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100th Anniversary of Macromolecular Science Viewpoint: Photochemical Reaction Orthogonality in Modern Macromolecular Science Nathaniel Corrigan and Cyrille Boyer*
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Centre for Advanced Macromolecular Design (CAMD) and Australian Centre for NanoMedicine (ACN), School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia ABSTRACT: The ability to perform multiple chemical reactions independently (orthogonally) in a single reaction vessel can allow simplified reaction protocols for intricate chemical syntheses. Light is an especially advantageous external stimuli to enact such orthogonal chemical reactions due to its independence with other stimuli, instantaneous spatiotemporal control, and material penetrability. The potential to combine orthogonal chemistry and polymerization is also very appealing, as these systems may open the door for polymeric materials to find applications in emerging and high-tech fields, including biotechnology, microelectronics, sensors, energy, and others. We highlight the use of light in orthogonal polymerization protocols, particularly for living and controlled polymerization, and explore potential future directions and challenges for this technology.
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groups.9 Merrifield described a system in which pentapeptides with t-butoxide (BOC) and dithiasuccinoyl (Dts) protecting groups could be attached to a resin with an o-nitrobenzyl (oNB) linker. Growth of the peptide was achieved via coupling reactions following deprotection of either the BOC group under acidic conditions or the Dts group via thiolysis, while the desired peptide could be released from the resin via photoactivation and cleavage of the oNB linker (Figure 1). Importantly, Merrifield’s three deprotection and cleavage reactions are mutually orthogonal and can thus be performed in an arbitrary order. The ability to independently control such chemical reactions allows significant simplification of multistep processes as purification and handling steps are reduced or eliminated. More importantly, because dormant chemical reactions can be activated on-demand in these systems, different chemical functionalities can be accessed at different times in a single system, as is frequently seen in nature for the regulation of complex biological functions. From a materials science perspective, orthogonal reaction systems can thus provide an avenue for the design of responsive functional materials. However, while there is a rich history of orthogonal reactions in solid-phase peptide synthesis, small molecule coupling reactions, and other organic transformations, application of these protocols in polymerization processes, especially living and controlled polymerizations, has been relatively underex-
t is well-known that polymerization and postmodification processes can be mediated by a variety of external stimuli, such as light, electrical current, mechanical force, and so on. A great deal of work has been performed in recent years to develop these systems and extend their scope of applications to advanced functional materials.1 For externally regulated chemical reactions, including polymerization and polymer postmodification, the ability to activate or deactivate chemical reactions via manipulation of the external stimulus is critical. For instance, in photochemical reactions, external switching of the light source between on and off states can often allow a chemical reaction to start and stop on-demand.2,3 The energy required to overcome the activation energy barrier for these chemical transformations is provided by photons from the light source, which are only generated when the light source is turned on. When the energy for a specific chemical reaction is absent from the system, the chemical reaction cannot be activated and is said to be dormant. Similarly, the reaction temperature,4 chemical (e.g., acid or base) concentrations,5,6 applied currents,7 and so on,8 can all be externally adjusted to induce chemical reactions from the dormant state. Interestingly, the external stimuli used to regulate chemical reactions can often act independently of one another. As a result, careful selection of the reaction conditions and external stimuli can allow specific chemical reactions to be activated in complex systems with multiple reagents; a chemical reaction is said to be orthogonal with another reaction when it can be performed independently of the other reaction in a single pot. An outstanding example of orthogonal chemical reactivity is seen in solid-phase peptide synthesis with multiple protecting © 2019 American Chemical Society
Received: April 19, 2019 Accepted: May 29, 2019 Published: June 19, 2019 812
DOI: 10.1021/acsmacrolett.9b00292 ACS Macro Lett. 2019, 8, 812−818
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Figure 1. Orthogonal deprotection for solid-phase peptide synthesis. Each reaction pathway (BOC removal under acidic conditions, Dts removal via thiolysis, and light-induced oNB cleavage) can be performed independently of the other two reactions.
