Shen, H.; Larsen, M. B.; Roessler, A.; Zimmerman, P.; Boydston, A. J. “Mechanochemical Release of N‐heterocyclic Carbenes from Flex‐Activated Mechanophores.” Angew. Chem. Int. Ed. 2021, Accepted. (DOI: 10.1002/anie.202100576).

We have discovered a new flex‐activated mechanophore that releases an N ‐heterocyclic carbene (NHC) under mechanical load. The mechanophore design is based upon NHC‐carbodiimide (NHC‐CDI) adducts, and demonstrates an important first‐step toward functional flex‐activated designs. Since the flex‐activation is non‐destructive to the main polymer chains, the material can be subjected to multiple compression cycles to achieve iterative increases in the activation percentage of mechanophores. Two different NHC structures were demonstrated, signifying the potential modularity of the mechanophore design.

Basu, A.; Wong, J.; Cao, B.; Boechler, N.; Boydston, A. J.; Nelson, A. “Mechanoactivation of Color and Autonomous Shape Change in 3D-Printed Ionic Polymer Networks.” ACS Appl. Mater. Interfaces, 2021, Accepted. (DOI: 10.1021/acsami.1c01166).

Stimuli-responsive materials can enhance the field of three-dimensional (3D) printing by generating objects that change shape in response to external cues. While temperature and pH are common inputs for initiating a response in a 3D-printed object, there are few examples of using a mechanical input to afford a response. Herein, we report a suite of mechanochromic ionic liquid gel inks that can be used to fabricate 3D-printed objects that use a single mechanoactivation event to elicit both a mechanochromic response and an autonomous shape change. Direct-ink write 3D printing was used to deposit ionic liquid gel inks to create multimaterial objects that underwent a predetermined mechanoactivated shape change (mechanomorphic) when the sample was pulled and then released. When spiropyran was incorporated into the inks, the onset of spiropyran isomerization into its purple merocyanine form occurred at strains dependent upon the particular ion gel ink formulation. We suggest that the color onset could be used as a simple indicator for when the strain required to achieve a predetermined change in shape has been reached, potentially serving as real-time visual cue for the user. Such morphing 3D-printed structures with integrated instructions have potential application to areas including stowable structures as well as multiresponsive autonomous components and sensors.

Yang, X.; Murphy, L. M.; Haque, F. M.; Grayson, S. M.; Boydston, A. J. “A highly efficient metal-free protocol for the synthesis of linear polydicyclopentadiene.” Polym. Chem., 2021, Advance Article. (DOI: 10.1039/D1PY00191D).

We have achieved breakthroughs for the efficient synthesis of linear polydicyclopentadiene (pDCPD) via photoredox mediated metal-free ring-opening metathesis polymerization (MF-ROMP). Molecular weight control from 1 to 16 kDa can be targeted in a straightforward manner. Assisted by this method, the TgMn dependence of linear pDCPD is investigated and reported for the first time.

Yang, X.; Gitter, S. R.; Roessler, A. G.; Zimmerman, P. M.; Boydston, A. J. “An Ion‐Pairing Approach to Stereoselective Metal‐Free Ring‐Opening Metathesis Polymerization.” Angew. Chem. Int. Ed. 2021, Accepted. (DOI: 10.1002/anie.202016393).

Stereochemistry can have profound impact on polymer and materials properties. Unfortunately, straightforward methods for realizing high levels of stereocontrolled polymerizations are often challenging to achieve. In a departure from traditional metal-mediated ring-opening metathesis polymerization (ROMP), we discovered a remarkably simple method for controlling alkene stereochemistry in photoredox mediated metal-free ROMP. Ion-pairing, initiator sterics, and solvation effects each had profound impact on the stereochemistry of polynorbornene (PNB). Simple modifications to the reaction conditions produced PNB with trans alkene content of 25 to >98%. High cis content was obtained from relatively larger counterions, toluene as solvent, low temperatures (-78 °C), and initiators with low Charton values. Conversely, smaller counterions, dichloromethane as solvent, and enol ethers with higher Charton values enabled production of PNB with high trans content. Data from a combined experimental and computational investigation are consistent with the stereocontrolling step of the radical cationic mechanism proceeding under thermodynamic control.

Schwartz, J. J.; Hamel, J.; Ekstrom, T.; Ndagang, L.; Boydston, A. J. “Not all PLA filaments are created equal: an experimental investigation.” Rapid Prototyping Journal, 2020, 26, 1263-1276. (DOI: 10.1108/RPJ-06-2019-0179).

Additive manufacturing (AM) methods such as material extrusion (ME) are becoming widely used by engineers, designers and hobbyists alike for a wide variety of applications. Successfully manufacturing objects using ME three-dimensional printers can often require numerous iterations to attain predictable performance because the exact mechanical behavior of parts fabricated via additive processes are difficult to predict. One of that factors that contributes to this difficulty is the wide variety of ME feed stock materials currently available in the marketplace. These build materials are often sold based on their base polymer material such as acrylonitrile butadiene styrene or polylactic acid (PLA), but are produced by numerous different commercial suppliers in a wide variety of colors using typically undisclosed additive feed stocks and base polymer formulations. This paper aims to present the results from an experimental study concerned with quantifying how these sources of polymer variability can affect the mechanical behavior of three-dimensional printed objects. Specifically, the set of experiments conducted in this study focused on following: several different colors of PLA filament from a single commercial supplier to explore the effect of color additives and three filaments of the same color but produced by three different suppliers to account for potential variations in polymer formulation.

