Duplex foldamers from assembly induced folding

Oligoamide strands 1, 2, and 3, consisting of 4-H-bond units, were originally designed to form noncovalent polymers based on the expectation that they would adopt an extended conformation. Instead of assembling into the expected supramolecular polymer through their 4-H-bond units, the 1:1 mixture of 1 and 2 was found to form a highly stable dimeric species. To dimerize, the H-bonding sequences of 1 and 2 can only adopt a folded (stacked) conformation. The self-assembly of 3 was also found to adopt a similar folded duplex conformation. These novel duplex foldamers are very stable. They were characterized by 1D and 2D 1H NMR, VPO, and mass spectral (ESI) studies.     

Crystallizability and thermal properties of phosphorus-containing polyamides

Bis(2-carb-n-butoxyethyl)methylphosphine oxide was prepared and condensed with 1,2-diaminoethane, 1,6-diaminohexane, 1,8-diaminooctane, 1,10-diaminodecane, and 1,12-diaminododecane to form polyamides with an increasing number of methylene groups between the amide moieties. Polyamides made from the 1,10- and 1,12-diaminoalkanes were shown by x-ray and DSC to be semicrystalline, while those based on the 1,6- and 1,8-diaminoalkanes were amorphous to x-ray and revealed no first-order transitions. The repeat distances of the crystalline polymers were determined, and an extended planar zigzag conformation was excluded as a possible conformation for these polymers. The thermal stability of the phosphorus-containing polyamides, in both oxygen and nitrogen, was studied by TGA and the results were correlated with the chemical structure.     

Conformational study of a single molecule of poly para phenylene ethynylenes in dilute solutions

The conformation of single molecules of dialkyl poly para phenylene ethynylenes (PPEs), electro-active polymers, is studied in solutions using molecular dynamics simulations. The conformation of conjugated polymers affects their electro-optical properties and therefore is critical to their current and potential uses, though only limited theoretical knowledge is available regarding the factors that control their configuration. The present study investigates the affects of molecular parameters including molecular weight of the polymer and chemical structure of the side chains of PPEs in different solvents on the conformation of the polymers. The PPEs are modeled atomistically where the solvents are modeled both implicitly and explicitly. The study finds that PPEs assume extended configuration which is affected by the length of the polymer backbone and the nature and length of substituting side chains. While the polymer remains extended, local dynamics is retained and no long range correlations are observed within the backbone. The results are compared with scattering experiments.     

Hard-segment polymorphism in MDI/diol-based polyurethane elastomers

X-ray analyses of methylene-bis(4)phenyl isocynate(MDI)/diol/poly(tetramethylene adipate) polyurethane elastomers prepared using hexanediol (HDO), butanediol (BDO), and propanediol (PDO) point to the development of a second crystalline structure in the hard segments as a result of thermal elongation and annealing. The HDO polymer crystallizes initially in the fully extended conformation, but a second crystal structure, a contracted form, develops with stretching and annealing at 130°C. In contrast, the BDO and PDO polymers crystallize initially in contracted conformation, but fully extended forms develop as a result of elongation and annealing. Only a contracted conformation has been seen so far in hard segments prepared using ethylene glycol (EDO) as the chain extender. These results correlate very well with DSC data for the same polymers. The development of the new crystal structures is accompanied by the appearance of new hard-segment melting peaks in the DSC traces. For the HDO polymer, the new peak for the contracted form appears at a higher temperature than that for the original extended form; for the BDO and PDO polymers the new peaks for the extended forms appear at lower temperatures than those for the contracted forms. Only a single peak is seen for the EDO polymer, for which only one crystal structure has been detected for the hard segments. These results indicate that the development of polymorphic crystal structures must be taken into account in interpreting the multiple melting phenomena seen for the hard domains in polyurethane elastomers.     

Influence of aggregates and solvent aromaticity on the emission of conjugated polymers

The influence of aggregates and solvent aromaticity on the photophysics and fluorescence dynamics of two conjugated polymers is studied. The two polymers are derivatives of poly(p-phenylene vinylene) (PPV) containing different kinked moieties along the main chain. The polymers contain 2,6-diphenylpyridine and m-terphenyl kinked moieties and they are abbreviated as PN and PC, respectively. The insertion of kinked segments along the main chain shifts the emission spectrum from the yellow-orange spectral region, common to PPV derivatives, to the blue-green spectral region. The results show that in dilute solutions the polymers decay monoexponentially, while in concentrated ones the fluorescence decays biexponentially, indicating fluorescence quenching. This is attributed to an energy transfer process from polymer chains to aggregates that occurs within a few tens of picoseconds. By comparing the photophysics and fluorescence dynamics of polymer PN in a nonaromatic and an aromatic solvent, we conclude that the polymer conformation adopted in the aromatic solvent leads to a higher fluorescence quantum yield and a longer fluorescence lifetime. Furthermore, the fluorescence quenching of PN because of aggregates is faster and more efficient in the aromatic than in the nonaromatic solvent. These results can be explained through a more extended chain conformation of PN in the aromatic solvent.     

