Synthetic hydrogels provide powerful material platforms to engineer cellular microenvironments with control over stiffness, viscoelasticity, porosity, degradability, and biochemical signals. Here, we demonstrate how orthogonal crosslinking reactions allow fabrication of covalent adaptable networks to tailor photopolymerizable bioresin formulations relevant for tissue engineering. Specifically, we synthesize multifunctional poly(ethylene glycol) (PEG) macromers containing dynamic boronate ester bonds and dithiolane and norbornene moieties that allow for photopolymerization and projection-based biofabrication. These materials are used to print human mesenchymal stromal cells (MSCs) in formulations where the ratio of elastic versus adaptable crosslinks is engineered to study and manipulate MSC spreading, actin structure, and macroscopic material-level deformation. We demonstrate how material and print parameters, peptide ligands, actomyosin-modulating drug treatments, and cell types influence cell-material interactions and emergence of morphogenesis that is uniquely enabled by viscoelasticity. The presented materials introduce a versatile strategy for spatiotemporal control over dynamic mechanical properties in cell-laden matrices.

Display technologies require optical adhesives that simultaneously provide high optical clarity, refractive index control, low birefringence and adhesive strength. However, many commercial adhesive systems rely on petroleum-derived acrylates and isocyanate-based urethanes. Herein, a fully biobased optical adhesive is reported that exploits the initiator-free photopolymerization of dithiolanes. Furthermore, hydrogen bond strength is modulated within the prepared adhesives by changing the chemistry with which the dithiolane is conjugated to a macromolecular core, allowing manipulation of properties including glass transition temperature, adhesion, refractive index, and other optical properties. Throughout these variations, all materials maintain high optical performance, exhibiting visible light transmittance above 98% and haze below 0.7%, coupled with low optical dispersion. These results demonstrate that dithiolanes are readily applied as initiator-free crosslinkers for the formulation of fully biobased optical adhesives.

Ensuring reproducibility should be a central priority for authors, editors, and reviewers throughout manuscript preparation and evaluation. Reports of photo-mediated processes are particularly susceptible to reproducibility challenges because their apparent simplicity often obscures how strongly reliable outcomes depend on carefully controlled and thoroughly reported experimental conditions. To assess current practice, 150 articles describing photo-mediated processes were randomly selected from high-impact journals in the Web of Science categories of "polymer science," "chemistry," and "physics." Each was evaluated for the minimum information necessary to reproduce a photo-induced process: exposure conditions (light source emission spectrum, incident irradiance, and irradiation duration), sample geometry (boundary conditions and thickness), and sample composition (identity and amount of the active chromophore and any other absorbing species). Only 8% of polymer science, 4% of chemistry, and 10% of physics papers provided sufficient detail to enable reproduction. The most significant deficiencies involved the light source; just 25% reported enough to reproduce both emission spectrum and irradiance. This work explains why each element is essential across diverse photo-induced processes and presents modeling and representative case studies illustrating the variability that arises when conditions are under-specified, using photopolymerization as a sensitive benchmark.

High-refractive-index (HRI) monomers swollen in a rubbery host matrix to form 2-stage composite materials play a crucial role in modern optics applications and are constantly being redesigned to produce enhanced material properties. To aid in this effort, we report the synthesis of two HRI halogenated acrylate monomers and characterize their affinity for high-spatial-frequency photopatterning when swollen in a polyurethane matrix. Through refractive index measurements and high-resolution photopatterning, we show that these 2-stage films support submicrometer spatial frequencies with refractive index modulation n 1 on the order of 0.01 and change in index upon polymerization Delta n of 0.0023. Photopolymerization kinetics from FTIR fit a three-species kinetic model, where the dependence of the polymerization rate on incident light intensity reveals that the dominant termination kinetics in these 2-stage films is unimolecular termination, as opposed to the standard bimolecular. Critically, we show that photopatterning does not induce undesirable optical or mechanical properties such as haze, discoloration, stiffening of the films, or increased dispersion, making these materials suitable for photopatterning applications. High-fidelity holographic diffraction gratings and 2D photo-patterns with micron-scale features are presented to show proof of concept.

