Unraveling Dynamic Helicity Inversion and Chirality Transfer through the Synthesis of Discrete Azobenzene Oligomers by an Iterative Exponential Growth Strategy.

Unraveling the chirality transfer mechanism of polymer assemblies and controlling their handedness is beneficial for exploring the origin of hierarchical chirality and developing smart materials with desired chiroptical activities. However, polydisperse polymers often lead to an ambiguous or statistical evaluation of the structure-property relationship, and it remains unclear how the iterative number of repeating units function in the helicity inversion of polymer assemblies. Herein, we report the macroscopic helicity and dynamic manipulation of the chiroptical activity of supramolecular assemblies from discrete azobenzene-containing oligomers (azooligomers), together with the helicity inversion and morphological transition achieved solely by changing the iterative chain lengths. The corresponding assemblies also differ from their polydisperse counterparts in terms of thermodynamic properties, chiroptical activities, and morphological control.

2,5-Dimethylfuran/Acrylonitrile as Latent Monomer for Sequence-Controlled Copolymer and Sequence-Dependent Thermo-Responsivity.

Sequence control has attracted increasing attention for its ability of regulating polymer property and performance. Herein, the sequence-controlled polymer containing acrylonitrile (AN) is achieved by using 2,5-dimethylfuran/acrylonitrile adduct as a latent monomer. The temperature-dependent retro Diels-Alder reaction is engaged in controlling the release of AN during RAFT polymerization, that is, regulating the instant AN concentration via a non-invasive and in situ manner. Such control over the instant AN concentration and particularly the molar ratio of comonomer pair leads to the simultaneous change of monomer units in "living" polymeric chain, thus resulting in the sequence-controlled polymeric structures. By delicately manipulating the polymerization temperature, diverse sequence-on-demand structures of AN-containing copolymers, such as poly(AN/methyl methacrylate), poly(AN/styrene), poly(AN/butyl acrylate), poly(AN/N,N-dimethylacrylamide), and poly(AN/N-isopropylacrylamide) are created. Meanwhile, this study presents an initial attempt in tuning the thermal responsivity of poly(AN/N-isopropylacrylamide), which is closely correlated to the sequence of polymer structure. More importantly, the polymer with averagely distributed AN units results in the higher thermal sensitivity. Therefore, the synthetic strategy proposed in this work offers a promising platform for accessing the sequence-controlled copolymers containing AN structures, thus expanding the investigation on the relationship between the polymer structures and correlated properties.

Transfer, Amplification, Storage, and Complete Self-Recovery of Supramolecular Chirality in an Achiral Polymer System.

Supramolecular chirality and its complete self-recovery ability are highly mystical in nature and biological systems, which remains a major challenge today. Herein, we demonstrate that partially cross-linked azobenzene (Azo) units can be employed as the potential chiral trigger to fully heal the destroyed helical superstructure in achiral nematic polymer system. Combining the self-assembly of Azo units and terminal hydroxyl groups in polymer side chains allows the vapor-induced chiral nematic phase and covalent fixation of the superstructure via acetal reaction. The induced helical structure of Azo units can be stored by inter-chain cross-linking, even after removal of the chiral source. Most interestingly, the stored chiral information can trigger perfect chiral self-recovery (CSR) behavior after being destroyed by UV light, heat, and solvents. The results pave a new way for producing novel chiroptical materials with reversible chirality from achiral sources.

Topological Glycopolymers as Agglutinator and Inhibitor: Cyclic versus Linear.

Carbohydrates play an important role in biological processes for their specific interactions with proteins. Cyclic glycopolymers are promising to mimic the topology of natural macrocycle-biomacromolecules due to their unique architecture of lacking chain ends. To systematically study the effect of glycopolymer architecture on the interactions with protein, the cyclic glycopolymers bearing galactose side-chain (cyclic PMAGn ) with three degrees of polymerization (n = 14, 24, 47) are prepared for the first time. The cyclic PMAGn exhibits unique properties in agglutinating and inhibiting proteins in subsequent studies by comparison with the linear precursor with the same molecular weights. More impressively, the cyclic PMAGn highlight the improved performance of cyclic architecture. For example, the cyclic PMAGn shows superior inhibition abilities to suppress amyloid formation from amyloid ß protein fragment 1-42 aggregation and block the specific interaction between bacteria and galactose-modified surface compared to that of respective linear counterpart. This interesting finding suggests that the architecture of cyclic glycopolymers may be capable of optimizing the ability to bind or inhibit proteins in biological processes.

Cooperation of Amphiphilicity and Smectic Order in Regulating the Self-Assembly of Cholesterol-Functionalized Brush-Like Block Copolymers.