Intramolecular van der Waals Interactions Challenge Anion Binding in perthio-Bambusurils

Bambusurils (BUs) are known to be rigid cavitands that feature an extended, jigger-like conformation, and the BU[6]s strongly bind anions within their hydrophobic cavity. These features are not necessarily shared by the family of perthio-BUs. This study reveals that perthio-BUs assume a compact conformation and perthio-BU[6]s are poor anion binders, crystallizing as anion-free species from solutions containing halide salts. Computational studies show that the equatorial sulfur atoms compete against guest anions for binding with the glycoluril methine groups via strong van der Waals (vdW) attractive interactions. These competitive contacts not only account for the diminished anion-binding of perthio-BUs, but also explain their compact conformation. The semithio- and perthio-BU[4]s form linear coordination polymers with Hg^II in the solid-state regardless of their intrinsic molecular conformation. The strong involvement of sulfur atoms in intramolecular interactions differentiates the equatorial from the axial (peripheral) heteroatoms, thus offering chemoselective and regioselective transformations.     

Para‐linked and meta‐linked cationic water‐soluble fluorene‐containing poly(aryleneethynylene)s: Conformational changes and their effects on iron–sulfur protein detection

Four para‐linked or meta‐linked cationic water‐soluble fluorene‐containing poly(aryleneethynylene)s (PAEs) were synthesized to investigate the solvent‐induced π‐stacked self‐assembly. These PAE backbones are composed of fluorenylene and phenylene units, which are alternatively linked by ethynylene bonds. UV–vis absorption and photoluminescence spectra were used to study their conformational changes as solvent was gradually changed from MeOH to H₂O. In pure water, with gradually increased meta‐phenylene content (0, 50, and 100%), they underwent a gradual transition process of conformation from disordered aggregate structure to helix structure, which was not compactly folded. Moreover, the polymer with an ammonium‐functionalized side chain on the meta‐phenylene unit appeared to adopt a more incompact or extended helix conformation than the corresponding one without this side chain. Furthermore, the conformational changes of these cationic PAEs in H₂O were used to study their effects on biological detection. Rubredoxin (Rd), a type of anionic iron–sulfur‐based electron transfer protein, was chosen to act as biological analyte in the fluorescence quenching experiments of these polymers. Preliminary results suggest that they all exhibit amplified fluorescence quenching, and that the polymer with more features of helix conformation tends to be quenched by Rd more efficiently. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5424–5437, 2006     

Micellization of telechelic associative polymers: self-consistent field modeling and comparison with scaling concepts

We present numerical results from self-consistent field calculations on the micellization of telechelic associative polymers and their mono-functional analogues. These results are confronted with relatively simple scaling concepts. The proportionality of the critical micelle concentration (CMC) with the hydrophilic backbone length, as found in the calculations, shows good correspondence with a scaling argument based on the entropic penalty of loop formation. It is also shown that models for the conformation of spherical brushes can be applied to predict the structure of the flowerlike micelles formed by these telechelic polymers. Furthermore, we find good agreement between the numerical dependence of the aggregation number upon both backbone and terminal hydrophobe length and an analytical expression derived from the well-known Daoud-Cotton model by introducing a correction for the finite size of the micellar core.     

Hybrid organo-inorganic clay with nonionic interlayers. mid- and near-IR spectroscopic studies

The intercalation of organic polymers molecules (i.e., PEGs and BRIJ) into a standard Ca-montmorillonite has been studied by XRD, TG, and IR spectroscopy. The polymer intercalation is confirmed by the increasing of the d(001) in XRD spectra as well as by the complex multisteps thermal decomposition behavior of the organo-clay materials. Mid-IR and diffuse reflectance near-IR spectra of the intercalated materials show the polymer diagnostic bands (CH stretching and deformation mode), shifted or changed in shape by the interaction with the clay matrix. Both PEG 1500 and PEG 4000 based materials are likely intercalated in an extended configuration, similar to the amorphous polymer form. BRIJ intercalated polymer spectra suggest the disordered conformation of the alkilic chain in a prevailing "gauche", poorly packed, conformation. Host montmorillonite IR bands, mainly OH and water stretching and deformation fundamentals, combination, and overtone bands, are reduced in intensity by polymer intercalation, pointing out an interaction, likely through H-bonding and/or a possible substitution of cations hydration water molecules.     

