Anthracene-Based Thiol–Ene Networks with Thermo-Degradable and Photo-Reversible Properties

New Advances in Covalent Network Polymers via Dynamic Covalent Chemistry

Covalent network polymers, as materials composed of atoms interconnected by covalent bonds in a continuous network, are known for their thermal and chemical stability. Over the past two decades, these materials have undergone significant transformations, gaining properties such as malleability, environmental responsiveness, recyclability, crystallinity, and customizable porosity, enabled by the development and integration of dynamic covalent chemistry (DCvC). In this review, we explore the innovative realm of covalent network polymers by focusing on the recent advances achieved through the application of DCvC. We start by examining the history and fundamental principles of DCvC, detailing its inception and core concepts and noting its key role in reversible covalent bond formation. Then the reprocessability of covalent network polymers enabled by DCvC is thoroughly discussed, starting from the significant milestones that marked the evolution of these polymers and progressing to their current trends and applications. The influence of DCvC on the crystallinity of covalent network polymers is then reviewed, covering their bond diversity, synthesis techniques, and functionalities. In the concluding section, we address the current challenges faced in the field of covalent network polymers and speculates on potential future directions.

Chemical Recycling and Physical Tuning of Necklace-Shaped Polydimethylsiloxanes Bearing Anthracene Dimer Units

The problem of plastic waste in the environment calls for the development of new polymeric materials designed specifically for easy recycling at the end of their life cycle. Herein, a green polymer system comprising a series of necklace-shaped polydimethylsiloxanes bearing anthracene dimer units is developed. The polymers have low environmental impact and are easily recycled. Further, their flexibility and glass transition temperatures are easy to control. These necklace-shaped inorganic polymers are synthesized by photopolymerizing (dimerizing) anthracene-terminated oligo-dimethylsiloxane monomers. A key achievement of the present work is the successful chemical recovery of the monomers from the polymers through thermal depolymerization, enabling monomer-polymer recycling. By applying equilibrium polymerization with base catalysts, monomers with a controlled distributed chain length are synthesized from monomers with a constant chain length. The necklace-shaped polymers synthesized from these randomized monomers have amorphous structures and readily form transparent films. It is possible to modulate the thermal and mechanical properties of the polymers by controlling the average chain length of the polydimethylsiloxane between the anthracene dimers. This investigation presents a method for the synthesis and cyclic utilization of polymer materials with a wide range of applications, including plastics and elastomers.

A renewably sourced, circular photopolymer resin for additive manufacturing

The additive manufacturing of photopolymer resins by means of vat photopolymerization enables the rapid fabrication of bespoke 3D-printed parts. Advances in methodology have continually improved resolution and manufacturing speed, yet both the process design and resin technology have remained largely consistent since its inception in the 1980s1. Liquid resin formulations, which are composed of reactive monomers and/or oligomers containing (meth)acrylates and epoxides, rapidly photopolymerize to create crosslinked polymer networks on exposure to a light stimulus in the presence of a photoinitiator2. These resin components are mostly obtained from petroleum feedstocks, although recent progress has been made through the derivatization of renewable biomass3-6 and the introduction of hydrolytically degradable bonds7-9. However, the resulting materials are still akin to conventional crosslinked rubbers and thermosets, thus limiting the recyclability of printed parts. At present, no existing photopolymer resin can be depolymerized and directly re-used in a circular, closed-loop pathway. Here we describe a photopolymer resin platform derived entirely from renewable lipoates that can be 3D-printed into high-resolution parts, efficiently deconstructed and subsequently reprinted in a circular manner. Previous inefficiencies with methods using internal dynamic covalent bonds10-17 to recycle and reprint 3D-printed photopolymers are resolved by exchanging conventional (meth)acrylates for dynamic cyclic disulfide species in lipoates. The lipoate resin platform is highly modular, whereby the composition and network architecture can be tuned to access printed materials with varied thermal and mechanical properties that are comparable to several commercial acrylic resins.

The Use of the Thiol-Ene Addition Click Reaction in the Chemistry of Organosilicon Compounds: An Alternative or a Supplement to the Classical Hydrosilylation?

