Cyclic Dimer Research Articles

Cocrystallization of monomer units of biobased and biodegradable Poly(l-lactic acid-co-glycolic acid) random copolymers

Random copolymers of l-lactic acid (LLA) and glycolic acid (GA) [P(LLA-GA)] with 0–100 mol% LLA units in the polymers were synthesized and their crystallization behavior was investigated by two representative crystallization methods of precipitation or melt-crystallization. P(LLA-GA) copolymers with LLA unit contents in the ranges of 0–20 and 73–100 mol% were crystallizable during precipitation or melt-crystallization. Surprisingly, these crystallizable ranges are similar to those reported for P(LLA-GA) copolymers synthesized by ring-opening polymerization of cyclic dimers (l-lactide and glycolide), which should have monomer sequence lengths twice those of the P(LLA-GA) copolymers synthesized by polycondensation of the monomers (LLA and GA) in the present study and are expected to have wider crystallizable ranges. The GA units of the LLA-rich P(LLA-GA) copolymers were incorporated in a lattice of LLA unit segments, whereas the LLA units of the GA-rich P(LLA-GA) copolymers were incorporated in a lattice of GA unit segments. Even the incorporation of relatively small-sized GA units in relatively large-sized LLA unit segments strongly induced structural disorder in the crystalline lattice. The crystalline size and melting temperature of the P(LLA-GA) copolymers decreased dramatically as the LLA unit content deviated from 0 to 100 mol%.

NIR Spectra Simulations by Anharmonic DFT-Saturated and Unsaturated Long-Chain Fatty Acids.

Spectra simulation based on quantum mechanical calculations is often an ultimate tool bringing decisive answers to spectroscopic problems, but in the case of NIR spectroscopy, such studies still remain very rare, particularly those on rather complicated molecules. In the present work we have employed fully anharmonic spectra simulation for saturated and unsaturated long-chain fatty acids (arachidic acid, palmitic acid, stearic acid, linoleic acid, linolenic acid, and oleic acid). The spectral features corresponding to the saturation of fatty acid were accurately reproduced by deperturbed vibrational second-order perturbation theory (DVPT2) throughout a wide NIR region (8000-4000 cm-1), which contains mostly combination bands, and detailed band assignments have been provided. The effect of the saturation of the alkyl chain and the dependency of the number of C═C bonds were reflected in the simulated NIR spectra. This allowed for drawing reliable conclusions about how exactly the existence of C═C bonds and their number in a molecule are translated into the observed spectra. The baseline elevation in the NIR spectra due to the combination bands involving OH stretching and bending modes of the long-chain fatty acid cyclic dimers were confirmed to be similar to those of short- and medium-chain fatty acids. Additionally, for two examples (linoleic and palmitic acid), highly anharmonic OH stretching modes were studied in detail by probing the relevant vibrational potentials over a dense grid for monomers and dimers. Subsequent solving of the time-independent Schrodinger equation by a generalized matrix Numerov method allowed for improving the inconsistency of the prediction by the DVPT2 route of the 2νOH modes of the monomers. For the cyclic dimers, the symmetric ( Ag) and antisymmetric ( Bu) OH stretching potential curves have been investigated as well. These observations were discussed in relation to the previous investigations of short- and medium-chain fatty acids.

Ring-Opening Polymerization of a New Diester Cyclic Dimer of Mandelic and Glycolic Acid: An Efficient Synthesis Method for Derivatives of Amorphous Polyglycolide with High Tg.

In this study, poly(mandelate-co-glycolate) (PMG), a modified polyglycolide (PGL), is prepared by ring-opening polymerization (ROP) of L-3-phenyl-1,4-dioxane-2,5-dione (PDD); the cyclic dimer of biobased mandelic acid and glycolic acid. The resulting polymer shows an increased glass transition temperature (Tg ) due to the incorporation of phenyl groups in the chain. High molecular weight PMG is obtained by bulk ROP at 150 °C, and it exhibits a glassy amorphous state with enhanced thermal properties such as a Tg being 35 °C higher than conventional PGL. PDD is also copolymerized with glycolide (GL) and lactide (LA), resulting in poly(mandelate-co-glycolate/glycolate) ((P(MG/GL)) with GL and poly(mandelate-co-glycolate/lactide) ((P(MG/LA)) with LA. The thermal properties of P(MG/GL) and P(MG/LA) are found to be distinctly different from PMG and conventional PGL and polylactide, and they are tunable with a changing molar ratio of PDD, GL, and LA. Therefore, PDD opens an elegant way to control and tailor the properties of biobased polyesters.

