Topology-Reset Execution: Repeatable Postcyclization Recyclization of Cyclic Polymers

Photo-Deactivation Strategy for Switchable ATRP with the Assistance of Molecular Switches

Light irradiation is an external stimulus, rapidly developed in switchable atom transfer radical polymerization (ATRP) via photo-activation methods in recent years. Herein, a photo-deactivation strategy is introduced to regulate ATRP with the assistance of photoswitchable hexaarylbiimidozole (HABI). Under visible light irradiation and in the presence of HABI, ATRP is greatly decelerated or quenched depending on the concentration of HABI. Interestingly, with visible light off, ATRP can proceed smoothly and follow a first-order kinetics. Moreover, photo-switchable ATRP alternatively with light off and on is demonstrated. Besides, the mechanism of photo-deactivation ATRP involving radical quenching is proposed in the presence of HABI.

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.

Acoustodynamic Covalent Materials Engineering for the Remote Control of Physical Properties Inside Materials

Advances in vat photopolymerization (VP) 3D printing (3DP) technology enable the production of highly precise 3D objects. However, it is a major challenge to create dynamic functionalities and to manipulate the physical properties of the inherently insoluble and infusible cross-linked material generated from VP-3DP without reproduction. The fabrication of light- and high-intensity focused ultrasound (HIFU)-responsive cross-linked polymeric materials linked with hexaarylbiimidazole (HABI) in polymer chains based on VP-3DP is reported here. Although the photochemistry of HABI produces triphenylimidazolyl radicals (TPIRs) during the process of VP-3DP, the orthogonality of the photochemistry of HABI and photopolymerization enables the introduction of reversible cross-links derived from HABIs in the resulting 3D-printed objects. While photostimulation cleaves a covalent bond between two imidazoles in HABI to generate TPIRs only near the surface of the 3D-printed objects, HIFU triggers cleavage in the interior of materials. In addition, HIFU travels beyond an obstacle to induce a response of HABI-embedded cross-linked polymers, which cannot be attainable with photostimulation. The present system would be beneficial for tuning the physical properties and recycling of various polymeric materials, but it will also open the door for pinpoint modification, healing, and reshaping of materials when coupled to various dynamic covalent materials.

Tailoring Polymerization Controllability and Dispersity Through a Photoswitchable Catalyst Strategy

Modulating on-demand polymerization is a challenge in synthetic macromolecules. Herein, tailoring polymerization controllability and dispersity during single-electron transfer mediated living radical polymerization (SET-LRP) of methyl methacrylate (MMA) is achieved. Hexaarylbiimidazole (HABI) is employed as a photoswitchable catalyst, allowing reversible control of catalytic activity between an active and inactive state. In the presence of HABI and with the light on (active state), control SET-LRP of MMA follows first-order kinetics, resulting in polymers with a narrow molecular weight distribution. In contrast, polymerization responds to light and reverts to their original uncontrolled state with light off (inactive state). Therefore, repeatable resetting polymerization can be easily performed. The key to photomodulating dispersity is to use an efficient molecular switch to tailor the breadths of dispersity. Besides, the mechanism of HABI-mediated SET-LRP with switchable ability is proposed.

Cyclic polymers: synthesis, characteristics, and emerging applications

Cyclic polymers with a ring-like topology and no chain ends are a unique class of macromolecules. In the past several decades, significant advances have been made to prepare these fascinating polymers, which allow for the exploration of their topological effects and potential applications in various fields. In this Review, we first describe representative synthetic strategies for making cyclic polymers and their derivative topological polymers with more complex structures. Second, the unique physical properties and self-assembly behavior of cyclic polymers are discussed by comparing them with their linear analogues. Special attention is paid to highlight how polymeric rings can assemble into hierarchical macromolecular architectures. Subsequently, representative applications of cyclic polymers in different fields such as drug and gene delivery and surface functionalization are presented. Last, we envision the following key challenges and opportunities for cyclic polymers that may attract future attention: large-scale synthesis, efficient purification, programmable folding and assembly, and expansion of applications.

