Synthesis and Thermoresponsive Behaviors of Thermo-, pH-, CO2-, and Oxidation-Responsive Linear and Cyclic Graft Copolymers

Smart and Efficient Synthesis of Cyclic Poly(N-isopropylacrylamide)s by Ring Expansion RAFT (RE-RAFT) Polymerization and Analysis of Their Unique Temperature-Responsive Properties

Cyclic polymers have many interesting properties compared to their linear analogs, but there are very few examples of their synthesis. This is because most cyclic polymers have been synthesized by stepwise processes, including synthesizing homo- or hetero-telechelic end-functionalized precursor polymers and consecutive intramolecularly coupling of both ends of the polymers. This requires a complicated synthesis, and the product yields are very low because the target cyclic polymers are usually synthesized under highly dilute conditions, consequently, making it difficult to systematically analyze the properties of cyclic polymers. In the present research, we have synthesized cyclic polymers using a ring expansion polymerization method. Particularly, the ring expansion RAFT polymerization (RE-RAFT polymerization) that we have developed using a cyclic chain transfer agent is a smart method that can synthesize cyclic polymers very efficiently. In this paper, we successfully synthesized cyclic-poly(N-isopropylacrylamide), which is widely known as a thermo-responsive polymer, by RE-RAFT polymerization. Furthermore, we have compared the thermo-responsive properties of the cyclic-poly(N-isopropylacrylamide)s with those of their linear analogs.

Predicting and screening high-performance polyimide membranes using negative correlation based deep ensemble methods

Polyimide polymer membranes have become critical materials in gas separation and storage applications due to their high selectivity and excellent permeability. However, with over 107 known types of polyimides, relying solely on experimental research means potential high-performance candidates are likely to be overlooked. This study employs a deep learning method optimized by negative correlation ensemble techniques to predict the gas permeability and selectivity of polyimide structures, enabling rapid and efficient material screening. We propose a deep neural network model based on negative correlation deep ensemble methods (DNN-NCL), using Morgan molecular fingerprints as input. The DNN-NCL model achieves an R2 value of approximately 0.95 on the test set, which is a 4% improvement over recent model performance, and effectively mitigates overfitting with a maximum discrepancy of less than 0.03 between the training and test sets. High-throughput screening of over 8 million hypothetical polymers identified hundreds of promising candidates for gas separation membranes, with 14 structures exceeding the Robeson upper bound for CO2/N2 separation. Visualization of high-throughput predictions shows that although the Robeson upper bound was never explicitly used as a model constraint, the majority of predictions are compressed below this limit, demonstrating the deep learning model's ability to reflect real-world physical conditions. Reverse analysis of model predictions using SHAP analysis achieved interpretability of the deep learning model's predictions and identified three key functional groups deemed important by the deep neural network for gas permeability: carbonyl, thiophene, and ester groups. This established a bridge between the structure and properties of polyimide materials. Additionally, we confirmed that two polyimide structures predicted by the model to have excellent CO2/N2 selectivity, namely 6-methylpyrimidin-5-amine and 1,4,5,6-tetrahydropyrimidin-2-amine, have been experimentally validated in previous studies. This research demonstrates the feasibility of using deep learning methods to explore the vast chemical space of polyimides, providing a powerful tool for discovering high-performance gas separation membranes.

Covalent adaptable polymer networks with CO2-facilitated recyclability

Cross-linked polymers with covalent adaptable networks (CANs) can be reprocessed under external stimuli owing to the exchangeability of dynamic covalent bonds. Optimization of reprocessing conditions is critical since increasing the reprocessing temperature costs more energy and even deteriorates the materials, while reducing the reprocessing temperature via molecular design usually narrows the service temperature range. Exploiting CO2 gas as an external trigger for lowering the reprocessing barrier shows great promise in low sample contamination and environmental friendliness. Herein, we develop a type of CANs incorporated with ionic clusters that achieve CO2-facilitated recyclability without sacrificing performance. The presence of CO2 can facilitate the rearrangement of ionic clusters, thus promoting the exchange of dynamic bonds. The effective stress relaxation and network rearrangement enable the system with rapid recycling under CO2 while retaining excellent mechanical performance in working conditions. This work opens avenues to design recyclable polymer materials with tunable dynamics and responsive recyclability.