Nanoparticle morphology significantly affects the application of nanometer-scale materials. Understanding nanoparticle formation mechanisms and directing morphological control in nanoparticle self-assembly processes have received wide attention. Herein, a series of brush-like amphiphilic liquid crystalline block copolymers, PChEMA m- b-POEGMA n, containing cholesteryl mesogens with different hydrophobic/hydrophilic block ratios were designed and synthesized. The self-assembly behaviors of the resulting PChEMA m- b-POEGMA n block copolymers in different solvents (tetrahydrofuran/H2O, 1,4-dioxane/H2O, and N, N-dimethylformamide) were investigated in detail. Desirable micellar aggregates with well-organized architectures, including short cylindrical micelles, nanofibers, fringed platelets, and ellipsoidal vesicles with smectic micellar cores, were observed in 1,4-dioxane/H2O with an increasing hydrophobic block ratio. Although both amphiphilicity and smectic order governed the self-assembly, these two factors were differently balanced in the different solvents. This unique supramolecular system provides a new strategy for the design of advanced functional nanomaterials with tunable morphologies.

Sequence-Controlled Polymers with Furan-Protected Maleimide as a Latent Monomer.

Herein, a novel methodology for preparing sequence-controlled polymers is illustrated by using a latent monomer, furan protected maleimide (FMI). At 110 °C, FMI is deprotected by retro Diels-Alder (rDA) reaction, and the released MI is immediately involved in the cross-polymerization with styrene (St) to deliver heterosegments. At 40 °C the rDA reaction does not proceed, therefore homo-poly(styrene) segments are produced. By implementing programmable temperature changes during polymerization of St and FMI, "living" polymers with tailored a sequence are created. A ternary copolymerization produces complex sequences as designed. Alkynyl-functionalized FMI, used as a latent monomer, leads to the desirable placement of functional groups along the polymer chain. This latent-monomer-based strategy opens a new avenue for fabricating sequence-controlled polymers.

Combining Orthogonal Chain-End Deprotections and Thiol-Maleimide Michael Coupling: Engineering Discrete Oligomers by an Iterative Growth Strategy.

Orthogonal maleimide and thiol deprotections were combined with thiol-maleimide coupling to synthesize discrete oligomers/macromolecules on a gram scale with molecular weights up to 27.4 kDa (128mer, 7.9 g) using an iterative exponential growth strategy with a degree of polymerization (DP) of 2n -1. Using the same chemistry, a "readable" sequence-defined oligomer and a discrete cyclic topology were also created. Furthermore, uniform dendrons were fabricated using sequential growth (DP=2n -1) or double exponential dendrimer growth approaches (DP=22n -1) with significantly accelerated growth rates. A versatile, efficient, and metal-free method for construction of discrete oligomers with tailored structures and a high growth rate would greatly facilitate research into the structure-property relationships of sophisticated polymeric materials.

A versatile cyclic 2,2'-azobenzenophane with a functional handle and its polymers: efficient synthesis and effect of topological structure on chiroptical properties.

Two novel cyclic azobenzenophanes (SC, RC) with functional handles have been synthesized efficiently by a Glaser coupling reaction. Through a Suzuki coupling reaction, alternating ring/linear polymers with rigid (conjugated)/flexible (unconjugated) bridges were obtained from the resultant cyclic azobenzenophanes. The optical activities of linear, cyclic, and macromolecular binaphethyl-azobenzene derivatives were investigated by UV/Vis and circular dichroism (CD) spectra and the time-dependent (TD)-DFT method. Experimental results and theoretical analyses indicated that the cyclic configurations exhibited better chiroptical features than the others, and the reverse conformation and difference of dextro-/levo-rotation of azobenzenophanes were detected by comparing linear and cyclic structures, which provides an opportunity for the optical-rotation-controlled "smart" materials systems in future.

Photoresponsive Amphiphilic Macrocycles Containing Main-Chain Azobenzene Polymers.

Herein, the first example of photosensitive cyclic amphiphilic homopolymers consisting of multiple biphenyl azobenzene chromophores in the cyclic main chain tethered with hydrophilic tetraethylene glycol monomethyl ether units is presented. The synthetic approach involves sequentially performed thermal catalyzed "click" step-growth polymerization in bulk, and Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) intramolecular cyclization from α-alkyne/ω-azide linear precursors. It is observed that such amphiphilic macrocycles exhibit increased glass transition temperatures (Tg ), slightly faster trans-cis-trans photoisomerization, and enhanced fluorescence emission intensity compared with the corresponding linear polymers. In addition, the cyclic amphiphilic homopolymers self-assemble into spherical nanoparticles with smaller sizes which possess slower photoresponsive behaviors in a tetrahydrofuran/water mixture compared with those of the linear ones. All these interesting observations suggest that the cyclic topology has a great influence on the physical properties and self-assembly behavior of these photoresponsive amphiphilic macrocycles in general.

