Multi-stimuli Responsive Polymers Research Articles

Sensing diversity in halogen-bonded multi-stimuli responsive materials: Light, pH, magnetism, and electron-rich species

The present study delves into the synthesis and characterization of multi-stimuli responsive polymers, demonstrating their sensitivity towards magnetic field, pH level, light irradiation, and electron-rich species through halogen bonding (XB). Detailed X-ray analyses of two azobenzene derivatives, each incorporating a tetrafluoroiodobenzene group, provided the compelling evidence of engagement of the building blocks in the formation of halogen bonds. Interestingly, the investigation of an ammonium analog, featuring a tetrahalogenoferrate(III) anion, not only showcased a magnetic response even upheld its ability to undergo isomerization under UV irradiation. These functional properties were subsequently harnessed in the form of multi-responsive compounds through the random quaternization of poly(2-(N,N-dimethylamino) ethyl methacrylate) (PDMAEMA), and by employing varying proportions of XB-donor dye (8, 11, 15, 21, and 43 %) followed by magnetic anion exchange. Leveraging the presence of free ternary amino groups and diazobenzene moieties, the resulting polymers demonstrated pronounced sensitivity towards a variation of pH and UV light stimuli, while a careful modulation of magnetic susceptibility was found to be directly proportional to the degree of quaternization. An NMR titration, conducted using a synthetic intermediate, revealed the formation of I⋯Cl‾ interactions in the solution state, thereby underscoring the materials' sensitivity to electron-rich species. Moreover, the electron microscopic analysis displayed an alteration in morphology upon the application of a magnetic field and UV irradiation. Thus, the presented strategic framework offers an avenue for the development of multi-stimuli responsive materials for remotely controlled smart devices to act in response to a diverse set of four stimuli, and heralding a significant advancement in the realm of responsive material design.

Synthesis of pH/redox-double-responsive ferrocene-containing diblock copolymers and their application in controlled release

A novel pH/redox-double-responsive ferrocene-containing diblock copolymer of poly(ethylene glycol)-block-poly[2-(2-((2-ferrocenyl ethyl ester) (methyl) amino) ethoxy) ethyl methacrylate] [mPEG113-b-P(Fe–N-EOMA)] was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. mPEG113-b-P(Fe–N-EOMA) is amphiphilic with PEG as hydrophilic block and the part containing ferrocene as hydrophobic block. For comparison, three other diblock copolymers were prepared by changing the molecular weight of the hydrophilic block and the microstructure of the hydrophobic block. The diblock copolymers have a critical pH in solution and increases with the increase of the hydrophilicity of the copolymers. The redox-response was confirmed by the shift of characteristic peak of ferrocene in the UV/vis absorption spectrum after adding FeCl3/VC. The self-assembly behavior of these diblock copolymers in solution were then investigated. It shows that they have low critical micelle concentration (cmc), and the cmc decreases as the hydrophobicity of copolymers increases. Their morphology is varied, including sphere, lamella, aggregated sphere, and can be reversible changed by the stimulation of pH and redox. Finally, mPEG113-b-P(Fe–N-EOMA) was applied to the release of Nile Red (NR), which showed a good release efficiency. The prepared diblock copolymers have sensitive and reversible response properties, which provided ideas for the design of multistimuli-responsive polymers, and have good application prospects in drug-release.

Photoswitchable and Solvent‐Controlled Directional Actuators: Supramolecular Assembly and Crosslinked Polymers

Front Cover: In article 2200547 by Jiun-Tai Chen and co-workers, multistimuli-responsive polymer actuators that can respond to light and solvent via structural changes with fast responses and high reproducibility are developed. The actuators are based on bilayer films of polydimethylsiloxane and azobenzene chromophore-crosslinked poly(diallyldimethylammonium chloride). The effects of different parameters on the bending angles and curvatures of the polymer films are also analyzed.

Tailoring a UCST-LCST-pH Multiresponsive Window through a Single Polymer Complex of Chitosan-Hyaluronic Acid.