Investigation of Soft Matter Nanomechanics by Atomic Force Microscopy and Optical Tweezers: A Comprehensive Review

Soft matter exhibits a multitude of intrinsic physico-chemical attributes. Their mechanical properties are crucial characteristics to define their performance. In this context, the rigidity of these systems under exerted load forces is covered by the field of biomechanics. Moreover, cellular transduction processes which are involved in health and disease conditions are significantly affected by exogenous biomechanical actions. In this framework, atomic force microscopy (AFM) and optical tweezers (OT) can play an important role to determine the biomechanical parameters of the investigated systems at the single-molecule level. This review aims to fully comprehend the interplay between mechanical forces and soft matter systems. In particular, we outline the capabilities of AFM and OT compared to other classical bulk techniques to determine nanomechanical parameters such as Young's modulus. We also provide some recent examples of nanomechanical measurements performed using AFM and OT in hydrogels, biopolymers and cellular systems, among others. We expect the present manuscript will aid potential readers and stakeholders to fully understand the potential applications of AFM and OT to soft matter systems.

Facile synthesis of monocyclic, dumbbell-shaped and jellyfish-like copolymers using a telechelic multisite hexablock copolymer

A heterofunctional hexablock copolymer comprising alternating reactive and non-reactive blocks is designed to generate cyclic, dumbbell-shaped and jellyfish-like copolymers.

Recent advances in the self-assembly of sparsely grafted amphiphilic copolymers in aqueous solution

This review describes the self-assembly of sparsely grafted amphiphilic copolymers and highlights the effects of structural factors and solvents on their self-assembly behaviour.

Multi-tunable aggregation behaviors of thermo/pH-responsive toothbrush-like and jellyfish-like copolymers

Rational design of comb-like and linear conjugates comprising PNIPAM and PDMAEMA segments allows the construction of a multi-tunable hierarchical self-assembly platform and insight into the topology effect.

Rational design of a multi-in-one heterofunctional agent for versatile topological transformation of multisite multisegmented polystyrenes

A “seven-in-one” initiating, coupling and stimuli-labile agent is designed to achieve topological transformations with reduced, similar and enhanced molar masses.

Well-Defined Thermo- and pH-Responsive Double Hydrophilic Graft Copolymer Bearing a Pyridine-Containing Backbone

Graft copolymers have extensive applications in material science because of their tunable composition of backbone and side chains and diverse morphologies of aggregates. Recent studies mainly focused on the amphiphilic...

Thermoresponsive Polymers Based on Tertiary Amine Moieties

Thermoresponsive polymers exhibiting unique reversible phase transition properties in aqueous solution in response to temperature stimuli have been extensively investigated. In the past two decades, thermoresponsive polymers based on tertiary amine moieties have achieved considerable progress and become an important family of thermoresponsive polymers, including tertiary amine functionalized poly((meth)acrylamide)s, poly((meth)acrylate)s, poly(styrene)s, poly(vinyl alcohol)s, and poly(ethylene oxide)s, which exhibit lower critical solution temperature and/or upper critical solution temperature in water or aliphatic alcohols. Their phase transition behavior can be modulated by the solution pH and CO₂ due to the protonation of tertiary amine moieties in acidic condition and deprotonation in alkaline condition and the charged ammonium bicarbonate formed by the tertiary amine moieties and CO₂ . The aim of this review is to summarize the recent progress in the thermoresponsive polymers based on tertiary amine moieties.

Phototunable Cloud Point Temperatures Stemming from Cyclic Topology: Synthesis and Thermal Phase Transition Behavior of Cyclic Poly(N-acryloylsarcosine methyl ester)

Cyclic polymers possess distinct properties compared with their linear counterparts, such as smaller hydrodynamic volume, lower viscosity, and higher glass-transition temperature, etc. To explore the impact of the cyclic topology on the thermo-induced phase transition behavior of poly(N-acryloylsarcosine methyl ester) (PNASME), the anthracene-terminated telechelic PNASMEs are synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of monomer NASME using a bifunctional chain transfer agent (CTA) with two anthryl groups. Subsequently, cyclic PNASMEs are prepared via UV-induced cyclization under 365 nm UV. There are considerable increases (up to 50 °C) for the cloud point temperatures (T_cp s) of cyclic PNASMEs compared with the linear counterparts. In view of the increment, the T_cp of PNASME is tuned by varying the cyclic/linear ratio (the molar ratio between cyclic PNASME and linear PNASME in the product) with different irradiation time.