Cyclic amphiphilic random copolymers bearing azobenzene side chains: facile synthesis and topological effects on self-assembly and photoisomerization.

In this article, well-defined cyclic amphiphilic random copolymers bearing azobenzene side chains and pendent carboxyl moieties, cyclic-P(BHMEm -co-AAn )s, are synthesized by combining atom transfer radical polymerization (ATRP) with Cu(I)-catalyzed azide/alkyne cycloaddition (CuAAC) "click" reaction and selective hydrolysis of tert-butyl ester. Successful synthesis of the cyclic-P(BHMEm -co-AAn )s is fully characterized and verified via conventional gel permeation chromatography, triple detection gel permeation chromatography, nuclear magnetic resonance, Fourier transform infrared, and matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. The cyclic topology induces profound effects on the glass transition temperatures, self-assembly behavior, and photoresponsive performance of the copolymers compared with their linear counterparts.

Well-Defined Oligo(azobenzene-graft-mannose): Photostimuli Supramolecular Self-Assembly and Immune Effect Regulation.

The immune system can recognize and respond to pathogens of various shapes. Synthetic materials that can change their shape have the potential to be used in vaccines and immune regulation. The ability of supramolecular assemblies to undergo reversible transformations in response to environmental stimuli allows for dynamic changes in their shapes and functionalities. A meticulously designed oligo(azobenzene-graft-mannose) was synthesized using a stepwise iterative method and "click" chemistry. This involved integrating hydrophobic and photoresponsive azobenzene units with hydrophilic and bioactive mannose units. The resulting oligomer, with its precise structure, displayed versatile assembly morphologies and chiralities that were responsive to light. These varying assembly morphologies demonstrated distinct capabilities in terms of inhibiting the proliferation of cancer cells and stimulating the maturation of dendritic cells. These discoveries contribute to the theoretical comprehension and advancement of photoswitchable bioactive materials.

A high-efficiency strategy for synthesizing cyclic polymers of methacryates in one pot.

An unprecedented strategy for the high-efficiency preparation of the cyclic polymers is developed. In this strategy, the atom transfer radical polymerization, the substitution of chain-end halide by azide group and Cu-catalyzed alkyne-azide cyclization, i.e., the frequently used three separated steps for the preparation of cyclic polymers, are integrated into a one-pot reaction by the introduction of a "regulator". The kernel of this novel strategy is the utilization of the different rates between the competitive ATRP propagation and SN 2 substitution of a tertiary-carbon halogen and secondary-carbon halogen. 0.55 g (yield = 59%) cyclic poly(methyl methacrylate) is obtained from 3.0 mL reaction solution. This work proposed a high-efficiency and bright promising strategy for the preparation of cyclic polymer, which would evoke more research interests on cyclic polymer.

Sequence effect on the self-assembly of discrete amphiphilic co-oligomers with fluorene-azobenzene semirigid backbones.

Sequences can have a dramatic impact on the unique properties and self-assembly in natural macromolecules, which has received increasing interest. Herein, we report a series of discrete amphiphilic co-oligomers with the same composition but different building blocks in a semirigid backbone. These sequence-defined oligomers possess two primary amine groups on the side chain of the azobenzene building block, and hence, they become amphipathic due to quaternization of the amine groups when protonated in acidic aqueous solution. These oligomer isomers assembled into different nanoparticles, including nanofibers, hollow vesicles and spherical micellar complexes, in a THF/water/HCl mixture under the same conditions. UV-vis absorption spectra, differential scanning calorimetry (DSC) and X-ray scattering (XRD) experiments combined with theoretical calculations reveal that the sequence-controlled co-oligomers induce different molecular packing conformations and arrangement modes of building blocks in self-assembly. Furthermore, these self-assembled nanoparticles demonstrate photoresponsive morphological transformation and fluorescence emission under UV light irradiation due to trans-to-cis photoisomerization of azobenzene. This work demonstrates that customizing functional nanoparticles can be achieved by controlling the sequence structure in synthetic co-oligomers.

Controlled bimodal molecular-weight-distribution polymers: facile synthesis by RAFT polymerization.

The RAFT agents RAFT-1 and RAFT-2 were used for RAFT polymerization to synthesize well-defined bimodal molecular-weight-distribution (MWD) polymers. The system showed excellent controllability and "living" characteristics toward both the higher- and lower-molecular-weight fractions. It is important that bimodal higher-molecular-weight (HMW) polymers and block copolymers with both well-controlled molecular weight (MW) and MWD could be prepared easily due to the "living" features of RAFT polymerization. The strategy realized a mixture of higher/lower-molecular-weight polymers at the molecular level but also preserved the features of living radical polymerization (LRP) of the RAFT polymerization.