ReaxFF molecular dynamics simulation of intermolecular structure formation in acetic acid-water mixtures at elevated temperatures and pressures.

The intermolecular structure formation in liquid and supercritical acetic acid-water mixtures was investigated using ReaxFF-based molecular dynamics simulations. The microscopic structures of acetic acid-water mixtures with different acetic acid mole fractions (1.0 ≥ xHAc ≥ 0.2) at ambient and critical conditions were examined. The potential energy surface associated with the dissociation of acetic acid molecules was calculated using a metadynamics procedure to optimize the dissociation energy of ReaxFF potential. At ambient conditions, depending on the acetic acid concentration, either acetic acid clusters or water clusters are dominant in the liquid mixture. When acetic acid is dominant (0.4 ≤ xHAc), cyclic dimers and chain structures between acetic acid molecules are present in the mixture. Both structures disappear at increased water content of the mixture. It was found by simulations that the acetic acid molecules released from these dimer and chain structures tend to stay in a dipole-dipole interaction. These structural changes are in agreement with the experimental results. When switched to critical conditions, the long-range interactions (e.g., second or fourth neighbor) disappear and the water-water and acetic acid-acetic acid structural formations become disordered. The simulated radial distribution function for water-water interactions is in agreement with experimental and computational studies. The first neighbor interactions between acetic acid and water molecules are preserved at relatively lower temperatures of the critical region. As higher temperatures are reached in the critical region, these interactions were observed to weaken. These simulations indicate that ReaxFF molecular dynamics simulations are an appropriate tool for studying supercritical water/organic acid mixtures.

High-dimensional fitting of sparse datasets of CCSD(T) electronic energies and MP2 dipole moments, illustrated for the formic acid dimer and its complex IR spectrum.

We present high-level, coupled-mode calculations of the infrared spectrum of the cyclic formic acid dimer. The calculations make use of full-dimensional, ab initio potential energy and dipole moment surfaces. The potential is a linear least-squares fit to 13 475 CCSD(T)-F12a/haTZ (haTZ means aug-cc-pVTZ basis set for O and C, and cc-pVTZ for H) energies, and the dipole moment surface is a fit to the dipole components, calculated at the MP2/haTZ level of theory. The variables of both fits are all (45) internuclear distances (actually Morse variables). The potential, which is fully permutationally invariant, is the one published recently and the dipole moment surface is newly reported here. Details of the fits, especially the dipole moment, and the database of configurations are given. The infrared spectrum of the dimer is calculated by solving the nuclear Schrödinger equation using a vibrational self-consistent field and virtual-state configuration interaction method, with subsets of the 24 normal modes, up to 15 modes. The calculations indicate strong mode-coupling in the C-H and O-H stretching region of the spectrum. Comparisons are made with experiments and the complexity of the experimental spectrum in the C-H and O-H stretching region is successfully reproduced.

A New Synthetic Route to Heteroanthracenes

In this report, we describe the synthesis of 2,3,4a,6,7,8a,9,10‐octaaza‐4,8‐dioxo‐3,4,4a,7,8,8a,9,9a,10,10a‐decahydroanthracene, an octaaza derivative of reduced anthracene, through the reaction of glyoxal bis(nitrosemicarbazone) with glyoxal bis(hydrazone) in aqueous medium at 70 °C as well as by the hydrolytic decomposition of glyoxal bis(nitrosemicarbazone) in aqueous medium at 70 °C in a one‐pot transformation. The structure of the resultant compound was confirmed by physicochemical analytical methods and X‐ray diffraction. We also obtained the first representative of the cyclic monohydrazone dimer 1,2,4,5,8,9,11,12‐octaazacyclotetradeca‐5,7,12,14‐tetraene‐3,10‐dione, which exists as lactam and lactim tautomers, through the reaction of glyoxal bis(nitrosemicarbazone) with glyoxal bis(hydrazone) in alkaline aqueous medium.

