Helical Polymers Research Articles

Control of Intramolecular Hydrogen Bonding in a Conformation-Switchable Helical-Spring Polymer by Solvent and Temperature.

A substituted poly(phenylacetylene) derivative (PPAHB ) with two hydroxymethyl groups at the meta position of the side phenyl ring was examined as a conformation-switchable helical spring polymer that responds to solvent and heat stimuli in a precisely controlled manner. Intramolecular hydrogen bonds, which cause the helical structure of the polymer, were broken and re-formed by adjusting the hydrogen-bonding strength values (pKHB ) of various combinations of solvents or by varying the temperature. In this process, a reversible conformational change from cis-cisoid to cis-transoid, accompanied by a phase transition in the form of a helix-coil transformation occurred, with the polymer exhibiting critical changes of color fading and recovery in specific environments. These results demonstrate that PPAHB can be used as either a pKHB indicator or a thermometer. The color changes of the polymer solution are described in detail based on spectroscopic analyses and thermodynamic considerations.

Control of Intramolecular Hydrogen Bonding in a Conformation‐Switchable Helical‐Spring Polymer by Solvent and Temperature

AbstractA substituted poly(phenylacetylene) derivative (PPAHB) with two hydroxymethyl groups at the meta position of the side phenyl ring was examined as a conformation‐switchable helical spring polymer that responds to solvent and heat stimuli in a precisely controlled manner. Intramolecular hydrogen bonds, which cause the helical structure of the polymer, were broken and re‐formed by adjusting the hydrogen‐bonding strength values (pKHB) of various combinations of solvents or by varying the temperature. In this process, a reversible conformational change from cis–cisoid to cis–transoid, accompanied by a phase transition in the form of a helix‐coil transformation occurred, with the polymer exhibiting critical changes of color fading and recovery in specific environments. These results demonstrate that PPAHB can be used as either a pKHB indicator or a thermometer. The color changes of the polymer solution are described in detail based on spectroscopic analyses and thermodynamic considerations.

Circular Intensity Differential Scattering Reveals the Internal Structure of Polymer Fibrils.

The optical and electronic properties of π-conjugated polymers in organic electronic devices depend on their intra- and interchain interactions, dictated by the internal arrangement of the polymer chains in an amorphous or semicrystalline aggregated state. Here, we discuss the utility of circular intensity differential scattering (CIDS) of circularly polarized light as a sensitive probe to identify the internal arrangement of the polymer chains in helical polymer aggregates. We advance existing theoretical models to utilize the CIDS response and extract structural properties such as the size, orientation, and periodicity of a polymer aggregate. As an example, we analyze the CIDS signatures of helically assembled fibrillar aggregates of a chiral polymer poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzothiadiazole)] (PFBT) in solution and reveal that PFBT fibrils incorporate at least five intertwined polymer chains. We anticipate our approach can be extended more generally to investigate the internal arrangement of supramolecular assemblies of a wide range of fibrillar aggregates of π-conjugated polymers.

From LC‐polymers to Nanomedicines: Different Aspects of Polymer Science from a Materials Viewpoint

AbstractThis essay compiles the work of the author on i) liquid crystalline (LC)‐polymers (including LC‐elastomers, ferroelectric LCs and LC‐particles), on ii) switchable helical polymers, on iii) polymer based opals, on iv) organic – inorganic hybrids for the improvement of quantum dot light emitting diodes or for battery applications as well as on v) polymeric carriers for biomedical applications (e.g. nanomedicine for tumor‐immune‐therapy). From the chemical side this work is based the use of various polymerization techniques including—as more special techniques—a) “surfactant‐free emulsion polymerization” to prepare extremely monodisperse polymer colloids, b) controlled radical polymerization (RAFT) to prepare functional block‐copolymers and c) photoinitiated polymerization, to start polymerization and/or crosslinking at a defined space and time during processing.

Coherent acoustic phonon dynamics in chiral copolymers.

