Chromatographic Separation: A Versatile Strategy to Prepare Discrete and Well-Defined Polymer Libraries.

ConspectusThe preparation of discrete and well-defined polymers is an emerging strategy for emulating the remarkable precision achieved by macromolecular synthesis in nature. Although modern controlled polymerization techniques have unlocked access to a cornucopia of materials spanning a broad range of monomers, molecular weights, and architectures, the word "controlled" is not to be confused with "perfect". Indeed, even the highest-fidelity polymerization techniques─yielding molar mass dispersities in the vicinity of D = 1.05─unavoidably create a considerable degree of structural and/or compositional dispersity due to the statistical nature of chain growth. Such dispersity impacts many of the properties that researchers seek to control in the design of soft materials.The development of strategies to minimize or entirely eliminate dispersity and access molecularly precise polymers therefore remains a key contemporary challenge. While significant advances have been made in the realm of iterative synthetic methods that construct oligomers with an exact molecular weight, head-to-tail connectivity, and even stereochemistry via small-molecule organic chemistry, as the word "iterative" suggests, these techniques involve manually propagating monomers one reaction at a time, often with intervening protection and deprotection steps. As a result, these strategies are time-consuming, difficult to scale, and remain limited to lower molecular weights. The focus of this Account is on an alternative strategy that is more accessible to the general scientific community because of its simplicity, versatility, and affordability: chromatography. Researchers unfamiliar with the intricacies of synthesis may recall being exposed to chromatography in an undergraduate chemistry lab. This operationally simple, yet remarkably powerful, technique is most commonly encountered in the purification of small molecules through their selective (differential) adsorption to a column packed with a low-cost stationary phase, usually silica. Because the requisite equipment is readily available and the actual separation takes little time (on the order of 1 h), chromatography is used extensively in small-molecule chemistry throughout industry and academia alike. It is, therefore, perhaps surprising that similar types of chromatography are not more widely leveraged in the field of polymer science as well.Here, we discuss recent advances in using chromatography to control the structure and properties of polymeric materials. Emphasis is placed on the utility of an adsorption-based mechanism that separates polymers based on polarity and composition at tractable (gram) scales for materials science, in contrast to size exclusion, which is extremely common but typically analyzes very small quantities of a sample (∼1 mg) and is limited to separating by molar mass. Key concepts that are highlighted include (1) the separation of low-molecular-weight homopolymers into discrete oligomers (D = 1.0) with precise chain lengths and (2) the efficient fractionation of block copolymers into high-quality and widely varied libraries for accelerating materials discovery. In summary, the authors hope to convey the exciting possibilities in polymer science afforded by chromatography as a scalable, versatile, and even automated technique that unlocks new avenues of exploration into well-defined materials for a diverse assortment of researchers with different training and expertise.

Heterotelechelic Silicones: Facile Synthesis and Functionalization Using Silane-Based Initiators.

The synthetic utility of heterotelechelic polydimethylsiloxane (PDMS) derivatives is limited due to challenges in preparing materials with high chain-end fidelity. In this study, anionic ring-opening polymerization (AROP) of hexamethylcyclotrisiloxane (D3) monomers using a specifically designed silyl hydride (Si-H)-based initiator provides a versatile approach toward a library of heterotelechelic PDMS polymers. A novel initiator, where the Si-H terminal group is connected to a C atom (H-Si-C) and not an O atom (H-Si-O) as in traditional systems, suppresses intermolecular transfer of the Si-H group, leading to heterotelechelic PDMS derivatives with a high degree of control over chain ends. In situ termination of the D3 propagating chain end with commercially available chlorosilanes (alkyl chlorides, methacrylates, and norbornenes) yields an array of chain-end-functionalized PDMS derivatives. This diversity can be further increased by hydrosilylation with functionalized alkenes (alcohols, esters, and epoxides) to generate a library of heterotelechelic PDMS polymers. Due to the living nature of ring-opening polymerization and efficient initiation, narrow-dispersity (D < 1.2) polymers spanning a wide range of molar masses (2-11 kg mol-1) were synthesized. With facile access to α-Si-H and ω-norbornene functionalized PDMS macromonomers (H-PDMS-Nb), the synthesis of well-defined supersoft (G' = 30 kPa) PDMS bottlebrush networks, which are difficult to prepare using established strategies, was demonstrated.

