Photoswitching Cationic and Radical Polymerizations: Spatiotemporal Control of Thermoset Properties

Two Material Properties from One Wavelength-Orthogonal Photoresin Enabled by a Monochromatic Laser Integrated Stereolithographic Apparatus (Mono LISA)

Multi-material printing has experienced critical advances in recent years, yet material property differentiation capabilities remain limited both with regard to the accessible properties - typically hard versus soft - and the achievable magnitude of differentiation. To enhance multi-material printing capabilities, precise photochemical control during 3D printing is essential. Wavelength-differentiation is a particularly intriguing concept yet challenging to implement. Notably, dual-wavelength printing to fabricate hard and soft sections within one object has emerged, where one curing process is insensitive to visible light, while UV irradiation inevitably activates the entire resin, limiting true spatio-temporal control of the material properties. Until now, pathway-independent wavelength-orthogonal printing has not been realized, where each wavelength exclusively triggers only one of two possible reactions, independent of the order in which the wavelengths are applied. Herein, a multi-wavelength printing technique is introduced employing a tunable laser to monochromatically deliver light to the printing platform loaded with a fully wavelength-orthogonal resin. Guided by photochemical action plots, two distinct wavelengths - each highly selective toward a specific photocycloaddtion reaction - are utilized to generate distinct networks within the photoresin. Ultimately, together with the printing technique, this orthogonally addressable photoresin allows fabricating multi-material objects with degradable and non-degradable properties, in a single fabrication step.

Rapid 3D printing of unlayered, tough epoxy-alcohol resins with late gel points via dual-color curing technology

Additive manufacturing technologies and, in particular, vat photopolymerization promise complex structures that can be made in a fast and easy fashion for highly individualized products. While the technology has upheld this promise many times already, some polymers are still out of reach or at least problematic to print reliably. High-performance epoxide-based resins, which are regulated by chain transfer via multifunctional alcohols, are a typical example of resins with late gel points, which require long irradiation times and high light intensities to print. Therefore, we have developed a dual-colour printing approach where rapid radical curing of a soft, wide-meshed polymer network facilitates fast and easy 3D structuring of the subsequently slow curing step-growth formulation at an orthogonal initiation-wavelength regime. Thereby the methacrylate system acts as a scaffold for an uncured epoxide alcohol system during the printing process, which is then cured with UV light post-printing. This way tough alcohol-regulated epoxy-systems become accessible to vat photopolymerization achieving outstanding high-resolution 3D printed parts without significant layering effects. The demonstrated wide-meshed matrix-assisted printing approach has the potential to make a multitude of slowly curing resins accessible to vat photopolymerization techniques, at low irradiation intensities and high curing speeds.

Acid-Enhanced Photoiniferter Polymerization under Visible Light

Photoiniferter (PI) is a promising polymerization methodology, often used to overcome restrictions posed by thermal reversible addition-fragmentation chain-transfer (RAFT) polymerization. However, in the overwhelming majority of reports, high energy UV irradiation is required to effectively trigger photolysis of RAFT agents and facilitate the polymerization, significantly limiting its potential, scope, and applicability. Although visible light PI has emerged as a highly attractive alternative, most current approaches are limited to the synthesis of lower molecular weight polymers (i.e. 10,000 g/mol), and typically suffer from prolonged reaction times, extended induction periods, and higher dispersities when high activity CTAs (photoiniferters), such as trithiocarbonates, are employed. Herein, an acid-enhanced PI polymerization is introduced that efficiently operates under visible light irradiation. The presence of small amounts of biocompatible citric acid fully eliminates the lengthy induction period (21 hours) by enhancing photolysis, rapidly consuming the CTA, and accelerating the reaction rate, yielding polymers with narrow molar mass distributions (Ð ~1.1), near-quantitative conversions (>97 %), and high end-group fidelity in just two hours. A particularly noteworthy aspect of this work is the possibility to target very high degrees of polymerization (i.e. DP=3,000) within short timescales (i.e. less than five hours) without compromising the control over the dispersity (Ð ~1.1). The versatility of the technique is further demonstrated through the synthesis of well-defined diblock copolymers and its compatibility to various polymer classes (i.e. acrylamides, acrylates, methacrylates), thus establishing visible-light PI as a robust tool for polymer synthesis.

