Generalizing the Aromatic δ-Amino Acid Foldamer Helix

A series of aromatic oligoamide foldamer sequences containing different proportions of three δ-amino acids derived from quinoline, pyridine, and benzene and possessing varying flexibility, for example due to methylene bridges, were synthesized. Crystallographic structures of two key sequences and ¹H NMR data in water concur to show that a canonical aromatic helix fold prevails in almost all cases and that helix stability critically depends on the ratio between rigid and flexible units. Notwithstanding subtle variations of curvature, i. e. the numbers of units per turn, the aromatic δ-peptide helix is therefore shown to be general and tolerant of a great number of sp³ centers. We also demonstrate canonical helical folding upon alternating two monomers that do not promote folding when taken separately: folding occurs with two methylenes between every other unit, not with one methylene between every unit. These findings highlight that a fine-tuning of helix handedness inversion kinetics, curvature, and side chain positioning in aromatic δ-peptidic foldamers can be realized by systematically combining different yet compatible δ-amino acids.     

Folding versus self-assembling

Model foldable polymers with sequences of rigid hydrophobic chromophores and flexible hydrophilic tetra(ethylene glycol) were synthesized and used as a paradigm for studying molecular-folding and self-assembly phenomena. Our results demonstrate that intramolecular association or folding prevails over intermolecular interaction or self-assembling in the concentration region from 1 microM to 0.1 M. Importantly, folded polymeric nanostructures have absorption and fluorescence properties that are distinct from those of unfolded polymers or free monomers. We hypothesize that the origins of folding and self-assembly come from interactions between molecular units, and that the key parameter that regulates the on-and-off of such interactions is the distance R separating the two molecular units. Each molecular unit produces a characteristic force field, and when another molecular unit enters this field, the probability that the two units will interact increases significantly. A preliminary estimate of the radius of such a force field for the perylene tetracarboxylic diimide chromophore is about 90-120 A. As a result, phenomena associated with folding or self-assembly of molecular species are observed when these conditions are met in solution.     

Rigid chain ribbon‐like metallopolymers

A set of new copolymers is here reported in which the repeating units are connected each other through Cu(II) metal centers. The coordination link is based on the bis‐chelating properties of salicylaldiminate groups of two different monomers. Due to their chemical structure, the two monomers afford, respectively, flexible and rigid repeating units in the metallocopolymers constitution upon coordination to copper centers. All the copolymers were soluble and easily processable. As shown by XRD analysis, rigid units' rich copolymers adopt a ribbon‐like structure in solid state in which highly planar strands of polymer stack thanks to π?π interactions, similarly to the polymer composed exclusively by rigid units. This behavior can be justified assuming the existence of a partial block character in copolymer constitution where long sequences of rigid units are alternated to sequences of flexible units. This assumption is supported also by DSC and UV–Vis analysis. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2412–2421     

Nematogenic block copolymers of rigid and flexible aromatic units. I. Synthesis and characterization

Three types of wholly aromatic block copolymers were synthesized using the phosphorylation reactions of Yamazaki and Higashi. Each copolymer contained blocks of rigid and flexible units. The first copolymer, PBA/PABH-T, contains blocks of poly(p-benzamide) and the polyterephthalamide of p-aminobenzhydrazide. The second copolymer, PBA/MPD-I, contains blocks of poly(p-benzamide) and poly(p-phenylene isophthalamide), whereas the third, PPD T/MPD-I, contains blocks of poly(p-phenylene terephthalamide) and poly(m-phenylene isophthalamide). Three synthetic routes were used for the preparation of the block copolymers. In the two-step polycondensation (A), monomers of the flexible block are added to the rigid prepolymer. The multistep method (B) differs in that the rigid prepolymer is carboxy-terminated prior to addition of the monomers of the flexible block. Carboxy-terminated prepolymer of the rigid block is reacted with amine-terminated prepolymer of the flexible block in the two-pot condensation (C). The presence of a considerable amount of the flexible homopolymer is indicated by viscosity, extraction, and NMR studies, particularly when methods A and C were used. The flexible homopolymer can be extracted by using a nonsolvent for the rigid blocks. Extraction of the rigid homopolymer (which may also be presumed to be produced) entails a more elaborate procedure. In principle, one can use these methods to obtain pure block copolymer for study of mixtures with the rigid and flexible homopolymers. Phase studies of some of these systems will be reported in a following paper.     

Synthons as building blocks in the synthesis of liquid crystalline multiblock copolymers

Synthetic approaches which allow to insert rather long fragments with exactly definite sequence of several single structural units into polymer chains are considered and summarized. The procedure consists in preliminary formation of such sequences in a form of bifunctional “complex monomer” (synthons) which may undergo a polycondensation with other single or complex monomer to form macromolecule with definite alternation of several single monomers. These synthetic approaches were used for the synthesis of LC multi-block copolymers with definite structure of rigid block.     

