Critical advances and future opportunities in upcycling commodity polymers.

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.

Regioselective Palladium-Catalyzed Chain-Growth Allylic Amination Polymerization of Vinyl Aziridines.

Organometallic-mediated chain growth polymerization of readily accessible chemical building blocks is responsible for important commercial and technological advances in polymer science, but the incorporation of heteroatoms into the polymer backbone through these mechanisms remains a challenge. Transition metal π-allyl complexes are well-developed organometallic intermediates for carbon-heteroatom bond formation in small-molecule catalysis yet remain underexplored in polymer science. Here, we developed a regioselective palladium-phosphoramidite-catalyzed chain-growth allylic amination polymerization of vinyl aziridines for the synthesis of novel nitrogen-rich polymers via ambiphilic π-allyl complexes. The polymerization accessed a linear microstructure with four carbons between each nitrogen, which is challenging to achieve through other chain-growth polymerization approaches. The highly regioselective allylic amination polymerization demonstrated the characteristics of a controlled polymerization and was able to achieve molar masses exceeding 20 kg mol-1 with low dispersities (D̵ < 1.3). The identification of the polymer structure and well-defined chain ends were supported by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and chain extension experiments demonstrate opportunities for building more complex materials from this method. A Hammett study was performed to understand the role of the catalyst and monomer structure on regioselectivity, and the data supported a mechanism wherein regioselectivity was primarily controlled by the ligand-metal complex. Postpolymerization desulfonylation provided access to a novel polyamine that demonstrated broad anticancer activity in vitro, which highlights the benefits of unlocking novel polyamine microstructures through regioselective chain-growth allylic amination polymerization.

C-H Functionalization of Polyolefins to Access Reprocessable Polyolefin Thermosets.

Upcycling plastic waste into reprocessable materials with performance-advantaged properties would contribute to the development of a circular plastics economy. Here, we modify branched polyolefins and postconsumer polyethylene through a versatile C-H functionalization approach using thiosulfonates as a privileged radical group transfer functionality. Cross-linking the functionalized polyolefins with polytopic amines provided dynamically cross-linked polyolefin networks enabled by associative bond exchange of diketoenamine functionality. A combination of resonant soft X-ray scattering and grazing incidence X-ray scattering revealed hierarchical phase morphology in which diketoenamine-rich microdomains phase-separate within amorphous regions between polyolefin crystallites. The combination of dynamic covalent cross-links and microphase separation results in useful and improved mechanical properties, including a ∼4.5-fold increase in toughness, a reduction in creep deformation at temperatures relevant to use, and high-temperature structural stability compared to the parent polyolefin. The dynamic nature of diketoenamine cross-links provides stress relaxation at elevated temperatures, which enabled iterative reprocessing of the dynamic covalent polymer network with little cycle-to-cycle property fade. The ability to convert polyolefin waste into a reprocessable thermoformable material with attractive thermomechanical properties provides additional optionality for upcycling to enable future circularity.

Circular Upcycling of Bottlebrush Thermosets.

The inability to re-process thermosets hinders their utility and sustainability. An ideal material should combine closed-loop recycling and upcycling capabilities. This trait is realized in polydimethylsiloxane bottlebrush networks using thermoreversible Diels-Alder cycloadditions to enable both reversible disassembly into a polymer melt and on-demand reconfiguration to an elastomer of either lower or higher stiffness. The crosslink density was tuned by loading the functionalized networks with a controlled fraction of dormant crosslinkers and crosslinker scavengers, such as furan-capped bis-maleimide and anthracene, respectively. The resulting modulus variations precisely followed the stoichiometry of activated furan and maleimide moieties, demonstrating the lack of side reactions during reprocessing. The presented circularity concept is independent from the backbone or side chain chemistry, making it potentially applicable to a wide range of brush-like polymers.

Stereoselective Helix-Sense-Selective Cationic Polymerization of N-Vinylcarbazole Using Chiral Lewis Acid Catalysis.

Helical polymers with a defined main-chain atropoisomeric conformation are important materials in high value applications such as nonlinear optics and chiral separations. Currently, no methods exist for the cationic helix-sense-selective polymerization of prochiral vinyl monomers, which limits access to a number of potentially valuable optically active helical polymers. Here, we demonstrate the first stereoselective cationic helix-sense-selective polymerization of a prochiral vinyl monomer, which provides access to optically active helices of poly(N-vinylcarbazole). Chiral bis(oxazoline)-scandium Lewis acids serve as chiral counterions to polymerize N-vinylcarbazole into highly isotactic (up to 94% meso triads) polymers. Mechanistic investigations uncovered the distinct phenomenon that are responsible for independent control of conformational (i.e., helicity) and configurational (i.e., tacticity) stereochemistry. Polymer helicity was strongly influenced by the stereoselectivity of the first monomer propagation, whereas polymer tacticity was dictated by the thermodynamically controlled conformation of the growing polymer chain end. Overall, this method expands the suite of accessible helical polymers through helix-sense-selective polymerization and provides mechanistic insight into how polymer tacticity and helicity can be controlled independently.

