain conformation or changes in polymer-polymer or polymer-solvent interactions. In recent years, there has been an effort to expand the scope of modifiable variables to include the covalent architecture of a polymer through the use of reversible covalent bonds. Polymers capable of architectural transformation are those that can undergo changes in their chain topology (e.g., linear to branched, star to comb, etc.) via rearrangement in the chain structure. This approach has proven particularly interesting because it allows access to materials capable of dramatic macromolecular property changes that cannot be replicated by the more traditional approaches to responsive polymer systems, which often rely on solubility or swelling transitions in solution. This review aims to highlight the main synthetic strategies to architecture-transformable polymers, including dynamic-covalent and supramolecular chemistry approaches. In addition, the properties and applications of those smart polymers are highlighted. It is clear from recent research in this area that macromolecules capable of undergoing transformations in topology represent a paradigm shift in the field of stimuli-responsive materials.
The translation of small molecule chemistries into efficient methodologies for polymer functionalization spans several decades, enabling critical advances in soft matter materials synthesis with ...tailored and adaptive property profiles. The present Perspective exploresbased on selected examples50 years of innovation in polymer functionalization chemistries. These span a diverse set of chemistries based on activated esters, thiol–ene/yne processes, nucleophilic systems based on isocyanates, reactions driven by the formation of imines and oximes, ring-opening processes, cycloadditions, andin a recent renaissancemulticomponent reactions. In addition, a wide variety of chain types and architectures have been modified based on the above chemistries, often with exquisite chemical control, highlighted by key examples. We conclude our journey through polymer functionalization with thein our viewmost critically required advances that have the potential to move from “science fiction” to “science fact”.
Cross-linked polymers constructed with dynamic-covalent boronic esters were synthesized via photoinitiated radical thiol–ene click chemistry. Because the reversibility of the boronic ester ...cross-links was readily accessible, the resulting materials were capable of undergoing bond exchange to covalently mend after failure. The reversible bonds of the boronic esters were shown to shift their exchange equilibrium at room temperature when exposed to water. Nevertheless, the materials were observed to be stable and hydrophobic and absorbed only minor amounts of water over extended periods of time when submerged in water or exposed to humid environments. The facile reversibility of the networks allowed intrinsic self-healing under ambient conditions. Highly efficient self-healing of these bulk materials was confirmed by mechanical testing, even after subjecting a single site to multiple cut–repair cycles. Several variables were considered for their effect on materials properties and healing, including cross-link density, humidity, and healing time.
Precision synthesis of advanced polymeric materials requires efficient, robust, and facile chemical reactions. Paradoxically, the synthesis of increasingly intricate macromolecular structures ...generally benefits from exploitation of the simplest reactions available. This idea, combined with requirements of high efficiency, orthogonality, and simplified purification procedures, has led to the rapid adoption of “click chemistry” strategies in the field of macromolecular engineering. This Perspective provides context as to why these newly developed or recently reinvigorated reactions have been so readily embraced for the preparation of polymers with advanced macromolecular topologies, increased functionality, and unique properties. By highlighting important examples that rely on click chemistry techniques, including copper(I)-catalyzed and strain-promoted azide−alkyne cycloadditions, Diels−Alder cycloadditions, and thiol−ene reactions, among others, we hope to provide a succinct overview of the current state of the art and future impact these strategies will have on polymer chemistry and macromolecular engineering.
Many recent developments in polymer chemistry have advanced the synthesis of materials in which synthetic polymers are immobilized to biological (macro)molecules to enhance the solubility, stability, ...activity, or therapeutic utility of the biological entity. In particular, the versatility and robust nature of controlled radical polymerization (CRP) has enabled access to a diverse family of new polymer bioconjugates. While nitroxide-mediated, atom transfer radical (ATRP), and reversible addition–fragmentation chain transfer (RAFT) polymerizations have all proven useful for the preparation of well-defined end-functional polymers capable of being efficiently conjugated to biological molecules, ATRP and RAFT have proven especially proficient for the synthesis of conjugates by direct polymerization of vinyl monomers from biological components functionalized to contain a group capable of initiating chain growth. This Viewpoint highlights several recent advances that have relied on grafting-from by CRP, with particular attention devoted to a recent report that seeks to facilitate the process of grafting-from proteins via ATRP under biologically relevant conditions.
Visible light‐mediated direct decarboxylation of carboxylic acids with an acridine photocatalyst is a convenient and powerful method to generate carbon‐centered radicals in polymer chains. ...Advantageously, this process proceeds under mild conditions, without preactivation of the acid groups. We utilize decarboxylation in the presence of a hydrogen atom donor to form statistical acrylate‐ethylene and acrylate‐propylene copolymers, which are challenging to obtain by direct polymerization. We additionally show that decarboxylation of methacrylic acid units within polymethacrylates can trigger degradation of the polymer backbones. Moreover, a dual catalytic approach, which combines the function of an acridine photocatalyst with that of a cobaloxime catalyst, is leveraged to furnish unique copolymers with pendent alkenes. Our work indicates that direct decarboxylation is a versatile technique for the synthesis of functional materials with tailored compositions and properties.
Visible light‐driven direct decarboxylation of carboxylic acid side chains in macromolecules was explored as a robust post‐polymerization modification strategy. Multiple transformations of copolymers were achieved, including derivatization with olefin units, functionalization with pendent alkenes, and backbone degradation. These decarboxylative processes were rapid, selective, and displayed high atom economy.