Self-assembly of cyclic grafted copolymers with rigid rings and their potential as drug nanocarriers

Enhancing the performance of polymer micelles by purposeful regulation of their structures is a challenging topic that receives widespread attention. In this study, we systematically conduct a comparative study between cyclic grafted copolymers with rigid and flexible rings in the self-assembly behavior via dissipative particle dynamics (DPD) simulation. With a focus on the possible stacking ways of rigid rings, we propose the energy-driven packing mechanism of cyclic grafted copolymers with rigid rings. For cyclic grafted copolymers with large ring size (14 and 21-membered rings), rigid rings present a novel channel-layer-combination layout, which is determined by the balance between the potential energy of micelles (E_micelle) and the interaction energy between water and micelles (E_int). Based on this mechanism, we further regulate a series of complex self-assembling structures, including curved rod-like, T-shape, annular and helical micelles. Compared with flexible copolymers, cyclic grafted copolymers with rigid rings provide a larger and loose hydrophobic core and higher structural stability with micelles due to the unique packing way of rigid rings. Therefore, their micelles have a great potential as drug nanocarriers. They possess a better drug loading capacity and disassemble more quickly than flexible counterparts under acidic tumor microenvironment. Furthermore, the endocytosis kinetics of rigid micelles is faster than the flexible counterparts for the adsorption and wrapping process. This study may provide a reasonable idea of structural design for polymer micelles to enhance their performance in biomedical applications.

Polyzwitterions with LCST Side Chains: Tunable Self-Assembly

Manipulation of self-assembly behavior of copolymers via environmental change is attractive in the fabrication of smart polymeric materials. We present tunable self-assembly behavior of graft copolymers, poly(sulfobetaine methacrylate)-graft-poly[oligo(ethylene glycol) methyl ether methacrylate)-co-di(ethylene glycol) methyl ether methacrylate] (PSBM-g-P(OEGMA-co-DEGMA)). Upon heating the aqueous solutions, the graft copolymers undergo a transition from micelles with PSBM cores to unimers (i.e., individual macromolecules) and then to reversed micelles with P(OEGMA-co-DEGMA) cores, thus demonstrating the tunability of the self-assembling through temperature change. In the presence of salt the temperature response of PSBM is eliminated, and the structure of the micelles with the P(OEGMA-co-DEGMA) core changes. Moreover, for the graft copolymer with long side chains, micelles with aggregation number ∼ 2 were formed with a PSBM core at low temperature, which is ascribed to the steric effect of the P(OEGMA-co-DEGMA) shell.

Synthesis of cyclic graft polymeric prodrugs with heterogeneous grafts of hydrophilic OEG and reducibly conjugated CPT for controlled release

Fabrication of cyclic graft (cg) copolymer-based polymeric prodrugs by conjugation of drug molecules to cg copolymers via a dynamic covalent bond capable of responding to biorelevant signals integrates simultaneously the merits of cg copolymers and polymeric prodrugs for enhanced stability of nanocarriers and precise modulation of drug release kinetics. To completely eliminate the compromised drug conjugation efficiency due to the steric hindrance of hydrophilic grafts, it will be useful to develop cg polymeric prodrugs with heterogeneous grafts composed of hydrophilic polymers and drug species, respectively. For this purpose, we reported in this study the synthesis of cyclic graft polymeric prodrugs with heterogeneous grafts of hydrophilic oligo (ethylene glycol) (OEG) and reducibly conjugated camptothecin (CPT), cg-poly(oligo(ethylene glycol) monomethyl ether methacrylate)-b-poly((2-hydroxyethyl methacrylate)-disulfide link-camptothecin) (cg-P(OEGMA)-b-P(HEMA-SS-CPT), cg-prodrugs), via an integrated strategy of a previously reported diblock copolymer-based template and post-polymerization intermolecular click conjugation of a reducible CPT prodrug. The micelles self-assembled from cg-prodrugs on one hand had sufficient salt stability due to the branched cg structure, and on the other hand showed a reduction-triggered cleavage of the disulfide link for a promoted CPT release. Most importantly, we uncovered two interesting phenomena of the cg-based polymeric prodrugs as delivery vehicles: (i) the dimensions of both self-assemblies formed by the cg and bottlegraft (bg) polymers depend substantially on the molecular size of the cg and bg polymers likely due to the steric hindrance of the grafted structures of the cg and bg molecules and relatively low aggregation number of the self-assembled structures, and (ii) cg-prodrug-based micelles exhibited greater in vitro cytotoxicity against cancer cells despite the lower drug loading content (DLC) than the bg-based analogues, which results primarily from the faster reduction-triggered degradation and drug release as well as the greater cellular uptake efficiency of the former micelle prodrugs. Taken together, the developed cg-prodrugs provide great potential for chemotherapy, and the aforementioned interesting results will definitely inspire more upcoming studies on the future design and development of novel cg polymers for biomedical applications.