Controlled-Radical Polymerization of α-Lipoic Acid: A General Route to Degradable Vinyl Copolymers.

Here, we present the synthesis and characterization of statistical and block copolymers containing α-lipoic acid (LA) using reversible addition-fragmentation chain-transfer (RAFT) polymerization. LA, a readily available nutritional supplement, undergoes efficient radical ring-opening copolymerization with vinyl monomers in a controlled manner with predictable molecular weights and low molar-mass dispersities. Because lipoic acid diads present in the resulting copolymers include disulfide bonds, these materials efficiently and rapidly degrade when exposed to mild reducing agents such as tris(2-carboxyethyl)phosphine (Mn = 56 → 3.6 kg mol-1). This scalable and versatile polymerization method affords a facile way to synthesize degradable polymers with controlled architectures, molecular weights, and molar-mass dispersities from α-lipoic acid, a commercially available and renewable monomer.

Improved Elastic Recovery from ABC Triblock Terpolymers.

The promise of ABC triblock terpolymers for improving the mechanical properties of thermoplastic elastomers is demonstrated by comparison with symmetric ABA/CBC analogs having similar molecular weights and volume fraction of B and A/C domains. The ABC architecture enhances elasticity (up to 98% recovery over 10 cycles) in part through essentially full chain bridging between discrete hard domains leading to the minimization of mechanically unproductive loops. In addition, the unique phase space of ABC triblocks also enables the fraction of hard-block domains to be higher (fhard ≈ 0.4) while maintaining elasticity, which is traditionally only possible with non-linear architectures or highly asymmetric ABA triblock copolymers. These advantages of ABC triblock terpolymers provide a tunable platform to create materials with practical applications while improving our fundamental understanding of chain conformation and structure-property relationships in block copolymers.

Tailoring Writability and Performance of Star Block Copolypeptides Hydrogels through Side-Chain Design.

Shear-recoverable hydrogels based on block copolypeptides with rapid self-recovery hold potential in extrudable and injectable 3D-printing applications. In this work, a series of 3-arm star-shaped block copolypeptides composed of an inner hydrophilic poly(l-glutamate) domain and an outer ß-sheet forming domain is synthesized with varying side chains and block lengths. By changing the ß-sheet forming domains, hydrogels with diverse microstructures and mechanical properties are prepared and structure-function relationships are determined using scattering and rheological techniques. Differences in the properties of these materials are amplified during direct-ink writing with a strong correlation observed between printability and material chemistry. Significantly, it is observed that non-canonical ß-sheet blocks based on phenyl glycine form more stable networks with superior mechanical properties and writability compared to widely used natural amino acid counterparts. The versatile design available through block copolypeptide materials provides a robust platform to access tunable material properties based solely on molecular design. These systems can be exploited in extrusion-based applications such as 3D-printing without the need for additives.

Building Tunable Degradation into High-Performance Poly(acrylate) Pressure-Sensitive Adhesives.

Pressure-sensitive adhesives (PSAs) based on poly(acrylate) chemistry are common in a wide variety of applications, but the absence of backbone degradability causes issues with recycling and sustainability. Here, we report a strategy to create degradable poly(acrylate) PSAs using simple, scalable, and functional 1,2-dithiolanes as drop-in replacements for traditional acrylate comonomers. Our key building block is α-lipoic acid, a natural, biocompatible, and commercially available antioxidant found in various consumer supplements. α-Lipoic acid and its derivative ethyl lipoate efficiently copolymerize with n-butyl acrylate under conventional free-radical conditions leading to high-molecular-weight copolymers (Mn > 100 kg mol-1) containing a tunable concentration of degradable disulfide bonds along the backbone. The thermal and viscoelastic properties of these materials are practically indistinguishable from nondegradable poly(acrylate) analogues, but a significant reduction in molecular weight is realized upon exposure to reducing agents such as tris (2-carboxyethyl) phosphine (e.g., Mn = 198 kg mol-1 → 2.6 kg mol-1). By virtue of the thiol chain ends produced after disulfide cleavage, degraded oligomers can be further cycled between high and low molecular weights through oxidative repolymerization and reductive degradation. Transforming otherwise persistent poly(acrylates) into recyclable materials using simple and versatile chemistry could play a pivotal role in improving the sustainability of contemporary adhesives.