Coherent phonon oscillations in the UV-Vis transient absorption and circular dichroism response of two chiral polyfluorene-based copolymer thin films are investigated. A slow oscillation in the hundred picosecond regime indicates the propagation of a longitudinal acoustic phonon with a frequency in the gigahertz range through cholesteric films of PFPh and PFBT, which allow for the optical determination of the longitudinal sound velocity in these polymers, with values of (2550 ± 140) and (2490 ± 150) m s−1, respectively. The oscillation is induced by a strain wave, resulting in a pressure-induced periodic shift of the electronic absorption bands, as extracted from a Fourier analysis of the transient spectra. The acoustic phonon oscillation is also clearly detected in the transient circular dichroism (TrCD) response of PFPh, indicating a transient pressure-induced shift of the CD spectrum and possibly also phonon-induced chirality changes via pitch length modulation of the cholesteric helical polymer stack.

Synthesis and asymmetric catalytic performance of one-handed helical poly(phenylacetylene)s bearing proline dipeptide pendants

One-handed helical substituted polyacetylene has received extensive attention due to its potential in chiral stationary phases and molecular recognition. Here, three one-handed helical poly(phenylacetylene)s bearing proline or proline dipeptide as the pendants (PPA-Pro, PPA-Pro-Pro and PPA-Pro-Hyp) were synthesized. As the catalyst, the three helical polymers separately catalyzed the asymmetric aldol reaction of acetone with p-nitrobenzaldehyde in a various media. The results indicate that PPA-Pro-Pro and PPA-Pro-Hyp bearing dipeptide pendants showed the higher catalytic activity and enantioselectivity than the PPA-Pro bearing proline pendants and the corresponding monomer catalysts. This indicates the synergistic effect of the dipeptide pendants with the one-handed helical mainchain of the polymer catalyst. The polymer catalysts can be recycled and maintain the catalytic performance after four circles. When imidazole was added to the reaction system, the reaction yield could be efficiently improved without affecting the enantioselectivity.

Trismaleimide Dendrimers: Helix-to-Superhelix Supramolecular Transition Accompanied by White-Light Emission.

Reported here are unprecedented fluorescent superhelices composed of primary, supramolecular polymers of the opposite helical twist. A new class of functional dendrimers was synthesized by amino-ene click reactions, and they demonstrate an alternating OFF/ON fluorescence with generation growth. A peripherally alkyl-modified dendrimer displays helix-sense-selective supramolecular polymerization, which predominantly forms right-handed (or left-handed) helical supramolecular polymers in the solution containing chiral solvents. With increasing the concentration, these primary helical supramolecular polymers spontaneously twist around themselves in the opposite direction to form superhelical structures. Atomic force microscopy and circular dichroism measurements were used to directly observe the helix-to-superhelix transition occurring with a reversal in the helical direction. Exceptional white-light emission was observed during superhelix formation.

Trismaleimide Dendrimers: Helix‐to‐Superhelix Supramolecular Transition Accompanied by White‐Light Emission

AbstractReported here are unprecedented fluorescent superhelices composed of primary, supramolecular polymers of the opposite helical twist. A new class of functional dendrimers was synthesized by amino‐ene click reactions, and they demonstrate an alternating OFF/ON fluorescence with generation growth. A peripherally alkyl‐modified dendrimer displays helix‐sense‐selective supramolecular polymerization, which predominantly forms right‐handed (or left‐handed) helical supramolecular polymers in the solution containing chiral solvents. With increasing the concentration, these primary helical supramolecular polymers spontaneously twist around themselves in the opposite direction to form superhelical structures. Atomic force microscopy and circular dichroism measurements were used to directly observe the helix‐to‐superhelix transition occurring with a reversal in the helical direction. Exceptional white‐light emission was observed during superhelix formation.

Synthesis and Self‐assembly of a Helical Polymer Grafted from a Foldable Polyurethane Scaffold

Herein a polyurethane graft poly-l-glutamate amphiphilic copolymer was synthesized from a polyurethane (PU)-based macro-initiator (containing pendant primary amine groups) through the ring opening polymerization of N-carboxy anhydride of γ-benzyl-l-glutamate (BLG-NCA). On average, twenty two l-glutamic acids were grafted from each amino group which was pendant on the polyurethane chain with 10 repeating units. The grafted polymer (PU-PP-1) exhibits self-assembly to produce a hydrogel in a wide pH window ranging from pH 5.0 to 8.0 with a critical gelation concentration (CGC) of 5.0 wt % (w/v) at pH 7.4. Furthermore, circular dichroism study revealed the transition of the α-helix to a random coil upon increasing the pH. Due to the protonation of side chains at pH 4.0, PU-PP-1 adopted an α-helical conformation whereas at pH >8.0 the side-chain carboxylic acid groups of the PLGAs were ionized, leading to the formation of an extended random coil conformation as a result of charge repulsion. Conformational switching was also supported by FTIR spectroscopy.

