Oligomer Approach Research Articles

Reactive end-group polybenzimidazoles for high temperature thermosets

Polybenzimidazoles (PBI) are heterocyclic polymers that possess high thermal and oxidative stability. The inherent strength and chemical and thermal stability of PBI make it ideal for use in extreme environments, however, the current processing of PBI molded parts makes the process practically and economically unfavorable. This work explores the improvement of PBI processability using a reactive oligomer approach. An easily processable PBI resin was synthesized, characterized, and tested in molding studies. The resin consists of PBI oligomers containing the p,p-OPBI chemistry (-O- linkage) with reactive end groups that crosslink at high temperature. The use of oligomers decreases the melt processing temperature of PBI, allowing for processability in less extreme conditions. The addition of reactive end groups allows for crosslinking during a post molding thermal cure, which imparts the high mechanical properties associated with PBI. The p,p-OPBI molded and cured articles exhibited tensile strengths of greater than 60 MPa, decreased water uptake and possessed high chemical and thermal stability.

An Oligomer Approach for Blue Thermally Activated Delayed Fluorescent Emitters Based on Twisted Donor-Acceptor Units.

The development of efficient blue donor-acceptor thermally activated delayed fluorescence (TADF) emitters remains a challenge. To enhance the efficiency of TADF-related processes of the emitter, we targeted a molecular design that would introduce a large number of intermediate triplet states between the lowest energy excited triplet (T1) and singlet (S1) excited states. Here, we introduce an oligomer approach using repetitive donor-acceptor units to gradually increase the number of quasi-degenerate states. In our design, benzonitrile (BN) moieties were selected as acceptors that are connected together via the amine donors, acting as bridges to adjacent BN acceptors. To preserve the photoluminescence emission wavelength across the series, we employed a design based on an ortho substitution pattern of the donors about the BN acceptor that induces a highly twisted conformation of the emitters, limiting the conjugation. Via a systematic photophysical study, we show that increasing the oligomer size allows for enhancement of the intersystem crossing and reverse intersystem crossing rates. We attribute the increasing intersystem crossing rate to the increasing number of intermediate triplet states along the series, confirmed by the time-dependent density functional theory. Overall, we report an approach to enhance the efficiency of TADF-related processes without changing the blue photoluminescence color.

Qualitative and quantitative analysis of agar in edible bird's nest and related products based on a daughter oligosaccharide-marker approach using LC-QTOF-MS

Agar is a common adulterant of edible bird's nest (EBN) and is often added in preparation of EBN products. Currently, there is no method that can be used to assess agar content in products because it is difficult to analyze a macromolecule that does not absorb UV. Herein, we report an efficient qualitative and quantitative method for detecting agar in food products based on a daughter oligosaccharide-marker approach. There are three steps: 1) acid hydrolysis of parent agar to release daughter oligosaccharides, 2) p-aminobenzoic ethyl ester (ABEE) labeling, 3) Liquid Chromatography Quadrupole Time-of-Flight Mass Spectrometry (LC-QTOF-MS) analysis. Twenty-four daughter oligosaccharides found to be specific to agar were selected as qualitative authentication markers. One of them, which showed high signal abundance and satisfactory linearity with standard agar, was selected as the assay marker; and it was identified as neoagarotetraose by comparing with standard references. Based on this marker, a qualitative and quantitative analytical method was successfully developed and well-validated in terms of linearity, precision, repeatability, and accuracy. This method revealed that agar is being used as a minor adulterant (one out of 12) in EBN raw material. In this study, 60% of commercial “agar” was, in fact, starch and a significant number of EBN products labeled as having agar either did not contain agar or did not contain the amount listed on the label. It is proved that this oligomer approach can be used to improve quality control of all commercial products labeled as or containing agar. Furthermore, this oligomer approach is efficient and reliable in the quality analysis of other polymers like agar.

Degree based and neighbourhood degree-sum based topological indices of PAH(Dimer 1) in graphene context

In this paper, twenty degree-based topological indices and seven neighbourhood degree-sum-based topological indices of Dimer 1 (two units of chrysene) [4] 0D & 1D in the graphene context are enumerated. The Oligomer Approach[3] is practiced here to explore the interconnection between PAH ( cove type periphery based on 11, 11’-dibromo-5,5’-bis chrysene as a key monomer-Dimer 1) and graphene numerically through the indices.

Polyaniline emeraldine salt as selective electrochemical sensor for HBr over HCl: a systematic density functional theory study through oligomer approach.