Hydrolytically Stable Ionic Fluorogels for High-Performance Remediation of Per- and Polyfluoroalkyl Substances (PFAS) from Natural Water.

PFAS are known bioaccumulative and persistent chemicals which pollute natural waters globally. There exists a lack of granular sorbents to efficiently remove both legacy and emerging PFAS at environmentally relevant concentrations. Herein, we report a class of polymer networks with a synergistic combination of ionic and fluorous components that serve as granular materials for the removal of anionic PFAS from water. A library of Ionic Fluorogels (IFs) with systematic variation in charge density and polymer network architecture was synthesized from hydrolytically stable fluorous building blocks. The IFs were demonstrated as effective sorbents for the removal of 21 legacy and emerging PFAS from a natural water and were regenerable over multiple cycles of reuse. Comparison of one IF to a commercial ion exchange resin in mini-rapid small-scale column tests demonstrated superior performance for the removal of short-chain PFAS from natural water under operationally relevant conditions.

Machine-Learning-Guided Discovery of 19F MRI Agents Enabled by Automated Copolymer Synthesis.

Modern polymer science suffers from the curse of multidimensionality. The large chemical space imposed by including combinations of monomers into a statistical copolymer overwhelms polymer synthesis and characterization technology and limits the ability to systematically study structure-property relationships. To tackle this challenge in the context of 19F magnetic resonance imaging (MRI) agents, we pursued a computer-guided materials discovery approach that combines synergistic innovations in automated flow synthesis and machine learning (ML) method development. A software-controlled, continuous polymer synthesis platform was developed to enable iterative experimental-computational cycles that resulted in the synthesis of 397 unique copolymer compositions within a six-variable compositional space. The nonintuitive design criteria identified by ML, which were accomplished by exploring <0.9% of the overall compositional space, lead to the identification of >10 copolymer compositions that outperformed state-of-the-art materials.

Brønsted Acid Catalyzed Stereoselective Polymerization of Vinyl Ethers.

Isotactic poly(vinyl ether)s (PVEs) have recently been identified as a new class of semicrystalline thermoplastics with a valuable combination of mechanical and interfacial properties. Currently, methods to synthesize isotactic PVEs are limited to strong Lewis acids that require a high catalyst loading and limit the accessible scope of monomer substrates for polymerization. Here, we demonstrate the first Brønsted acid catalyzed stereoselective polymerization of vinyl ethers. A single-component imidodiphosphorimidate catalyst exhibits a sufficiently low pKa to initiate vinyl ether polymerization and acts as a chiral conjugate base to direct the stereochemistry of monomer addition to the oxocarbenium ion reactive chain end. This Brønsted acid catalyzed stereoselective polymerization enabled an expanded substrate scope compared to previous methods, the use of chain transfer agents to lower catalyst loading, and the capability to recycle the catalyst for multiple polymerizations.

Mechanistic Insight into the Stereoselective Cationic Polymerization of Vinyl Ethers.

The control of the tacticity of synthetic polymers enables the realization of emergent physical properties from readily available starting materials. While stereodefined polymers derived from nonpolar vinyl monomers can be efficiently prepared using early transition metal catalysts, general methods for the stereoselective polymerization of polar vinyl monomers remain underdeveloped. We recently demonstrated asymmetric ion pairing catalysis as an effective approach to achieve stereoselective cationic polymerization of vinyl ethers. Herein, we provide a deeper understanding of stereoselective ion-pairing polymerization through comprehensive experimental and computational studies. These findings demonstrate the importance of ligand deceleration effects for the identification of reaction conditions that enhance stereoselectivity, which was supported by computational studies that identified the solution-state catalyst structure. An evaluation of monomer substrates with systematic variations in steric parameters and functional group identities established key structure-reactivity relationships for stereoselective homo- and copolymerization. Expansion of the monomer scope to include enantioenriched vinyl ethers enabled the preparation of an isotactic poly(vinyl ether) with the highest stereoselectivity (95.1% ± 0.1 meso diads) reported to date, which occurred when monomer and catalyst stereochemistry were fully matched under a triple diastereocontrol model. The more complete understanding of stereoselective cationic polymerization reported herein offers a foundation for the design of improved catalytic systems and for the translation of isotactic poly(vinyl ether)s to applied areas.