This review presents the main achievements in the use of the thiol-ene reaction in the chemistry of silicones. Works are considered, starting from monomers and ending with materials.The main advantages and disadvantages of this reaction are demonstrated using various examples. A critical analysis of the use of this reaction is made in comparison with the hydrosilylation reaction.

Intrinsically Re-curable Photopolymers Containing Dynamic Thiol-Michael Bonds

The development of photopolymers that can be depolymerized and subsequently re-cured using the same light stimulus presents a significant technical challenge. A bio-sourced terpenoid structure, l-carvone, inspired the creation of a re-curable photopolymer in which the orthogonal reactivity of an irreversible thioether and a dynamic thiol-Michael bond enables both photopolymerization and thermally driven depolymerization of mechanically robust polymer networks. The di-alkene containing l-carvone was partially reacted with a multi-arm thiol to generate a non-crosslinked telechelic photopolymer. Upon further UV exposure, the photopolymer crosslinked into a mechanically robust network featuring reversible Michael bonds at junction points that could be activated to revert, or depolymerize, the network into a viscous telechelic photopolymer. The regenerated photopolymer displayed intrinsic re-curability over two recycles while maintaining the desirable thermomechanical properties of a conventional network: insolubility, resistance to stress relaxation, and structural integrity up to 170 °C. Our findings present an on-demand, re-curable photopolymer platform based on a sustainable feedstock.

Photo-Controlled [4+4] Cycloaddition of Anthryl-Polymer Systems: A Versatile Approach to Fabricate Functional Materials

The reversible photo-induced [4+4] cycloaddition reaction of anthracene enables multiple cycles of dimerization and scission, allowing phototunable linkage of molecular fragments for the synthesis of polymer scaffolds. New functional materials ranging from hydrogels to shape-memory polymers were designed from anthryl-polymer systems because of their diverse photochemical reactivity and responsiveness. Light as an external stimulus allows for the remote and precise spatiotemporal control of materials without the need for additional reagents. Depending on how the photoreactive anthracene moieties were introduced, the interaction of anthryl-polymer systems with light results in various processes such as polymerization, cyclization, and cross-linking. Structural modifications of anthracene derivatives could shift their absorption from the ultraviolet to the visible light region, widening their range of applications including biologically relevant studies. These applications are further diversified and enhanced by the reversibility of the dimerization reaction using light and heat as stimuli. In this review, current developments in the synthesis and photodimerization of anthracene-containing polymers and their emerging applications in the fabrication of new materials are discussed.

Exchangeable Liquid Crystalline Elastomers and Their Applications

This Review presents and discusses the current state of the art in "exchangeable liquid crystalline elastomers", that is, LCE materials utilizing dynamically cross-linked networks capable of reprocessing, reprogramming, and recycling. The focus here is on the chemistry and the specific reaction mechanisms that enable the dynamic bond exchange, of which there is a variety. We compare and contrast these different chemical mechanisms and the key properties of their resulting elastomers. In the conclusion, we discuss the most promising applications that are enabled by dynamic cross-linking and present a summary table: a library of currently available materials and their main characteristics.

Photoswitchable Covalent Adaptive Networks Based on Thiol-Ene Elastomers

Covalent adaptive networks combine the advantages of cross-linked elastomers and dynamic bonding in a single system. In this work, we demonstrate a simple one-pot method to prepare thiol-ene elastomers that exhibit reversible photoinduced switching from an elastomeric gel to fluid state. This behavior can be generalized to thiol-ene cross-linked elastomers composed of different backbone chemistries (e.g., polydimethylsiloxane, polyethylene glycol, and polyurethane) and vinyl groups (e.g., allyl, vinyl ether, and acrylate). Photoswitching from the gel to fluid state occurs in seconds upon exposure to UV light and can be repeated over at least 180 cycles. These thiol-ene elastomers also exhibit the ability to heal, remold, and serve as reversible adhesives.