Multistimuli-responsive polymers are important for controlled release. Owing to the fact that these polymers are derived from vinyl-based monomers, their decoration with other molecules is limited. Polysaccharides, especially chitosan (CS) and hyaluronic acid (HA), are pH-responsive biopolymers, whose chemical structures contain reactive functional groups for feasible chemical modifications to obtain add-on functions. The present work demonstrates the introduction of polymers with upper critical solution temperature (UCST) and lower critical solution temperature (LCST) performances onto CS and HA, respectively. By simply varying the mole ratio between the CS-containing UCST polymer and the HA-containing LCST polymer along with adjusting the pH, a polymer system with a UCST-LCST-pH multiresponsive window can be obtained. This multiresponsive window enables us to control the encapsulation and release with repeatability as evidenced from a model study on lysozyme. The present work, for the first time, shows a simple approach to obtain multiresponsive biodegradable polymers through the formation of a single polymer complex to tailor a specific multiresponsive window.

Reversible Ratiometric Mechanochromic Fluorescence Switching in Highly Stretchable Polyurethane Elastomers with Ultratoughness Enhanced by Polyrotaxane

Multistimuli-responsive polymers containing mechanophoric motifs are highlighted as promising mechano-luminescent materials upon stretching via optical fluorescence signals. Herein, we report a distinct ratiometric force-induced fluorescence changes from green-emissive napthalimide stopper (donor) embedded in polyrotaxane (PR) cross-linkers to red-emissive rhodamine mechanophore (acceptor) incorporated in polyurethane (PU) backbones. The utilization of PR (1 wt %) can significantly enhance both toughness and stretchability in our targeted PU elastomers, where the mechanical work is largely dissipated by the ring-sliding motion and pulley effects of PR cross-linkers during stretching. Therefore, the simple soft robotic behavior with ultrafast shape memory and reversible ratiometric mechanochromic fluorescence switching by heating (60 °C) in PR-based PU films provide a distinctive strategy for the construction of molecular machines in the high-performance elastomers, which also offer pathways for practical applications of stimuli-responsive polymers in highly stretchable artificial muscles featuring both outstanding pulley effects on enhanced mechanical properties and novel signal variations of mechanochromic fluorescence simultaneously.

Playing construction with the monomer toy box for the synthesis of multi‐stimuli responsive copolymers by reversible deactivation radical polymerization protocols

AbstractIn this review article, we survey the 2016–June 2021 scientific literature on the synthesis of multi‐stimuli responsive (MSR) polymers, the main focus being on reversible deactivation radical polymerization techniques (RDRPs, also known as controlled radical polymerizations). In fact, along more than 40 years of extensive research, RDRPs have boosted the synthesis of stimuli‐responsive polymers. RDRPs are now robust, versatile, relatively user‐friendly and even interconvertible, thus allowing control over composition, sequence, and topology of polymers. Such control can afford materials with well‐defined responses to physical, chemical, and biological external stimuli. Furthermore, “click” reactions are used to combine macromolecular precursors or to introduce specific functional groups in the target structure. As a result, MSR polymers are obtained from diverse combinations of commercial or specially synthesized building blocks arranged at will into desired sequences and architectures. Thanks to this versatility, self‐assembling polymeric structures are designed either to respond to triggers and perform specific applicative tasks, or to investigate the influence of structural variables on the responsivity of polymers. The “green” trend emerging in the field of responsive polymers and RDRPs is also briefly discussed.

The Study of pH Effects on Phase Transition of Multi-Stimuli Responsive P(NiPAAm-co-AAc) Hydrogel Using 2D-COS.

The temperature and mechanism of phase transition of poly(N-isopropylacrylamide-co-acrylic acid) [P(NiPAAm-co-AAc)], which is one of the multi-stimuli responsive polymers, were investigated at various pHs using infrared (IR) spectroscopy, two-dimensional (2D) gradient mapping, and two-dimensional correlation spectroscopy (2D-COS). The determined phase transition temperature of P(NiPAAm-co-AAc) at pH 4, 3, and 2 based on 2D gradient mapping and principal component analysis (PCA) showed that it decreases with decreasing pH, because COOH group in AAc changes with variation of pH. The results of 2D-COS analysis indicated that the phase transition mechanism of P(NiPAAm-co-AAc) hydrogel at pH4 is different from that at pH2 due to the effect of COOH group of AAc.