Triboelectric Nanogenerators: Enhancing Performance by Increasing the Charge-Generating Layer Compressibility.

Triboelectric nanogenerators (TENGs) have received significant attention for next-generation wearable electronics due to their simple device structure and low cost. Although the performance of TENGs is intimately tied to compressibility effects in the charge-generating layer, achieving high compressibility with conventional elastomers is challenging because molecular entanglements place a lower bound on the softness of cross-linked networks. Here, we demonstrate that bottlebrush elastomers are efficient charge-generating layers that improve the output performance of TENGs, including voltage, current, and surface potential, by minimizing entanglements and decreasing the compressive modulus (E). For example, a cross-linked bottlebrush with poly(dimethylsiloxane) side chains yielded TENGs with an output voltage (120 V) more than two times larger than a linear PDMS network (55 V). In conclusion, this study highlights the advantage of designing new charge-generating layers with improved compressibility to enhance TENG performance.

Superlubricity of pH-responsive hydrogels in extreme environments.

Poly(acrylamide-co-acrylic acid) (P(AAm-co-AA)) hydrogels are highly tunable and pH-responsive materials frequently used in biomedical applications. The swelling behavior and mechanical properties of these gels have been extensively characterized and are thought to be controlled by the protonation state of the acrylic acid (AA) through the regulation of solution pH. However, their tribological properties have been underexplored. Here, we hypothesized that electrostatics and the protonation state of AA would drive the tribological properties of these polyelectrolyte gels. P(AAm-co-AA) hydrogels were prepared with constant acrylamide (AAm) concentration (33 wt%) and varying AA concentration to control the amount of ionizable groups in the gel. The monomer:crosslinker molar ratio (200:1) was kept constant. Hydrogel swelling, stiffness, and friction behavior were studied by systematically varying the acrylic acid (AA) concentration from 0-12 wt% and controlling solution pH (0.35, 7, 13.8) and ionic strength (I = 0 or 0.25 M). The stiffness and friction coefficient of bulk hydrogels were evaluated using a microtribometer and borosilicate glass probes as countersurfaces. The swelling behavior and elastic modulus of these polyelectrolyte hydrogels were highly sensitive to solution pH and poorly predicted the friction coefficient (µ), which decreased with increasing AA concentration. P(AAm-co-AA) hydrogels with the greatest AA concentrations (12 wt%) exhibited superlubricity (µ = 0.005 ± 0.001) when swollen in unbuffered, deionized water (pH = 7, I = 0 M) and 0.5 M NaOH (pH = 13.8, I = 0.25 M) (µ = 0.005 ± 0.002). Friction coefficients generally decreased with increasing AA and increasing solution pH. We postulate that tunable lubricity in P(AAm-co-AA) gels arises from changes in the protonation state of acrylic acid and electrostatic interactions between the probe and hydrogel surface.

Carbon Nanotube Composites with Bottlebrush Elastomers for Compliant Electrodes.

Wearable electronics and biointerfacing technology require materials that are both compliant and conductive. The typical design strategy exploits polymer composites containing conductive particles, but the addition of a hard filler generally leads to a substantial increase in modulus that is not well-matched to biological tissue. Here, we report a new class of supersoft, conductive composites comprising carbon nanotubes (CNT) embedded in bottlebrush polymer networks. By virtue of the bottlebrush polymer architecture, these materials are several orders of magnitude softer than comparable composites in the literature involving linear polymer networks. For example, a CNT content of 0.25 wt % yields a shear modulus of 66 kPa while maintaining a typical conductivity for a CNT composite (ca. 10-2 S/m). An added benefit of this bottlebrush matrix chemistry is the presence of dynamic polyester bonds that facilitate thermal (re)processing. This unique strategy of designing soft composites provides new opportunities to tailor the structure and properties of sustainable advanced materials.

