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The vast majority of commodity plastics do not degrade and therefore they permanently pollute the environment. At present, less than 20% of post-consumer plastic waste in developed countries is recycled, predominately for energy recovery or repurposing as lower-value materials by mechanical recycling. Chemical recycling offers an opportunity to revert plastics back to monomers for repolymerization to virgin materials without altering the properties of the material or the economic value of the polymer. For plastic waste that is either cost prohibitive or infeasible to mechanically or chemically recycle, the nascent field of chemical upcycling promises to use chemical or engineering approaches to place plastic waste at the beginning of a new value chain. Here state-of-the-art methods are highlighted for upcycling plastic waste into value-added performance materials, fine chemicals and specialty polymers. By identifying common conceptual approaches, we critically discuss how the advantages and challenges of each approach contribute to the goal of realizing a sustainable plastics economy. Methods for the transformation of plastics into materials with value, known as plastic waste upcycling, are outlined, and their advantages and challenges in terms of a sustainable plastics economy are discussed.

We report a semiautomated synthesis of sequence and architecturally defined, unimolecular macromolecules through a marriage of multistep flow synthesis and iterative exponential growth (Flow-IEG). The Flow-IEG system performs three reactions and an in-line purification in a total residence time of under 10 min, effectively doubling the molecular weight of an oligomeric species in an uninterrupted reaction sequence. Further iterations using the Flow-IEG system enable an exponential increase in molecular weight. Incorporating a variety of monomer structures and branching units provides control over polymer sequence and architecture. The synthesis of a uniform macromolecule with a molecular weight of 4,023 g/mol is demonstrated. The user-friendly nature, scalability, and modularity of Flow-IEG provide a general strategy for the automated synthesis of sequence-defined, unimolecular macromolecules. Flow-IEG is thus an enabling tool for theory validation, structure–property studies, and advanced applications in biotechnology and materials science.

Chemists have long sought sequence-controlled synthetic polymers that mimic nature's biopolymers, but a practical synthetic route that enables absolute control over polymer sequence and structure remains a key challenge. Here, we report an iterative exponential growth plus side-chain functionalization (IEG+) strategy that begins with enantiopure epoxides and facilitates the efficient synthesis of a family of uniform >3 kDa macromolecules of varying sequence and stereoconfiguration that are coupled to produce unimolecular polymers (>6 kDa) with sequences and structures that cannot be obtained using traditional polymerization techniques. Selective side-chain deprotection of three hexadecamers is also demonstrated, which imbues each compound with the ability to dissolve in water. We anticipate that these new macromolecules and the general IEG+ strategy will find broad application as a versatile platform for the scalable synthesis of sequence-controlled polymers. Regioselective epoxide opening of an enantiopure epoxy–alkyne results in the stereospecific introduction of functional side-chains into growing macromolecules. This process—in combination with 'click' chemistry and orthogonal deprotection of terminal alkynes—underpins an iterative exponential growth methodology that enables the efficient synthesis of >6-kDa stereo- and sequence-controlled polymers.

The stereochemistry of polymers has a profound impact on their properties. Despite the well-developed stereoselective methods for prochiral vinyl monomers, current methods for racemic monomers are limited. Conventional approaches treat sp 3 chiral centers as immutable, resulting in poor atom-economical processes and limited control over enantioselectivity. This contrasts with stereoconvergent catalysis in small molecules, which has revolutionized synthesis by interrupting the transfer of chiral information from the substrate to the product, providing a clear platform for catalysts to access enantiopure compounds from racemic mixtures in up to 100% yield. Here we designed a catalyst that converges stereochemical information during polymerization, enabling access to asymmetric, isotactic polymers with quantitative atom economy from racemic feedstocks. The mechanism of stereoconvergence is accomplished by the catalyst ablating chiral information, followed by a stereoselective propagation event to control both tacticity and enantioselectivity. Using this method, we accessed both enantiomers of an isotactic polymer from a single enantiomer of monomer and identified a novel stereocomplex. These results represent a conceptual framework to expand stereoconvergent polymerization into additional monomers and mechanisms.