plored.10 As living and controlled polymerization techniques are undeniably useful for producing well-defined and structurally diverse macromolecules,11,12 the amalgamation of these techniques with orthogonal chemistry will expand the scope of the resulting functional materials to emerging hightech applications. Herein, we highlight some recent polymerization systems mediated by orthogonal external stimuli, with a focus on living and controlled polymerization systems (ionic polymerization, reversible deactivation radical polymerization (RDRP), etc.) and systems that utilize light to regulate chemical reactions. Indeed, light is an exceptional choice of external stimuli in orthogonal reaction protocols due to its independence with other stimuli, and often with itself via the appropriate choice of multiple discrete irradiation wavelengths. Additionally, light’s inherent spatiotemporal characteristics, material penetrability, and temperature independence can open additional opportunities for these systems to be applied in biotechnology, microelectronics, and 3D printing, among many others.13 These opportunities and some future challenges for the progression of this technology are explored toward the end of this viewpoint. In an elegant example of chemical reaction orthogonality in living and controlled polymerization, You, Wu, Hong, and coworkers presented a polymerization system capable of reversible switching by alternating two orthogonal external stimuli, that is, heat and light (Figure 2a).14 The authors outlined the use of “high” temperatures (50 °C) to induce anionic ring opening polymerization (AROP) catalyzed by quaternary onium salts (i.e., tetraphenylphosphonium chloride), and irradiation with blue light to activate Ir(ppy)3catalyzed photoinduced electron/energy transfer-reversible addition−fragmentation chain transfer (PET-RAFT) polymer-
ization. Critically, AROP of 2-(phenoxymethyl) thiirane (POMT) did not occur at low temperatures (20 °C), even after 48 h, while PET-RAFT polymerization of N,Ndimethylacrylamide (DMAm) did not occur in the absence of light. Due to the independent polymerization mechanisms, multiblock copolymers with alternating blocks of POMT and DMAm were synthesized in a single pot by simply alternating the temperature between high and low and the light source between ON and OFF states (Figure 2b,c). Additionally, more complex multiblock structures were synthesized by simply adjusting the lengths of exposure to each orthogonal stimuli. For example, symmetrical gradient copolymers were synthesized by considering the rates of reaction for both the AROP and PET-RAFT polymerizations and programming the external stimuli accordingly, as shown in Figure 2d,e. The seamless switching between the polymerization mechanisms allowed structurally diverse and welldefined copolymers via a one-pot approach; such a strategy may prove useful for generating sequence-defined synthetic polymers, which currently suffer from tedious handling and purification steps during synthesis. Other interesting systems have taken advantage of light and heat as orthogonal external stimuli for living and controlled polymerization and polymer postmodification.15,16 For instance, Johnson and co-workers developed thermoresponsive N-isopropylacrylamide (NIPAm) polymer networks containing a photoredox catalyst (phenothiazine) capable of activating photoinduced RAFT polymerization under irradiation with a compact fluorescent lamp.17 When the reaction temperature was increased to 50 °C, the catalyst-functionalized network underwent a phase transition from a translucent swollen gel to an opaque shrunken network, which restricted photopolymerization from occurring due to significantly decreased light 813
DOI: 10.1021/acsmacrolett.9b00292 ACS Macro Lett. 2019, 8, 812−818
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ability to regulate a single polymerization through multiple orthogonal stimuli allows a greater degree of control over the reaction. The fine-tuning of reaction conditions and orthogonal stimuli should thus allow more complex reaction sequences and cascade reactions to be performed in a simplified manner. In another example, we utilized the change in photocatalyst absorbance at different pH to regulate a PETRAFT polymerization under 650 nm irradiation. The photocatalyst, Zn(II) meso-tetra (4-sulfonatophenyl) porphyrin (ZnTPPS4−), presents a low absorbance at 650 nm in acidic environments (pH ∼ 3.5), which results in a very slow polymerization rate. By switching the pH from 3.5 to 8.5, polymerization was regulated between dormant (OFF) to active (ON) states.18 Orthogonal reaction protocols with light and a chemical stimulus have also recently been developed to exploit the independent reaction mechanisms of different polymerization processes. Our group demonstrated the orthogonal synthesis of diblock copolymers through Ir(ppy)3 catalyzed PET-RAFT polymerization and diphenyl phosphate (DPP) catalyzed ROP from a bifunctional initiator (hydroxy terminated trithiocarbonate).19 The ROP of ε-caprolactone from the hydroxyl group only proceeded in the presence of DPP, while PETRAFT polymerization of methyl acrylate (MA) via the trithiocarbonate moiety only occurred under blue light irradiation; as such, manipulation of the DPP concentration and the light source (ON/OFF) throughout the reaction allowed the radical and cationic reactions to be selectively activated (Figure 3b). Although previous strategies for ROP and radical polymerization in a single-pot had been