Doerr, A. M.; Burroughsm J. M.; Gitter, S. R.; Yang, X.; Boydston, A. J.*; Long, B. K.*  “Advances in Polymerizations Modulated by External Stimuli.” ACS Catal. 2020, 10, 14457–14515. (DOI: 10.1021/acscatal.0c03802). (*Corresponding authors)

Synthetic polymer chemistry endeavors to imitate the spatial and temporal control exhibited within biological systems to obtain well-defined polymeric materials with unique structures, properties, and applications. This is often approached through the development of dynamic catalyst (or initiator) systems that use external stimuli to elicit discrete, site-specific transformations that impact the polymerization. Herein we highlight developments in polymerizations that are modulated by external stimuli, with particular focus on those systems that enable notable changes in kinetics, monomer selectivity, polymer architecture, or tacticity. Examples of external stimuli include chemical oxidants or reductants, light, applied voltage, and mechanical force.

Ghanem, M. A.; Basu, A.; Behrou, R.; Boechler, N.*; Boydston, A. J.*; Craig, S. L.*; Lin, Y.; Lynde, B. E.; Nelson, A.*; Shen, H.; Storti, D. W.*  “The role of polymer mechanochemistry in responsive materials and additive manufacturing.” Nat. Rev. Mater., 2020. (DOI: 10.1038/s41578-020-00249-w). (*Corresponding authors)

The use of mechanical forces to chemically transform polymers dates back decades. In recent years, the use of mechanochemistry to direct constructive transformations in polymers has resulted in a range of engineered molecular responses that span optical, mechanical, electronic and thermal properties. The chemistry that has been developed is now well positioned for use in materials science, polymer physics, mechanics and additive manufacturing. Here, we review the historical backdrop of polymer mechanochemistry, give an overview of the existing toolbox of mechanophores and associated theoretical methods, and speculate as to emerging opportunities in materials science for which current capabilities are seemingly well suited. Non-linear mechanical responses and internal, amplifying stimulus–response feedback loops, including those enabled by, or coupled to, microstructured metamaterial architectures, are seen as particularly promising.

Lu, P.; Kensy, V. K.; Tritt, R. L.; Seidenkranz, D. T.; Boydston, A. J.* ” Metal-Free Ring-Opening Metathesis Polymerization: From Concept to Creation” Acc. Chem. Res., 2020, 53, 10, 2325–2335. (DOI: 10.1021/acs.accounts.0c00427). (*Corresponding author)

Ring-opening metathesis polymerization (ROMP), which is derived from transition-metal-based olefin metathesis, has evolved into one of the most prevalent technologies for making functional polymeric materials in academia and in industry. The initial discovery of and advances in ROMP used ill-defined mixtures of metal salts to initiate polymerization. The initiators most commonly used today, developed with tremendous efforts, are well-defined metal−alkylidene complexes that have enabled a good mechanistic understanding of the polymerization as well as improvement of the initiators’ activity, stability, and functional group tolerance.

The evolution of ROMP has been decidedly metal-centric, with the path to accolades being paved primarily in ruthenium-, molybdenum-, and tungstenbased systems. Our departure from the ROMP trailhead was inspired in part by recent breakthroughs in radical-mediated polymerizations, whereby their mechanisms were leveraged to develop metal-free reaction conditions. Inventing a metal-free complement to traditional ROMP would essentially involve stepping away from decades of inorganic and organometallic developments, but with the promise of crossing new synthetic capabilities and curiosities. Driven by this motivation, as well as a community-inspired desire to develop “greener” controlled polymerizations, our team pioneered the search for, and discovery of, a wholly organic alternative to traditional metal-mediated ROMP. In this Account, we review our recent efforts to develop metal-free ring-opening metathesis polymerization (MF-ROMP), which is inspired by previous reports in electro- and photo-mediated organic transformations.

This work began with an exploration of the direct oxidation of enol ethers and the propensity of the ensuing radical cations to initiate ROMP. To overcome limitations of the electrochemical conditions, a photoredox-mediated method was investigated next, using photoexcited pyrylium salts to oxidize the enol ethers. With this system, we demonstrated the ability to produce ROMP products and temporally control the polymerization.

Further investigations into different aspects of the reaction included monomer scope, functional group tolerance, the impact of changing photocatalyst properties, and the ability to control molecular weight. The unique mechanism of MF-ROMP, along with the relative ease of synthesizing enol ether initiators, enabled the preparation of numerous polymeric materials that are hard to access through traditional metal-mediated pathways. At the end of this Account, we provide a perspective on future opportunities in this emerging area.

Xiang, Y.; Schilling, C.; Arora, N.; Boydston, A. J.; Rudykh, S.* “Mechanical characterization and constitutive modeling of visco-hyperelasticity of photocured polymers” Additive Manufacturing, 2020, 30, 101511. (DOI: 10.1016/j.addma.2020.101511). (*Corresponding author)

In this work, we study the nonlinear behavior of soft photocured polymers typically used in 3D-printing. We perform experimental testing of 3D-printed samples cured at various controlled light intensities. The experimental data show the dependency of the material elasticity and rate-sensitivity on the curing light intensity. To elucidate these relations, we develop a physically-based visco-hyperelastic model in the continuum thermodynamics framework. In our model, the macroscopic viscoelastic behavior is bridged to the microscopic molecular chain scale. This approach allows us to express the material constants in terms of polymer chain physical parameters. We consider different physical mechanisms governing hyperelasticity and rate-dependent behaviors. The hyperelastic behavior is dictated by the crosslinked network; whereas, the viscous part originates in the free and dangling chains. Based on our experimental data, we illustrate the ability of the new constitutive model to accurately describe the influence of the light intensity on photocured polymer viscoelasticity.