Helicity Induction and Its Static Memory of Poly(biphenylylacetylene)s Bearing Pyridine N‐Oxide Groups and Their Use as Asymmetric Organocatalysts

ABSTRACTNovel poly(biphenylylacetylene) derivatives carrying different types of pyridine N‐oxide units with a bulky or less‐bulky substituent at a different position as the functional pendant groups (poly‐2a and poly‐2b) were synthesized by the rhodium‐catalyzed polymerization of the corresponding monomers. The influence of the steric environment around the catalytically active pyridine N‐oxide sites on the helicity induction and its static memory as well as the asymmetric catalytic activities of the resulting helical polymers with a macromolecular helicity memory was investigated. The polyacetylenes formed an excess one‐handed helical conformation upon noncovalent interactions with optically active alcohols and the induced macromolecular helicities of the polyacetylenes were efficiently memorized after the removal of the chiral inducers. Poly‐2b with the macromolecular helicity memory showed an enantioselectivity for the catalytic asymmetric allylation of benzaldehydes, producing optically active allyl alcohols, although their enantioselectivities were low. On the other hand, poly‐2a exhibited a negligible catalytic activity probably due to the bulky substituent at the o‐position of the pyridine N‐oxide residues, while poly‐2a underwent a unique helix‐inversion with the increasing concentration of chiral alcohols and the opposite helicity of poly‐2a was further successfully memorized. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2481–2490

Temperature Control of Sequential Nucleation-Growth Mechanisms in Hierarchical Supramolecular Polymers.

Upon cooling in solution, chiral triarylamine tris-amide unimers produce organogels by stacking into helical supramolecular polymers, which subsequently bundle into larger fibers. Interestingly, circular dichroism, vibrational circular dichroism, and AFM imaging of the chiral self-assemblies revealed that monocolumnar P-helical fibrils formed upon fast cooling, whereas bundled M-superhelical fibers formed upon slow cooling. The mechanistic study of this structural bifurcation reveals the presence of a strong memory effect, reminiscent of a complex stepwise combination of primary and secondary nucleation-growth processes. These results highlight the instrumental role of sequential self-assembly processes to control supramolecular architectures of multiple hierarchical order.

Catalysts Supported by Homochiral Molecular Helices: A New Concept to Implement Asymmetric Amplification in Catalytic Science

AbstractConnecting intrinsically achiral catalytic centers to helical polymers provides a new class of asymmetric catalysts whose selectivity exclusively stems from the chiral environment provided by the helical polymer. Chirality amplification phenomena allow to control the handedness and the helical sense excess of these polymers which in turn dictate the stereochemical direction and the extent of enantioinduction, respectively, obtained in the course of the asymmetric reaction. This minireview is focused on such helical catalysts built either on a covalent or a non‐covalent scaffold and for which the handedness is controlled by an asymmetric polymerization reaction, by enantiopure monomers or by chiral additives. The selectivity of catalysts based on a dynamic helical backbone can be tuned by changing the solvent or with additives. The fact that the enantiomeric state of such asymmetric catalysts can be switched in a predictable way is particularly emphasized.

Synthesis and Light‐Mediated Structural Disruption of an Azobenzene‐Containing Helical Poly(isocyanide)

Helical poly(isocyanide)s are an important class of synthetic polymers possessing a static helical structure. Since their initial discovery, numerous examples of these helices have been fabricated. In this contribution, the synthesis of a chiral, azobenzene (azo)-containing isocyanide monomer is reported. Upon polymerization with nickel(II) catalysts, a well-defined circular dichroism (CD) trace is obtained, corresponding to the formation of a right-handed polymeric helix. The helical polymer, dissolved in chloroform and irradiated with UV light (365 nm), undergoes a cis to trans isomerization of the azobenzene side-chains. After the isomerization, a change in conformation of the helix occurs, as evidenced by CD spectroscopy. When the solution is irradiated with LED light, the polymer returns to a right-handed helical conformation. To open up the possibility for chain-end post-polymerization modification of this light-responsive system, an alkyne-functionalized nickel(II) catalyst is also used in the polymerization of the azobenzene monomer, resulting in a stimuli-responsive, terminal-alkyne-containing helical poly(isocyanide).

