Backbone Units Research Articles

Self-consistent-field modeling of hydrated unsaturated lipid bilayers in the liquid-crystal phase and comparison to molecular dynamics simulations.

A molecular-level self-consistent-field (SCF) theory is applied to model the lipid bilayer structures composed of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (18:0/18:1 omega 9cis PC) and 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphatidylcholine (18:0/22:6 omega 3cis PC). As compared to earlier attempts to model (saturated) PC membranes several additional features are implemented: (i) A water model is used which correctly leads to low water concentration in the bilayers. (ii) Free volume is allowed for, which is important to obtain bilayers in the fluid state. (iii) A polarization term is included in the segment potentials; this new feature corrects for a minor thermodynamic inconsistency present in (all) earlier results for charged bilayers. (iv) The CH3 groups in the lipid molecules are assumed to have twice the volume of a CH2 group; this leads to stable noninterdigitated bilayers. (v) A cis double bond is simulated by forcing gauche conformations along the sn-2 acyl chain. Results of an all-atom molecular dynamics (MD) simulation, using the collision dynamics method, on the same system are presented. Both SCF and MD prove, in accordance with experimental facts, that acyl unsaturation effectively reduces the length of the chain which counteracts interdigitation. It is also found that the phosphatidylcholine head group is lying almost flat on the membrane surface and the water penetrates into the bilayer upto the glycerol backbone units. From the SCF results it further followed that the free volume is not exactly evenly distributed over the bilayer. There is a small increase in free volume in the center of the bilayer as well as in the glycerol backbone region.

Surface Morphology, Molecular Reorientation, and Liquid Crystal Alignment Properties of Rubbed Nanofilms of a Well-Defined Brush Polyimide with a Fully Rodlike Backbone

Nanoscaled films of poly{p-phenylene 3,6-bis[4-(n-butoxy)phenyloxy]pyromellitimide} (C4-PMDA-PDA PI), a well-defined brush polyimide (PI) composed of aromatic−aliphatic bristles set into a fully rodlike polymer backbone (two bristles per chemical repeat unit of the polymer backbone), were studied by atomic force microscopy, optical retardation, prism coupling, and linearly polarized IR spectroscopy before and after mechanical rubbing with velvet fabric, and their liquid crystal (LC) aligning abilities were investigated. Uniform, homogeneous LC alignment was achieved at the rubbed film surfaces of PIs with positively birefringent characteristics. Surprisingly, however, the LC alignment director for this PI is perpendicular to the rubbing direction as well as to the reorientation direction of the polymer main chains. This is the first time that LCs on a PI surface have been induced to align in the direction perpendicular to the rubbing direction, a significant departure from the LC alignment observed for al...

Didentate phosphine ligands with alkenyl and alkynyl linker units as building blocks for dendrimer fixation

Using diethyl-2-methylmalonate as a starting material, the two diphosphines HCCCH2C(CH3)(CH2PPh2)2 (7) and H2CCHCH2C(CH3)(CH2PPh2)2 (13), containing alkynyl and alkenyl unit in the ligand backbone were prepared in multistep syntheses. These two ligands were employed in the synthesis of [{(CH3)3SiCCCH2C(CH3)(CH2PPh2)2}PdCl2] (8) and [{(CH3)3SiCCCH2C(CH3)(CH2PPh2)2}PtCl2] (9) as well as [{H2CCHCH2C(CH3)(CH2PPh2)2}PdI2] (14b), all of which were characterized by X-ray diffraction. Reaction of Si[(CH2)3SiMe2Cl]4 (= “G[0]-[Cl]4”) with four molar equivalents of the in situ lithiated alkynyl diphosphine 7 cleanly yielded the four-fold functionalized derivative G[0]-[CCCH2C(CH3)(CH2PPh2)2]4 (15) which was converted to the metallated derivative G[0]-[CCCH2C(CH3)(CH2PPh2)2PdCl2]4 (16). Since the fixation of the alkenyl-functionalized diphosphine ligand 13 to the SiH-silane Si[(CH2)3SiMe2H]4 (= “G[0]-[H]4”) by Pt-catalyzed hydrosilation proved to be unsuccessful due to the negative interference of the phosphine with Karstedt's catalyst, the chloro-substituted precursor of 13, H2CCHCH2C(CH3)(CH2Cl)2 (12) was used. Hydrosilation of G[0]-[H]4 with four molar equivalents of 12 readily gave the functionalized dendrimer G[0]-[CH2CH2CH2C(CH3)(CH2Cl)2]4 (17) which was then reacted with eight equivalents of LiPPh2 to yield the phosphinated derivative G[0]-[CH2CH2CH2C(CH3)(CH2PPh2)2]4 (18).