Synthesis of dual-responsive polymer via convertible RAFT and ring-opening polymerizations in one-pot

The synthesis and applications of dual- or multi stimuli-responsive polymers have gained increasing attention. One-pot strategy by combination two different polymerization mechanisms is promising for the synthesis of dually stimuli-responsive polymers in a rapid route. In this work, we report a convertible anionic ring-opening and RAFT polymerization by using asymmetrical trithiocarbonate as both an initiator for the anionic ring-opening polymerization and a chain transfer agent for RAFT polymerization to produce poly(propylene sulfide) (PPS)- poly(N-isopropylmethacrylamide) block-like copolymers. Furthermore, the ROS and temperature responsibility of the formed copolymer are also evaluated.

Tetracarboxylated Azobenzene/Polymer Supramolecular Assemblies as High-Performance Multiresponsive Actuators.

Multistimuli-responsive polymers are materials of emerging interest but synthetically challenging. In this work, supramolecular assembly was employed as a facile and effective approach for constructing 3,3',5,5'-azobenzenetetracarboxylic acid (H4abtc)/poly(diallyldimethylammonium chloride) (PDAC) supramolecules. Structural transformations of H4abtc can be induced by light, mechanical force, and heat and influenced by free volume. Thus, the fabricated free-standing H4abtc/PDAC film underwent bending/unbending movements upon treatment with light, humidity, or temperature, as asymmetric structural transformations on either side of the film generated asymmetric contraction/stretching forces. Fast rates of shape recovery were achieved for the film on exposure to gently flowing humid nitrogen. The bending/unbending motions are controllable, reversible, and repeatable. Hence, this light-, humido-, and thermo-responsive film has great potential in device applications for advanced functions.

Triple-stimuli responsive polymers with fine tuneable magnetic responses

The formation of multi-stimuli responsive polymers exhibiting magnetic, pH and light sensitivity is reported.

Photo-, Thermo-, and Piezochromic Nafion Film Incorporating Cationic Spiropyran

Multistimuli-responsive polymer films incorporating cationic spiropyran are demonstrated based on a Nafion membrane. The films exhibit color changes between yellow and red under photoirradiation, temperature changes, and application of pressure. These characteristic behaviors are based on the stabilization of the zwitterionic merocyanine cation in the polar environment of Nafion.

Folic acid-conjugated pH/temperature/redox multi-stimuli responsive polymer microspheres for delivery of anti-cancer drug

The folic acid (FA)-conjugated pH/temperature/redox multi-stimuli responsive poly(methacrylic acid-co-N,N-bis(acryloyl)cystamine/poly(N-isopropylacrylamide-co-glycidyl methacrylate-co-N,N-bis(acryloyl)cystamine) microspheres were prepared by a two-stage distillation–precipitation polymerization with subsequent surface modification with FA. The microspheres were characterized by transmission electron microscopy, dynamical light scattering, Fourier-transform infrared spectra, UV–vis spectra and elemental analysis. The degradation of the functional microspheres could be triggered by a reductive reagent, such as glutathione, due to presence of BAC crosslinker. The drug-loaded microspheres exhibited a pH/temperature/redox multi-stimuli responsive drug release character for doxorubicin hydrochloride as a model anti-cancer drug, which was efficiently loaded into the microspheres with a high loading capacity of 208.0% and an encapsulation efficiency of 85.4%. In vitro drug delivery study indicated that the FA-conjugated microspheres could deliver Dox into MCF-7 cells more efficiently than the microspheres without functionalization of FA. Furthermore, WST-1 assay showed that the microspheres had no obvious toxicity to MCF-7 cells even at a high concentration of 2000μgmL−1. The resultant microsphere may be a promising vector for delivery of anti-cancer drugs as it exhibits a low cytotoxicity and degradability, precise molecular targeting property and multi-stimuli responsively controlled drug release.