Boydston, A. J.; Cui, J.; Lee, C.-U.;* Lynde, B. E.; Schilling, C. A. “100th Anniversary of Macromolecular Science Viewpoint: Integrating Chemistry and Engineering to Enable Additive Manufacturing with High-Performance Polymers” ACS Macro Lett., 2020, 1119–1129. (DOI: 10.1021/acsmacrolett.0c00390). (*Corresponding author)

Additive manufacturing (AM) with high-performance polymers (HPPs) represents simultaneously one of the most desirable and challenging feats in the AM arena. The very properties that make HPPs so attractive in a broad range of applications also make them nearly impossible to process using common AM equipment. Furthermore, when AM is achieved, it often brings the caveat of compromised mechanical properties of the final parts, in comparison with those made via injection molding. The demand to have advanced fabrication methods, rapid prototyping, and customization of parts while maintaining high performance in the finished products has inspired creative innovations that integrate chemical synthesis, materials science, mechanical engineering, and other fields into a multidisciplinary approach to advance AM with the seemingly “unprintable” HPPs. In this Viewpoint, we summarize several standout developments in the area and offer our perspective on future directions and challenges.

Lee, D. C.; Sellers, D. L.; Liu, F.; Boydston, A. J.; Pun, S.* “Synthesis and characterization of anionic poly(cyclopentadienylene vinylene) and its use in conductive hydrogels.” Angew. Chem. Int. Ed., 2020, 59, 13430-13436. (DOI: 10.1002/anie.202004098). (*Corresponding author)

The use of π‐conjugated polymers (CPs) in conductive hydrogels remains challenging due to the water‐insoluble nature of most CPs. Conjugated polyelectrolytes (CPEs) are promising alternatives because they have tunable electronic properties and high water‐solubility, but they are often difficult to synthesize and thus have not been widely adopted. Herein, we report the synthesis of an anionic poly(cyclopentadienylene vinylene) (aPCPV) from an insulating precursor under mild conditions and in high yield. Functionalized aPCPV is a highly water‐soluble CPE that exhibits low cytotoxicity, and we found that doping hydrogels with aPCPV imparts conductivity. We also anticipate that this precursor synthetic strategy, due to its ease and high efficiency, will be widely used to create families of not‐yet‐explored π‐conjugated vinylene polymers.

Kensy, V.; Tritt, R.; Haque, F.; Murphy, L.; Knorr, D.; Grayson, S.; Boydston, A. J.* “Molecular Weight Control via Cross Metathesis in Photo‐Redox Mediated Ring‐Opening Metathesis Polymerization” Angew. Chem. Int. Ed., 2020, 59, 9074–9079. (DOI: 10.1002/anie.202000434). (*Corresponding author)

Photo‐redox mediated ring‐opening metathesis polymerization (photo‐ROMP) is an emerging ROMP technique that uses an organic redox mediator and a vinyl ether initiator, in contrast to metal‐based initiators traditionally used in ROMP. The reversibility of the redox‐mediated initiation and propagation steps enable spatiotemporal control over the polymerization. Herein, we explore a simple, inexpensive means of controlling molecular weight, using alpha olefins as chain transfer agents. This method enables access to low molecular weight oligomers, and molecular weights between 1 and 30 kDa can be targeted simply by altering the stoichiometry of the reaction. This method of molecular weight control was then used to synthesize a functionalized norbornene copolymer in a range of molecular weights for specific materials applications.

Schwartz, J. J.; Behrou, R.; Cao, B.; Bassford, M.; Mendible, A.; Shaeffer, C.; Boydston, A. J.*; Boechler, N.* “Reduced Strain Mechanochemical Activation Onset in Microstructured Materials” Polym. Chem., 2020, 11, 1122–1126. (DOI: 10.1039/C9PY01875A).

In this study, we show that mechanochemical activation in responsive materials with designed, periodic microstructures can be achieved at lower applied strains than their bulk counterparts. Furthermore, by characterizing two responsive polymeric materials, we have developed a computational model capable of quantitatively predicting mechanochemical activation in geometrically complex structures.

Lynde, B. E.; Maust, R. L.; Li, P.; Lee, D. C.; Jasti, R.*; Boydston, A. J.* “Ring-opening metathesis polymerization of a strained stilbene-based macrocyclic monomer” Mater. Chem. Front. 2020, 4, 252–256. (DOI: 10.1039/C9QM00604D).

We report the synthesis of a new class of strained macrocycle that performs well in ring-opening metathesis polymerization (ROMP). The polymerization displays chain growth characteristics with evidence of secondary metathesis in the form of chain transfer. The unique structure enables access to stilbene-based polymers that are traditionally prepared via uncontrolled polymerizations.

Krappitz, T.; Jovic, K.; Feist, F.; Frisch, H.; Rigoglioso, V. P.; Blinco, J. P.; Boydston, A. J.*; Barner-Kowollik, C.* “Hybrid Photo-induced Copolymerization of Ring-Strained and Vinyl Monomers Utilizing Metal-Free Ring-Opening Metathesis Polymerization ConditionsJ. Am. Chem. Soc. 2019141, 16605-16609. (DOI: 10.1021/jacs.9b09025). (*Co-corresponding authors)

Abstract ImageWe introduce the hybrid copolymerization of two disparate monomer classes (vinyl monomers and ring-strained cyclic olefins) via living photopolymerization. The living character of the polymerization technique (metal-free photo-ROMP) is demonstrated by consecutive chain-extensions. Further, we propose a mechanism for the copolymerization and analyze the copolymer structure in detail by high-resolution mass spectrometry.

Lee, C.-U.; Vandenbrande, J.; Goetz, A. E.; Ganter, M. A.; Storti, D. W.; Boydston, A. J. “Room Temperature Extrusion 3D Printing of Polyether Ether Ketone Using a Stimuli-Responsive Binder” Addit. Manuf. 201928, 430-438. (DOI: 10.1016/j.addma.2019.05.008).