Dinitrogen as a Sensor for Metastable Carboxylic Acid Dimers and a Weak Hydrogen Bond Benchmarking Tool.

Molecular nitrogen as a weak hydrogen bond acceptor is added to formic and acetic acid and their monodeuterated isotopologues. FTIR spectroscopy of supersonic expansions in the O-H stretching region reveals the formation of the weakly bound N2-carboxylic acid complexes. Their respective spectral downshifts from the monomer fundamental vibration are used to benchmark electronic structure calculations and vibrational perturbation theory. The small size of the investigated systems allows for a wide range of electronic structure levels to be explored. The O-H stretching vibration of an open dimer of acetic acid can be discriminated from the cyclic dimer vibrations by its higher susceptibility to coexpanded nitrogen.

Quantum chemical study of the impact of protective association on the chemoselective synthesis of carboxybetaine from 2-(dimethylamino)ethanol and acrylic acid

Abstract Targeted carboxybetaine synthesis is an important process that one can apply to prepare polymeric materials with various unique properties or low molecular weight products, such as deep eutectic solvents based on acrylic acid formulated for the specific tasks. Analyzing the mechanism of carboxybetaine formation, it was found that with an excess of amine, the H-bond formed between amine nitrogen and the acidic proton of the acrylic acid was strong enough to protect the intermediate obtained after carbanion protonation from the interaction with acrylate anion, thus facilitating the formation of quaternary ammonium salts as the final product at the beginning of the reaction instead of the desired carboxybetaine. Similarly, a threefold excess of acrylic acid should also lead to the preferential formation of the quaternary salt due to the protection of the half-product from the interaction with acrylate anion via cyclic dimer formation with two strong H-bonds. From the other side, calculations revealed that a twofold excess of acrylic acid should provide formation of carboxybetaine as the dominant product. The discussed energy profiles were supplemented with extensive natural bond order analysis in the course of which delocalization energies were calculated and used to discuss the strength of intermolecular donor–acceptor interactions within the considered molecular systems.

Well-Defined Selenium-Containing Aliphatic Polycarbonates via Lipase-Catalyzed Ring-Opening Polymerization of Selenic Macrocyclic Carbonate Monomer.

The synthesis of well-defined, biodegradable selenium-containing polymers remains a formidable challenge in polymer chemistry. Herein, a selenic cyclic carbonate dimer monomer (MSe) was developed to generate well-defined, biodegradable aliphatic polycarbonates with selenide functionality on the backbone. The monomer was synthesized via the intermolecular cyclization of di(1-hydroxyethylene) selenide and diphenyl carbonate with lipase CA as catalysts in a mass of anhydrous toluene with very dilute monomer concentration. Then living ring-opening polymerization (ROP) was executed by solution method using the same lipase CA as catalysts. Similarly, the copolymerizations with commercial trimethylene carbonate (TMC) generated random copolymers demonstrated by 13C NMR, regulating the density of selenium functional groups. The resulting polymers exhibited a living polymerization characteristic, as evidenced by polymerization kinetics, predictable molecular weights, narrow molecular-weight distribution, and controlled copolymer compositions. Using hydrophilic macroinitiators (PEG), amphiphilic di/triblock copolymers could be obtained, suggesting their potential as controlled drug delivery system (DDS) and hydrogel scaffolds for tissue engineering.