Catalyzing PET-RAFT Polymerizations Using Inherently Photoactive Zinc Myoglobin

Protein photocatalysts provide a modular platform to access new reaction pathways and affect product outcomes, but their use in polymer synthesis is limited because co-catalysts and/or co-reductants are required to complete catalytic cycles. Herein, we report using zinc myoglobin (ZnMb), an inherently photoactive protein, to mediate photoinduced electron/energy transfer (PET) reversible addition-fragmentation chain transfer (RAFT) polymerizations. Using ZnMb as the sole reagent for catalysis, photomediated polymerizations of N,N-dimethylacrylamide in PBS were achieved with predictable molecular weights, dispersity values approaching 1.1, and high chain-end fidelity. We found that initial apparent rate constants of polymerization increased from 4.6×10-5 s-1 for zinc mesoporpyhrin IX (ZnMIX) to 6.5×10-5 s-1 when ZnMIX was incorporated into myoglobin to yield ZnMb, indicating that the protein binding site enhanced catalytic activity. Chain extension reactions comparing ZnMb-mediated RAFT polymerizations to thermally-initiated RAFT polymerizations showed minimal differences in block copolymer molecular weights and dispersities. This work enables studies to elucidate how protein modifications (e.g., secondary structure folding, site-directed mutagenesis, directed evolution) can be used to modulate polymerization outcomes (e.g., selective monomer additions towards sequence control, tacticity control, molar mass distributions).

Multimaterial Thermoset Synthesis: Switching Polymerization Mechanism with Light Dosage

The synthesis of polymeric thermoset materials with spatially controlled physical properties using readily available resins is a grand challenge. To address this challenge, we developed a photoinitiated polymerization method that enables the spatial switching of radical and cationic polymerizations by controlling the dosage of monochromatic light. This method, which we call Switching Polymerizations by Light Titration (SPLiT), leverages the use of substoichiometric amounts of a photobuffer in combination with traditional photoacid generators. Upon exposure to a low dose of light, the photobuffer inhibits the cationic polymerization, while radical polymerization is initiated. With an increased light dosage, the buffer system saturates, leading to the formation of a strong acid that initiates a cationic polymerization of the dormant monomer. Applying this strategy, patterning is achieved by spatially varying light dosage via irradiation time or intensity allowing for simple construction of multimaterial thermosets. Importantly, by the addition of an inexpensive photobuffer, such as tetrabutylammonium chloride, commercially available resins can be implemented in grayscale vat photopolymerization 3D printing to prepare sophisticated multimodulus constructs.

Combining photocontrolled-cationic and anionic-group-transfer polymerizations using a universal mediator: enabling access to two- and three-mechanism block copolymers

An ongoing challenge in polymer chemistry is accessing diverse block copolymers from multiple polymerization mechanisms and monomer classes. One strategy to accomplish this goal without intermediate compatibilization steps is the use of universal mediators. Thiocarbonyl thio (TCT) functional groups are well-known mediators to combine radical with either cationic or anionic polymerization, but a sequential cationic-anionic universal mediator system has never been reported. Herein, we report a TCT universal mediator that can sequentially perform photocontrolled cationic polymerization and thioacyl anionic group transfer polymerization to access poly(ethyl vinyl ether)-block-poly(thiirane) polymers for the first time. Thermal analyses of these block copolymers provide evidence of microphase separation. The success of this system, along with the established compatibility of radical polymerization, enabled us to further chain extend the cationic-anionic diblock using radical polymerization of N-isopropylacrylamide. The resulting terpolymer represents the first example of a triblock made from three different monomer classes incorporated via three different mechanisms without any end-group modification steps. The development of this simple, sequential synthesis using a universal mediator approach opens up new possibilities by providing facile access to diverse block copolymers of vinyl ethers, thiiranes, and acrylamides.

Multi-color dual wavelength vat photopolymerization 3D printing via spatially controlled acidity

Dual wavelength vat photopolymerization (DW-VP) has emerged as a powerful approach to create multimaterial objects. However, only a limited range of properties have been showcased. In this work, we report the 3D printing (3DP) of multi-color objects from a single resin vat using DW-VP. This was accomplished by concurrently curing resin with visible light and modulating local resin color with 365-nm ultraviolet (UV) light. The key advance was to use a photoacid generator (PAG) in combination with pH responsive dyes in the 3DP resins. The specific color is dictated by the extent of reaction, or local acidity in our case, and controlled by the light dosage and pattern of UV light applied. Multi-color object formation was implemented in two-step processes involving first 3DP to set the object structure, followed by UV exposure, as well as single processes that leveraged DW-VP to create a broad range of vibrant colors and patterns.