One-Step Synthesis of One-Dimensional Supramolecular Assemblies Composed of Helical Macromolecular Building Blocks.

Living systems achieve sophisticated functions using supramolecular protein assemblies, in which the protein building blocks possess a specific secondary structure and are noncovalently arranged in a preprogrammed manner. Herein, we demonstrate the one-step synthesis of one-dimensional macromolecular assemblies by simply mixing a glycine-based isocyanide with a nickel catalyst, in which helical constituent polymers are linked end-to-end through multiple hydrogen bonds. The applicable scope of this approach is not confined to a particular monomer bearing a specially designed pendant, but covers a wide range of glycine-based isocyanides with or without aromatic and other functional groups. Surprisingly, copolymerization with an analogous chiral isocyanide (1 mol %) afforded an almost perfect one-handed helical supramolecular fiber owing to intramolecular/intermolecular dual chiral amplifications. The simplicity and broad applicability of this approach, which can also afford exquisite chiral amplification, enable the creation of a wide variety of functional supramolecular assemblies and provide access to new supramolecular materials.

Chiral Conflict as a Method to Create Stimuli-Responsive Materials Based on Dynamic Helical Polymers.

A new multi-sensor material based on helical copolymers showing the chiral conflict effect have been prepared. It can successfully detect and identify diverse metal cations in solution. The design of this material has taken into account not only the opposite helical senses induced by the two chiral monomers in the copolymer, but also their dynamic behavior. The induced helical sense can thus be enhanced, diminished, or inverted by interaction with different stimuli (that is, metal ions). Thus, depending on both the copolymer compositions (such as monomer ratios and absolute configurations) and the nature of the metal ion, the response of these dynamic helical copolymers to adopt a single-handed P or M helix is unique, making it possible not only to detect their presence, but also to identify them individually. New multi-sensors materials based and inspired on this effect should arise in the future choosing appropriate monomers and stimuli.

Chiral Conflict as a Method to Create Stimuli‐Responsive Materials Based on Dynamic Helical Polymers

AbstractA new multi‐sensor material based on helical copolymers showing the chiral conflict effect have been prepared. It can successfully detect and identify diverse metal cations in solution. The design of this material has taken into account not only the opposite helical senses induced by the two chiral monomers in the copolymer, but also their dynamic behavior. The induced helical sense can thus be enhanced, diminished, or inverted by interaction with different stimuli (that is, metal ions). Thus, depending on both the copolymer compositions (such as monomer ratios and absolute configurations) and the nature of the metal ion, the response of these dynamic helical copolymers to adopt a single‐handed P or M helix is unique, making it possible not only to detect their presence, but also to identify them individually. New multi‐sensors materials based and inspired on this effect should arise in the future choosing appropriate monomers and stimuli.

Structural characterization and biological properties of silver(I) tris(pyrazolyl)methane sulfonate

The water-soluble 1D helical coordination polymer [Ag(Tpms)]n (1) [Tpms = tris(pyrazolyl)methane sulfonate, −O3SC(pz)3; pz = pyrazolyl] was synthesized and fully characterized, its single-crystal X-ray diffraction analysis revealing the ligand acting as a bridging chelate N3-donor ligand. The antiproliferative potential of 1 was performed on two human tumour cell lines, A2780 and HCT116, and in normal fibroblasts, with a much higher effect in the former cell line (IC50 of 0.04 μM) as compared to the latter cell line and to normal fibroblasts. Compound 1 does not alter cell cycle progression but interferes with the adherence of A2780 cells triggering cell apoptosis. Apoptosis appears to occur via the extrinsic pathway (no changes in mitochondria membrane potential, reactive oxygen species (ROS) and pro-apoptotic (B-cell lymphoma 2 (BCL-2) associated protein (BAX))/anti-apoptotic (BCL-2) ratio) being this hypothesis also supported by the presence of silver mainly in the supernatants of A2780 cells. Results also indicated that cell death via autophagy was triggered. Proteomic analysis allowed us to confirm that compound 1 is able to induce a stress response in A2780 cells that is related with its antiproliferative activity and the trigger of apoptosis.