Cyclic Perfluoropolyether: Distinct Film Formability and Thermostabilization Upon Recyclable Cyclic-Linear Topological Transformation

Perfluoropolyether (PFPE) is an industrially important fluoropolymer and has great industrial importance due to its flexible, noncombustible, and chemically robust properties. However, exploration and application of chemically modified homogeneous PFPEs are hampered by their immiscibility against nonfluorine-containing molecules. Here, the synthesis is reported of cyclic PFPE with hexaarylbiimidazoles (HABIs) in chains from linear PFPE having 2,4,5-triphenylimidazole (lophine) end groups. While phase separation between the end groups and main chains took place for linear PFPE, HABIs and main chains in cyclic PFPE are miscible to form transparent glass films. The design of cyclic PFPE also enables cyclic to linear topological transformation based on conversion of HABIs into lophines upon mild heating in the glass film state. Sequential linear-to-cyclic and cyclic-to-linear topological transformations enable fabrication of thermostabilized transparent films derived from PFPE.

Photocleavable Regenerative Network Materials with Exceptional and Repeatable Viscoelastic Manipulability

The development of solventless system for modulating properties of network materials is imperative for the next generation sustainable technology. Utilization of photostimulation is important owing to its spatial and temporal locality, yet designing photoresponsive network materials exhibiting repeatable and dramatic change in their properties remains a challenge. Here, the authors report a photocleavable regenerative network (PRN) linked with photoresponsive hexaarylbiimidazoles (HABIs) synthesized from narrow dispersity star-shaped poly(dimethylsiloxane)s (PDMSs) having 2,4,5-triphenylimidazole end groups. The use of urea anion as a catalyst for ring opening polymerization (ROP) of cyclic siloxane initiated from silanols enables control of molecular weight and dispersity. The rheological measurements for the synthesized PRNs exhibit drastic changes in storage and loss moduli (G' and G″) upon photoirradiation in the solid state (G' > G″). This photocontrolled change in viscoelasticity with retaining solidity enables application of PRNs as a remotely-controlled photo-melt adhesive and photo-scissible string. The developed PRNs will enable a wide variety of applications such as industrially important next-generation sustainable adhesive, sealant, and reversibly-deformable 3D printing materials with their spatially and temporally local manipulability, solventless handleability, and excellent reversibility.

Electrochemical topological transformation of polysiloxanes

Coupling reactions between polymers are an important class of chemical modifications for changing, enhancing, and tuning the properties of polymeric materials. In particular, transformation of polymer topologies based on efficient, facile and less wasted coupling reactions remains a significant challenge. Here, we report coupling reactions based on electrochemical oxidation of 2,4,5-triphenylimidazole into a 2,4,5-triphenylimidazolyl radical and its spontaneous dimerization into hexaarylbiimidazole. Based on this chemistry, electrochemical topological transformation (ETT) and electrochemical chain extension have been realized with siloxane-based oligomers and polymers. Moreover, this approach enables one step ETT of star-shaped poly(dimethyl siloxane)s (PDMSs) into network PDMSs, running in an ionic liquid solvent and requiring no purification steps.

Synthesis, Properties, and Applications of Bio-Based Cyclic Aliphatic Polyesters

Cyclic polymers have long been reported in the literature, but their development has often been stunted by synthetic difficulties such as the presence of linear contaminants. Research into the synthesis of these polymers has made great progress in the past decade, and this review covers the synthesis, properties, and applications of cyclic polymers, with an emphasis on bio-based aliphatic polyesters. Synthetic routes to cyclic polymers synthesized from bioderived monomers, alongside mechanistic descriptions for both ring closure and ring expansion polymerization approaches, are reviewed. The review also highlights some of the unique physical properties of cyclic polymers together with potential applications. The findings illustrate the substantial recent developments made in the syntheses of cyclic polymers, as well as the progress which can be made in the commercialization of bio-based polymers through the versatility this topology provides.