Reaction of PdCl2 with Diphenylacetylene Revisited: Effect of Solvents and Ball Milling on Cyclization and Crystal Structures of Dimeric Cyclobutenylpalladium Chloride Complexes

AbstractIn an attempt to establish the structures of the product formed in the reaction of PdCl2 and diphenylacetylene in polar organic solvents, the reactions were carried out in alcohols, acetonitrile and tetrahydrofuran at room temperature. The reactions in protic solvents such as primary and secondary alcohols resulted in the cyclic dimerization of diphenylacetylene to yield the corresponding dimeric endo‐4‐alkoxy‐π‐1,2,3,4‐tetraphenylcyclobutenypalladium(II) chloride complexes 1–4 in good yields. In these complexes the tetraphenylcyclobutenyl moieties are coordinated to palladium atoms in η3‐mode and are bridged by two chlorides as confirmed by the single crystal X‐ray analysis of complexes 1, 2 and 4. In all these complexes, the coordination of cyclobutenyl moieties to palladium atoms are similar but a marked difference was observed for the Pd2Cl2 core which is planar in complex 1 but puckered in complexes 2 and 4 resulting in shorter Pd⋅⋅⋅Pd distances. The same reaction under solvent free ball milling condition yielded hexaphenylbenzene by the cyclic trimerization while that in the presence of ethanol and n‐butanol produced complexes 1 and 3, respectively, as major and hexaphenylbenzene as minor product. An efficient catalytic synthesis of hexaphenylbenzene using PdCl2 was developed using ball milling condition which has several advantages over the existing catalytic systems.

Impact of chirality on peculiar ibuprofen molecular dynamics: hydrogen bonding organization and syn vs. anti carboxylic group conformations.

Studies of the impact of chirality on amorphous states are scarce. Here, we present combined dielectric relaxation spectroscopy (DRS) experiments and molecular dynamics (MD) simulation investigations of homochiral and racemic ibuprofen in the liquid, undercooled liquid and glassy states. The influence of chirality is particularly investigated on the syn and anti conformations of the -COOH moiety of the ibuprofen molecule and its link to the peculiar Debye-like dynamical process detected in this compound. Most of the studied properties are found to be nearly identical in the homochiral and racemic systems. But the polarity and intensity of the Debye-like process are clearly found to be more intense in the racemic mixture than in the enantiomerically pure ibuprofen. The difference is explained by the higher population of the anti conformation (with the higher dipole moment) and the lower population of hydrogen bonded cyclic dimers that can be transiently formed in the racemic mixture.

A helically-twisted ladder based on 9,9′-bifluorenylidene: synthesis, characterization, and carrier-transport properties

We synthesized a ladder-shaped 9,9′-bifluorenylidene cyclic dimer (CBF), in which the two 9,9′-bifluorenylidene units are connected directly with two covalent bonds.

In English

The structural features of surface of the nano-dimensional bor-azo-graphenic powders (t-BNg) after previous washing in boiling water were researched. The results showed that after process of purifier (washing) the powder’s surface of t-BNg characterized as slit-like micro-, mesoporous (monodispersed) structure with a narrow porous distribution in the range of 3.82 - 4.17 nm. The outer surface specific area of the powders of t-BNg according to “t - method” is 28.3 m2/g. The inner specific surface area of the mesopores is 141 m2/g (BJH method). The residues of boron oxonitride in the form of a purified sublimate, a white powder, extracted from a washed and dried sample of t-BNg at a temperature of 540 K and a pressure of ≤ 1.0 Pa. The sublimate, according to chemical analysis and infrared spectroscopy, was identified on the assumption of the cyclic dimer of di-hydro-di-hydroxo-di-bor-ox-azole of the composition of H(OH)[(BON)2](OH)H. The model of carbamide synthesis of boron nitride, as a sequence of chemical transformations of borate-carbamide precursors in a freely radical boron-pair (> B - N <), have proposed.

Linear and cyclic oligomers in polybutylene terephthalate for food contact materials