Partially Fluorinated Copolymers as Oxygen Sensitive 19 F MRI Agents.

Effective diagnosis of disease and its progression can be aided by 19 F magnetic resonance imaging (MRI) techniques. Specifically, the inherent sensitivity of the spin-lattice relaxation time (T1 ) of 19 F nuclei to oxygen partial pressure makes 19 F MRI an attractive non-invasive approach to quantify tissue oxygenation in a spatiotemporal manner. However, there are only few materials with the adequate sensitivity to be used as oxygen-sensitive 19 F MRI agents at clinically relevant field strengths. Motivated by the limitations in current technologies, we report highly fluorinated monomers that provide a platform approach to realize water-soluble, partially fluorinated copolymers as 19 F MRI agents with the required sensitivity to quantify solution oxygenation at clinically relevant magnetic field strengths. The synthesis of a systematic library of partially fluorinated copolymers enabled a comprehensive evaluation of copolymer structure-property relationships relevant to 19 F MRI. The highest-performing material composition demonstrated a signal-to-noise ratio that corresponded to an apparent 19 F density of 220 mm, which surpasses the threshold of 126 mm 19 F required for visualization on a three Tesla clinical MRI. Furthermore, the T1 of these high performing materials demonstrated a linear relationship with solution oxygenation, with oxygen sensitivity reaching 240×10-5  mmHg-1 s-1 . The relationships between material composition and 19 F MRI performance identified herein suggest general structure-property criteria for the further improvement of modular, water-soluble 19 F MRI agents for quantifying oxygenation in environments relevant to medical imaging.

Chemo- and Regioselective Functionalization of Isotactic Polypropylene: A Mechanistic and Structure-Property Study.

Polyolefins represent a high-volume class of polymers prized for their attractive thermomechanical properties, but the lack of chemical functionality on polyolefins makes them inadequate for many high-performance engineering applications. We report a metal-free postpolymerization modification approach to impart functionality onto branched polyolefins without the deleterious chain-coupling or chain-scission side reactions inherent to previous methods. The identification of conditions for thermally initiated polyolefin C-H functionalization combined with the development of new reagents enabled the addition of xanthates, trithiocarbonates, and dithiocarbamates to a variety of commercially available branched polyolefins. Systematic experimental and kinetic studies led to a mechanistic hypothesis that facilitated the rational design of reagents and reaction conditions for the thermally initiated C-H xanthylation of isotactic polypropylene (iPP) within a twin-screw extruder. A structure-property study showed that the functionalized iPP adheres to polar surfaces twice as strongly as commercial iPP while demonstrating similar tensile properties. The fundamental understanding of the elementary steps in amidyl radical-mediated polyolefin functionalization provided herein reveals key structure-reactivity relationships for the design of improved reagents, while the demonstration of chemoselective and scalable iPP functionalization to realize a material with improved adhesion properties indicates the translational potential of this method.

C-H Functionalization of Commodity Polymers.

Synthetic manipulation of polymer substrates is one of the oldest and most reliable methods to increase the functional diversity of soft materials. Modifying the chemical structure of polymers that are already produced on a commodity scale leverages the current high-volume and low-cost production of commodity plastics for the discovery of modern materials. A myriad of polymer C-H functionalization methods have been developed which enable the modification of material properties on both a laboratory and industrial scale. More recently, driven by advances in C-H activation, photoredox catalysis, and radical chemistry, chemoselective approaches have emerged as a means to impart precise functionality onto commodity polymer substrates. This Review discusses the historical significance of and contemporary advances in the C-H functionalization of commodity polymers. The conceptual approach outlined herein presents exciting new directions for the field, including increasing the value of otherwise pervasive materials, uncovering entirely new material properties, and a viable path to upcycle post-consumer plastic waste.

Scalable synthesis of sequence-defined, unimolecular macromolecules by Flow-IEG.

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.

Regioselective C-H Xanthylation as a Platform for Polyolefin Functionalization.

Polyolefins that contain polar functional groups are important materials for next-generation lightweight engineering thermoplastics. Post-polymerization modification is an ideal method for the incorporation of polar groups into branched polyolefins; however, it typically results in chain scission events, which have deleterious effects on polymer properties. Herein, we report a metal-free method for radical-mediated C-H xanthylation that results in the regioselective functionalization of branched polyolefins without coincident polymer-chain scission. This method enables a tunable degree of polymer functionalization and capitalizes on the versatility of the xanthate functional group to unlock a wide variety of C-H transformations previously inaccessible on branched polyolefins.

Photoswitching using visible light: a new class of organic photochromic molecules.