presented,20 the use of light as external stimulus in this case provided facile temporal control over the poly(methyl acrylate) (poly(MA)) chain growth, which provided an additional level of reaction control. Other systems that use light in conjunction with another stimuli to switch the polymerization mechanism present additional opportunities to prepare complex macromolecular structures with varied functionality that would not be possible through traditional single-pot approaches.21,22 The structure of molecules dictates their ability to absorb light of different wavelengths and subsequently induce selected chemical reactions; molecules with structurally distinct light absorbing groups (chromophores) can thus be used in orthogonal reaction protocols that operate based on differences in the emission wavelengths of the light source. The term chromatic orthogonality, often called λ-orthogonality, was introduced in the early 2000s to define systems in which reaction orthogonality was imparted through different wavelengths of light. For instance, Bochet and Blanc demonstrated that irradiation with 420 nm light induced cleavage of a nitroveratyl ester, while irradiation with 254 nm light induced the cleavage of a benzoin ester with high selectivity.23 Although the nitroveratyl ester was also capable of photolysis under 254 nm light, careful consideration of reagent concentrations and reaction times allowed discrimination of each reaction pathway. Since this early work there have been several examples of chromatic orthogonality in living and controlled polymerization protocols. For instance, Kaji, Goto, and co-workers demonstrated that iodine-mediated polymerization of methacrylates and ROP of δ-valerolactone could be performed in an independent and sequential manner by switching the irradiation wavelength.24 Irradiation under 550−750 nm light excited a benzopyran derivative that was capable of activating
Figure 2. Dually switchable AROP and PET-RAFT polymerization mediated by orthogonal external stimuli (heat and light): (a) reaction components and mechanism; (b) production of multiblock copolymer with similar block lengths; (c) alternating dual ON/OFF stimuli for undecablock copolymers with even block lengths; (d) alternating dual ON/OFF stimuli for symmetrically gradient undecablock copolymers; (e) production of symmetrically gradient undecablock copolymers. Reproduced from ref 14. Copyright 2018 Nature Publishing.
penetration within the network (Figure 3a). Alternatively, polymerization could be effectively stopped or started by switching the light ON or OFF, while the temperature was low, and the network was translucent. As shown in this example, the
Figure 3. Combination of light and a second stimuli to control living and controlled polymerization. (a) Logic gated photopolymerization through light (blue shaded area) and heat (orange shaded area); (b) selective control of ROP by catalyst concentrations (white shaded area, light OFF) and RAFT polymerization by light ON (pink shaded area). Reproduced from refs 17 and 19. Copyright 2017 and 2016 American Chemical Society. 814
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choice of monomers as well as the concentrations of the catalyst allowed each polymerization mechanism to be independently activated. Compared to the previously described processes where each polymerization occurred at a distinct initiating site, for example, alcohol for ROP and trithiocarbonate for RAFT polymerization, this protocol allowed both vinyl ether and acrylate monomers to be inserted into the polymer chain at the same reactive site. As a result, this process provides the possibility to synthesize more complex linear copolymers (multiblock copolymers, gradient copolymers, etc.) with traditionally incompatible monomer families via simple switching of the light source. Other topologically complex polymers have been synthesized via protocols that leverage reactivity differences under discrete irradiation wavelengths.27,28 Orthogonal reaction protocols for living and controlled polymerization have also been used in applications that tailor toward materials science. For instance, Cui and co-workers demonstrated that polymer brushes could be grown from a surface under blue light (460 nm) and subsequently detached under UV light (350 nm).29 In this work, an o-nitrophenyl ethyl containing ATRP initiator was fixed to a surface via dopamine functionalities (Figure 6a); the polymer brushes
iodine mediated polymerization; subsequent irradiation of the reaction mixture with 350−380 nm light induced photolysis of a photoacid generator, which allowed ROP to proceed with high selectivity. Our group also employed a dual wavelength strategy for switching between photoinduced radical (RAFT) polymerization and photomediated ROP.25 In this work, a merocyanine photoacid capable of reversible proton dissociation was used, which allowed ROP to be activated under blue light and deactivated in the dark, while red light irradiation activated zinc tetraphenylporphyrin (ZnTPP) catalyzed PETRAFT polymerization. As such, an ionic or radical polymerization mechanism could be alternatively induced by switching the light source between blue and red, and block copolymers could be formed by using a bifunctional initiator (Figure 4).
Figure 4. Diblock copolymer synthesis via selective activation of photomediated ROP and PET-RAFT under two different irradiation wavelengths. Adapted with permission from ref 25. Copyright 2016 The Royal Society of Chemistry.