Processing of Self-Healing Polymers for Soft Robotics

Soft robots are, due to their softness, inherently safe and adapt well to unstructured environments. However, they are prone to various damage types. Self-healing polymers address this vulnerability. Self-healing soft robots can recover completely from macroscopic damage, extending their lifetime. For developing healable soft robots, various formative and additive manufacturing methods have been exploited to shape self-healing polymers into complex structures. Additionally, several novel manufacturing techniques, noted as (re)assembly binding techniques that are specific to self-healing polymers, have been created. Herein, the wide variety of processing techniques of self-healing polymers for robotics available in the literature is reviewed, and limitations and opportunities discussed thoroughly. Based on defined requirements for soft robots, these techniques are critically compared and validated. A strong focus is drawn to the reversible covalent and (physico)chemical cross-links present in the self-healing polymers that do not only endow healability to the resulting soft robotic components, but are also beneficial in many manufacturing techniques. They solve current obstacles in soft robots, including the formation of robust multi-material parts, recyclability, and stress relaxation. This review bridges two promising research fields, and guides the reader toward selecting a suitable processing method based on a self-healing polymer and the intended soft robotics application.

Reversible hetero-Diels–Alder amine hardener as drop-in replacement for healable epoxy coatings

Replacing commercial hardeners with bio-sourced fatty acids linked by hetero Diels–Alder (HDA) motifs enabled epoxy-amine coatings with intrinsic self-healing properties. The HDA-based coatings demonstrate scratch healing at 95 °C within 15 min.

Ambient Temperature Cross-Linking of a Sustainable, Cardanol-Based Cyanate Ester Via Synergistic Thiol-ene Copolymerization

Cardanol, a low-cost component of cashew nut shell oil, is a phenolic compound with a 15-carbon unsaturated chain in the position meta to the hydroxyl group. This biorenewable substrate was...

Polyether-Based Diblock Terpolymer Micelles with Pendant Anthracene Units-Light-Induced Crosslinking and Limitations Regarding Reversibility

The synthesis of 9-methylanthracenyl glycidyl ether (AnthGE) as a crosslinkable monomer that can be applied in anionic ring opening polymerization is reported. Diblock terpolymers of the composition methoxy-poly(ethylene oxide)-block-poly(2-ethylhexyl glycidyl ether-co-9-methylanthracenyl glycidyl ether) (mPEO-b-P(EHGE-co-AnthGE) with 10 to 24 wt% of AnthGE are synthesized and characterized. Their micellization behavior, as well as their light-induced core-crosslinking via irradiation with UV light (λ = 365 nm) is studied. The results are compared with studies on the dimerization, and the dimer cleavage via irradiation with UV-C light (λ = 254 nm), of the same diblock terpolymer in organic solution, and the small-molecule model compound 9-methoxymethylanthracene. Differences in ¹ H NMR spectra of the crosslinked or dimerized compounds and reaction kinetics of the dimerization reactions under different conditions suggest possible side reactions for the case of the core-crosslinking of micelles in aqueous solution. These side reactions limit the reversibility of the anthracene dimerization reaction in aqueous solutions, even if the anthracene molecule is encapsulated within the hydrophobic core of a polymeric micelle.

The Influence of the Furan and Maleimide Stoichiometry on the Thermoreversible Diels-Alder Network Polymerization

In recent work, the thermoreversible Diels–Alder reaction between furan and maleimide functional groups has been studied extensively in the context of self-healing elastomers and thermosets. To elaborate the influence of the stoichiometric ratio between the maleimide and furan reactive groups on the thermomechanical properties and viscoelastic behavior of formed reversible covalent polymer networks, a series of Diels–Alder-based networks with different stoichiometric ratios was synthesized. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and dynamic rheology measurements were performed on the reversible polymer networks, to relate the reversible network structure to the material properties and reactivity. Such knowledge allows the design and optimization of the thermomechanical behavior of the reversible networks for intended applications. Lowering the maleimide-to-furan ratio creates a deficit of maleimide functional groups, resulting in a decrease in the crosslink density of the system, and a consequent decrease in the glass transition temperature, Young’s modulus, and gel transition temperature. The excess of unreacted furan in the system results in faster reaction and healing kinetics and a shift of the reaction equilibrium.

A Novel Photoinduced Ligation Approach for Cross-Linking Polymerization, Polymer Chain-End Functionalization, and Surface Modification Using Benzoyl Azides

Various ligation processes have recently become a powerful tool in synthetic polymer chemistry. Herein, the use of a new photochemical ligation process as a versatile approach for the cross-linking polymerization, functionalization of polymer chain ends, and surface modification of various materials such as silica and graphene oxide, is demonstrated. The process is based on the formation of urethane linkages by the reaction of photochemically in situ generated isocyanates from benzoyl azides with hydroxyl moieties in the presence of organobase, bicyclo[2.2.2]-1,4-diazaoctane (DABCO) under ambient conditions. The intermediates and obtained materials are characterized by NMR, FTIR, TGA, and TEM analyses. It is believed that this simple and efficient ligation process will expand future applications to fabricate complex macromolecular structures, biomaterials, and gels.