PhotoROMP: The Future Is Bright

Since the earliest investigations of olefin metathesis catalysis, light has been the choice for controlling the catalyst activity on demand. From the perspective of energy efficiency, temporal and spatial control, and selectivity, photochemistry is not only an attractive alternative to traditional thermal manufacturing techniques but also arguably a superior manifold for advanced applications like additive manufacturing (AM). In the last three decades, pioneering work in the field of ring-opening metathesis polymerization (ROMP) has broadened the scope of material properties achievable through AM, particularly using light as both an activating and deactivating stimulus. In this Perspective, we explore trends in photocontrolled ROMP systems with an emphasis on approaches to photoinduced activation and deactivation of metathesis catalysts. Recent work has yielded a myriad of commercial and synthetically accessible photosensitive catalyst systems, although comparatively little attention has been paid to achieving precise control over polymer morphology using light. Metal-free, photophysical, and living ROMP systems have also been relatively underexplored. To take fuller advantage of both the thermomechanical properties of ROMP polymers and the operational simplicity of photocontrol, clear directions for the field are to improve the reversibility of activation and deactivation strategies as well as to further develop photocontrolled approaches to tuning cross-link density and polymer tacticity.

Lamp vs Laser: A Visible Light Photoinitiator That Promotes Radical Polymerization at Low Intensities and Cationic Polymerization at High Intensities

A visible light absorbing anthraquinone derivative 1-tosyloxy-2-methoxy-9,10-anthraquinone (QT) mediates both cationic and radical polymerizations depending on the intensity of visible light used. A previous study showed that this initiator generates para-toluenesulfonic acid through a stepwise, two-photon excitation mechanism. Thus, under high-intensity irradiation, QT generates acid in sufficient quantities to catalyze the cationic ring-opening polymerization of lactones. However, under low-intensity (lamp) conditions, the two-photon process is negligible, and QT photooxidizes DMSO, generating methyl radicals which initiate the RAFT polymerization of acrylates. This dual capability was utilized to switch between radical and cationic polymerizations to synthesize a copolymer using a one-pot procedure.

Fast Living 3D Printing via Free Radical Promoted Cationic RAFT Polymerization

The application of reversible deactivation radical polymerization techniques in 3D printing is emerging as a powerful method to build "living" polymer networks, which can be easily postmodified with various functionalities. However, the building speed of these systems is still limited compared to commercial systems. Herein, a digital light processing (DLP)-based 3D printing system via photoinduced free radical-promoted cationic reversible addition-fragmentation chain transfer polymerization of vinyl ethers, which can build "living" objects by a commercial DLP 3D printer at a relatively fast building speed (12.99 cm h^-1 ), is reported. The polymerization behavior and printing conditions are studied in detail. The livingness of the printed objects is demonstrated by spatially controlled postmodification with a fluorescent monomer.

Organocatalyzed Controlled Radical Copolymerization toward Hybrid Functional Fluoropolymers Driven by Light

Photo-controlled polymerizations are attractive to tailor macromolecules of complex compositions with spatiotemporal regulation. In this work, with a convenient synthesis for trifluorovinyl boronic ester (TFVB), we report a light-driven organocatalyzed copolymerization of vinyl monomers and TFVB for the first time, which enabled the controlled synthesis of a variety of hybrid fluorine/boron polymers with low dispersities and good chain-end fidelity. The good behaviors of "ON/OFF" switch, chain-extension polymerizations and post-modifications further highlight the versatility and reliability of this copolymerization. Furthermore, we demonstrate that the combination of fluorine and boron could furnish copolymer electrolytes of high lithium-ion transference number (up to 0.83), bringing new opportunities of engineering high-performance materials for energy storage purposes.