Photo-alignment of low-molecular mass nematic liquid crystals on photoreactive polyimide and polymethacrylate film by irradiation of a linearly polarized UV light

Photocrosslinkable polyimide and polymethacrylate comprising chalcone moieties in the side chain were synthesized for fabricating the alignment layer of liquid crystals. Chalcone group was introduced into the side chain unit of the polymer backbone. We observed a photodimerization behavior either in solution or in film by irradiation of a UV light. Linearly polarized UV irradiation on the chalcone-polymer films induced optical anisotropy of the polymer film and the resultant film can be used for alignment layers for low molecular weight nematic liquid crystals.

Synthesis and Characterization of New Highly Soluble Polyamides Derived from α, α’-Bis[3,5-dimethyl-4-(4-aminophenoxy)phenyl]-1,4-Diisopropylbenzene

The five benzene rings-containing diamine, α,α’-bis[3,5-dimethyl-4-(4-aminophenoxy)phenyl]-1,4-diisopropylbenzene (BDAPD) was prepared by the aromatic nucleophilic substitution of α,α’-bis(4-hydroxy-3,5-dimethylphenyl)-1,4-diisopropylbenzene with 1-chloro-4-nitrobenzene, and subsequent hydrogenation of the intermediate dinitro compound. The diamine was reacted with various aromatic dicarboxylic acids to prepare a series of new polyamides. The polyamides were produced with high yield and inherent viscosities of 0.68–0.94 dL g-1. The wide-angle X-Ray diffraction diagrams revealed that all the polyamides showed amorphous character. All of the polyamides showed excellent solubility in a variety of solvents such as N-methyl-2-pyrrolidinone, N,N-dimethylacetamide, N,N-dimethylformade, dimethyl sulfoxide, pyridine, cyclohexanone, and tetrahydrofuran. These five benzene rings-containing polyamides had better solubility than those containing only one isopropylidene unit or a hexafluoroisopropylidene linkage in the repeating unit of polyamide backbone. These polyamides had glass transition temperatures (Tg’s) between 237–256°C. The thermogravimetric analyses demonstrated that almost all of the polymers were stable up to 400°C, and the 10% weight loss temperatures were recorded in the range 437–452°C and 447–463°C in nitrogen and air atmosphere, respectively.

Novel poly(arylene ether) as membranes for gas separation

Fluorine containing polymers are of special interest because of their possible use as gas selective membranes, their remarkably low water absorption and their low dielectric constant and enhanced flame resistance. In this work, the gas transport and separation properties of H 2, CO 2, O 2, N 2 and CH 4 were studied for four novel poly(arylene ether)s (PAEs) membranes, two containing the 2,6-bis(trifluoromethylphenylene)pyridine unit (6FPPy series) and the others containing 2,5-bis(3-trifluoromethylphenylene)thiophene unit (6FPT series) in the polymer backbone. The preparation of this new class of semifluorinated polymers relied on the aromatic substitution polymerization of trifluoromethyl activated bishalo monomers with different bis(phenol)s. It was found that the introduction of CF 3 groups into the bisphenol unit increases the gas permeabilities and decreases slightly the selectivities for both series of membranes. The gas permeability coefficients of 6FPPy membranes are higher than those of 6FPT membranes while their selectivities are comparable. These results were discussed based on the gas diffusion and solubility in the membranes and could be ascribed to the polymer structure. The effect of temperature on the gas transport properties of these membranes was measured and the corresponding activation energies for gas permeation and diffusion were calculated.