Helix-Sense-Specific and Enantiomer-Specific Living Polymerizations of Phenyl Isocyanides Using Chiral Palladium(II) Catalysts

Controlled synthesis of a single-handed helical polymer from an achiral monomer is still a great challenge. In this work, we report two chiral Pd(II) catalysts bearing commercially available R- or S-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (R- or S-BINAP) ligands. Polymerization of achiral phenyl isocyanide (A-1) by S-BINAP/Pd(II) afforded a single right-handed helical poly(phenyl isocyanide)s in high yields with controlled molar mass (Mns) and narrow molar mass distributions (Mw/Mns). While the R-BINAP/Pd(II) catalyst leads to the formation of a single left-handed helix under the same conditions. The single-handed helices were determined by circular dichroism, UV–vis analyses, and direct atomic force microscopy (AFM) observations as well. Moreover, the chiral Pd(II) catalysts showed enantiomer specificity on polymerizations of isocyanide enantiomers (l- and d-1), although the chiral center was remote from the polymerization site. S-BINAP/Pd(II) can effectively promote the living polymerization of l-1, afforded a right-handed helical poly-l-1m(S), while it failed on the polymerization of the d-1 enantiomer and the d/l-1 racemate under the same conditions. Accordingly, R-BINAP/Pd(II) can only catalyze the polymerization of the d-1 enantiomer.

N-Substituted Benzene-1-Urea-3,5-Biscarboxamide (BUBA): Easily Accessible C2 -Symmetric Monomers for the Construction of Reversible and Chirally Amplified Helical Assemblies.

Non-C3 -symmetric supramolecular helices are gaining interest for the design of hierarchical assemblies, for the compartmentalisation or the self-assembly of polymer chains and for application in asymmetric catalysis. Herein, N-substituted benzene-1-urea-3,5-biscarboxamide (BUBA) monomers, which consist of one urea and two carbon-connected amide functions linked to an aromatic ring, are introduced as an easily accessible class of C2 -symmetric supramolecular synthons. In apolar solvents, BUBA monomers assemble into long helical assemblies by means of hydrogen-bonding and aromatic interactions, as assessed by several analytical techniques. To probe the influence of the urea function, BUBA and related benzene-1,3,5-tricarboxamide (BTA) helical polymers have been compared, in terms of their thermodynamics of formation, stability, reversibility and chiral amplification properties. Similar to BTA, BUBA monomers form long helices reversibly through a highly cooperative mechanism and the helicity of their assemblies is governed by chiral amplification effects. However, precise quantification of their properties reveals that BUBA monomers assemble in a more cooperative manner. Also, chiral amplification operates to a higher extent in BUBA helices, as probed by both sergeants-and-soldiers and majority-rules experiments. Compatibility between urea and amide functions also allows the formation of co-assemblies that incorporate both BUBA and BTA monomers. Importantly, a small amount of chiral BUBA monomers in these co-assemblies is sufficient to obtain single-handed helices; thus paving the way towards the development of functional supramolecular helices.

Structural Variability in the RLR-MAVS Pathway and Sensitive Detection of Viral RNAs.

Cells need high-sensitivity detection of non-self molecules in order to fight against pathogens. These cellular sensors are thus of significant importance to medicinal purposes, especially for treating novel emerging pathogens. RIG-I-like receptors (RLRs) are intracellular sensors for viral RNAs (vRNAs). Their active forms activate mitochondrial antiviral signaling protein (MAVS) and trigger downstream immune responses against viral infection. Functional and structural studies of the RLR-MAVS signaling pathway have revealed significant supramolecular variability in the past few years, which revealed different aspects of the functional signaling pathway. Here I will discuss the molecular events of RLR-MAVS pathway from the angle of detecting single copy or a very low copy number of vRNAs in the presence of non-specific competition from cytosolic RNAs, and review key structural variability in the RLR / vRNA complexes, the MAVS helical polymers, and the adapter-mediated interactions between the active RLR / vRNA complex and the inactive MAVS in triggering the initiation of the MAVS filaments. These structural variations may not be exclusive to each other, but instead may reflect the adaptation of the signaling pathways to different conditions or reach different levels of sensitivity in its response to exogenous vRNAs.