Importantly, the discrete light absorption by the two chromophores (merocyanine photoacid and ZnTPP) under blue and red light allowed polymerization mechanisms to be independently controlled. Fors and co-workers also demonstrated that polymerization mechanisms could be switched between cationic and radical by switching the light source from green to blue (Figure 5).26
Figure 6. Wavelength orthogonality for growth and detachment of polymer chains from a surface. Reproduced with permission from ref 29. Copyright 2018 American Chemical Society.
were grown by surface-initiated ICAR-ATRP using dimanganese decacarbonyl (Mn2(CO)10) as radical generator under blue light irradiation. The polymer chains were subsequently detached on-demand by photolyzing the o-nitrophenyl ethyl group under UV light. Importantly, this dual wavelength strategy allowed patterning of arbitrary surface patterns in a two-step approach. Irradiation of the surface with a photomask in the first step allowed polymer brushes to be grown only in the irradiated areas; similarly, UV irradiation of the polymer patterned surface with a photomask detached the polymer brushes only in the previously patterned and now irradiated regions (Figure 6b,c). Notably, our group has presented a procedure for polymerization-induced self-assembly (PISA) under red light irradiation to form nanoparticles and subsequent UV light-induced
Figure 5. Interconversion of cationic and radical polymerization via switching of the irradiation wavelength. Reprinted with permission from ref 26. Copyright 2017 American Chemical Society.
Under blue light irradiation, the photocatalyst Ir(ppy)3 was activated, leading to reduction of the trithiocarbonate and polymerization of acrylates via a radical mechanism; alternatively, green light irradiation excited a pyrylium photocatalyst, which oxidized the trithiocarbonate and led to cationic polymerization of isobutyl vinyl ether (IBVE). Although the pyrylium catalyst also absorbed blue light, the 815
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quickly diversified by increasing the number of external stimuli used in combination, and heat, light, electrochemistry, mechanical force, chemical concentrations, and so on, can theoretically all be used together to perform orthogonal chemical reactions for living and controlled polymerizations. Although some systems have been developed that use light in conjunction with another stimuli for reaction orthogonality, the full scope and combination of external stimuli are yet to be realized. For instance, living and controlled polymerizations can be mediated by two different wavelengths of light and two different electrical currents,42 however, the combination of electrical current and light as external stimuli for living and controlled polymerizations has not been explored. While there is great potential for expanding the scope of macromolecular syntheses by combining multiple external stimuli, further developments in terms of orthogonal reaction chemistry will be required. The expansion of orthogonal chemistry will provide polymer chemists with an opportunity to streamline complex multistep macromolecular syntheses; as the need for complex macromolecular products increases, so too does the need for simplified synthetic protocols. Orthogonal chemical reactions can assist here, as multistep reactions can be performed in a single reaction vessel by simple adjustment of external stimuli. Although multistep processes in living and controlled polymerization systems have certainly been well-explored to date, repeated purification and handling steps often limit the practicality of production, except in some cases.43−45 By reducing numerous isolation steps, orthogonal chemistry can simplify syntheses and provide a more attractive alternative for complex macromolecular production. Furthermore, as orthogonal chemistry expands, more systems that can operate concurrently (rather than sequentially) under multiple stimuli will be realized. This is particularly pertinent for the advancement of accessible but complex material systems, such as multimaterial 3D printing systems, which typically suffer from repetitive synthetic steps (vide infra). Additionally, reducing isolation steps may improve the cost-effectiveness and environmental impact of macromolecular synthesis to a point where the production is economically viable. Such approaches may be critical for producing precision polymers on industrial scales for commercial applications. There also exists a very promising opportunity to more broadly apply orthogonal chemical reactions in materials science; the ability to activate specific reaction pathways on-demand will allow access to new stimuli responsive functional materials for various high-tech applications. As mentioned above, orthogonal reaction protocols have already been employed in materials science, however, more complex systems that tailor for specific applications can be envisioned. Particularly, the rapid expansion of photomediated 3D printing technology has allowed the production of polymeric materials with endless geometries; the combination of these manufacturing processes with orthogonal chemistry will allow nonexperts to produce geometrically complex objects that can undergo on-demand functionality changes via an applied external stimuli.46 Indeed, some outstanding systems have been developed recently that allow polymeric materials to be 3D printed using light-based approaches and subsequently altered under a second stimulus.47−49 While these materials were fabricated through conventional polymerization, the possibility to combine living and controlled polymerization into similar systems will only
nanoparticle photodissociation by photocleavage of nitrophenylester groups.30 Such a strategy could prove useful for on-demand cargo delivery in situ. In a related process, polymeric nanoparticles were initially formed via the PISA approach under red light and subsequently exposed to UV light irradiation to induce cross-linking and permanently fix the nanoparticle morphologies.31 Chromatic orthogonality has also been applied for the reversible formation and dissociation of cross-linked polymer networks.32−34 While the cited systems do not use living or controlled polymerization techniques, the wavelength-gated orthogonal chemistry used in these transformations may hold great promise for other systems, particularly in materials science. The previous examples show the current capacity of reaction orthogonality in living and controlled polymerization processes; however, there are still many challenges that remain for the full potential of these systems to be realized (Figure 7).