刚性链侧链型液晶高分子合成与研究

以自由基聚合方法，合成了一系列含有３个苯环通过酯键相联的液晶性单体及其聚合物，这类刚性液晶基元不通过柔性间隔基而直接竖挂在聚丙烯酸酯大分子主链上，具有很高的Ｔｇ·ＤＳＣ及偏光显微镜结果表明，所有的单体和聚合物均为向列型热致性液晶。     

Intermolecular potentials with flexible monomers

Most available intermolecular potentials assume rigid monomers. Such an assumption is a reflection of a trivial observation that if monomer degrees of freedom are considered, the dimensionality of an intermolecular potential increases dramatically even for few-atom monomers. This puts strict limits on the sizes of clusters with flexible monomers that can be treated by ab initio methods since the number of grid points needed to fit a potential surface quickly becomes enormous. Most of the literature flexible-monomer potentials were obtained from the so-called site-site rigid-monomer potentials by simply allowing the sites to move with atomic nuclei as the monomers are deformed. This simple atom-following approach has been investigated in the present work, using water dimer and Ar-HF as test systems, and shown to be not adequate. A method has been proposed which uses properties of isolated flexible monomers to improve upon the atom-following approach without performing any dimer calculations outside the rigid-monomer grid. The method is shown to give for Ar-HF a 3D representation of the dispersion energy that should be adequate for HF motion in the ground and first excited vibrational states.     

Synthesis, structure and characterization of side chain cholesteric liquid crystalline polysiloxanes

A series of liquid crystalline homopolysiloxanes and copolysiloxanes were synthesized. The chemical structures of the monomers M₁-M₇ were confirmed by FTIR and ¹H NMR spectroscopy. The structure-property relationships of the monomers and polymers are discussed; their phase behaviour and optical properties were investigated by differential scanning calorimetry, thermogravimetric analysis, and polarizing optical microscopy. All the monomers, except M₂ and M₇ showed smectic and nematic phases; the copolymers P₈-P₁₅ displayed cholesteric phases. The homopolymers P₁-P₇ exhibited smectic phases. The selective reflection of cholesteric monomers and copolymers shifted to longer wavelengths with increasing length of the rigid mesogenic core, with decreasing length of the flexible spacer, or with increasing content of nematic units. Experimental results demonstrated that a flexible polymer backbone, a rigid mesogenic core and a long flexible spacer tended to produce a lower glass transition temperature, higher thermal stability, and wider mesophase temperature range.     

一类新的刚性链侧链型液晶高分子的合成

以自由基聚合方法，合成了一系列含有三个苯环通过酯键相联的液晶性单体及其聚合物．在这类新的液晶高分子中，刚性液晶基元不通过柔性间隔基而直接竖挂在聚甲基丙烯酸酯大分子主链上．这些高分子有很高的玻璃化转变温度，表明其分子链刚性较大，因而代表了一类新的刚性链侧链型液晶高分子．它们的液晶行为用ＤＳＣ、偏光显微镜和Ｘ－光衍射进行了表征．发现所有单体和聚合物均为向列型热致性液晶．     

2′ Substituted sergeants influencing the deoxy soldiers in folded oligonucleotides

We demonstrate that conformational‐biased monomers can alter neighboring monomer conformation leading to globe changes in polymer topology. This work is compared to the “sergeants and soldiers” effect observed when a small percentage of optically active monomers control a polymer's helical handedness. Specifically, we show that one biased monomer in a sequence of flexible monomers yields an oligonucleotide block copolymer that folds into two topologies with competitive free energies and activation barriers (from an unfolded state). Double substitutions, on the other hand, have varying degrees of influence depending on the position of the second substitution. Substitutions within the same guanosine block have a lesser effect relative to substitutions in different blocks. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3271–3278, 2006     

60Co-initiated copolymerization of styrene with vinyl acetate

The kinetic behavior of the ⁶⁰Co-initiated copolymerization at 25°C of styrene with vinyl acetate at 1100 and 2000 rad/hr was studied. As in the case of thermal and photochemical copolymerizations of these monomers, the growing chains are particularly rich in styrene units, and the overall rate is affected by a diluent effect due to the vinyl acetate monomer. However, in the case of the radiation copolymerization, this effect is partially counterbalanced by an increase of the initiation rate with the vinyl acetate concentration; the polymerization rate curve shows a maximum at a vinyl acetate molar fraction of 0.25. This effect is due to the very different free radical yields of these two monomers. The experimental results may be understood on the basis of a kinetic scheme which involves an energy transfer process from the excited vinyl acetate molecules to the styrene monomer and a termination reaction of the growing chains by very short styrene radicals when the mixture is rich in vinyl acetate.     