Engineering Li/Na selectivity in 12-Crown-4-functionalized polymer membranes.

Lithium is widely used in contemporary energy applications, but its isolation from natural reserves is plagued by time-consuming and costly processes. While polymer membranes could, in principle, circumvent these challenges by efficiently extracting lithium from aqueous solutions, they usually exhibit poor ion-specific selectivity. Toward this end, we have incorporated host-guest interactions into a tunable polynorbornene network by copolymerizing 1) 12-crown-4 ligands to impart ion selectivity, 2) poly(ethylene oxide) side chains to control water content, and 3) a crosslinker to form robust solids at room temperature. Single salt transport measurements indicate these materials exhibit unprecedented reverse permeability selectivity (∼2.3) for LiCl over NaCl-the highest documented to date for a dense, water-swollen polymer. As demonstrated by molecular dynamics simulations, this behavior originates from the ability of 12-crown-4 to bind Na+ ions more strongly than Li+ in an aqueous environment, which reduces Na+ mobility (relative to Li+) and offsets the increase in Na+ solubility due to binding with crown ethers. Under mixed salt conditions, 12-crown-4 functionalized membranes showed identical solubility selectivity relative to single salt conditions; however, the permeability and diffusivity selectivity of LiCl over NaCl decreased, presumably due to flux coupling. These results reveal insights for designing advanced membranes with solute-specific selectivity by utilizing host-guest interactions.

Emergence of Hexagonally Close-Packed Spheres in Linear Block Copolymer Melts.

The hexagonally close-packed (HCP) sphere phase is predicted to be stable across a narrow region of linear block copolymer phase space, but the small free energy difference separating it from face-centered cubic spheres usually results in phase coexistence. Here, we report the discovery of pure HCP spheres in linear block copolymer melts with A = poly(2,2,2-trifluoroethyl acrylate) ("F") and B = poly(2-dodecyl acrylate) ("2D") or poly(4-dodecyl acrylate) ("4D"). In 4DF diblocks and F4DF triblocks, the HCP phase emerges across a substantial range of A-block volume fractions (circa fA = 0.25-0.30), and in F4DF, it forms reversibly when subjected to various processing conditions which suggests an equilibrium state. The time scale associated with forming pure HCP upon quenching from a disordered liquid is intermediate to the ordering kinetics of the Frank-Kasper σ and A15 phases. However, unlike σ and A15, HCP nucleates directly from a supercooled liquid or soft solid without proceeding through an intermediate quasicrystal. Self-consistent field theory calculations indicate the stability of HCP is intimately tied to small amounts of molar mass dispersity (D); for example, an HCP-forming F4DF sample with fA = 0.27 has an experimentally measured D = 1.04. These insights challenge the conventional wisdom that pure HCP is difficult to access in linear block copolymer melts without the use of blending or other complex processing techniques.

Epstein-Barr-virus-positive large B-cell lymphoma associated with breast implants: an analysis of eight patients suggesting a possible pathogenetic relationship.

Breast implant anaplastic large cell lymphoma (ALCL) is a T-cell neoplasm arising around textured breast implants that was recognized recently as a distinct entity by the World Health Organization. Rarely, other types of lymphoma have been reported in patients with breast implants, raising the possibility of a pathogenetic relationship between breast implants and other types of lymphoma. We report eight cases of Epstein-Barr virus (EBV)-positive large B-cell lymphoma associated with breast implants. One of these cases was invasive, and the other seven neoplasms were noninvasive and showed morphologic overlap with breast implant ALCL. All eight cases expressed B-cell markers, had a non-germinal center B-cell immunophenotype, and were EBV+ with a latency type III pattern of infection. We compared the noninvasive EBV+ large B-cell lymphoma cases with a cohort of breast implant ALCL cases matched for clinical and pathologic stage. The EBV+ large B-cell lymphoma cases more frequently showed a thicker capsule, and more often were associated with calcification and prominent lymphoid aggregates outside of the capsule. The EBV+ B-cell lymphoma cells were more often arranged within necrotic fibrinoid material in a layered pattern. We believe that this case series highlights many morphologic similarities between EBV+ large B-cell lymphoma and breast implant ALCL. The data presented suggest a pathogenetic role for breast implants (as well as EBV) in the pathogenesis of EBV+ large B-cell lymphoma. We also provide some histologic findings useful for distinguishing EBV+ large B-cell lymphoma from breast implant ALCL in this clinical setting.