We report our efforts toward 3D printing of polyether ether ketone (PEEK) at room temperature by direct-ink write technology. The room-temperature extrusion printing method was enabled by a unique formulation comprised of commercial PEEK powder, soluble epoxy-functionalized PEEK (ePEEK), and fenchone. This combination formed a Bingham plastic that could be extruded using a readily available direct-ink write printer. The initial green body specimens were strong enough to be manipulated manually after drying. After printing, thermal processing at 230 °C resulted in crosslinking of the ePEEK components to form a stabilizing network throughout the specimen, which helped to preclude distortion and cracking upon sintering. A final sintering stage was conducted at 380 °C. The final parts were found to have excellent thermal stability and solvent resistance. The Tg of the product specimens was found to be 158 °C, which is 13 °C higher than commercial PEEK as measured by DSC. Moreover, the thermal decomposition temperature was found to be 528 °C, which compares well against commercial molded PEEK samples. Chemical resistance in trifluoroacetic acid and 8 common organic solvents, including CH2Cl2 and toluene, were also investigated and no signs of degradation or weight changes were observed from parts submerged for 1 week in each solvent. Test specimens also displayed desirable mechanical properties, such as a Young’s modulus of 2.5 GPa, which corresponds to 63% of that of commercial PEEK (reported to be 4.0 GPa).

Shafranek, R. T.; Millik, S. C.; Smith, P. T.; Lee, C.-U.; Boydston, A. J.*; Nelson, A.* “Stimuli-Responsive Materials in Additive Manufacturing” Prog. Polym. Sci., 2019, 93, 36-67. (DOI: 10.1016/j.progpolymsci.2019.03.002). (*Co-corresponding authors)

Additive manufacturing (AM) technologies are expanding the boundaries of materials science and providing an exciting forum for interdisciplinary research. The ability to fabricate arbitrarily complex objects has made AM technologies indispensable in personalized healthcare, soft electronics, and renewable energy. At the intersection of AM technologies and materials chemistry are stimuli-responsive polymers, which change their chemical and physical properties in response to specific environmental cues. The responsiveness of these “smart” polymers makes them suitable for AM and provides functionality to the additively manufactured objects. Furthermore, the type and degree of stimulus response of smart polymers can be regulated through precise synthetic design or via incorporation of additives. Herein, we review recently reported stimuli-responsive polymers used in AM, with a focus on the design and chemistry of the polymers. The materials are broadly classified by type of printing, and more specifically classified by type of stimulus response. Finally, we briefly consider existing challenges that stimuli-responsive materials in AM can address in the future.

Schwartz, J. J.; Boydston, A. J. “Multimaterial Actinic Spatial Control 3D and 4D Printing” Nat. Commun. 2019, 10, 791. (DOI: 10.1038/s41467-019-08639-7).

Production of objects with varied mechanical properties is challenging for current manufacturing methods. Additive manufacturing could make these multimaterial objects possible, but methods able to achieve multimaterial control along all three axes of printing are limited. Here we report a multi-wavelength method of vat photopolymerization that provides chemoselective wavelength-control over material composition utilizing multimaterial actinic spatial control (MASC) during additive manufacturing. The multicomponent photoresins include acrylate- and epoxide-based monomers with corresponding radical and cationic initiators. Under long wavelength (visible) irradiation, preferential curing of acrylate components is observed. Under short wavelength (UV) irradiation, a combination of acrylate and epoxide components are incorporated. This enables production of multimaterial parts containing stiff epoxide networks contrasted against soft hydrogels and organogels. Variation in MASC formulation drastically changes the mechanical properties of printed samples. Samples printed using different MASC formulations have spatially-controlled chemical heterogeneity, mechanical anisotropy, and spatially-controlled swelling that facilitates 4D printing.

Lee, D. C.; Kensy, V. K.; Maroon, C. R.; Long, B. L.; Boydston, A. J. “The Intrinsic Mechanochemical Reactivity of Vinyl-Addition Polynorbornene” Angew. Chem. Int. Ed. 2019, 58, 5639-5642. (DOI: 10.1002/anie.201900467).

Herein we report the discovery of the intrinsic mechanochemical reactivity of vinyl-addition polynorbornene (VA-PNB), which has strained bicyclic ring repeat units along the polymer backbone. VA-PNBs with three different side chains were found to undergo ring-opening olefination upon sonication in dilute solutions. The sonicated polymers exhibited spectroscopic signatures consistent with conversion of the bicyclic norbornane repeat units to the ring-open isomer typical of polynorbornene made by ring-opening metathesis polymerization (ROMP-PNB). Thermal analysis and evaluation of chain scission kinetics suggest that sonication of VA-PNB results in chain segments containing a statistical mixture of vinyl-added and ROMP-type repeat units.

Lu, P.; Boydston, A. J. “Integration of Metal-Free Ring-Opening Metathesis Polymerization and Organocatalyzed Ring-Opening Polymerization through a Bifunctional Initiator” Polym. Chem. 2019, 10, 2975-2979. (DOI: 10.1039/C8PY01417E).

We have investigated the use of metal-free ring-opening metathesis polymerization (MF-ROMP) in combination with organocatalyzed ring-opening polymerization (o-ROP) to produce diblock copolymers with highly disparate block compositions via exclusively metal-free methods. Use of a bifunctional initiator bearing a vinyl ether as organic initiator for MF-ROMP and an alcohol for initiation of o-ROP allowed for investigation of three synthetic approaches: 1) sequential polymerization with isolation of the intermediate macroinitiators, 2) simultaneous bidirectional polymerizations, and 3) “one-pot” sequential monomer addition. Macroinitiators formed by first conducting o-ROP were successfully used in subsequent MF-ROMP to prepare diblock copolymers. Simultaneous MF-ROMP and o-ROP was thwarted by incompatible cross-combinations of catalysts and monomers. Finally, a straightforward “one-pot” synthesis of block copolymers, using o-ROP followed by MF-ROMP, was realized by sequential addition of each monomer-catalyst combination.

Lee, D. C.; Lamm, R. J.; Prossnitz, A. N.; Boydston, A. J.; Pun, S. H. “Dual Polymerizations: Untapped Potential for Biomaterials” Adv. Healthcare Mater. 2019, 8, 1800861. (DOI: 10.1002/adhm.201800861).