The performance of holographic photopolymers, one of the most promising material platforms for addressing the critical needs in emerging holographic applications, is governed by spatial variations of chemical composition, which have rarely been assessed at length scales commensurate with the grating pitch. Here, the chemical specificity of AFM-IR, an infrared technique capable of nanoscale resolution, is leveraged to map the local composition of a holographic grating recorded in a thiol-ene/polyurethane photopolymer film. Quantitative analysis enabled by calibration of photopolymer (1584 cm-1) to polyurethane (1532 cm-1) intensity ratios in flood-cured films with systematically varied photopolymer loading reveals variations of the grating local peak-to-valley concentration (averaged through the grating thickness) of ≈1.7% g/g. In addition, the grating structure was probed at different depths by measuring the grating angular diffraction efficiency upon sequential, layer-by-layer material removal with a cryo-microtome. The evolution of the diffraction peak intensity and position with depth (i.e., upon thinning of the grating) indicates through-thickness heterogeneity in both the refractive index modulation and the grating geometry. In combination, these approaches offer a framework for assessing the composition of holographic gratings and for understanding their performance.

ABSTRACT Volumetric additive manufacturing (VAM) has vast potential for simultaneous preparation of 3D parts but has been inherently limited by the implementation of materials designed for traditional vat polymerization. Such resins are highly nonoptimal due to gravitational settling, low reaction conversion contrast, and limited green part mechanics. Here, the rationally designed VAM materials platform, in the form of a tunable, degradable, and covalently‐crosslinked polyurethane scaffold, enables access to a broad library of low‐viscosity monomers corresponding ultimately to a range of thermomechanical properties. In combination with a radical inhibitor, this library of materials significantly advances the capabilities and applications of VAM, facilitating the transition from prototyping to manufacturing by significantly expanding the accessible range of thermomechanical properties. This approach achieves high conversion contrast, quantitatively characterized by an order‐of‐magnitude modulus differential between in‐part and out‐of‐part regions, utilizing the solid‐like scaffold to prevent settling even over the extended timescale necessitated by high inhibitor concentrations, and empowers green part moduli up to tens of MPa. Upon print completion, thioester degradation in the scaffold reveals the final printed part with minimal property deterioration and less than 4% change in part mass. Together, this degradable‐scaffold photoresist establishes a generalizable and versatile materials strategy for high‐contrast volumetric additive manufacturing.

Light-induced, initiatorless homopolymerization of 1,2-dithiolanes is widely attributed to radical propagation. However, several experimental observations, including copolymerization behavior and EPR measurements, are inconsistent with this assignment. Here, this reaction is examined using in situ photoNMR, revealing first-order monomer consumption that is incompatible with a propagating radical mechanism. PhotoNMR further enables direct observation of a sulfonium intermediate attributed to a charge-transfer complex, corroborated through steady-state and transient absorption spectroscopy. Together, these data establish an activated-monomer-type polymerization, proceeding through a cationic rather than radical pathway. This same intermediate also enables controlled chain growth and is leveraged as a photoinitiator for cationic vinyl ether polymerization, extending the scope of dithiolane photochemistry beyond homopolymer formation and opening new opportunities for dithiolane-based photopolymerization strategies.

Control over network chemistry and connectivity of hydrogels is critical for the generation of tunable material properties, including material degradation for applications such as tissue scaffolding and drug delivery. Here, the degradation of hydrogels employing different hydrolytically cleavable groups including benzamide and syringic acid-derived carbamates, kojic acid-derived carbonates, and kojic acid-derived esters under physiological conditions was studied. Tunability of the hydrogel network degradation was demonstrated by varying the hydrolytically degradable moiety, macromer functionality, and copolymerization with hydrolytically stable macromers. These hydrolytically labile macromers were introduced and cross-linked intracellularly to induce transient cellular quiescence in MCF10A cells, resulting in a highly tunable degradation mechanism that is shown to be capable of inducing reversible biostasis of cells with 60% of cells treated with the carbonate macromer returning to their proliferative state and rebounding in translational activity after 72 h, while the biological activity of the carbamate macromer-treated cells remained suppressed.

Although thermoplastic and thermoset materials have revolutionized modern living, their widespread use has also led to significant environmental challenges due to limited recyclability and persistence in the environment. To address these issues, the development of new materials that are both functional and sustainable is crucial. In this work, an acetal-based polymerization is demonstrated, wherein dichloromethane (DCM) serves as a methylene synthon for forming acetals between alcohol end groups in the presence of a strong base, facilitating the production of fully degradable polymers and networks, as well as facile recovery of repolymerizable monomers. The incorporation of functional groups via primary alcohols, such as norbornene methanol, enables simultaneous functionalization of the polymer structure. Glycol-based linear polymers and macromers are utilized as primary building blocks, offering a versatile platform for generating tunable polymer architectures. Additionally, acetal-based oligomerization of decanediol is achieved at increased temperature and pressure, broadening the scope of potential materials accessible through this method. This historically understudied reaction holds great promise for the design of functional, degradable, and recyclable polymers.