ABSTRACTPolybutylene terephthalate (PBT) is a polyester (PES) gaining more importance on the food contact material (FCM) market. However, little is known about the potential migration of PBT oligomers which are formed during polymer production. In this work, PBT pellets and a slotted spoon manufactured from this material by injection moulding were analysed on extractable oligomers and their migration potential into hydrophilic foods. Overall 27 oligomers (cycles and linears) could be identified in different extracts by HPLC-DAD/ESI-MS data, but without confirmation by reference substances. The oligomers were quantified by HPLC-DAD using bis(2-hydroxyethyl) terephthalate (BHET) as external standard and the total amount of oligomers isolated by reprecipitation from the pellets and the spoon were 0.69 and 0.71%, respectively. While cyclic oligomers made up for approximately 90% of the extractable oligomers, linear oligomers proved to be more relevant for migration into aqueous foodstuffs. Furthermore, it was shown that hydrolysis of oligomers can take place in water at elevated temperatures. Consequently, the qualitative and quantitative composition of PBT oligomers in aqueous foods from FCMs does not only depend on migration but also on hydrolysis. Migration testing of the PBT spoon under repeat use conditions with water at 100°C for 2 h resulted in 0.29 mg item−1 of linear oligomers and 0.05 mg item−1 of the cyclic PBT dimer in the third migrate.

Dimeric cholaphanes with disulfide spacers

ABSTRACTNew dimeric cholaphanes with disulfide spacers were prepared as potential supramolecular hosts. The spacers bind two identical steroidal subunits through C3, C3′ and C24, C24′ positions (head-to-head isomer – ‘cis-dimer’; 5) or C3, C24′ and C3′, C24 positions (head-to-tail isomer – ‘trans-dimer’; 4). Another cyclic dimer containing both disulfide and sulfide spacers in its structure (6) was also obtained.

Experimental and simulation study of nylon 6 solid–liquid extraction process

Abstract The solid–liquid extraction process of nylon 6 to eliminate small molecules, i.e., caprolactam (CL), cyclic dimers (CD) and cyclic trimers (CT), is investigated in detail by both batch extraction experiments and numerical simulations. In the batch extraction experiments, due to the small molecules attaching to the polymeric surface, the basic physical mechanism shifts from surface diffusion to internal diffusion as the extraction proceeded. The experimental data are well reproduced by a diffusion model consisting of two distinct steps, characterized as surface diffusion and internal diffusion. Furthermore, based on the established mass transfer mechanism and diffusion model of the two distinct steps, the equilibrium constants and internal diffusion coefficients of CL, CD and CT are acquired. An industrial countercurrent extraction tower is further simulated. It is found that the extraction efficiency of CL can be significantly improved by increasing the temperature at the bottom portion of the tower. The elimination of CD, which can be greatly promoted by a high-concentration CL-water solution, is controlled by mass transfer resistance, whereas the removal of CL is mainly affected by the equilibrium.

Mathematical Modeling of Nylon 6/6,6 Copolymerization in Batch Reactor: Investigating Recipes without Water and with Cyclic Dimer

AbstractA model is used to simulate batch copolymerization of caprolactam with hexamethylene diamine (HMD) and adipic acid (ADA) to produce nylon 6/6,6. Four different recipes are considered: a recipe containing caprolactam and an aqueous solution of HMD and ADA, a recipe containing caprolactam and dry HMD/ADA salt, and two recipes with a portion of the caprolactam replaced by nylon 6 cyclic dimer (CD). Consuming CD would be advantageous because CD is an undesirable side product from nylon 6 production. Simulation results lead to three important findings: (i) operation using dry salt rather than aqueous salt solution leads to higher degree of polymerization, (ii) substantial quantities of CD can be consumed to produce nylon 6/6,6 copolymer, and (iii) including water in the recipe is beneficial for achieving improved consumption of CD. The results of this study will be helpful in designing experiments aimed at improving industrial nylon 6/6,6 copolymerization processes.

Racemic crystals of trolox derivatives compared to their chiral counterparts: Structural studies using solid-state NMR, DFT calculations and X-ray diffraction

Trolox C (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) is a water-soluble vitamin E analogue that is available in enantiomeric forms R or S. Enantiomerically pure Trolox 1, its derivatives 2, 3 (R and S enantiomers) and racemic forms 1–3 were studied using solid-state 13C cross-polarisation (CP) magic angle spinning (MAS) NMR (13C CPMAS NMR). Gauge-including projector-augmented wave density functional theory (GIPAW DFT) calculations of the shielding constants supported the assignment of 13C resonances in the solid-state NMR spectra. For the 13C CPMAS NMR spectra of 1, resonances of pure enantiomers were significantly broader than those of the racemic R/S form. In order to explain these effects, five of the available crystal structures were analysed (1R/S, 3R/S, 2S and the newly measured 2R/S and 3S). Cyclic dimers with one R and one S enantiomer linked by two OHOC2b hydrogen bonds were formed in 1R/S. Similar hydrogen-bonded dimers were present in 3S but not in 3R/S, in which interactions are water-mediated. A comparison of X-ray diffraction, CPMAS NMR data and the DFT GIPAW calculations of racemic forms and pure enantiomers was conducted for the first time. Our results, particularly the solid-state NMR data, were discussed in relation to Wallach's rule, that the racemic crystal appears as more ordered than its chiral counterpart.