Block copolymers with unique architectures and those that can self‐assemble into supramolecular structures are used in medicine as biomaterial scaffolds and delivery vehicles for cells, therapeutics, and imaging agents. To date, much of the work relies on controlling polymer behavior by varying the monomer side chains to add functionality and tune hydrophobicity. Although varying the side chains is an efficient strategy to control polymer behavior, changing the polymer backbone can also be a powerful approach to modulate polymer self‐assembly, rigidity, reactivity, and biodegradability for biomedical applications. There are many developments in the syntheses of polymers with segmented backbones, but these developments are not widely adopted as strategies to address the unique constraints and requirements of polymers for biomedical applications. This review highlights dual polymerization strategies for the synthesis of backbone‐segmented block copolymers to facilitate their adoption for biomedical applications.

Boydston, A. J.*; Cao, B.; Nelson, A.*; Ono, R. J.; Saha, A.; Schwartz, J. J.; Thrasher, C. J. “Additive Manufacturing with Stimuli-Responsive Materials” J. Mater. Chem. A. 2018, 6, 20621-20645. (*Co-corresponding authors)

Additive manufacturing, commonly referred to as 3D printing (3DP), has ushered in a new era of advanced manufacturing that is seemingly limited only by imagination. In actuality, the fullest potentials of 3DP can only be realized through innovative breakthroughs in printing technologies and build materials. Whereas equipment for 3DP has experienced considerable development, molecular-scale programming of function, adaptivity, and responsiveness in 3DP is burgeoning. This review aims to summarize the state-of-the-art in stimuli-responsive materials that are being explored in 3DP. First, we discuss stimuli-responsiveness as it is used to enable 3DP. This highlights the diverse ways in which molecular structure and reactivity dictate energy transduction that in turn enables 3D processability. Second, we summarize efforts that have demonstrated the use of 3DP to create materials, devices, and systems that are in their final stage stimuli-responsive. This section encourages the artistic license of advanced manufacturing to be applied toward leveraging, or enhancing, energy transduction to impart device function across multiple length scales.

Leben, L. M.; Schwartz, J. J.; Boydston, A. J.; D’Mello, R. J.; Waas, A. M. “Optimized Heterogeneous Plates with Holes Using 3D Printing via Vat Photopolymerization.” Addit. Manuf. 2018, 24, 210-216. (DOI: 10.1016/j.addma.2018.09.018)

New advancements in 3D printing enable manufacturing a solid part with spatially controlled and varying material properties; this research seeks to establish techniques for finding optimal designs that use this new technology for the greatest structural benefit. We describe the use of a sequential quadratic programming based optimization solver to find an optimal distribution of material properties that minimize strain energy gradients, as calculated using finite element analysis. This design method is applied to the case of a flat thin plate with a hole, and has been proven to successfully reduce strain energy gradients and therefore stress concentrations. The optimally designed plates are 3D printed using a novel technology that uses vat polymerization technology. The computational model is validated with experiments. Enabling design engineers to customize material properties around geometric discontinuities will provide greater flexibility in reducing stress concentrations without modifying geometry or adding additional supports.

Cao, B.; Boechler, N.; Boydston, A. J. “Additive Manufacturing with a Flex Activated Mechanophore for Nondestructive Assessment of Mechanochemical Reactivity in Complex Object Geometries” Polymer 2018, 152, 4-8.

We used digital light processing additive manufacturing (DLP-AM) to produce mechanochemically responsive test specimens from custom photoresin formulations, wherein designer, flex activated  mechanophores enable quantitative assessment of the total mechanophore activation in the specimen. The manufactured object geometries included an octet truss unit cell, a gyroid lattice, and an “8D cubic lattice”. The mechanophore activation in each test specimen was measured as a function of uniaxial compressive strain applied to the structure. Full shape recovery after compression was exhibited in all cases. These proof-of-concept results signify the potential to use flex activated mechanophore for nondestructive, quantitative volumetric assessment of mechanochemistry in test specimens with complex geometries. Additionally, the integration of DLP-AM with flex activated mechanophore build materials enabled the creation of customizable, three-dimensional mechanochemically responsive parts that exhibit small molecule release without undergoing irreversible deformation or fracture.

Lee, Chang-Uk, Khalifehzadeh, R.; Ratner, B.*; Boydston, A. J.* “Facile Synthesis of Fluorine-Substituted Polylactides and Their Amphiphilic Block Copolymers” Macromolecules 201851, 1280-1289. (*Co-corresponding authors)

We report facile synthesis of 3-trifluoromethyl-6-methyl-1,4-dioxane-2,5-dione and ring opening polymerization of the fluoro-lactide monomer to prepare polylactides composed of  trifluoromethyl and methyl pendent groups on each repeat unit (FPLA). Molecular weights of the prepared polymers correlated well with the initial molar ratio of monomer to initiator, and were found to range from 6.6 to 22.5 kDa as determined by 1H NMR spectroscopy. GPC analysis revealed an Mn of up to 16.5 kDa. 1H, 13C, and 19F NMR spectroscopy were consistent with the structures of the lactide monomer isomers, and 1H NMR analysis was consistent with polymer backbones of alternating trifluoromethyl- and methyl-substituted lactate constituents. Glass transition temperature (Tg) and decomposition temperature (Td) of the new FPLA were found to be 39 °C and 225 °C by DSC and TGA, respectively. Additionally, we prepared amphiphilic block copolymers of FPLA and polyethylene glycol (PEG). Specifically, FPLA-b-PEG diblocks and FPLA-PEG-FPLA triblocks were synthesized by using PEG monomethyl ether (mPEG) or PEG as alcohol initiators, respectively. We observed the formation of vesicles or worm-like micelles from the particles of FPLA-PEG-FPLA in dilute aqueous solution by transmission electron microscopy (TEM), suggesting potential applications for drug delivery.

Thrasher, C. J.; Schwartz, J. J.; Boydston, A. J. “Modular Elastomer Photoresins for Digital Light Processing Additive Manufacturing” ACS Applied Mater. Interfaces 20179, 39707-39716.