Dynamic Polymer Networks: A New Avenue towards Sustainable and Advanced Soft Machines

While the fascinating field of soft machines has grown rapidly over the last two decades, the materials they are constructed from have remained largely unchanged during this time. Parallel activities have led to significant advances in the field of dynamic polymer networks, leading to the design of three-dimensionally cross-linked polymeric materials that are able to adapt and transform through stimuli-induced bond exchange. Recent work has begun to merge these two fields of research by incorporating the stimuli-responsive properties of dynamic polymer networks into soft machine components. These include dielectric elastomers, stretchable electrodes, nanogenerators, and energy storage devices. In this Minireview, we outline recent progress made in this emerging research area and discuss future directions for the field.

Substituent effect on the thermophysical properties and thermal dissociation behaviour of 9-substituted anthracene derivatives

The chemical structure and location of substituents on anthracene derivatives influence the electron balance of the aromatic system, thus determining the wavelengths at which light is absorbed, which results in the photochemically induced dimerization or monomerization. Here, the thermal dissociation kinetics of 7 photodimers of 9-substituted anthracene derivatives are studied using a combination of spectroscopic and calorimetric techniques in the condensed state and compared to scarce literature data on thermal dissociation of other anthracene derivatives. The length and chemical structure of the substituent chains have a clear impact on the melting temperatures of the anthracene derivatives and corresponding photodimers. The crystallinity of the photodimers and monomers in turn influences the thermal dissociation kinetics. The thermal dissociation behaviour and previously published photochemistry data are related to the electronic effects of the substituents by means of the Hammett parameters. Stronger electron-withdrawing effects result in larger red shifts of the maximum wavelength λmax for the photodimerization of the anthracene derivatives. It is also shown that for the studied substitutions on the 9-position of anthracene, the higher the magnitude of the electronic effect - both electron-donating and electron-withdrawing - the faster the thermal dissociation kinetics and thus the lower the thermal stability. The strong electronic effects of the substituents on the thermal and photochemical reactivity of the anthracene derivatives and their photodimers allow tuning of the thermal or photochemical responsiveness, e.g. for polymer networks.

Environmentally friendly recycling system for epoxy resin with dynamic covalent bonding

Recycling of epoxy resin and its composites is extremely difficult due to its thermoset nature. In this study, we proposed the environmentally-friendly recycling system of epoxy resin with dynamic covalent bonding in the assistance of cysteine-containing tripeptide, so-called glutathione. The glutathione attached on the epoxy resin and resulted in the cleavage of dynamic disulfide bonds of epoxy resin through thiol-disulfide exchange reaction between the thiol group of glutathione and disulfide bonding of epoxy resin, followed by the scission of epoxy networks. Therefore, the degraded epoxy residue was dissolved into chloroform. Finally, this resulting product could be reused as reagent for preparation the new epoxy materials with approximately 90% of initial mechanical strength via regeneration of disulfide bonding through heating. This work demonstrated the different aspect to understand the decomposition and recycling of thermosetting networks and the wide application under more environmentally friendly condition.

Combined light- and heat-induced shape memory behavior of anthracene-based epoxy elastomers

The development of multi-stimuli-responsive shape memory polymers has received increasing attention because of its scientific and technological significance. In this work, epoxy elastomers with reversible crosslinks are synthesized by polymerizing an anthracene-functionalized epoxy monomer, a diepoxy comonomer, and a dicarboxylic acid curing agent. The synthesized elastomers exhibit active responses to both light and heat enabled by the incorporated anthracene groups. When exposed to 365 nm UV light, additional crosslinking points are created by the photo-induced dimerization of pendant anthracene groups. The formation of the crosslinking points increases modulus and glass transition temperature of the elastomers, allowing for the fixation of a temporary shape at room temperature. The temporary shape remains stable until an external heat stimulus is applied to trigger the scission of the dimerized anthracene, which reduces the modulus and glass transition temperature and allows the elastomers to recover their original shapes. The effects of external stimuli on the thermal and dynamic mechanical properties of the elastomers are investigated experimentally and are correlated with molecular dynamics simulations that reveal the changes of structure and dynamics of the anthracene molecules and flexible chains.