Flow-Induced Concentration Nonuniformity and Shear Banding in Entangled Polymer Solutions.

Recent models have predicted entangled polymer solutions could shear band due to unstable flow-induced demixing. This work provides the first experimental probe of the in situ concentration profile of entangled polymer solutions under shear. At shear rates above a critical value, we show that the concentration and velocity profiles can develop bands, in quantitative agreement with steady-state model predictions. These findings highlight the critical importance of flow-concentration coupling in entangled polymer solutions.

Light-Mediated Synthesis and Reprocessing of Dynamic Bottlebrush Elastomers under Ambient Conditions.

We introduce a novel grafting-through polymerization strategy to synthesize dynamic bottlebrush polymers and elastomers in one step using light to construct a disulfide-containing backbone. The key starting material-α-lipoic acid (LA)-is commercially available, inexpensive, and biocompatible. When installed on the chain end(s) of poly(dimethylsiloxane) (PDMS), the cyclic disulfide unit derived from LA polymerizes under ultraviolet (UV) light in ambient conditions. Significantly, no additives such as initiator, solvent, or catalyst are required for efficient gelation. Formulations that include bis-LA-functionalized cross-linker yield bottlebrush elastomers with high gel fractions (83-98%) and tunable, supersoft shear moduli in the ∼20-200 kPa range. An added advantage of these materials is the dynamic disulfide bonds along each bottlebrush backbone, which allow for light-mediated self-healing and on-demand chemical degradation. These results highlight the potential of simple and scalable synthetic routes to generate unique bottlebrush polymers and elastomers based on PDMS.

Light-Switchable and Self-Healable Polymer Electrolytes Based on Dynamic Diarylethene and Metal-Ion Coordination.

Self-healing polymer electrolytes are reported with light-switchable conductivity based on dynamic N-donor ligand-containing diarylethene (DAE) and multivalent Ni2+ metal-ion coordination. Specifically, a polystyrene polymer grafted with poly(ethylene glycol-r-DAE)acrylate copolymer side chains was effectively cross-linked with nickel(II) bis(trifluoromethanesulfonimide) (Ni(TFSI)2) salts to form a dynamic network capable of self-healing with fast exchange kinetics under mild conditions. Furthermore, as a photoswitching compound, the DAE undergoes a reversible structural and electronic rearrangement that changes the binding strength of the DAE-Ni2+ complex under irradiation. This can be observed in the DAE-containing polymer electrolyte where irradiation with UV light triggers an increase in the resistance of solid films, which can be recovered with subsequent visible light irradiation. The increase in resistance under UV light irradiation indicates a decrease in ion mobility after photoswitching, which is consistent with the stronger binding strength of ring-closed DAE isomers with Ni2+. 1H-15N heteronuclear multiple-bond correlation nuclear magnetic resonance (HMBC NMR) spectroscopy, continuous wave electron paramagnetic resonance (cw EPR) spectroscopy, and density functional theory (DFT) calculations confirm the increase in binding strength between ring-closed DAE with metals. Rheological and in situ ion conductivity measurements show that these polymer electrolytes efficiently heal to recover their mechanical properties and ion conductivity after damage, illustrating potential applications in smart electronics.

Origins of Lithium/Sodium Reverse Permeability Selectivity in 12-Crown-4-Functionalized Polymer Membranes.