Endgroup-based separation and quantitation of polyamide-6,6 by means of critical chromatography

Polyamide-6,6 is a polycondensation product from the two monomers adipic acid and 1,6-hexamethylenediamine. Depending on the reacted amount of these monomers, different ratios of amine and carboxylic acid endgroups can be formed. Besides linear chains, cyclic polyamides will also be formed. Using critical chromatography, polyamide-6,6 can be separated independently of molar mass. Retention is based solely on endgroup functionality. It is demonstrated that high-molecular-mass polyamide-6,6 ( M w≈20 000–30 000) can be separated using this approach. The separation was optimized by using different parameters, such as percentage modifier, temperature and pressure. The concentration of phosphoric acid was used for selective retention of the different endgroup functionalities. Using this property, a new method called critical gradient chromatography was performed where the mobile phase changes from a weak to a strong solvent with respect to the endgroup functionality, while retaining the critical conditions of the backbone unit. Quantification using UV detection is discussed.

Synthesis and molecular structures of dimeric assemblies of telluronium salts derived from o-C 6H 4(CH 2TeMe) 2 and PhMeTe

The ditelluroether, o-C 6H 4(CH 2TeMe) 2 reacts with excess MeI in acetone to afford the bis(triorganotelluronium iodide), o-C 6H 4(CH 2TeMe 2I) 2, in high yield which has been characterised by 1H-, 13C{ 1H}- and 125Te{ 1H}-NMR spectroscopy, microanalysis and X-ray crystallography. The crystal structure of this species reveals a weakly associated dimer, assembled through a series of secondary Te⋯I interactions to give a pseudo-cubane Te 4I 4 core, involving three-coordinate (pyramidal) iodine and six-coordinate (distorted octahedral) tellurium. The o-xylyl backbone units are oriented across the diagonal of two opposite faces of the cubane. This is the first crystallographic study of a triorganotelluronium halide salt derived from a ditelluroether. The crystal structure of PhTeMe 2I shows a weakly associated μ 2-diiodo bridged dimer, in this case with two-coordinate iodine and five-coordinate, distorted square pyramidal tellurium. The stereochemical activity of the Te-based lone pair is discussed for each system and the structures are compared with other related species.

Crystal Structure of Poly(hexamethylene 2,6-naphthalate)

The crystal structure of poly(hexamethylene 2,6-naphthalate) (PHN) was identified by X-Ray diffraction and molecular modeling. The possibility of stress- or temperature-induced crystal transformation is also examined. The unit cell of PHN was found to be triclinic (P1 space group) with dimensions of a = 6.70, b = 5.66, c = 18.40 A, α = 79.0°, β = 145.0°. γ = 90.0°, and crystal density was 1.313 g cm -3 . It was assumed from the observed density (1.298 g cm -3 ) that the unit cell contains one repeating unit of a polymer chain. The hexamethylene unit in PHN backbone takes nearly all-trans conformation. The packing mode is of the face-to-face type relative to the naphthalene rings. The PHN polymer does not undergo crystal transformation by temperature and/or stress.

Large and ultrafast third-order optical nonlinearity of novel copolymers containing fluorene and tetraphenyldiaminobiphenyl units in backbones

A femtosecond time-resolved optical Kerr gate method, using 115 fs laser pulses at 830 nm, has been applied to investigate the third-order nonlinearity of two novel copolymers containing fluorene and tetraphenyldiaminobiphenyl units in their backbones. Ultrafast off-resonant optical Kerr responses have been observed and the magnitude of the second-order hyperpolarizability was measured as large as 10 −30 esu. The origin of the extraordinary large value was explored and compared to other organic materials.