Function matters in materials science, and methodologies that provide paths to multiple functionality in a single step are to be prized. Therefore, we introduce a robust and efficient strategy for exploiting the versatile reactivity of ketenes in polymer chemistry. New monomers for both radical and ring-opening metathesis polymerization have been developed, which take advantage of Meldrum's acid as both a synthetic building block and a thermolytic precursor to dialkyl ketenes. The ketene-functionalized polymers are directly detected by their characteristic infrared absorption and are found to be stable under ambient conditions. The inherent ability of ketenes to provide crosslinking via dimerization and to act as reactive chemical handles via addition, provides simple methodology for application in complex materials challenges. Such versatile characteristics are illustrated by covalently attaching and patterning a dye through microcontact printing. The strategy highlights the significant opportunities afforded by the traditionally neglected ketene functional group in polymer chemistry. The ability to rapidly functionalize polymers is vital for application development. Here, a method for the introduction of masked ketenes into monomers for both ring-opening metathesis and radical-type polymerizations is described. These ketenes — a group previously underexploited in polymer chemistry — allow both crosslinking and post-polymerization functionalization of the polymers.

Finding alternative fates for plastics that would otherwise end up in landfills requires innovative chemistry. Now, poly(acrylic acid) from diaper waste has been converted into valuable pressure-sensitive adhesives through an open-loop recycling method that is cost-effective and environmentally competitive.

A traditionally irreversible reaction that led to very stable chemical bonds could be reversed by pulling the molecule apart in solution with mechanical forces. When synthetic chemists are confronted with sluggish chemical transformations, they typically try to make the reaction go faster by increasing the temperature, pumping in more photons, or turning to specially designed catalysts. In many cases, these solutions also lead to unwanted side products that waste valuable reactants, which has led to a search for simpler strategies for speeding up chemical reactions. On page 1606 of this issue, Brantley et al. ( 1 ) report the selective activation of covalent bonds through mechanical force and observed totally different reactivity compared with thermal or photochemical activation of the same reaction. By pushing, or in this case literally pulling, the reactions down different pathways, they explore novel concepts for synthesizing organic molecules.

To migrate efficiently through tissues, cells must transit through small constrictions within the extracellular matrix. However, in vivo environments are geometrically, mechanically, and chemically complex, and it has been difficult to understand how each of these parameters contribute to the propulsive strategy utilized by cells in these diverse settings. To address this, we employed a sacrificial micromolding approach to generate polymer substrates with tunable stiffness, controlled adhesivity, and user-defined microscale geometries. We combined this together with live-cell imaging and three-dimensional traction force microscopy (TFM) to quantify the forces that cells use to transit through constricting channels. Surprisingly, we observe that cells migrating through compliant constrictions take longer to transit and experience greater nuclear deformation than those migrating through more rigid constrictions. TFM reveals that this deformation is generated by inwardly directed contractile forces that decrease the size of the opening and pull the walls closed around the nucleus. These findings show that nuclear deformation during confined migration can be accomplished by internal cytoskeletal machinery rather than by reactive forces from the substrate, and our approach provides a mechanism to test between different models for how cells translocate their nucleus through narrow constrictions. The methods, analysis, and results presented here will be useful to understand how cells choose between propulsive strategies in different physical environments. Cell migration is critical for both physiological events like wound healing and pathological events like metastasis. Understanding how cells move through complex environments will assist efforts to enhance or inhibit such processes. We developed a method to quantify the forces that cells use to move through multidimensional environments, including through narrow constrictions like those in tissues. Surprisingly, we find that cells transiting through soft constrictions take longer and deform more than those transiting through rigid constrictions, and we connect this finding to inwardly directed contractile forces generated by migrating cells. Together, this work reveals a key role for substrate rigidity to regulate cell transit through confining geometries and provides a quantitative platform to investigate similar processes in other settings.

This work details a method to make efficacious field-effect transistors from monolayers of polycyclic aromatic hydrocarbons that are able to sense and respond to their chemical environment. The molecules used in this study are functionalized so that they assemble laterally into columns and attach themselves to the silicon oxide surface of a silicon wafer. To measure the electrical properties of these monolayers, we use ultrasmall point contacts that are separated by only a few nanometers as the source and drain electrodes. These contacts are formed through an oxidative cutting of an individual metallic single-walled carbon nanotube that is held between macroscopic metal leads. The molecules assemble in the gap and form transistors with large current modulation and high gate efficiency. Because these devices are formed from an individual stack of molecules, their electrical properties change significantly when exposed to electron-deficient molecules such as tetracyanoquinodimethane (TCNQ), forming the basis for new types of environmental and molecular sensors.