A series of photoresins suitable for production of elastomeric objects via digital light processing additive manufacturing are reported. Notably, the printing procedure is readily accessible using only entry-level equipment under ambient conditions using visible light projection. The photoresin formulations were found to be modular in nature and straightforward adjustments to the resin components enabled access to a range of compositions and mechanical properties. Collectively, the series includes silicones, hydrogels, and hybrids thereof. Printed test specimens displayed maximum elongations of up to 472% under tensile load, tunable swelling behavior in water, and Shore A hardness values from 13.7 to 33.3. A combination of the resins was used to print a functional multi-material three-armed pneumatic gripper. These photoresins could be transformative to advanced prototyping applications such as simulated human tissues, stimuli-responsive materials, wearable devices, and soft robotics.

Lu, P.; Alrashdi, N. M.; Boydston, A. J. “Bidirectional Metal-Free ROMP from Difunctional Organic Initiators” J. Polym. Sci. A, Polym. Chem. 201755, 2977–2982. [Invited for special issue in honor of Prof. Robert H. Grubbs]

Ditopic initiators were evaluated for bidirectional organocatalyzed ROMP. Incorporation of monomer was found to be successful for both inward and outward polymer growth, stemming from divinyl ethers with different relative orientation of alkoxy moieties. Macroinitiators were also used to prepare triblock and graft copolymers that were found to be easily cleaved with acid catalyst.

Pascual, L. M. M.; Goetz, A. E.; Roehrich, A. M.; Boydston, A. J. “Investigation of Tacticity and Living Characteristics of Photoredox-Mediated Metal-Free Ring-Opening Metathesis  Polymerization” Macromol. Rapid Commun. 201738, 1600766 (1–6).

We have investigated the microstructures of polymers produced via photoredox-mediated metal-free ring-opening metathesis polymerization (ROMP). Polynorbornene, poly(exo-dihydrodicyclopentadiene), and poly(endo-dicyclopentadiene) were found to have cis olefin contents of 23%, 24%, and 28%, respectively. Additionally, the cis/trans ratio remained consistent during the course of norbornene polymerization. Polymer tacticity was evaluated by quantitative 13C NMR spectroscopy, which revealed each polymer to be largely atactic. Specifically, the three polymers were estimated to be 33%, 58%, and 55% syndiotactic, respectively. In parallel, we also explored the ability to produce diblock copolymers from norbornene and exo-dihydrodicyclopentadiene. Successful diblock copolymerization was achieved using either monomer order. In each case, however, we observed results consistent with chain-chain coupling (increased molecular weight) and irreversible termination (dead chains observed during attempted chain extension) when reaction times were extended.

Church, D. C.; Nourian, S.; Lee, C.-U.; Yakelis, N. A.; Toscano, J. P.; Boydston, A. J. “Amphiphilic Copolymers Capable of Concomitant Release of HNO and Small Molecule Organics” ACS Macro Lett. 20176, 46-49.

We demonstrate concomitant release of HNO and small molecule organics from amphiphilic poly(norbornene)-based copolymers. This key function was achieved by incorporation of thermally-labile oxazine units within random and block copolymer architectures. Upon  thermolysis, we observed generation of HNO and release of a small molecule conjugate. Importantly, the release kinetics of HNO and a UV-active small molecule (4-nitroaniline) were found to be 1:1, signifying an ability to monitor HNO production indirectly, or to simultaneously release organic therapeutics (e.g., nonsteroidal anti-inflammatory drugs)) along with HNO. To our knowledge, these are the first reported polymeric materials demonstrating HNO release from covalently attached HNO donors.

Peterson, G. I.; Schwartz, J. S.; Zhang, D.; Weiss, B. M.; Ganter, M. A.; Storti, D. W.*; Boydston, A. J.* “Production of Materials with Spatially-Controlled Crosslink Density via Vat Photopolymerization” ACS Appl. Mater. Interfaces 20168, 29037–29043. (*Co-corresponding authors)

We describe an efficient method to produce objects comprising spatially controlled and graded crosslink densities using vat photopolymerization additive manufacturing (AM). Using a commercially available diacrylate-based photoresin, 3D printer, and digital light processing (DLP) projector, we projected grayscale images to print objects in which the varied light intensity was correlated to controlled crosslink densities and associated mechanical properties. Cylinder and bar test specimens were used to establish correlations between light intensities used for printing and crosslink density in the resulting specimens. Mechanical testing of octet truss unit cells in which the properties of the crossbars and vertices were independently modified, revealed unique mechanical responses from the different compositions. From the various test geometries, we measured changes in mechanical properties such as increased strain-to-break in inhomogeneous structures in comparison with homogeneous variants.

Kensy, V. K.; Peterson, G. I.; Church, D. C.; Yakelis, N. A.; Boydston, A. J. “Investigation of the Dynamic Nature of 1,2-Oxazines Derived from Peralkylcyclopentadiene and Nitrosocarbonyl Species” Org. Biomol. Chem. 2016, 14, 5617-5621.

Goetz, A. E.; Pascual, L. M. M.; Dunford, D. G.; Ogawa, K. A.; Knorr, D. B., Jr.; Boydston, A. J. “Expanded Functionality of Polymers Prepared Using Metal-Free Ring-Opening Metathesis Polymerization” ACS Macro Lett. 20165, 579-582.

Pascual, L. M. M.; Dunford, D. G.; Goetz, A. E.; Ogawa, K. A.; Boydston, A. J. “Comparison of Pyrylium and Thiopyryium Photo-oxidants in Metal-Free Ring-Opening Metathesis Polymerization” Synlett 201627, 759-762. (invited communication).

Wang, C. E.; Wei, H.; Tan, N.; Boydston, A. J.; Pun, S. H. “Sunflower Polymers for Folate-Mediated Drug Delivery” Biomacromolecules 201617, 69-75.