Figure 7. Challenges and opportunities for photochemical reaction orthogonality in modern macromolecular science.
Arguably, the biggest challenge is the development of orthogonal/selective chemistry; more diverse orthogonal chemistry that can be applied to living and controlled polymerization is needed, particularly for systems that exploit chromatic orthogonality as these systems are relatively underdeveloped. In this regard, polymer scientists can draw inspiration from orthogonal organic chemical reactions.35,36 Indeed, some groups have already applied orthogonal small molecule chemical reactions for macromolecular transformations.37,38 For instance, Barner-Kowollik, Blinco, and coworkers demonstrated that star polymers could be synthesized via λ-orthogonal coupling reactions, where the irradiation wavelength and reaction sequence dictated the structure of the final polymers.39 Notably, Barner-Kowollik and co-workers have developed other λ-orthogonal chemical reaction protocols for organic and polymer syntheses.40,41 The development of orthogonal chemistry can also be aided by computational chemistry. Current orthogonal approaches often suffer from nonperfect selective activation of reaction pathways; in these cases, fine control over the reaction conditions is required to suppress the unwanted chemical reaction. To expand the scope of orthogonal reactions in living and controlled polymerization and allow broader, less stringent reaction conditions, more precise and selective chemical reactions are required. Indeed, a great challenge for this technology is the development of specific chemical reactions that can be externally and independently activated under wide ranging conditions. Considering this, rapid screening and optimization of reaction pathways and reagents via computational chemistry may prove invaluable in reducing the time required to find suitable reactions that operate completely independently of one another under wide ranging conditions. Another challenge is the combination of multiple external stimuli for reaction orthogonality; reaction complexity can be 816
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(7) Magenau, A. J. D.; Strandwitz, N. C.; Gennaro, A.; Matyjaszewski, K. Electrochemically Mediated Atom Transfer Radical Polymerization. Science 2011, 332 (6025), 81. (8) Piermattei, A.; Karthikeyan, S.; Sijbesma, R. P. Activating catalysts with mechanical force. Nat. Chem. 2009, 1, 133. (9) Merrifield, R. B. Solid Phase Synthesis (Nobel Lecture). Angew. Chem., Int. Ed. Engl. 1985, 24 (10), 799−810. (10) Wong, C.-H.; Zimmerman, S. C. Orthogonality in organic, polymer, and supramolecular chemistry: from Merrifield to click chemistry. Chem. Commun. 2013, 49 (17), 1679−1695. (11) Boyer, C.; Corrigan, N. A.; Jung, K.; Nguyen, D.; Nguyen, T.K.; Adnan, N. N. M.; Oliver, S.; Shanmugam, S.; Yeow, J. CopperMediated Living Radical Polymerization (Atom Transfer Radical Polymerization and Copper(0) Mediated Polymerization): From Fundamentals to Bioapplications. Chem. Rev. 2016, 116 (4), 1803− 1949. (12) Kamigaito, M.; Satoh, K.; Uchiyama, M. Degenerative chaintransfer process: Controlling all chain-growth polymerizations and enabling novel monomer sequences. J. Polym. Sci., Part A: Polym. Chem. 2019, 57 (3), 243−254. (13) Corrigan, N.; Yeow, J.; Judzewitsch, P.; Xu, J.; Boyer, C. Seeing the Light: Advancing Materials Chemistry through Photopolymerization. Angew. Chem., Int. Ed. 2019, 58 (16), 5170−5189. (14) Zhang, Z.; Zeng, T.-Y.; Xia, L.; Hong, C.-Y.; Wu, D.-C.; You, Y.-Z. Synthesis of polymers with on-demand sequence structures via dually switchable and interconvertible polymerizations. Nat. Commun. 2018, 9 (1), 2577. (15) Kaupp, M.; Hiltebrandt, K.; Trouillet, V.; Mueller, P.; Quick, A. S.; Wegener, M.; Barner-Kowollik, C. Wavelength selective polymer network formation of end-functional star polymers. Chem. Commun. 