Headspace solid-phase microextraction — a tool for new insights into the long-term thermo-oxidation mechanism of polyamide 6.6

Low-molecular-mass products formed during thermo-oxidation of polyamide 6.6 at 100°C were extracted by headspace solid-phase microextraction and identified by GC–MS. A total of 18 degradation products of polyamide 6.6 were identified. In addition some low-molecular-mass products originating from the lubricants were detected. The identified degradation products were categorized into four groups where compounds within each group contain the same structural feature. In groups A, B and C several new thermo-oxidation products of polyamide 6.6 were identified including cyclic imides, pyridines and structural fragments from the original polyamide chain. 1-Pentyl-2,5-pyrrolidinedione (pentylsuccinimide) showed the largest increase in abundance during oxidation. The cyclopentanones in group D were already present in the un-aged material. Their amounts decreased during ageing and they are thus not formed during thermo-oxidation of polyamide 6.6 at 100°C. The identified thermo-oxidation products can be formed as a result of extensive oxidation of the hexamethylenediamine unit in the polyamide backbone. The degradation products pattern shows that the long-term thermo-oxidative degradation, just like thermal degradation and photo-oxidation of polyamide 6.6, starts at the N-vicinal methylene groups.

Synthesis and Characterization of Highly Soluble Polyamide-Imides Derived from α,α′-Bis[3,5-Dimethyl-4-(4-Trimellitimidophenoxy)-Phenyl]-1,4-Diisopropylbenzene

A new diimide-dicarboxylic acid, α,α′-bis[3,5-dimethyl-4-(4-trimellitimidophenoxy)phenyl]-1,4-diisopropylbenzene (BDTPD), was synthesized by the condensation reaction of α,α′-bis[3,5-dimethyl-4-(4-aminophenoxy)phenyl]-1,4-diisopropylbenzene (BDAPD) with trimellitic anhydride in refluxing glacial acetic acid. A series of new polyamide-imide (PAIs) were prepared by direct polycondensation of BDAPD and various aromatic diamines in N-methyl-2-pyrrolidinone (NMP) using triphenyl phosphite and pyridine as condensing agents. The polymers were produced with high yield and moderate to high inherent viscosities of 0.77–1.03 dl g−1. Wide-angle x-ray diffractograms revealed that the polymers were amorphous. All the PAIs exhibited excellent solubility and could be readily dissolved in various solvents such as NMP, N,N-dimethylacetamide (DMAc), N,N-dimethylformamide, dimethyl sulfoxide, pyridine, cyclohexanone, tetrahydrofuran and chloroform at room temperature or upon heating at 70°C. Such PAIs had better solubility than those containing a hexafluoroisopropylidene linkage in the repeating unit of a PAI backbone. These PAIs had glass transition temperatures between 252 and 269°C and 10% weight loss temperatures in the range of 453–482°C and 462–485°C in nitrogen and air atmospheres, respectively. The tough polymer films, obtained by solution casting from DMAc solution, had a tensile strength range of 80–103 MPa and a tensile modulus range of 1.6–2.0 GPa.

An NMR study of mobility in a crystalline side‐chain comblike polymer

AbstractCarbon‐13 spin–lattice relaxation times are measured for poly(octadecyl acrylate) above and below the melting point of the crystalline side chains. The chain backbone has long spin–lattice relaxation times below the melting point that shorten by more than an order of magnitude as the melting point range is traversed. Below the melting point, the backbone is nearly immobilized with spin–lattice relaxation changing very slowly with temperature. Above the melting point, the shorter spin–lattice relaxation times are typical of a rubber above the glass transition and decrease with increasing temperature. The methylene groups in the side chain are quite mobile well below the melting point, indicating fairly rapid anisotropic motion within the crystal. The methyl group at the end of the chain and the adjacent methylene group have longer spin–lattice relaxation times, indicating the greatest side‐chain mobility at the end, which is in the middle of the crystal structure. The side‐chain carbon adjacent to the carbonyl group is as mobile as the majority of the side‐chain carbon, indicating side‐chain mobility extends to all of the side‐chain CH2 groups. The abrupt transition in the mobility of the backbone is not typical of the amorphous phase in a semicrystalline polymer where the backbone units can crystallize. The close proximity of every backbone segment to the crystalline domain locks backbone segmental motion below the melting point. Melting and crystallization of the side chains switch segmental motion of the backbone on and off. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1548–1552, 2001