Larsen, M. B.; Boydston, A. J. “Investigations in Fundamental and Applied Polymer Mechanochemistry” Macromol. Chem. Phys. 2016217, 354-364. (Selected by the Editorial Board as one of the best papers for 2016.)

Ogawa, K. A.; Goetz, A. E.; Boydston, A. J. “Developments in Externally Regulated Ring-Opening Metathesis Polymerization” Synlett 201627, 203-214. [Invited Accounts article]

Wei, H.; Wang, C. E.; Tan, N.; Boydston, A. J.; Pun, S. H. “ATRP Synthesis of Sunflower Polymers Using Cyclic Multimacroinitiators” ACS Macro Lett.20154, 938-941.

Goetz, A. E.; Boydston, A. J. “Metal-Free Preparation of Linear and Crosslinked Polydicyclopentadiene” J. Am. Chem. Soc. 2015137, 7572-7575.

Peterson, G. I.; Yurtoglu, M.; Larsen, M. B.; Craig, S. L.; Ganter, M. A.; Storti, D. W.; Boydston, A. J. “Additive Manufacturing of Mechanochromic Polycaprolactone on Entry-Level Systems” Rapid Prototyping J. 201521, 520-527.

Ogawa, K. A.; Goetz, A. E.; Boydston, A. J. “Metal-Free Ring-Opening Metathesis Polymerization” J. Am. Chem. Soc. 2015137, 1400-1403.

Peterson, G. I.; Larsen, M. B.; Ganter, M. A.; Storti, D. W.; Boydston, A. J. “3D-Printed Mechanochromic Materials” ACS Appl. Mater. Interfaces 20157, 577-583.

Ogawa, K. A.; Boydston, A. J. “Recent Developments in Organocatalyzed Electro-organic Chemistry” Chem. Lett. 201544, 10-16. (invited Highlight Review)

Peterson., G. I.; Church, D. C.; Yakelis, N. A.*; Boydston, A. J.* “1,2-Oxazine Linker as a Thermal Trigger for Self-Immolative Polymers” Polymer201455, 5980-5985. (*Co-corresponding authors)

Peterson, G. I.; Boydston, A. J. “Kinetic Analysis of Mechanochemical Chain Scission of Linear Poly(phthalaldehyde)” Macromol. Rapid Commun.201435, 1611-1614.

Peterson, G. I.; Boydston, A. J. “Modeling the Mechanochemical Degradation of Star Polymers” Macromol. Theory Simul. 201423, 555-563. (Journal Cover)

Church, D. C.; Peterson, G. I.; Boydston, A. J. “Comparison of Mechanochemical Chain Scission Rates for Linear versus Three-Arm Star Polymers in Strong Acoustic Fields” ACS Macro Lett20143, 648-651.

Diesendruck, C. E.; Peterson, G. I.; Kulik, H. J.; Kaitz, J. A.; Mar, B. D.; May, P. A.; White, S. R.; Martínez, T. J.; Boydston, A. J.; Moore, J. S. “Mechanically-Triggered Heterolytic Unzipping of a Low Ceiling Temperature Polymer” Nature Chem. 20146, 623-628.

Ogawa, K. A.; Boydston, A. J. “Electrochemical Characterization of Azolium Salts” Chem. Lett. 201443, 907-909.

Ogawa, K. A.; Boydston, A. J. “Anodic Oxidation of Aldehydes to Thioesters” Org. Lett. 201416, 1928-1931.

Larsen, M. B.; Boydston, A. J. “Successive Mechanochemical Activation and Small Molecule Release in an Elastomeric Material” J. Am. Chem. Soc. 2014136, 1276-1279.

Larsen, M. B.; Boydston, A. J. “Flex-Activated Mechanophores: Using Polymer Mechanochemistry to Direct Bond Bending Activation” J. Am. Chem. Soc. 2013135, 8189-8192.

Peterson, G. I.; Larsen, M. B.; Boydston, A. J. “Controlled Depolymerization: Stimuli-Responsive Self-Immolative Polymers” Macromolecules 201245, 7317-7328. (invited Perspective Article, GIP and MBL contributed equally, Journal Cover)

Finney, E. E.; Ogawa, K. A.; Boydston, A. J. “Organocatalyzed Anodic Oxidation of Aldehydes” J. Am. Chem. Soc. 2012134, 12374-12377.

AJB publications from undergraduate, graduate, and postdoctoral work:

  • Momčilović, N.; Clark, P. G.; Boydston, A. J.*; Grubbs, R. H.* “One-Pot Synthesis of Polyrotaxanes via Acyclic Diene Metathesis Polymerization of Supramolecular Monomers” J. Am. Chem. Soc. 2011133, 19087-19089. (*Co-corresponding authors)
  • Xia, Y.; Boydston, A. J.; Grubbs, R. H. “Synthesis and Direct Imaging of Ultrahigh Molecular Weight Cyclic Brush Polymers” Angew. Chem. Int. Ed. 201150, 5882-5885.
  • Boydston, A. J.; Holcombe, T. W.; Unruh, D. A.; Fréchet, J. M. J.; Grubbs, R. H. “A Direct Route to Cyclic Organic Nanostructures via Ring-Expansion Metathesis Polymerization of a Dendronized Macromonomer” J. Am. Chem. Soc. 2009131, 5388-5389.
    • Highlighted by C&E News 2009April 14, 32.
    • Highlighted by Nature Chemistry 20091, 178-179.
  • Xia, Y.; Boydston, A. J.; Yao, Y.; Kornfield, J. A.; Gorodetskaya, I. A.; Spiess, H. W.; Grubbs, R. H. “Ring-Expansion Metathesis Polymerization: Catalyst Dependent Polymerization Profiles” J. Am. Chem. Soc. 2009131, 2670-2677.
  • Boydston, A. J.; Xia, Y.; Kornfield, J. A.; Gorodetskaya, I. A.; Grubbs, R. H. “Cyclic Ruthenium-Alkylidene Catalysts for Ring-Expansion Metathesis Polymerization” J. Am. Chem. Soc. 2008130, 12775-12782.
  • Tang, T.; Coady, D. J.; Boydston, A. J.; Dykhno, O. L.; Bielawski, C. W. “Pro-Ionomers: An Anion Metathesis Approach to Amphiphilic Block Ionomers from Neutral Precursors” Adv. Mater. 200820, 3096-3099.
  • Boydston, A. J.; Vu, P. D.; Dykhno, O. L.; Chang, V.; Wyatt, A. R., II; Stockett, A. S.; Ritschdorff, E. T.; Shear, J. B.; Bielawski, C. W. “Modular Fluorescent Benzobis(imidazolium) Salts: Syntheses, Photophysical Analyses, and Applications” J. Am. Chem. Soc. 2008130, 3143-3156.
  • Boydston, A. J.; Pecinovsky, C. S.; Chao, S. T.; Bielawski, C. W. “Phase-Tunable Fluorophores Based Upon Benzobis(imidazolium) Salts” J. Am. Chem. Soc. 2007129, 14550-14551.
  • Vu, P. D.; Boydston, A. J.; Bielawski, C. W. “Ionic Liquids via Efficient, Solvent-Free Anion Metathesis” Green Chem. 20079, 1158-1159.
  • Williams, K. A.; Boydston, A. J.; Bielawski, C. W. “Towards Electrically Conductive, Self-Healing Materials” J. R. Soc. Interface 2007, 4, 359-362.
  • Boydston, A. J.; Rice, J. D.; Sanderson, M. D.; Dykhno, O. L.; Bielawski, C. W. “Mild and Efficient Syntheses of Main-Chain Organometallic Polymers Containing Bis(bidentate) Benzobis(imidazolylidene)s and a Related Bis(benzimidazolylidene)Ni(II) Complex” Organometallics 200625, 6087-6098.
  • Khramov, D. M.; Boydston, A. J.; Bielawski, C. W. “Synthesis and Study of Janus Bis(carbene)s and Their Transition Metal Complexes” Angew. Chem., Int. Ed. 200645, 6186-6189.
  • Boydston, A. J.; Khramov, D. M.; Bielawski, C. W. “An Alternative Synthesis of Benzobis(imidazolium) Salts Via a “One-pot” Cyclization/Oxidation Reaction Sequence” Tetrahedron Lett. 200647, 5123-5125.
  • Khramov, D. M.; Boydston, A. J.; Bielawski, C. W. “Highly Efficient Synthesis and Solid-State Characterization of 1,2,4,5-Tetrakis(alkyl– and arylamino)benzenes and Cyclization to Their Respective Benzobis(imidazolium) Salts” Org. Lett. 20068, 1831-1834.
  • Sartin, M. M.; Boydston, A. J.; Pagenkopf, B. L.; Bard, A. J. “Electrochemistry, Spectroscopy, and Electrogenerated Chemiluminescence of Silole-Based Chromophores” J. Am. Chem. Soc. 2006128, 10163-10170.
    • Highlighted by the ACS: Chemical Innovation Heart Cut: August 28, 2006.
  • Hinrichs, H.; Boydston, A. J.; Jones, P. G.; Hess, K.; Herges, R.; Haley, M. M.; Hopf, H. “The Phane Properties of [2.2]Paracyclophane/ Dehydrobenzoannulene Hybrids” Chem. Eur. J. 200612, 7103-7115.
  • Boydston, A. J.; Williams, K. A.; Bielawski, C. W. “A Modular Approach to Main-Chain Organometallic Polymers” J. Am. Chem. Soc. 2005127, 12496-12497.
    • Highlighted by C&E News 2005Sept. 5, 32.
  • Boydston, A. J.; Pagenkopf, B. L. “Improving Quantum Efficiencies of Siloles and Silole-derived Butadiene Chromophores through Structural Tuning” Angew. Chem., Int. Ed. 200443, 6336-6338.
    • Highlighted by the ACS: Chemical Innovation Heart Cut: December 13, 2004.
  • Boydston, A. J.; Yin, Y.; Pagenkopf, B. L. “A Controlled, Iterative Synthesis of Oligo[(p-phenyleneethynylene)-alt-(2,5-siloleneethynylene)]s” J. Am. Chem. Soc. 2004126, 10350-10354.
  • Boydston, A. J.; Yin, Y.; Pagenkopf, B. L. “Synthesis and Electronic Properties of Donor-Acceptor p-Conjugated Siloles” J. Am. Chem. Soc. 2004126, 3724-3725.
    • Highlighted by the ACS: Chemical Innovation Heart Cut: April 19, 2004.
  • Boydston, A. J.; Haley, M. M.; Williams, R. V.; Armantrout, J. R. “Diatropicity of 3,4,7,8,9,10,13,14-Octadehydro[14]annulenes: A Combined Experimental and Theoretical Investigation” J. Org. Chem. 200267, 8812-8819.
  • Boydston, A. J.; Laskoski, M.; Bunz, U. H. F.; Haley, M. M. “Evaluation of Ring-Strain Effects in Cycloalkene-Fused Octadehydro[14]annulenes” Synlett2002, 981-983.
  • Boydston, A. J.; Haley, M. M. “Diatropicity of Dehydrobenzo[14]annulenes: Comparative Analysis of the Bond-Fixing Ability of Benzene on the Parent 3,4,7,8,9,10,13,14-Octadehydro[14]annulene” Org. Lett. 20013, 3599-3601.
  • Boydston, A. J.; Bondarenko, L.; Dix, I.; Weakley, T. J. R.; Hopf, H.; Haley, M. M. “[2.2]Paracyclophane/Dehydrobenzoannulene Hybrids: Transannular Delocalization in Open-Circuited Conjugated Macrocycles” Angew. Chem., Int. Ed. 200140, 2986-2989.