Crystal structure of a looped-chain CoII coordination polymer: catena-poly[[bis-(nitrato-κO)cobalt(II)]bis-[μ-bis-(pyridin-3-ylmeth-yl)sulfane-κ2N:N']

The asymmetric unit of the title compound, [Co(NO3)2(C12H12N2S)2] n , contains a bis-(pyridin-3-ylmeth-yl)sulfane (L) ligand, an NO3- anion and half a CoII cation, which lies on an inversion centre. The CoII cation is six-coordinated, being bound to four pyridine N atoms from four symmetry-related L ligands. The remaining coordination sites are occupied by two O atoms from two symmetry-related nitrate anions in a monodentate manner. Thus, the CoII centre adopts a distorted octa-hedral geometry. Two symmetry-related L ligands are connected by two symmetry-related CoII cations, forming a 20-membered cyclic dimer, in which the CoII atoms are separated by 10.2922 (7) Å. The cyclic dimers are connected to each other by sharing CoII atoms, giving rise to the formation of an infinite looped chain propagating along the [101] direction. Inter-molecular C-H⋯π (H⋯ring centroid = 2.89 Å) inter-actions between one pair of corresponding L ligands and C-H⋯O hydrogen bonds between the L ligands and the nitrate anions occur in the looped chain. In the crystal, adjacent looped chains are connected by inter-molecular π-π stacking inter-actions [centroid-to-centroid distance = 3.8859 (14) Å] and C-H⋯π hydrogen bonds (H⋯ring centroid = 2.65 Å), leading to the formation of layers parallel to (101). These layers are further connected through C-H⋯O hydrogen bonds between the layers, resulting in the formation of a three-dimensional supra-molecular architecture.

Safety assessment of the substance [3-(2,3-epoxypropoxy)propyl]trimethoxy silane, for use in food contact materials.

This scientific opinion of the EFSA Panel on Food Contact Materials, Enzymes, Flavourings and Processing Aids (CEF Panel) deals with the safety evaluation of [3‐(2,3‐epoxypropoxy)propyl]trimethoxy silane as a component for sizing glass fibres used for manufacturing glass‐fibre‐reinforced plastics. The substance is typically applied at up to around 0.2% related to the final plastic. The resulting food contact materials are intended for various scenarios of use, i.e. long‐term contact at ambient temperature (e.g. storage tanks) or short‐term contact at elevated temperatures (e.g. kitchen utensils). In extracts of treated fibres, neither the substance was detectable at 10 μg/kg fibre nor its hydrolysis product and oligomers at 60 μg/kg fibre. Based on the detection limits, modelling for the plastics and scenarios of intended use resulted in maximum migrations of 0.05 μg/kg food for the substance and 0.15 μg/kg food for the sum of the reaction products. The Panel concludes that the substance has a genotoxic potential. This may also apply to some of its reaction products which contain the epoxy function. However, due to the very low exposure, if any, [3‐(2,3‐epoxypropoxy)propyl]trimethoxy silane does not raise safety concern if used as a component of sizing agents to treat glass fibres imbedded into low diffusivity plastics (polyethylene terephthalate, polycarbonate, polybutylene terephthalate, thermoset polyesters and epoxy bisphenol vinylester) in contact with all foodstuffs. In addition, the residues in the treated glass fibres must not be detectable at 10 μg/kg for the substance and 60 μg/kg for each of the reaction products (hydrolysed monomers and epoxy‐containing cyclic dimer, trimer and tetramer).