Resolution of the excitation-emission spectra of FMN in rigid poly(vinyl alcohol) matrices

The excitation-emission spectra of flavin mononucleotide (FMN) were measured in rigid PVA films for concentrations ranging from 6.92 x 10(-4)M to 1.03 M. The theoretical three-linear decomposition of the excitation-emission spectra indicated the presence of two absorption and emission centers corresponding to FMN monomer and dimer, respectively. The component of the fluorescence profile corresponding to the FMN monomer has a large negative part which is the mirror image of the emission band profile of the dimer. The elimination of this part by taking a linear combination of the emission components of the monomer and of the dimer resulted in emission spectrum, which is in a very good agreement with the monomer spectrum measured directly. The appearance of a negative part of the monomer emission profile obtained by trilinear decomposition of the emission-absorption spectra of FMN can be explained in terms of the non-radiative reverse energy transfer from the FMN dimers to the FMN monomers. The presented results confirm that the FMN molecules in rigid PVA form dimers but not higher order aggregates. Moreover, they enable to obtain fluorescence spectra of dimers and suggest that FMN dimers may take part in the process of non-radiative energy transfer occurring in photoreception phenomena.     

Formation of A2B2 supramolecular porphyrin co-polymers

New supramolecular A2B2 co-polymers formed in solution from a rigid diporphyrin monomer (the A2 unit) and a short flexible dipyridine monomer (the B2 unit) are reported; NMR experiments show that complete binding occurs at mM concentrations; UV titrations reveal that the dipyridine unit and a monomeric control ligand have identical binding constants, confirming that linear polymers were generated (in preference to small cyclic oligomers); at 2 x 10(-2) M polymers with an average molecular weight of 17,100 g mol-1 and containing approximately 14 porphyrin units have formed.     

Thermotropic behavior of aromatic copolyesters based on 3,4′-substituted diphenyl ether

Recently, flexible rod-like monomers based on conformational isomerism have been used to synthesize fully aromatic thermotropic polymers of reduced processing temperature and good thermal stability. 3,4′-Dihydroxy and 3,4′-dicarboxydiphenyl ethers HE and DE, obtained by modified Ullmann condensations between proper hydroxy and bromo derivatives, are investigated as basic components for homo- and copolyesters with the above-mentioned characteristics. Homopolymers from HE and terephthaloyl chloride or DE chloride and 2-methyl (or phenyl) hydroquinone, synthesized by low temperature solution polycondensation, melt to an isotropic fluid; the chain packing is frustrated in the latter by asymmetrical substitution on the aromatic rings, which causes a sequence randomization as well as an increased chain separation. The incorporation of linear rigid units, by partial substitution of the flexible monomers with 1,4 aromatic moieties, leads to thermotropicity, but the critical content of 1,4 units varies with the steric hindrance of the diphenol. Variation of chain rigidity, arising from the chemical structure, composition and sequence distribution, can account for the thermal behavior of the samples and, in particular, for their different abilities to give liquid crystalline behavior.     

Internal stress in plasma polymer films prepared by cascade arc torch polymerization

Thin plasma polymer films were deposited from several liquid monomers (mainly siloxane‐type monomers) in a low‐temperature cascade arc torch (CAT) reactor. The effects of monomer structures and plasma parameters on internal stress in the films were experimentally studied. By appropriately adjusting these factors, the internal stress in the film was reduced nearly two orders of magnitude from 10⁹ to 10⁷ dyn/cm². It was noted that the polymer films prepared from siloxane‐type monomers showed lower internal stress than their hydrocarbon counterpart. Fourier transform‐infrared spectroscopy (FTIR) studies indicated that a large amount of Si O Si structure from siloxane monomers, which are very flexible bonds, was preserved in the resultant plasma polymers. Ellipsometry results suggested that the internal stress can be qualitatively correlated with the refractive index of the plasma polymer film. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1577–1587, 1999     

Computer simulation study of the global phase behavior of linear rigid Lennard-Jones chain molecules: comparison with flexible models

The global phase behavior (i.e., vapor-liquid and fluid-solid equilibria) of rigid linear Lennard-Jones (LJ) chain molecules is studied. The phase diagrams for three-center and five-center rigid model molecules are obtained by computer simulation. The segment-segment bond lengths are L = sigma, so that models of tangent monomers are considered in this study. The vapor-liquid equilibrium conditions are obtained using the Gibbs ensemble Monte Carlo method and by performing isobaric-isothermal NPT calculations at zero pressure. The phase envelopes and critical conditions are compared with those of flexible LJ molecules of tangent segments. An increase in the critical temperature of linear rigid chains with respect to their flexible counterparts is observed. In the limit of infinitely long chains the critical temperature of linear rigid LJ chains of tangent segments seems to be higher than that of flexible LJ chains. The solid-fluid equilibrium is obtained by Gibbs-Duhem integration, and by performing NPT simulations at zero pressure. A stabilization of the solid phase, an increase in the triple-point temperature, and a widening of the transition region are observed for linear rigid chains when compared to flexible chains with the same number of segments. The triple-point temperature of linear rigid LJ chains increases dramatically with chain length. The results of this work suggest that the fluid-vapor transition could be metastable with respect to the fluid-solid transition for chains with more than six LJ monomer units.     