Ru-Se Coordination: A New Dynamic Bond for Visible-Light-Responsive Materials

Photodynamic bonds are stable in the dark and can reversibly dissociate/form under light irradiation. Photodynamic bonds are promising building blocks for responsive or healable materials, photoactivated drugs, nanocarriers, extracellular matrices, etc. However, reactive intermediates from photodynamic bonds usually lead to side reactions, which limit the use of photodynamic bonds. Here, we report that the Ru-Se coordination bond is a new photodynamic bond that reversibly dissociates under mild visible-light-irradiation conditions. We observed that Ru-Se bonds form via the coordination of a selenoether ligand with [Ru(tpy)(biq)(H₂O)]Cl₂ (tpy = 2,2':6',2″-terpyridine, biq = 2,2'-biquinoline) in the dark, while the Ru-Se bond reversibly dissociates under visible-light irradiation. No side reaction is detected in the formation and dissociation of Ru-Se bonds. To demonstrate that the Ru-Se bond is applicable to different operating environments, we prepared photoresponsive amphiphiles, surfaces, and polymer gels using Ru-Se bonds. The amphiphiles with Ru-Se bonds showed reversible morphological transitions between spherical micelles and bowl-shaped assemblies for dark/light irradiation cycles. The surfaces modified with Ru-Se-bond-containing compounds showed photoswitchable wettability. Polymer gels with Ru-Se cross-links underwent photoinduced reversible sol-gel transitions, which can be used for reshaping and healing. Our work demonstrates that the Ru-Se bond is a new type of dynamic bond, which can be used for constructing responsive, reprocessable, switchable, and healable materials that work in a variety of environments.

Reversible cyclic-linear topological transformation using a long-range rotaxane switch

A reversible linear-cyclic topological transformation of polymers facilitated by a long-range rotaxane switch.

Synthesis of reversibly photocleavable pseudo-ladder polymers

Pseudo-ladder polymers composed of stiff ladder and reversibly photocleavable backbone linkages and their graft copolymers have been synthesized. Micelles constructed from the graft copolymers exhibited growth in their sizes upon photoirradiation.

Wavelength-gated photoreversible polymerization and topology control

We exploit the wavelength dependence of [2 + 2] photocycloadditions and -reversions of styrylpyrene to exert unprecedented control over the photoreversible polymerization and topology of telechelic building blocks. Blue light (λ max = 460 nm) initiates a catalyst-free polymerization yielding high molar mass polymers (M n = 60 000 g mol-1), which are stable at wavelengths exceeding 430 nm, yet highly responsive to shorter wavelengths. UVB irradiation (λ max = 330 nm) induces a rapid depolymerization affording linear oligomers, whereas violet light (λ max = 410 nm) generates cyclic entities. Thus, different colors of light allow switching between a depolymerization that either proceeds through cyclic or linear topologies. The light-controlled topology formation was evidenced by correlation of mass spectrometry (MS) with size exclusion chromatography (SEC) and ion mobility data. Critically, the color-guided topology control was also possible with ambient laboratory light affording cyclic oligomers, while sunlight activated the linear depolymerization pathway. These findings suggest that light not only induces polymerization and depolymerization but that its color can control the topological outcomes.

A Strategy toward Cyclic Topologies Based on the Dynamic Behavior of a Bis(hindered amino)disulfide Linker

A simple and efficient method to generate macrocyclic structures has been developed based on the dynamic behavior of the linker bis(2,2,6,6-tetramethylpiperidin-1-yl)disulfide (BiTEMPS). The prime linear structure was transformed into a (macro)cycle using the following sequence: 1) thiol-ene reaction with a BiTEMPS derivative to afford the linear precursor, then 2) an entropy-driven transformation induced by diluting and heating. The radicals generated from BiTEMPS upon heating are highly tolerant toward a variety of chemical species, including oxygen and olefins, but they exhibit high reactivity in exchange reactions, which can be applied to the topology transformation of various skeletons. The advantages of the present method, namely, its procedural simplicity and substrate versatility, are demonstrated by the gram-scale synthesis of cyclic compounds with low and relatively high molecular weight.