一种手性甲壳型液晶高分子的稀溶液行为及链刚性

用激光光散射和粘度法研究了一系列窄分子量分布的新的手性甲壳型液晶聚合物P1～P6的溶液行为及其链刚性.研究发现四氢呋喃是聚合物P1～P6的良溶剂.在四氢呋喃溶液中,聚合物P1～P6的特性粘数[η]和均方根旋转半径z12对重均分子量Mw的依赖关系分别是[η]=(2.75±0.05)×10-3Mw0.78±0.02和z12=(1.53±0.04)×10-2Mw0.60±0.01.按照YamakawaFujiiYoshizaki蠕虫状圆筒模型的粘度理论和BohdaneckyBushin表达式,求得聚合物的单位围长摩尔质量ML=(29.8±1.0)×102nm,构象保持长度q=(15.4±3.0)nm.q和MHS方程指数α的值说明这类在结构上属于侧链型液晶高分子的聚合物在良溶剂四氢呋喃中呈现比较伸展的刚性链构象,其链刚性与半刚性主链液晶高分子的相似.     

Spectroscopic studies of poly(vinylamine) containing optically active thymine derivatives as grafted pendants

The optical properties of poly(vinylamine) containing optically active (+)- or (?)-2-(thymin-1-yl) propionyl groups as grafted pendants (PT) and the related monomer (MT) and dimer models (DT) were investigated by UV, circular dichroism (CD), and NMR spectroscopy. Highly syndiotactic PT has a smaller hypochromicity versus MT and a larger optical rotation than the less syndiotactic PT in various solutions. These results are attributed to an interaction between the configurational arrangement of thymines, the conformation of the polymers, and base stacking between thymines. The interactions of these polymers with poly(adenylic acid) (polyA) were also studied and the results compared with other vinyl-type nucleic acid model polymers. The isomers of the optically active dimer models [prepared from meso and (dl)-2,4-diaminopentane] were separated. The CD spectra of (+)-D(?)T in CHCl₃ and trifluoroethanol (TFE) displayed extremely solvent-dependent exciton coupling of the π–π^* (B_2u) transition of the base chromophore, which was not observed in the other models or polymers, except the meso-type dimer model (meso)-D-(?)T. This coupling decreased with increasing solvent dielectric constant, while UV hypochromicity increased. This behavior as well as the 360-MHz NMR spectra suggest that (+)-D(?)T exists in an extended form in solvents of low dielectric constant and gradually assumes a stacked conformation as the dielectric constant increases.     

Conformational disorder of conjugated polymers

Conformational disorder of conjugated polymers is an important issue to be understood and quantified. In this paper we present a new method to assess the chain conformation of conjugated polymers based on measurements of intrachain energy transfer. The chain conformation is modeled on the basis of monomer-monomer interactions, such as torsion, bending, and stretching of the connecting bond. The latter two potentials are assumed to be harmonic, while the torsional potential was calculated by density functional theory using B3-LYP functional with the SVP basis set. The energy transfer dynamics of excitons on these chains are quantitatively simulated using Forster-type line-dipole energy transfer. This allows us to compare the simulated ground state conformation of single polymer chains to ultrafast depolarization experiments of poly [3-(2,5-dioctylphenyl)thiophene] in solution. We identify torsional rotation as the main contributor to conformational disorder and find that this disorder is mainly controlled by the energy difference between syn and anti bonds.     

Adsorption and flocculation by polymers and polymer mixtures

Polymers of various types are in widespread use as flocculants in several industries. In most cases, polymer adsorption is an essential prerequisite for flocculation and kinetic aspects are very important. The rates of polymer adsorption and of re-conformation (relaxation) of adsorbed chains are key factors that influence the performance of flocculants and their mode of action. Polyelectrolytes often tend to adopt a rather flat adsorbed configuration and in this state their action is mainly through charge effects, including ‘electrostatic patch’ attraction. When the relaxation rate is quite low, particle collisions may occur while the adsorbed chains are still in an extended state and flocculation by polymer bridging may occur. These effects are now well understood and supported by much experimental evidence. In recent years there has been considerable interest in the use of multi-component flocculants, especially dual-polymer systems. In the latter case, there can be significant advantages over the use of single polymers. Despite some complications, there is a broad understanding of the action of dual polymer systems. In many cases the sequence of addition of the polymers is important and the pre-adsorbed polymer can have two important effects: providing adsorption sites for the second polymer or causing a more extended adsorbed conformation as a result of ‘site blocking’.     