2016, 52 (9), 1975−1978. (16) Carmean, R. N.; Figg, C. A.; Becker, T. E.; Sumerlin, B. S. Closed-System One-Pot Block Copolymerization by TemperatureModulated Monomer Segregation. Angew. Chem., Int. Ed. 2016, 55 (30), 8624−8629. (17) Chen, M.; Deng, S.; Gu, Y.; Lin, J.; MacLeod, M. J.; Johnson, J. A. Logic-Controlled Radical Polymerization with Heat and Light: Multiple-Stimuli Switching of Polymer Chain Growth via a Recyclable, Thermally Responsive Gel Photoredox Catalyst. J. Am. Chem. Soc. 2017, 139 (6), 2257−2266. (18) Shanmugam, S.; Xu, J.; Boyer, C. A logic gate for external regulation of photopolymerization. Polym. Chem. 2016, 7 (42), 6437− 6449. (19) Fu, C.; Xu, J.; Kokotovic, M.; Boyer, C. One-Pot Synthesis of Block Copolymers by Orthogonal Ring-Opening Polymerization and PET-RAFT Polymerization at Ambient Temperature. ACS Macro Lett. 2016, 5 (4), 444−449. (20) Duxbury, C. J.; Wang, W.; de Geus, M.; Heise, A.; Howdle, S. M. Can Block Copolymers Be Synthesized by a Single-Step Chemoenzymatic Route in Supercritical Carbon Dioxide? J. Am. Chem. Soc. 2005, 127 (8), 2384−2385. (21) 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, DOI: 10.1039/C8PY01417E. (22) Keyes, A.; Basbug Alhan, H. E.; Ha, U.; Liu, Y.-S.; Smith, S. K.; Teets, T. S.; Beezer, D. B.; Harth, E. Light as a Catalytic Switch for Block Copolymer Architectures: Metal−Organic Insertion/Light Initiated Radical (MILRad) Polymerization. Macromolecules 2018, 51 (18), 7224−7232. (23) Blanc, A.; Bochet, C. G. Wavelength-Controlled Orthogonal Photolysis of Protecting Groups. J. Org. Chem. 2002, 67 (16), 5567− 5577. (24) Ohtsuki, A.; Lei, L.; Tanishima, M.; Goto, A.; Kaji, H. Photocontrolled Organocatalyzed Living Radical Polymerization Feasible over a Wide Range of Wavelengths. J. Am. Chem. Soc. 2015, 137 (16), 5610−5617.
increase the control over network architectures and thus materials properties. Furthermore, new materials with disparate properties will be produced via reaction orthogonality, especially using chromatic orthogonal chemistry. Already, some exceptional systems have been developed that take advantage of wavelength selective chemistry to produce materials with chemically distinct domains and novel mechanical properties.50−53 Such λorthogonal chemistry opens the door for the production of multimaterial objects via 3D printing approaches, which is currently a significant challenge. Indeed, as recently shown by Hawker and co-workers,52 as well as Schwartz and Boydston,53 alteration of the light source in λ-orthogonal systems can be used to control the mechanical properties of multimaterials manufactured via 3D printing approaches. The use of light in these approaches is critical as different domains can be spatially resolved; this provides access to patterned materials with tailored mechanical and chemical properties, often via simplified one-step approaches. Such systems begin to approach the complexity of natural materials and present significantly more variability in terms of composition and function compared to traditional single component materials. Again, the amalgamation of these techniques with living and controlled polymerization will allow increased control over the reactions and resulting materials. Undoubtedly, the unbridled potential of orthogonal chemistry will provide new possibilities for macromolecular syntheses. Furthermore, the combination of photochemistry, living and controlled polymerization, and reaction orthogonality will drive innovation for new advanced materials.
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AUTHOR INFORMATION
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[email protected]. ORCID
Cyrille Boyer: 0000-0002-4564-4702 Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acsmacrolett.9b00292 ACS Macro Lett. 2019, 8, 812−818
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DOI: 10.1021/acsmacrolett.9b00292 ACS Macro Lett. 2019, 8, 812−818