Synthesis and characterization of new organosoluble polyimides based on flexible diamine

The new five benzene rings-containing diamine, α,α′-bis[3,5-dimethyl-4-(4-aminophenoxy)phenyl]-1,4-diisopropylbenzene ( BDAPD) was prepared by the aromatic nucleophilic substitution of α,α′-bis(4-hydroxy-3,5-dimethylphenyl)-1,4-diisopropylbenzene with 1-chloro-4-nitrobenzene, and subsequent hydrogenation of the intermediate dinitro. The diamine was reacted with various aromatic dianhydrides to prepare a series of new polyimides (IPs) via the poly(amic acid) precursors and thermal or chemical imidization. The polyimides were produced with inherent viscosities of 0.60–0.85 dl g −1 by chemical imidization. All the poly(amic acid)s films could be obtained by solution-cast from N, N-dimethylacetamide (DMAc) solutions and thermally converted into transparent, flexible, and tough polyimide films. The wide-angle X-ray diffraction diagrams revealed that all the polyimides showed amorphous character. All of the polyimides, except PI-1, showed excellent solubility in a variety of solvents such as N-methyl-2-pyrrolidinone, DMAc, N, N-dimethylformade, dimethyl sulfoxide, pyridine, cyclohexanone and tetrahydrofuran. Such polyimides had better solubility than those containing only one isopropylidene unit or a hexafluoroisopropylidene linkage in the repeating unit of polyimide backbone. These polyimides had glass transition temperatures ( T g) between 241 and 270°C, which were higher than the T g value of Ultem 1000 (215°C). The thermogravimetric analyses demonstrated that almost all of the polymers were stable up to 450°C, and the 10% weight loss temperatures were recorded in the range 466–488°C in air atmosphere.

Novel Fluoropolymers for Use in 157nm Lithography.

Unexpectedly good UV transmittance at 157nm of poly(norbornene sulfone) bearing a pendant hexafluoroisopropanol functionality has prompted us to employ this fluoroalcohol as an acid group for the design of chemical amplification resists for use in 157nm lithography. The backbone structures to which the hexafluoroalcohol group is attached are polynorbornene and polystyrene. Furthermore, our discovery that poly(methyl α-trifluoromethylacrylate) is adequately transparent at 157nm has led us to incorporate the α-trifluoromethylacrylic unit in the polymer backbone by radical copolymerization with styrenes and norbornenes. Thus, four platforms are currently available to us in preparation of 157nm resist polymers; 1) all-acrylic, 2) all-norbornene, 3) acrylic-norbornene, and 4) acrylic-styrenic systems.

Double-helical cyclic peptides: design, synthesis, and crystal structure of figure-eight mirror-image conformers of adamantane-constrained cystine-containing cyclic peptide cyclo (Adm-Cyst)(3).

A large number of macrocycles containing alternating repeats of cystine diOMe(-NH-CH(CO(2)Me)-CH(2)-S-)(2) and either a conformationally rigid aromatic/alicyclic moiety or a flexible polymethylene unit (X) in the cyclic backbone with ring size varying from 13- to 78-membered have been examined by spectral ((1)H NMR, FT-IR, CD) and X-ray crystallography studies for unusual conformational preferences. While (1)H NMR measurements indicated a turnlike conformation for all macrocycles, stabilized by intramolecular NH.CO hydrogen bonding, as also supported by FT-IR spectra in chloroform, convincing proof for beta-turn structures was provided by circular dichroism studies. Single-crystal X-ray studies on 39-membered cyclo (Adm-L-Cyst)(3) revealed a double-helical fold (figure-eight motif) for the macrocycle. Only a right-handed double helix was seen in the macrocycle constructed from L-cystine. The mirror-image macrocycle made up of D-cystine units exhibited a double helix with exactly the opposite screw sense, as expected. The enantiomeric figure-eights were stabilized by two intramolecular NH. CO hydrogen bonds and exhibited identical (1) H NMR and FT-IR spectra. The CD spectra of both isomers had a mirror-image relationship. The present results have clearly brought out the importance of cystine residues in inducing turn conformation that may be an important deciding factor for the adoption of topologically important structures by macrocycles containing multiple S-S linkages.