Photopolymerization-driven additive manufacturing (AM) is a well-established technique to generate polymeric 3D structures with both high resolution and formation in complex geometries. Recent approaches focus on AM techniques that enable multiproperty architectures using wavelength orthogonal photochemistry. Herein, a dual-cure, single-vat resin was developed, based on the radical photopolymerization of a thiol-methacrylate monomer system containing covalently bound chalcone moieties as dimerizable cross-linkers. Thermo-mechanical properties were spatially and systematically controlled via the wavelength-selective [2 + 2] cycloaddition reaction of the chalcone groups. Reaction kinetics were studied with infrared and ultraviolet-visible spectroscopy to ensure sequence-dependent lambda-orthogonality during the two-stage illumination process. 3D-structures were fabricated by dynamic light processing (DLP), imprinting, and two-photon lithography (TPL). In particular, the ability to excite both the radical photoinitiator and the chalcone groups separately with TPL in high spatial resolution enabled the production of multifunctional microstructures and represents a versatile concept for the fabrication of soft active devices along various length scales.

The haze developed during the recording of holographic gratings was investigated and mitigated in a typical two-stage holographic photopolymer system where a high-refractive-index acrylate (1,3-bis(phenylthio)-2-propyl acrylate, i.e., BPTPA) was utilized as a writing monomer. An acrylate writing monomer (1,3-bis(phenylthio)-2-propyl urethane ethyl acrylate, i.e., BPTPUA) was proposed and synthesized to achieve a lower interaction parameter (chi) between the matrix and the writing monomer. Confirmed by theoretical predictions and experimental photolithography results, the formulation incorporating BPTPUA exhibited a significant improvement in miscibility between the two phases as compared to the analogue compound without a urethane, BPTPA. The higher miscibility led to dramatically lower haze of holographic gratings, due to which the quality of angular playback curves from holographic gratings improved from being strongly distorted to being well-matched with the Kogelnik coupled wave theory. Additionally, a higher light intensity reduced the haze remarkably due to the potential kinetic restriction for phase separation and lower molecular weight of the photopolymer formed. The molecular weight decrease at higher light intensity was confirmed experimentally, and the trend was fit by a theoretical calculation of the kinetic chain length. Using the fitted relationship, the free energy change of mixing was calculated and suggested that a higher light intensity improved the miscibility between the photopolymer and the matrix thermodynamically. Moreover, a reactive matrix was introduced to mitigate the haze development by forming covalent bonds between the two phases. With contributions from covalent attachment and BPTPUA as the writing monomer, haze as low as 0.5% was achieved in a 50 mu m recording medium at an extremely high light intensity (200 mW/cm2) for holographic grating.

Covalent hydrogel networks suffer from a stiffness-toughness conflict, where increased crosslinking density enhances the modulus of the material but also leads to embrittlement and diminished extensibility. Recently, strategies have been developed to form highly entangled hydrogels, colloquially referred to as tanglemers, by optimizing polymerization conditions to maximize the density and length of polymer chains and minimize the crosslinker concentration. It is challenging to assess entanglements in crosslinked networks beyond approximating their theoretical contribution to mechanical properties; thus, in this work, we synthesize and characterize polyacrylamide tanglemers using a photolabile crosslinker, which allows for direct assessment of covalent trapping of entanglements under tension. Further, this chemistry allows tuning of the modulus in situ by crosslink photocleavage (from tensile modulus (ET) = 100 kPa to <25 kPa). Beyond cleavage of crosslinks, we demonstrate that even non-degradable tanglemer formulations can be photo-softened and completely degraded through Fe3+-mediated oxidation of the polyacrylamide backbone. While both photodegradation methods are useful for spatial patterning and result in softer gels with reduced fracture strength, only crosslink photocleavage improves gel extensibility via light-induced chain lengthening (epsilon F = 700% to >1500%). Crosslink photocleavage in tanglemers also affords significant control over localized swelling and diffusivity. In sum, we introduce a simple and user-directed approach for probing entanglements and asserting spatiotemporal control over stress-strain responses and small molecule diffusivity in polyacrylamide tanglemers, suggesting a multitude of potential soft matter applications including controlled release and tunable bioadhesive interfaces.