Multi-stimuli responsive polymers – the all-in-one talents

The integration of several responsive moieties within one polymer yields smart polymers exhibiting a multifaceted responsive behaviour.

Multi-stimuli responsive poly(azodibenzo-18-crown-6-ether)s

Multi-stimuli responsive polymers based on dibenzo-18-crown-6-ethers.

Flavylium-Supported Poly(N-isopropylacrylamide): A Class of Multistimuli Responsive Polymer

A new multistimuli responsive polymer was prepared by means of the free-radical copolymerization of N-isopropylacrylamide and a vinyl flavylium derivative. The polymer shows response to temperature (conferred by the N-isopropylacrylamide units) and pH and light stimuli (conferred by the flavylium moieties). The polymer follows the typical pH dependent network of chemical reactions of flavylium compounds in water that was studied by means of UV–vis spectroscopy and stationary fluorescence emission. Regarding the temperature response, the presence of the flavylium cation in the polymeric chains reduces the LCST, and it is possible to take advantage of this effect to photo-collapse partially the polymer by irradiating the system in defined ranges of pH and temperature.

An on–off biosensor based on multistimuli-responsive polymer films with a binary architecture and bioelectrocatalysis

Abstract The films with binary architecture combining electropolymerized poly(4-vinylpyridine) (P4VP) as the inner layers and chemically polymerized poly(N,N′-diethylacrylamide) (PDEA) hydrogels containing glucose oxidase (GOD) as the outer layers were successfully prepared on electrode surface, designated as P4VP/PDEA-GOD. Cyclic voltammetric response of ferrocenedicarboxylic acid (Fc(COOH) 2 ) at P4VP/PDEA-GOD film electrodes showed reversible on–off behaviors to environmental pH, temperature, ClO 4 − , and SO 4 2− concentration. In particular, the films exhibited a very sensitive ClO 4 − -responsive behavior toward Fc(COOH) 2 with the critical concentration as low as 0.01 M. The responsive mechanism of the films to different stimuli was discussed and explored by comparative experiments. The pH- and ClO 4 − -sensitive properties were mainly attributed to the electrostatic interaction between P4VP inner layers and Fc(COOH) 2 under different conditions, and the thermo- and SO 4 2− -responsive behaviors should be ascribed to the structure change of PDEA hydrogels in PDEA-GOD outermost layers under different conditions. The multiple stimuli-responsive films could be further used to realize multiply switchable electrochemical oxidization of glucose catalyzed by GOD immobilized in the films with Fc(COOH) 2 in solution as the mediator. The films with the unique binary architecture may open a new way to establish a foundation for fabricating a novel type of multi-controllable biosensors based on bioelectrocatalysis with immobilized enzymes.

Photoisomerization-Induced Tunable LCST Phase Separation of Azobenzene-Containing Polymers in an Ionic Liquid

4-phenylazophenyl methacrylate (AzoMA) and benzyl methacrylate (BnMA) were copolymerized to produce multistimuli-responsive polymers (P(AzoMA-r-BnMA)s) in a hydrophobic ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([C2mim][NTf2]), as the solvent. P(AzoMA-r-BnMA)s with a maximum of ca. 4 mol % AzoMA were soluble in [C2mim][NTf2] at low temperatures, and they underwent lower critical solution temperature (LCST) phase separation with an increase in temperature. Under UV and visible light irradiation, P(AzoMA-r-BnMA)s underwent reversible photochromism of trans-to-cis and cis-to-trans isomerization, respectively. The LCST temperature differences between trans- and cis-form polymers in the IL were as large as 22 degrees C. Reversible photoinduced phase separation of the polymers was achieved at a certain temperature; at this temperature, the cis-form polymers were soluble in the IL, but the trans-form polymers were not.