Polymeric multimaterials by photochemical patterning of crystallinity

An organized combination of stiff and elastic domains within a single material can synergistically tailor bulk mechanical properties. However, synthetic methods to achieve such sophisticated architectures remain elusive. We report a rapid, facile, and environmentally benign method to pattern strong and stiff semicrystalline phases within soft and elastic matrices using stereo-controlled ring-opening metathesis polymerization of an industrial monomer, cis-cyclooctene. Dual polymerization catalysis dictates polyolefin backbone chemistry, which enables patterning of compositionally uniform materials with seamless stiff and elastic interfaces. Visible light-induced activation of a metathesis catalyst results in the formation of semicrystalline trans polyoctenamer rubber, outcompeting the formation of cis polyoctenamer rubber, which occurs at room temperature. This bottom-up approach provides a method for manufacturing polymeric materials with promising applications in soft optoelectronics and robotics.

Monophosphoniums as Effective Photoredox Organocatalysts for Visible Light-Regulated Cationic RAFT Polymerization

Visible light-regulated metal-free polymerizations have attracted considerable attention for macromolecular syntheses in recent years. However, few organic photocatalysts show high efficiency and strict photocontrol in cationic polymerizations. Herein, we introduce monophosphonium-doped polycyclic arenes as an organic photocatalyst, which features the high tunability, broad redox window, long excited state lifetime, and excellent temporal control in the cationic reversible addition-fragmentation chain transfer polymerization of vinyl ethers. A correlation of the catalytic performance and the photophysical and electrochemical properties of photocatalysts is also discussed.

Photoredox Catalysis in Photocontrolled Cationic Polymerizations of Vinyl Ethers

ConspectusAdvances in photocontrolled polymerizations have expanded the scope of polymer architectures and structures that can be synthesized for various applications. The majority of these polymerizations have been developed for radical processes, which limits the diversity of monomers that can be used in macromolecular design. More recent developments of photocontrolled cationic polymerizations have taken a step toward addressing this limitation and have expanded the palette of monomers that can be used in stimuli-regulated polymerizations, enabling the synthesis of previously inaccessible polymeric structures. This Account will detail our group's studies on cationic polymerization processes where chain growth is regulated by light and highlight how these methods can be combined with other stimuli-controlled polymerizations to precisely dictate macromolecular structure.Photoinitiated cationic polymerizations are well-studied and important processes that have control over initiation. However, we wanted to develop systems where we had spatiotemporal control over both polymer initiation and chain growth. This additional command over the reaction provides the ability to manipulate the growing polymer with an external stimulus during a polymerization, which can be used to control structure. To achieve this goal, we set out to develop a method to photoreversibly generate a cation at a growing chain end that could participate in a controlled polymerization process. We took inspiration from previous work on cationic degenerate chain transfer polymerizations of vinyl ethers that used thiocarbonylthio chain transfer agents. These polymerizations were initiated by a strong acid and gave well-defined poly(vinyl ether)s. We posited that we could remove the acid initiator in these systems and reversibly oxidize the thiocarbonylthio chain ends in these reactions with a photocatalyst to give a photocontrolled cationic polymerization of vinyl ethers. This Account will focus on our journey to discover cationic photocontrolled polymerizations. We will summarize our initial developments and detail our mechanistic understanding of these reactions using both organic and inorganic based photocatalysts, and we will outline more recent efforts to expand cationic degenerate chain transfer polymerizations to other thioacetal initiators. Finally, we will detail how these photocontrolled cationic polymerizations can be used to switch monomer selectivity in situ using light to control polymer structure. At the end of the Account, we will discuss our vision for future potential applications of these photocontrolled cationic polymerizations in the synthesis of novel block copolymers and next generation cross-linked networks.

NIR-sensitized hybrid radical and cationic photopolymerization of several cyanines in combination with diaryliodonium bis(trifluoromethyl)sulfonyl imide

A series of cyanines exhibiting absorption between 750 and 930 nm reacted after NIR excitation with the bis(t-butylphenyl) iodonium cation comprising the [(CF3SO2)2N]− anion (NTf2)−, resulting in the generation of free radicals and conjugate acids.

One-pot synthesis of structure-controlled temperature-responsive polymer gels

The simultaneous use of metal Lewis acids and photo-radical generators for dithioesters, which are the common dormant species for cationic and radical polymerization, made it possible to convert a cationic species into a radical by photoirradiation.

The challenges of controlling polymer synthesis at the molecular and macromolecular level

In this Perspective, we outline advances and challenges in controlling the structure of polymers at various size regimes in the context of structural features such as molecular weight distribution, end groups, architecture, composition and sequence.