100th Anniversary of Macromolecular Science Viewpoint: Poly(N-isopropylacrylamide)-Based Thermally Responsive Micelles

Poly(N-isopropylacrylamide) (PNIPAAm)-based thermally responsive micelles are of great importance as smart materials for a number of applications such as drug delivery and biosensing, owing to their tunable lower critical solution temperature (LCST). Their design and synthesis in the nanoscale size range have been widely studied, and research interest in their structural and physic-chemical properties is continually growing. In this Viewpoint, representative research on the construction of PNIPAAm-based thermally responsive micelles as well as their applications are highlighted and discussed, which would serve as a good start for newcomers in this field and a positive guide for future research.

CO2-Responsive Nano-Objects with Assembly-Related Aggregation-Induced Emission and Tunable Morphologies

CO₂-responsive polymeric nano-objects with assembly-related aggregation-induced emission (AIE) are obtained via polymerization-induced self-assembly (PISA) of 2-(dimethylamino)ethyl methacrylate (DMAEMA), 2-(4-formylphenoxy)ethyl methacrylate (MAEBA), and 4-(1,2,2-triphenylvinyl)phenyl methacrylate (TPEMA). These nano-objects exhibit, depending on the feed of MAEBA, a morphology evolution process from spherical micelles to vesicles. Due to the presence of DMAEMA units, CO₂ promotes morphology transformation of the nano-objects from spheres to a mixture of "jellyfish" and vesicles and vesicles to complex vesicles. Moreover, TPEMA endows the AIE feature to these nano-objects, offering a strategy to monitor the morphology evolution process in real time. Thus, this approach is significant for exploring the assembly mechanism of copolymer in polymerization-induced self-assembly and designing multistimuli-responsive polymeric nanomaterials with tunable morphologies and sizes.

The difluoromethylthio moiety lowers the LCST of oligo(ethylene glycol)-based homopolymers

Introduction of difluoromethylthio moiety could significantly lower LCSTs of oligo(ethylene glycol)-based thermo-responsive homopolymers.

Rational design of nonlinear crystalline-amorphous-responsive terpolymers for pH-guided fabrication of 0D–3D nano-objects

Crystallization/pH-induced self-assembly of starlike and tadpole-linear terpolymers allowed the formation of 0D spheres/vesicles, 1D cylinders, 2D platelets/nanosheets and 3D tadpoles/dendritic vesicles.

Thermo- and oxidation-sensitive poly(meth)acrylates based on alkyl sulfoxides: dual-responsive homopolymers from one functional group

Alkyl sulfoxide side groups introduce thermo- and oxidation-sensitivity into poly(meth)acrylates, thus realizing new dual-responsive homopolymers based on one functional group.

Synthesis and properties of pH-cleavable toothbrush-like copolymers comprising multi-reactive Y junctions and a linear or cyclic backbone

Y-junction-bearing toothbrush-like copolymers can exhibit unique physical properties and hierarchical (co)assembly behaviors dependent on topology, external stimuli and hydrolysis.

Synthesis, thermoresponsivity and multi-tunable hierarchical self-assembly of multi-responsive (AB)mC miktobrush-coil terpolymers

Stimuli-responsive miktobrush-coil terpolymers can exhibit unique physical properties and hierarchical self-assembly behaviors dependent on composition, concentration and external stimuli.

Evaluation of Ring Expansion-Controlled Radical Polymerization System by AFM Observation

We here present a direct link between the reaction mechanisms for the ring-expansion "vinyl" polymerization system and atomic force microscopy (AFM) observations. The brush-modification clearly discriminates the desired cyclic species with the contour lengths (L_c) of 28-132 nm and molar masses (M_AFM) of 60.2-283 kg mol^-1 from the other linear ones. The 293 polymer blushes observed in a 1.0 μm × 1.0 μm AFM image are individually characterized, eventually providing clear answers about the mechanisms of this rare polymerization system, which include ring-expansion vinyl polymerizations to generate cyclic polymers, fusions of the generated cycles to form multimers, and their scission to form linear or ring-opened species. The relationship between the molecular chain lengths and the cyclic versus linear morphologies is highlighted.