Leveraging the kinetic selectivity of various thiol-based chemistries, sequential thiol-Michael and thiol-ene reactions were applied semiorthogonally toward holographic recording, thereby expanding the available toolbox for developing thiol-ene-based optical recording media. In a unique ternary mixture, thiol glycolates are highly favored kinetically due to the higher stability and therefore enhanced reactivity of the thiolate anion as compared with aliphatic thiols in the thiol-Michael reaction. The thiol glycolate is base-catalyzed to react with a Michael acceptor, i.e., an electron-deficient double bond, to form the first-stage matrix, leaving most of the aliphatic thiol unreacted and available for the successive thiol-ene photopolymerization. Through product ratios obtained from 1H NMR, the high kinetic selectivity was demonstrated in small molecule model studies, in which a significant excess loading of aliphatic thiol monomer was utilized (up to five-fold excess of thiol functional groups). Furthermore, the two-stage behavior was evaluated in a bulk material system comprised of multifunctional monomers through photorheology. The resultant films, which are robust elastomers, exhibit high spatiotemporal control in photopatterning. Taking advantage of the decoupled choice of thiol monomers to realize a higher theoretical refractive index contrast between two stages, transmission holographic gratings were recorded in similar formulations, yielding a peak-to-mean refractive index contrast of 0.0064 with high fidelity of spatial resolution even at a size scale of 620 nm period.

In this work, we develop a tetrafunctional monomer incorporating 1,2-dithiolanes as the reactive group, lipoic acid pentaerythritol ethoxylate, which is capable of photopolymerization and is suitable for light-based additive manufacturing with high spatial resolution across various length scales. This monomer polymerizes in either the presence or absence of exogenous photoinitiator. Using dynamic light processing and two photon lithography techniques, parts were printed on size scales ranging from multiple cm to μm, with resolution as small as 1 μm. As a result of the dithiolane polymerization, linear disulfides are formed, forming covalent adaptable networks directly from the polymerization reaction. Furthermore, through heating and dilution in solvent, the network was recycled back to the lipoic acid functional monomer with approximately 95% monomer recovery, which was subsequently repolymerized to achieve nearly identical modulus evolution as a function of exposure time. This work represents an advance in the development of multifunctional dithiolane monomers, as well as recyclable resins for additive manufacturing that are capable of polymerization with or without exogenous photoinitiators.

Degradable polymers hold promise for their recyclability and their potential to facilitate the transition toward a circular economy of plastics. As such, a consecutive di-SN2 reaction between dithiols and polyhalogenated compounds was used to produce degradable polymers with tunable amounts of redox-enabled dithioacetal and disulfide linkages. The reaction employed bases of varying strengths and different thiol types, allowing for control over molecular weights ranging from <1 kDa to 16 kDa, with tunable degrees of polymerization ranging from 3 to over 75. The ratio of dithioacetals to disulfides was adjusted by manipulating the concentration of the methylene source, enabling the formation of polymers with up to 100% disulfide linkages or up to 93% dithioacetal linkages. This tunability resulted in polymers with multiple redox degradation pathways, capable of being degraded by either reduction or oxidation. Vinyl ether functionalized, telechelic oligomers were used as crosslinkers in network formation, yielding materials with multiple degradation pathways. This di-SN2 approach provides a promising and straightforward method for creating materials with tailored degradation characteristics.

In the writing of holographic photopolymers, the addition of a third-stage cure to the typical polyurethane matrix and acrylate writing monomer steps is used here to modify the ultimate thermomechanical properties of the final holographic photopolymer. Inclusion of a thermally latent, low-refractive-index epoxide homopolymerization increases the Tg from a value of -22 °C during the writing step to a final Tg of 101 °C after the epoxide cure. Critically, the diffraction grating structure is retained with high fidelity, an index contrast of 0.0057, and a diffraction efficiency of 89% achieved in these materials. Ultimately, the 3-stage design and final glassy nature of these materials promote thermal and dimensional stability of the final holographic material.