Direct lithium extraction via membrane separations has been fundamentally limited by lack of monovalent ion selectivity exhibited by conventional polymeric membranes, particularly between sodium and lithium ions. Recently, a 12-Crown-4-functionalized polynorbornene membrane was shown to have the largest lithium/sodium permeability selectivity observed in a fully aqueous system to date. Using atomistic molecular dynamics simulations, we reveal that this selectivity is due to strong interactions between sodium ions and 12-Crown-4 moieties, which reduce sodium ion diffusivity while leaving lithium ion mobility relatively unaffected. Moreover, the ion diffusivities in the membrane, when scaled by their respective solution diffusivities and free ion fractions, can be collapsed to an almost universal relationship depending on solvent volume fraction.

Chemical and Mechanical Tunability of 3D-Printed Dynamic Covalent Networks Based on Boronate Esters.

As the scope of additive manufacturing broadens, interest has developed in 3D-printed objects that are derived from recyclable resins with chemical and mechanical tunability. Dynamic covalent bonds have the potential to not only increase the sustainability of 3D-printed objects, but also serve as reactive sites for postprinting derivatization. In this study, we use boronate esters as a key building block for the development of catalyst-free, 3D-printing resins with the ability to undergo room-temperature exchange at the cross-linking sites. The orthogonality of boronate esters is exploited in fast-curing, oxygen-tolerant thiol-ene resins in which the dynamic character of 3D-printed objects can be modulated by the addition of a static, covalent cross-linker with no room-temperature bond exchange. This allows the mechanical properties of printed parts to be varied between those of a traditional thermoset and a vitrimer. Objects printed with a hybrid dynamic/static resin exhibit a balance of structural stability (residual stress = 18%) and rapid exchange (characteristic relaxation time = 7 s), allowing for interfacial welding and postprinting functionalization. Modulation of the cross-linking density postprinting is enabled by selective hydrolysis of the boronate esters to generate networks with swelling capacities tunable from 1.3 to 3.3.

Room temperature 3D printing of super-soft and solvent-free elastomers.

Super-soft elastomers derived from bottlebrush polymers show promise as advanced materials for biomimetic tissue and device applications, but current processing strategies are restricted to simple molding. Here, we introduce a design concept that enables the three-dimensional (3D) printing of super-soft and solvent-free bottlebrush elastomers at room temperature. The key advance is a class of inks comprising statistical bottlebrush polymers that self-assemble into well-ordered body-centered cubic sphere phases. These soft solids undergo sharp and reversible yielding at 20°C in response to shear with a yield stress that can be tuned by manipulating the length scale of microphase separation. The addition of a soluble photocrosslinker allows complete ultraviolet curing after extrusion to form super-soft elastomers with near-perfect recoverable elasticity well beyond the yield strain. These structure-property design rules create exciting opportunities to tailor the performance of 3D-printed elastomers in ways that are not possible with current materials and processes.

Efficient Synthesis of Asymmetric Miktoarm Star Polymers.

Asymmetric miktoarm star polymers comprising an unequal number of chemically-distinct blocks connected at a common junction produce unique material properties, yet existing synthetic strategies are beleaguered by complicated reaction schemes that are restricted in both monomer scope and yield. Here, we introduce a new synthetic approach coined "µSTAR" - Miktoarm Synthesis by Termination After Ring-opening metathesis polymerization - that circumvents these traditional synthetic limitations by constructing the block-block junction in a scalable, one-pot process involving (1) grafting-through polymerization of a macromonomer followed by (2) in-situ enyne-mediated termination to install a single mikto-arm with exceptional efficiency. This modular µSTAR platform cleanly generates AB n and A(BA') n miktoarm star polymers with unprecedented versatility in the selection of A and B chemistries as demonstrated using many common polymer building blocks: poly(siloxane), poly(acrylate), poly(methacrylate), poly(ether), poly(ester), and poly(styrene). The average number of B or BA' arms (n) is easily controlled by the molar equivalents of macromonomer relative to Grubbs catalyst in the initial ring-opening metathesis polymerization step. While these materials are characterized by dispersity in n that arises from polymerization statistics, they self-assemble into mesophases that are identical to those predicted for precise miktoarm stars as evidenced by small-angle X-ray scattering experiments and self-consistent field theory simulations. In summary, the µSTAR technique provides a significant boost in design flexibility and synthetic simplicity while retaining the salient phase behavior of precise miktoarm star materials.