Synthesis of Novel pH-Tunable Thermoresponsive Hydroxyl-Terminated Hyperbranched Polyether

In this study, a new pH-tunable thermoresponsive hydroxyl-terminated hyperbranched polyether (HTHP 2) was successfully prepared via a one-pot cationic polymerization technique and postmodification. In the first step, hydroxyl-terminated hyperbranched polyether containing double bonds (HTHP 1) were synthesized. Then, through thiol-ene "click" reaction, pH-responsive carboxyl groups were introduced to the target polymer of HTHP 2. The products were characterized via Fourier-transform infrared spectra (FTIR), nuclear magnetic resonance (NMR), and size-exclusion chromatography-multiangle laser light scattering (SEC-MALLS). Moreover, dynamic light scattering (DLS) and UV-Vis spectroscopy was employed to study the pH- and thermoresponsiveness in detail. Results showed that HTHP 2 possessed typical pH-controllable thermoresponsive behavior. By regulating the solution pH value range 3.0-5.4, LCST of HTHP 2 could be changed from 12.8 to 68.0 °C. Meanwhile, the cell viabilities of A549 cells were more than 80% for in vitro cytotoxicity tests of HTHP 2, suggested that HTHP 2 polymers are of good biocompatibility for up to 24 h.

A polyelectrolyte-containing copolymer with a gas-switchable lower critical solution temperature-type phase transition

A polyelectrolyte-containing copolymer with a CO₂/N₂-switchable cloud point, resulting from the gas-induced alternation of hydrophilicity, was prepared.

Temperature and solvent isotope dependent hierarchical self-assembly of a heterografted block copolymer

A heterografted block copolymer with doubly thermoresponsive grafts is designed to address the challenge in hierarchical self-assembly. Upon heating, solvent isotope dependent morphology transitions from spheres to nanobowls, vesicles, disks, nanosheets, nanoribbons and hyperbranched micelles can be achieved. The strategy provides a general platform to prepare diverse thermoreversible nano-objects.

Photo-responsive gels based on cyclic/linear polymers: efficient synthesis and properties

Azobenzene-induced photoresponsive gels based on cyclic polymers were prepared and the properties of the gels formed from these cyclic polymers were investigated by comparison with gels made from the polymeric linear precursors.

Ring-Expansion/Contraction Radical Crossover Reactions of Cyclic Alkoxyamines: A Mechanism for Ring Expansion-Controlled Radical Polymerization

Macrocyclic polymers present an important class of macromolecules, displaying the reduced radius of gyration or impossibility to entangle. A rare approach for their synthesis is the ring expansion-controlled radical "vinyl" polymerization, starting from a cyclic alkoxyamine. We here describe ring-expansion radical crossover reactions of cyclic alkoxyamines which run in parallel to chain-propagation reactions in the polymerization system. The radical crossover reactions extensively occurred at 105⁻125 °C, eventually producing high molecular weight polymers with multiple inherent dynamic covalent bonds (NOC bonds). A subsequent ring-contraction radical crossover reaction and the second ring-expansion radical crossover reaction are also described. The major products for the respective three stages were shown to possess cyclic morphologies by the molecular weight profiles and the residual ratios for the NOC bonds (φ in %). In particular, the high φ values ranging from ca. 80% to 98% were achieved for this cyclic alkoxyamine system. This result verifies the high availability of this system as a tool demonstrating the ring-expansion "vinyl" polymerization that allows them to produce macrocyclic polymers via a one-step vinyl polymerization.

Photoregulating of Stretchability and Toughness of a Self-Healable Polymer Hydrogel

Supramolecular hydrogels that are assembled through dynamic host-guest interactions have presented apparent potential in the construction of materials with promising performance. Herein, a photoregulated hydrogel cross-linked by host-guest interactions with multifunctions of high stretchability, strong toughness, and rapid self-healing property is reported. The hydrogel exhibits unique light-responsive property due to the introduction of two photoisomerized groups. For example, the stress-strain curve of the original hydrogel indicates 1020% rupture strain with the maximum tensile strain value of 214 kPa. Upon 365 nm light irradiation for an hour, its tensile strain increases to 15 times with lower tensile stress indicating a better stretchability. Moreover, the hydrogel is photochromic and surface patternable, where it can reversibly switch color between luminous yellow and brown while exposed to 365 and 440 nm light irradiation. It holds great promise for applying in self-recovering optically controlled and labeled elastic mechanical materials.

Ultraviolet light-, temperature- and pH-responsive fluorescent sensors based on cellulose nanocrystals

Intelligent CNC-g-P(AzoC₆MA-co-DMAEMA) fluorescent nanosensors present ultraviolet light-, temperature- and pH-responsive properties.