Self-Healing in Mobility-Restricted Conditions Maintaining Mechanical Robustness: Furan-Maleimide Diels-Alder Cycloadditions in Polymer Networks for Ambient Applications

Two reversible polymer networks, based on Diels–Alder cycloadditions, are selected to discuss the opportunities of mobility-controlled self-healing in ambient conditions for which information is lacking in literature. The main methods for this study are (modulated temperature) differential scanning calorimetry, microcalorimetry, dynamic rheometry, dynamic mechanical analysis, and kinetic simulations. The reversible network 3M-3F630 is chosen to study the conceptual aspects of diffusion-controlled Diels–Alder reactions from 20 to 65 °C. Network formation by gelation is proven and above 30 °C gelled glasses are formed, while cure below 30 °C gives ungelled glasses. The slow progress of Diels–Alder reactions in mobility-restricted conditions is proven by the further increase of the system’s glass transition temperature by 24 °C beyond the cure temperature of 20 °C. These findings are employed in the reversible network 3M-F375PMA, which is UV-polymerized, starting from a Diels–Alder methacrylate pre-polymer. Self-healing of microcracks in diffusion-controlled conditions is demonstrated at 20 °C. De-gelation measurements show the structural integrity of both networks up to at least 150 °C. Moreover, mechanical robustness in 3M-F375PMA is maintained by the poly(methacrylate) chains to at least 120 °C. The self-healing capacity is simulated in an ambient temperature window between −40 and 85 °C, supporting its applicability as self-healing encapsulant in photovoltaics.

N-Acyl Dibenzazepine Chemistry as Versatile Approach for Photoreversible Thiol-Ene Networks

It is herein reported that a facile application of N-acyl dibenzazepine (ADBA) photochemistry for preparing photoreversible ADBA based thiol-ene networks. Crosslinking of the ADBA thiol-ene networks is successfully achieved by UV induced dimerization of ADBA groups at wavelengths above 300 nm while a subsequent deep UV exposure (λ = 250 nm) results in a well-defined cleavage of the crosslinks. The photochemical bonding and cleavage of the process has been determined and studied in detail by spectroscopic measurements.

Thermal dissociation of anthracene photodimers in the condensed state: kinetic evaluation and complex phase behaviour

Thermally and photochemically reversible functional groups, such as photodimers of anthracene derivatives, offer interesting stimuli-responsive behaviour. To evaluate their potential for application in reversible polymer networks, accurate kinetic parameters and knowledge of their thermophysical behaviour are required. Accurate kinetic studies of the thermal dissociation of the photodimers in the condensed state, thus without the influence of solvents on their reactivity, is still lacking. A methodology was set up to accurately evaluate the chemical reaction kinetics and complex phase behaviour during the thermal dissociation of photodimers into their corresponding monomers. Temperature-controlled time-resolved FTIR spectroscopy was used to determine the reaction progress, while non-isothermal DSC measurements were used to study the thermophysical changes, resulting from the thermal dissociation reaction. The thermal dissociation behaviour in the condensed state is more challenging than in the solution state due to the crystallinity of the dimers, stabilizing the dimers and thus slowing down the initial dissociation rates. Distinctly different sets of kinetic parameters were found for the dissociation from the molten and the crystalline state. For experiments performed below the melting temperature of the photodimer, the reaction rate changes abruptly as the dimer is partly dissociated and partly dissolved into the formed monomer. This methodology provides an accurate assessment of the reaction kinetics with detailed knowledge about the complex phase behaviour of the mixture of the anthracene photodimer and monomer during thermal dissociation.