Cyclic side-chain phenylazo naphthalene polymers: enhanced fluorescence emission and surface relief grating formation.

Well-defined cyclic-polymers (cyclic-PAzoMMAs), bearing side-chain phenylazo naphthalene chromophore, were successfully synthesized by the combination of atom transfer radical polymerization (ATRP) and copper(I)-catalyzed azide/alkyne cycloaddition "click" reaction, as verified by GPC, (1) H NMR, FTIR, and MALDI-TOF mass spectrometry. The cyclic-PAzoMMA showed higher glass transition temperatures than the linear-PAzoMMA with the same molecular weight. Interestingly, the cyclic-PAzoMMA exhibited deeper modulation depth (M.D.) induced by SRG, larger value of the photoinduced birefringence, increased fluorescence emission, and longer fluorescence lifetime in comparison with its linear counterpart.

Azobenzene/Acid Binary Systems for Colorimetric Humidity Sensing with Reversibility, High Sensitivity, and Tunable Colors.

Humidity sensors are important for humidity detection in many storage and manufacturing processes. Issues like sensibility, response rate, controllability, and material and preparation process costs need to be taken into consideration for practical applications. Herein, we report an investigation on a series of azobenzene/acid binary systems using easily accessible compounds, whose thin films display reversible and widely tunable color changes in response to humidity stimulation, with high sensitivity, fast color change, and recovery speed. The interesting properties for colorimetric humidity sensing are showcased with potential applications in dynamic art painting, smart windows, and respiration monitoring.

Smart supramolecular nanofibers and nanoribbons from uniform amphiphilic azobenzene oligomers.

A series of self-assembled 1D nanostructures, including straight and helix nanofibers, nanoribbons, and nanobelts, were fabricated from uniform amphiphilic azobenzene oligomers with tunable molecular weight and side chain functionality, promoted by multiple and cooperative supramolecular interactions. Additionally, the morphological transformation of the nanofibers was achieved during the photoisomerization process.

A novel and universal route to SiO2-supported organic/inorganic hybrid noble metal nanomaterials via surface RAFT polymerization.

Polymer-encapsulated gold or silver nanoparticles were synthesized and sterically stabilized by a shell layer of poly(4-vinylpyridine) (P4VP) grafted on SiO(2) nanoparticles that acts as a scaffold for the synthesis of hybrid noble metal nanomaterials. The grafting P4VP shell was synthesized via surface reversible addition-fragmentation chain transfer (RAFT) polymerization of 4-vinylpyridine (4VP) using SiO(2)-supported benzyl 9H-carbazole-9-carbodithioate (SiO(2)-BCBD) as the RAFT agent. The covalently tethered P4VP shell can coordinate with various transition metal ions such as Au(3+) or Ag(+) and therefore stabilize the corresponding Au or Ag nanoparticles reduced in situ by sodium borohydride (NaBH(4)) or trisodium citrate. The SiO(2)-supported RAFT agent and the Au or Ag nanoparticles embedded in the P4VP shell layer were characterized by UV-vis spectrophotometer, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and surface-enhanced Raman spectroscopy (SERS).

The first example of main-chain cyclic azobenzene polymers.

A novel main-chain azobenzene cyclic polymer, cyclic-PEHPA, has been successfully synthesized by 'click' cyclization of the α-alkyne-ω-azido hetero-difunctional linear precursors (linear-PEHPA), which is synthesized by a step-growth polymerization of the 3'-ethynylphenyl[4-hexyl-(2-azido-2-methyl- propionate) phenyl] azobenzene (EHPA). Gel permeation chromatography, and (1) H NMR and FT-IR spectra confirmed the complete transformation of linear-PEHPA into cyclic-PEHPA. With the same molecular weights, the cyclic-PEHPAs are found to have higher glass transition temperatures than the linear-PEHPAs, but almost the same decomposition temperatures. In addition, the obtained cyclic azobenzene polymer with lower molar mass shows a slightly better trans-cis-trans photoisomerization ability than the corresponding linear-PEHPA.

Smart azobenzene-containing tubular polymersomes: fabrication and multiple morphological tuning.

A fundamental challenge in nanomaterial science is to facilely fabricate nonspherical polymersomes. Here, several kinds of novel tubular polymersomes were fabricated via self-assembly of amphiphilic azobenzene-containing block copolymers. Besides, their shape could be tuned by multiple approaches including changes in the chemical structure, self-assembly conditions and external stimuli.