Interfacial Properties of Asymmetric Polymer Mixtures

Using a Monte‐Carlo simulation of a continuous space Rod Bead Model the interface properties of systems of flexible polymer chains with different sizes of monomers are investigated. An immiscible polymer blend in the strong segregation state is modeled by a double sandwich system of chains differing by an factor of two in the size of the beads and the interfacial tension is calculated by a virial theorem method. The simulation data are compared to self‐consistent mean field and experimental data. The results show that the simulation data agree very satisfactory with mean‐field results. The interfacial tension decreases for asymmetric systems in comparison to symmetric systems with comparable volume contents of monomers and interaction strengths due to a decrease of the effective interaction. The parameters of the investigated systems are close to the properties of PS, PMMA and PI melts. A comparison with experimental results yields a very good agreement with data for PS/PMMA and less satisfactory for PS/PI. Additionally to the interfacial tension we have studied the interfacial width, the deformation of polymer chains near the interface, distributions of chain ends, monomer densities and distributions of centers of mass of chains.

Snapshot of a typical configuration for chains with different monomer sizes and equal number of monomers per chain.     

Chain-length-dependent helical motifs and self-association of beta-peptides with constrained side chains

Homo-oligomers constructed by using trans-2-aminocyclohexanecarboxylic acid monomers without protecting groups were studied. Both ab initio theory and NMR measurements showed that the tetramer tends to adopt a 10-helix motif, while the pentamer and hexamer form the known 14-helix. It was concluded that the conformationally constrained backbone is flexible enough to afford both 10-helical and 14-helical motifs, this observation in turn providing evidence of the true folding process. Self-association of the helical units was also detected, and the results of variable-temperature diffusion NMR measurements strongly suggested the presence of helical bundles in methanol solution.     

Thermal properties of non-symmetric bibenzoate liquid crystalline dimers

The liquid crystal behaviour of a family of non-symmetric liquid crystalline dimers is reported. These systems contain two bibenzoate rigid units that are linked to distinct terminal groups at one end, and to a flexible interconnecting spacer at the other. Several systems having different terminal and central chains are studied using calorimetric, microscopic and diffraction techniques. All the samples form phases with variable degrees of order (from low ordered smectic to crystalline phases) depending on the chemical constitution of the different segments. The influence of the length, parity and lateral substitution of the spacers on the transitional properties and the symmetry of the mesophases that are formed is analysed. It is found that a decrease in the transition temperatures and enthalpies occurs when the length of the flexible spacers increases, when lateral methyl substituents are introduced, or when the parity of the central spacer changes from an even to odd number of carbon atoms or ether groups. The arrangement of the mesogens and dissimilar flexible groups within the ordered structure is discussed with respect to the observed L/d ratios. Different values were obtained depending on the parity of the central spacer and on the degree of order. Interpenetrated structures, in which the flexible groups of different lengths are mixed, seem to be compatible with low ordered smectic phases, but sterically disfavoured when constructing crystalline phases.     

Polymer compatibility - a consequence of repulsive groups within monomer units

The majority of compatible polymer mixtures known so far has been rationalized by assuming special interactions between the different polymers or by specific repellent forces within certain types of copolymers. Thus, the compatibility of PMMA with special copolymers of styrene and acrylonitrile is explained by an intramolecular repulsion within the copolymer between the two comonomers styrene and acrylonitrile. In our opinion this phenomenon is not limited to copolymers, but also holds for explaining the compatibility of various homopolymers such as PVDF/PMMA. To this end we simply regard the homopolymer PVDF (-CH₂-CF₂-) as a copolymer of -CH₂- and -CF₂- “monomers”. By the same token it is possible to assume repellent forces within the PMMA monomer units (i.e. repulsion between the CH₂-C(CH₃) and the polar carbonyl group). Those strong repellent forces within the PVDF and PMMA homopolymers can be reduced by mixing the two species. This concept of repulsive forces within monomers as a driving force for polymer-polymer compatibility can be used to search for new classes of compatible polymers. This will be demonstrated with the polystyrene/polyalkylmethacrylate blends. Thus, with comparable geometry in the side chains, polystyrene and polymethacrylic esters will form compatible blends, as born out in the systems polystyrene/polycyclohexyl methacrylate or poly-tert-butyl styrene/poly-3,3,5-trimethylcyclohexylmethacrylate which are compatible in all proportions. Taking into account certain steric prerequisites, one can even obtain compatible blends lacking exothermic interaction.