A facile synthesis, preparation, and integration of a diacrylate-functionalized mechanophore is presented to demonstrate that a single, identical mechanophore is readily incorporated into polymer networks of various origins as a tool for identifying differences and similarities in the mechanical behavior of these networks. This work investigates polymer mechanical properties during and following stimulation of mechanofluorophore-incorporated step and chain growth crosslinked polymers. This is achieved with an acrylate-functionalized benzoxazole mechanofluorophore polymerized via a nucleophile-mediated thiol-Michael polymerization (step growth) or a photoinitiated radical acrylate homopolymerization (chain growth). These systems were tuned to have similar storage moduli by incorporating a monofunctional acrylate as a chain extender and crosslink density diluent in the chain growth network. Uniaxial tensile tests indicate that mechanophore incorporation in both systems results in an increase in the percent strain at break compared to control materials without the benzoxazole mechanophore but otherwise maintained monomer composition. Integration of the mechanofluorophore was also implemented in dynamic networks formed from step and chain growth polymerizations where the presence of disulfide bonds facilitated thermally activated bond rearrangement for stress relaxation. The presence of the mechanofluorophore in these networks did not inhibit the dynamic nature of the networks as evidenced by comparable stress-relaxation traces.

A reaction kinetic model incorporating free volume theory was developed to elucidate and predict diffusion-controlled kinetics and attainment of limited functional group conversions in radical-mediated thiol-ene photopolymerizations. The glass transition temperature (T g) of the reaction mixture was experimentally correlated with conversion by using a combination of photo-DSC and temperature-sweep DSC measurements. The subsequently determined T g values of the thiol-ene monomer mixture (0% conversion) and fully polymerized photopolymer (100% conversion) were incorporated into the kinetic model via free volume theory, enabling a highly accurate prediction of both the kinetics and the ultimate conversion, within 5% of experimental values. Leveraging this accurate description of the kinetics, systematic simulations were conducted to evaluate the impact of the cross-linking density, reaction temperature and plasticizer loading on thiol-ene photopolymerization kinetics. Notably, the model predicts thiol-ene reaction mixtures at limited conversions with T g at or below the reaction temperature, a phenomenon rarely observed in chain-growth polymerizations, where T g typically exceeds the reaction temperature under diffusion-controlled conditions. These findings provide critical insights into thiol-ene photopolymerizations and highlight potential strategies for thiol-ene photopolymer development in diverse applications where diffusion-limited kinetics constrains the material performance.

Control of thin film surface features is critical in the fields of optics, biologics, electronics, and microfluidics, among others. Although facile in method, implementation of mechanophotopatterning has been chemically constrained, resulting in an undesired evolution or a limited processing window. This work overcomes these limitations by combining dynamic covalent chemistry to alter the surface relief with a dual-cure approach that increases the cross-link density and glass transition temperature following patterning to permanently fix the structure. The inclusion of a photosensitive dynamic covalent moiety, in the form of an allyl sulfide, allows for spatiotemporal stress relaxation control, and the associated formation of topographic patterns when the elastomer is exposed to light under strain. Typically, the resulting topography remains susceptible to undesirable evolution as the network's dynamic capacity persists. To mitigate the residual dynamics, 65 wt % bisphenol A diglycidyl ether is included, in combination with a thermally latent acid, to facilitate a post-topography altering cationic polymerization which permanently fixes the topography through large changes in cross-link density and glass transition. Feature height of films fixed by this cure remain within 100 nm (<1% change), of their original dimensions.

Dual-thiol holographic photopolymers have been approached from various material schemes; however, optimal performance has yet to be developed, understood and explored. Exploiting the orthogonal nature of the base catalyzed thiol-Michael reaction and the radical mediated thiol-ene photopolymerization, two-stage dual-thiol materials were applied to form holographic photopolymers. Through thiol monomer selection, kinetic selectivity was applied to the thiol-Michael reaction pathway opening the possibility of wider monomer selection. This approach enables one thiol to be chosen for the low refractive index matrix crosslinking agent, and a different thiol to be chosen for the high refractive index photopolymer. The flexibility of this technique facilitates one-pot synthesis of multiple different thiols while maintaining control and separation of the two thiol-click reactions. Herein, we expanded upon a previous approach using kinetic selectivity by pushing the performance of dual-thiol holographic photopolymers through driving up the refractive index contrast of the thiol monomers while exploring the fidelity of thiol-Michael selectivity. High refractive index thiol monomers previously developed are natural candidates to improve holographic photopolymer performance; however, the molecular composition that affects refractive index also impacts the reaction kinetics. By exploring the performance as indicated by the refractive index contrast while understanding the impact on the kinetic selectivity of such monomers, we present a more comprehensive approach to this dual-thiol holographic photopolymer approach.