Room temperature crystallization of amorphous polysiloxane using photodimerization

Bulk crystallization in flexible polymeric systems is difficult to control due to the random orientation of the chains. Here we report a photo cross-linking strategy that results in simultaneous cross-linking and crystallization of polysiloxane chains into millimeter sized leaf-like polycrystalline structures. Polymers containing pendant anthracene groups are prepared and undergo [4+4] photocycloaddition under 365 nm irradiation at room temperature. The growth and morphology of the crystalline structures is studied using polarized optical microscopy (POM) and atomic force microscopy and is found to progress through three unique stages of nucleation, growth, and constriction. The mobility of the individual chains is probed using pulsed-field gradient (PFG) NMR to provide insights into the diffusion processes that may govern chain transport to the growing crystal fronts. The room temperature crystallization of this conventionally amorphous polymer system may allow for a new level of morphological control for silicone materials.

Polymer actuators based on covalent adaptable networks

Advances in polymer actuators containing covalent adaptable networks (CANs) are summarized and discussed in this review.

Visible-light reversible photopolymerisation: insights via online photoflow – electrospray ionisation – mass spectrometry

Herein, we introduce a scalable photopolyaddition polymerisations using the pyrene-chalcone [2+2]-cycloaddition and monitor the photodepolymerisation process via an online photoflow – electrospray ionisation mass spectrometry setup.

Tailoring the Mechanical Properties of 3D Microstructures Using Visible Light Post-Manufacturing

The photochemistry of anthracene, a new class of photoresist for direct laser writing, is used to enable visible-light-gated control over the mechanical properties of 3D microstructures post-manufacturing. The mechanical and viscoelastic properties (hardness, complex elastic modulus, and loss factor) of the microstructures are measured over the course of irradiation via dynamic mechanical analysis on the nanoscale. Irradiation of the microstructures leads to a strong hardening and stiffening effect due to the generation of additional crosslinks through the photodimerization of the anthracene functionalities. A relationship between the loss of fluorescence-a consequence of the photodimerization-and changes in the mechanical properties is established. The fluorescence thus serves as a proxy read-out for the mechanical properties. These photoresponsive microstructures can potentially be used as "mechanical blank slates": their mechanical properties can be readily adjusted using visible light to serve the demands of different applications and read out using their fluorescence.

Light-Healable Epoxy Polymer Networks via Anthracene Dimer Scission of Diamine Crosslinker

Two anthracene-based diamine crosslinkers were used to cure a range of commercially available monomers to produce four highly photoreversible crosslinked epoxy polymers. Through careful selection of the epoxy monomers used, the properties of the resultant polymer networks were varied to create a coating material that possessed room-temperature light-stimulated healing. Of the four coatings created, the best healing performance was exhibited by the two most flexible systems, both of these also exhibited the thermal and mechanical performance necessary for coatings. By using anthracene, the utilization of a wide range of wavelengths in the healing process is possible, which in applications such as industrial coatings would be of significant benefit.

Mechanically Robust and Recyclable Cross-Linked Fibers from Melt Blown Anthracene-Functionalized Commodity Polymers

Melt blowing combines extrusion of a polymer melt through orifices and attenuation of the extrudate with hot high-velocity air jets to produce nonwoven fibers in a single step. Due to its simplicity and high-throughput nature, melt blowing produces more than 10% of global nonwovens (∼$50 billion market). Semicrystalline thermoplastic feedstock, such as poly(butylene terephthalate), polyethylene, and polypropylene, have dominated the melt blowing industry because of their facile melt processability and thermal/chemical resistance; other amorphous commodity thermoplastics (e.g., styrenics, (meth)acrylates, etc.) are generally not employed because they lack one or both characteristics. Cross-linking commodity polymers could enable them to serve more demanding applications, but cross-linking is not compatible with melt processing, and it must be implemented after fiber formation. Here, cross-linked fibers were fabricated by melt blowing linear anthracene-functionalized acrylic polymers into fibers, which were subsequently cross-linked via anthracene-dimerization triggered by either UV light or sunlight. The resulting fibers possessed nearly 100% gel content because of highly efficient anthracene photodimerization in the solid state. Compared to the linear precursors, the anthracene-dimer cross-linked acrylic fibers exhibited enhanced thermomechanical properties suggesting higher upper service temperatures (∼180 °C), showing promise for replacing traditional thermoplastic-based melt blown nonwovens in certain applications. Additionally, given the dynamic nature of the anthracene-dimer cross-links at elevated temperatures (> ∼180 °C), the resulting cross-linked fibers could be effectively recycled after use, providing new avenues toward sustainable nonwoven products.