Preparation and properties of a new aromatic polyamide with an ethoxycarbonyl pendant group: Poly(4,4?-diamino-2?-ethoxycarbonyl-benzanilide terephthalamide)

A new aromatic polyamide containing a pendant ethoxycarbonyl group was successfully synthesized from the reaction between 4,4′-diamino-2′-ethoxycarbonylbenzanilide and terephthaloyl chloride. The new polymer was soluble in organic solvents such as N-methyl-2-pyrrolidone and dimethylacetamide, and a tough and transparent film was cast from the polymer solution with viscosities ranging from 2.6 to 5.6 dL/g. When the polymer film was heat-treated at a temperature greater than 300 °C, a cyclization reaction occurred between the ethoxycarbonyl group and the adjacent amide bond to form a benzoxazinone unit in the polymer backbone. The thermal decomposition onset temperature of the cyclized film was about 523 °C, which was somewhat lower than that of poly(p-phenylene terephthalamide) (PPTA; 566 °C); however, the decomposition rate was slower than that of PPTA to yield a higher char residue. The dispersion temperature of the uncyclized poly(4,4′-diamino-2′-ethoxycarbonylbenzanilide terephthalamide) (PDEBTA) was about 340 °C, whereas that of the cyclized PDEBTA was not clear. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 936–942, 2000

The sarcosine effect on protein stability: a case of nonadditivity?

We have used differential scanning calorimetry to determine the effect of low concentrations (C = 0-2 M) of the osmolyte sarcosine on the Gibbs energy changes (deltaG) for the unfolding of hen-egg-white lysozyme, ribonuclease A, and ubiquitin, under the same buffer and pH conditions. We have also computed this effect on the basis of the additivity assumption and using published values of the transfer Gibbs energies for the amino acid side chains and the peptide backbone unit. The values thus predicted for the slope delta deltaG/deltaC agree with the experimental ones, but only if the unfolded state is assumed to be compact (that is, if the accessibility to solvent of the unfolded state is modeled using segments excised from native structures). The additivity-based calculations predict similar delta deltaG/deltaC values for the three proteins studied. We point out that, to the extent that this approximate constancy of delta deltaG/deltaC holds, osmolyte-induced increases in denaturation temperature will be larger for proteins with low unfolding enthalpy (small proteins that bury a large proportion of apolar surface). The experimental results reported here are consistent with this hypothesis.

Diketopiperazine-based polymers from common amino acids

High-melting-point polymers with heterocyclic backbone units were synthesized from common amino acids. Acids were first converted to methyl ester salts and then were dimerized to form diketopiperazines, which were polymerized in solution. Aspartic acid, serine, lysine, glycine, and tyrosine were used as starting materials for the diketopiperazines. Aromatic and aliphatic comonomers were used. Thus, we demonstrate the technical feasibility of producing strong, durable resins from abundant domestic resources. These resins show great potential for high-performance polymers for use in films, fibers, and composites. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2213–2218, 2000

Incorporation and characterization of abasic and phenyl residues in peptide nucleic acids

The N-(2-aminoethyl)glycine backbone unit of PNA has been derivatized with phenylacetic acid and acetic acid moieties to produce monomers for incorporation into PNAs. As expected, oligomers containing these residues form PNA·DNA double helices that are destabilized compared to the unsubstituted duplexes. However, unlike the analogous substitutions in DNA, these residues show very little sequence specificity.