Folding of coordination polymers into double-stranded helical organization

Self-assembling coordination polymers based on Pd II and Cu II metal ions were prepared from complexation of a bent-shaped bispyridine ligand and a corresponding transition metal. These coordination polymers were observed to self-assemble into supramolecular structures that differ significantly depending on the coordination geometry of the metal center. The polymer based on Pd II self-assembles into a layer structure formed by bridging bispyridine ligands connected in a trans-position of the square-planar coordination geometry of metal center. In contrast, the polymer based on Cu II adopts a double-helical conformation with regular grooves, driven by interstranded, copper-chloride dimeric interaction. The double-stranded helical organization is further confirmed by structure optimization from density functional theory with aromatic framework, showing that the optimized double-helical structure is energetically favorable and consistent with the experimental results. These results demonstrate that weak metal-ligand bridging interactions can provide a useful strategy to construct stable double-stranded helical nanotubes.     

The effect of chemical constitution of mesogenic monomer unit on liquid crystalline properties of polymers

Glass transition temperatures and thermodynamic parameters of mesophase melting of polymers with mesogenic side-groups of the phenyl benzoate class depend on the length of the alkoxy substitutent, the mobility of the mesogenic groups and the nature of the main chain. Polymers with mesogenic groups connected directly to the main chain, obtained by precipitation from solution, have equilibrium liquid crystalline structures in which the macromolecules exist in a compact coil conformation with an ordered arrangement of the mesogenic groups. The nature of the main chain of these polymers affects the liquid crystalline structure. Mesophase melting parameters of the polymers with mesogenic groups, connected by flexible spacer groups to the main chain, are almost independent of the thermal history of the samples. These polymers in the isotropic melts are assumed to contain aggregates of the mesogenic groups.     

Photochemistry Highlights on On-Surface Synthesis

On-surface chemistry is a promising way to achieve the bottom-up construction of covalently-bonded molecular precursors into extended atomically-precise polymers adsorbed on surfaces. These polymers exhibit unprecedented physical or chemical properties which are of great interest for various potential applications. These nanostructures were mainly obtained in ultra-high vacuum (UHV) on noble metal single-crystal surfaces by thermal annealing as stimulus to provoke the polymerization with a catalytic role of the surface adatoms. Nevertheless, photons are also a powerful source of energy to induce the formation of covalent architectures, even if it is less-used on surfaces than in solution. In this minireview, we discuss the photo-induced on-surface polymerization from the basic mechanisms of photochemistry to the formation of extended polymers on different kinds of surfaces, which are characterized by scanning probe microscopies.     

Helical polymers based on intramolecularly hydrogen-bonded aromatic polyamides

Inspired by arylamide-based oligomeric foldermers that are stabilized by intramolecular hydrogen bonding, a series of polyamides with intramolecular hydrogen-bonding motifs were synthesized via polycondensation reactions. These polymers can fold into helical conformation different from their linear control. The chirality of helical conformation can further be tuned via acid-base complexation using chiral residues.     

Optically Active Helical Vinylterphenyl Polymers: Chiral Teleinduction in Radical Polymerization and Tunable Stereomutation

Helical vinyl aromatic polymers are emerging as interesting chiral materials due to their dynamic tailorability, synthetic simplicity, and outstanding chemical and physical stabilities. This Personal Account discusses long‐range chirality transfer in the radical polymerization of vinylterphenyl monomers and tunable stereomutation of the resultant polymers. It begins with a general introduction to the design, synthesis, and characterization of helical poly{(+)‐2,5‐bis[4′‐((S)‐2‐methylbutyloxy)phenyl]styrene}, the first one of this series of polymers. Then, long‐range chirality transfer during radical polymerization of terphenyl‐based vinyl monomers is explained. After that, the chiroptical property control of the resultant polymers by means of the transition from kinetically controlled conformation to thermodynamically controlled conformation and external stimulus is described. This Personal Account concludes by discussing the advantages and disadvantages of the strategy of using vinylterphenyls to obtain optically active helical polymers and providing a short outlook, especially emphasizing the importance of tacticity on the chiroptical properties of polymers.     

C−N Coupling Reactions on Graphene with Aromatic Macromolecules and the Spatial Conformation of Grafted Macromolecules

The fabrication of advanced graphene-based nanocomposites with high-performance polymers requires covalent modification of graphene with aromatic macromolecules. Herein, C−N coupling reactions between fluorinated graphene (FG) and aromatic polyamides containing the benzimidazole moiety are successfully achieved. The optimized conditions are presented based on the nucleophilic behavior of the C−N coupling reaction on graphene. Different from the C−N coupling reaction between two small aromatic molecules, the conformation of grafted aromatic polyamide after reaction changes from torsional to paralleled alignment on graphene with the molecular length increment. Non-covalent interactions between graphene and aromatic polyamides result in this conformational change owing to the extended π systems of graphene and aromatic polyamides, and the synergistic effect of covalent and non-covalent interactions is put forward. As a consequence, graphene dispersibility is greatly enhanced in the solution of aromatic polyamide.