Reversible Surface Patterning by Dynamic Crosslink Gradients: Controlling Buckling in 2D

Harnessing the self-organization of soft materials to make complex, well-ordered surface patterns in a noninvasive manner is challenging. The wrinkling of thin films provides a compelling strategy to achieve this. Despite much attention, however, a simple, single-step, reversible method that gives rise to controlled, two-dimensional (2D) ordered, continuous, and discontinuous patterns has proven to be elusive. Here a novel, robust method is described to achieve this using an ultraviolet-light-sensitive anthracene-containing polymer thin film. The origin of the patterns is the local buckling of the thin film, where the control over the topology is given by laterally patterning out-of-plane gradients in the crosslink density of the film. The underlying buckling mechanics and formation of the surface features are well-described by finite element analysis. By illuminating the film with a photomask, local and long-range patterns that can be both continuous and discontinuous are able to be written. Furthermore, the patterning is fully reversible over multiple cycles. The results demonstrate a simple strategy for erasable storage of information in a surface topography that has applications in memory, anticounterfeiting, and plasmonics.

UV induced reversible chain extension of 1-(2-anthryl)-1-phenylethylene functionalized polyisobutylene

The synthesis of novel 1-(2-anthryl)-1-phenylethylene (APE) di-telechelic polyisobutylenes is described. Utilization of a difunctional cationic initiator and the in situ addition of the non-homopolymerizable APE lead to the formation of di-anthryl telechelic polyisobutylenes. Products were characterized by ¹H NMR spectroscopy and Size Exclusion Chromatography. The polymers were UV irradiated at 365 and 254 nm and the reversible photocycloaddition of anthryl moieties was investigated. The chain extension of di-anthryl telechelic PIBs through photocoupling at 365 nm produced higher molecular weight products from low molecular weight precursors. The effect of precursor polymer concentration on the degree of chain extension was investigated, and intermolecular interactions leading to the formation of tetramers was observed. The photocoupled products were UV irradiated at 254 nm to induce the reversal of photocycloaddition of anthryl groups and to follow the consequent photoscission of polymers.

Poly(thioether) Vitrimers via Transalkylation of Trialkylsulfonium Salts

Vitrimers are permanently cross-linked organic polymers that can be reshaped, molded, and recycled without loss of network integrity. Herein, we report poly(thioether) networks, prepared through a straightforward thiol-ene photopolymerization, that can be turned into catalyst-free vitrimer materials by partial alkylation of the thioethers (1-10%) to the corresponding trialkylsulfonium salts. Based on a classical S_N2-type substitution, the resulting polyionic networks can be reshaped upon heating via swift transalkylation reactions. This novel exchange reaction for the design of vitrimers was studied both on low MW model compounds as well as on a material level. In addition, we demonstrated the recycling of these networks without significant loss of mechanical properties.

Wavelength-Selective Sequential Polymer Network Formation Controlled with a Two-Color Responsive Initiation System

We report a wavelength-selective polymerization process controlled by visible/UV light, whereby a base is generated for anion-mediated thiol-Michael polymerization reaction upon exposure at one wavelength (400-500 nm), while radicals are subsequently generated for a second stage radical polymerization at a second, independent wavelength (365 nm). Dual wavelength, light controlled sequential polymerization not only provides a relatively soft intermediate polymer that facilitates optimum processing and modification under visible light exposure but also enables a highly cross-linked, rigid final material after the UV-induced second stage radical polymerization. A photobase generator, NPPOC-TMG, and a photo-radical initiator, Irgacure 2959, were selected as the appropriate initiator pair for sequential thiol-Michael polymerization and acrylate homopolymerization. FT-IR and rheological tests were utilized to monitor the dual cure photo-polymerization process, and mechanical performance of the polymer was characterized at each distinct stage by dynamic mechanical analysis (DMA). By demonstrating complete light control in another sequential polymerization system (thiol-Michael and thiol-ene hybrid polymerization), this initiator pair exhibits great potential to regulate many other coupled anion and radical hybrid polymerizations in both a sequential and controllable manner.

Light- and heat-triggered reversible luminescent materials based on polysiloxanes with anthracene groups

Reversible silicone elastomers were successfully developed by light-triggered dimerization and heat depolymerization which happened to the anthryl groups.