A versatile new class of organic photochromic molecules that offers an unprecedented combination of physical properties including tunable photoswitching using visible light, excellent fatigue resistance, and large polarity changes is described. These unique features offer significant opportunities in diverse fields ranging from biosensors to targeted delivery systems while also allowing non-experts ready synthetic access to these materials.

Synthetic aptamer-polymer hybrid constructs for programmed drug delivery into specific target cells.

Viruses have evolved specialized mechanisms to efficiently transport nucleic acids and other biomolecules into specific host cells. They achieve this by performing a coordinated series of complex functions, resulting in delivery that is far more efficient than existing synthetic delivery mechanisms. Inspired by these natural systems, we describe a process for synthesizing chemically defined molecular constructs that likewise achieve targeted delivery through a series of coordinated functions. We employ an efficient "click chemistry" technique to synthesize aptamer-polymer hybrids (APHs), coupling cell-targeting aptamers to block copolymers that secure a therapeutic payload in an inactive state. Upon recognizing the targeted cell-surface marker, the APH enters the host cell via endocytosis, at which point the payload is triggered to be released into the cytoplasm. After visualizing this process with coumarin dye, we demonstrate targeted killing of tumor cells with doxorubicin. Importantly, this process can be generalized to yield APHs that specifically target different surface markers.

Design and synthesis of donor-acceptor Stenhouse adducts: a visible light photoswitch derived from furfural.

The development of an easily synthesized, modular, and tunable organic photoswitch that responds to visible light has been a long-standing pursuit. Herein we provide a detailed account of the design and synthesis of a new class of photochromes based on furfural, termed donor-acceptor Stenhouse adducts (DASAs). A wide variety of these derivatives are easily prepared from commercially available starting materials, and their photophysical properties are shown to be dependent on the substituents of the push-pull system. Analysis of the switching behavior provides conditions to access the two structural isomers of the DASAs, reversibly switch between them, and use their unique solubility behavior to provide dynamic phase-transfer materials. Overall, these negative photochromes respond to visible light and heat and display an unprecedented level of structural modularity and tunabilty.

Stereoselective Polymerization of 3,6-Disubstituted N-Vinylcarbazoles.

Poly(N-vinylcarbazole) (PNVC-H) is a valuable nonconjugated photoconductive polymer, but the free radical polymerization conditions typically used for its synthesis do not control polymer stereochemistry and are not tolerant to many substituted N-vinylcarbazoles. Here, we report the stereoselective cationic polymerization of a series of 3,6-disubtituted N-vinylcarbazole derivatives using a chiral scandium-bis(oxazoline) Lewis acid catalyst. The combination of asymmetric ion-pairing catalysis and inherent monomer stereoelectronics facilitated stereoselective polymerization at room temperature, which enabled the polymerization of less soluble 3,6-disubstituted-N-vinylcarbazole derivatives. Isotactic halogen-substituted PNVCs demonstrated self-assembly in solution through halogen-halogen bonding, which was not observed in their atactic counterparts. Initial spectral characterization displayed a wide range of excitation-emission profiles for substituted PNVCs, which demonstrate the promise of these materials as a new class of nonconjugated photoconductive polymers for optoelectronic applications. Overall, these results showcase a diverse class of isotactic poly(N-vinylcarbazoles), highlight the benefits of identifying alternative stereocontrol mechanisms for polymerization, and expand the suite of accessible nonconjugated hole-transport materials.

Continuous Polymer Synthesis and Manufacturing of Polyurethane Elastomers Enabled by Automation.

Connecting polymer synthesis and processing is an important challenge for streamlining the manufacturing of polymeric materials. In this work, the automated synthesis of acrylate-capped polyurethane oligomers is integrated with vat photopolymerization 3D printing. This strategy enabled the rapid manufacturing of a library of polyurethane-based elastomeric materials with differentiated thermal and mechanical properties. The automated semicontinuous batch synthesis approach proved enabling for resins with otherwise short shelf lives because of the intimate connection between synthesis, formulation, and processing. Structure-property studies demonstrated the ability to tune properties through systematic alteration of cross-link density and chemical composition.

External regulation of controlled polymerizations.

Polymer chemists, through advances in controlled polymerization techniques and reliable post-functionalization methods, now have the tools to create materials of almost infinite variety and architecture. Many relevant challenges in materials science, however, require not only functional polymers but also on-demand access to the properties and performance they provide. The power of such temporal and spatial control of polymerization can be found in nature, where the production of proteins, nucleic acids, and polysaccharides helps regulate multicomponent systems and maintain homeostasis. Here we review existing strategies for temporal control of polymerizations through external stimuli including chemical reagents, applied voltage, light, and mechanical force. Recent work illustrates the considerable potential for this emerging field and provides a coherent vision and set of criteria for pursuing future strategies for regulating controlled polymerizations.