Density functional theory (DFT) calculations are performed to rationalize the experimentally observed sensitivity and selectivity of polyaniline emeraldine for hydrogen bromide over hydrogen chloride. The interaction behaviour is studied at UB3LYP method of density functional theory through oligomer approach. The properties for polymers are obtained extrapolation through second-degree polynomial fit. Optimized of geometries, interaction energies, Mulliken and natural bond orbital charges are analysed to study the sensing behaviour. In the preferred orientation modes, emeraldine salt acts as a hydrogen donor, whereas HBr and HCl are hydrogen acceptors (nucleophile). Basis set superposition error corrected interaction energies are calculated for accurate determination of interaction strength between sensor and analyte. The basis set superposition error is significant for HBr complex compared to HCl. The decrease in energy gap between highest occupied and lowest unoccupied molecular of conducting polymer (sensor) on complex formation with both analytes has been studied. The results of computational study show that polyaniline emeraldine salt shows more selectivity towards HBr as compared to HCl and this outcome is in agreement with reported results based on experimental observations. Graphical abstract.

The oligomer approach: An effective strategy to assess phenylene vinylene systems as organic heterogeneous photocatalysts in the degradation of aqueous indigo carmine dye

Four oligo-phenylenevinylenes (OPVs) were synthesized by the Mizoroki-Heck cross-coupling reaction to provide a deeper understanding of the mechanism and fate of phenylenevinylene systems when applied to the heterogeneous photocatalytic degradation of indigo carmine (IC) dye in aqueous media. OPVs displayed visible light absorption near 500 nm and appreciable emission properties. The stability, mechanism, and photodegradation activity of the OPV systems over aqueous indigo carmine were investigated using radical scavengers, a singlet oxygen (1O2) trap, and ESI-IT-MS. It was confirmed that superoxide radical (O2·−), 1O2, and direct oxidation are responsible for dye degradation. Hydroxide radical formation, under neutral pH conditions, does not occur, and it was corroborated by the HOMO and LUMO levels of the photocatalysts. Additionally, the use of oxalic acid as an electron sacrificial donor was demonstrated as an effective approach to enhance the OPVs photocatalytic activity. However, a significant decrease in activity during the first three irradiation cycles was observed, indicating that the enhancement in photo- and chemical stability is required for further dye-contaminated water treatment applications.

Fluorene-Based Donor-Acceptor Copolymers Containing Functionalized Benzotriazole Units: Tunable Emission and their Electrical Properties.

Monomers 4,7-dibromo-2H-benzo[d]1,2,3-triazole (m1) and 4,7-(bis(4-bromophenyl)ethynyl)-2H-benzo[d]1,2,3-triazole (m2) have been synthesized in good yields using different procedures. Monomers m1 and m2 have been employed for building new copolymers of fluorene derivatives by a Suzuki reaction under microwave irradiation using the same conditions. In each case different chain lengths have been achieved, while m1 gives rise to polymers for m2 oligomers have been obtained (with a number of monomer units lower than 7). Special interest has been paid to their photophysical properties due to excited state properties of these D-A units alternates, which have been investigated by density functional theory (DFT) calculations using two methods: (i) An oligomer approach and (ii) by periodic boundary conditions (PBC). It is highly remarkable the tunability of the photophysical properties as a function of the different monomer functionalization derived from 2H-benzo[d]1,2,3-triazole units. In fact, a strong modulation of the absorption and emission properties have been found by functionalizing the nitrogen N-2 of the benzotriazole units or by elongation of the π-conjugated core with the introduction of alkynylphenyl groups. Furthermore, the charge transport properties of these newly synthesized macromolecules have been approached by their implementation in organic field-effect transistors (OFETs) in order to assess their potential as active materials in organic optoelectronics.

Intermolecular modulation of IR intensities in the solid state. The role of weak interactions in polyethylene crystal: A computational DFT study.

Density functional theory calculations with periodic boundary conditions are exploited to study the infrared spectrum of crystalline polyethylene. Spectral changes lead by the intermolecular packing in the orthorhombic three-dimensional crystal are discussed by means of a careful comparison with calculations carried out for an isolated polymer chain in the all-trans conformation, described as an ideal one-dimensional crystal. The results are analyzed in the framework of the "oligomer approach" through the modelling of the IR spectrum of n-alkanes of different lengths. The study demonstrates that a relevant absorption intensity modulation of CH2 deformation transitions takes place in the solid state. This finding suggests a new interpretation for the experimental evidences collected in the past by means of IR intensity measurement during thermal treatment. Moreover, the comparison between calculations for 3-D crystal and for the isolated polyethylene chain (1-D crystal) allows to put in evidence the effect of the local electric field on the computed infrared intensities. This observation provides guidelines for the comparison between infrared absorption intensities predicted for an isolated unit and for a molecule belonging to a crystal, through the introduction of suitable correction factors based on the refraction index of the material and depending on the dimensionality of such units (0D-molecule; 1D-polymer; 2D-slab).

A comparative study of one and two dimensional π-conjugated systems

The comparative study of one (1D) and two dimensional (2D) π-conjugated systems has been carried out at D3 dispersion corrected B3LYP and restricted active space (RAS) levels of theory using oligomer approach. Two sets of systems differeing by the connection density have been studied. The first one is poly-m-phenylene (1D-PMP) and the corresponding 2D analogue, porous graphene (2D-PMP). The second one is trans-polyacetylene (1D-PA) and graphene nanoflakes(2D-PA). Both 1D- and 2D-PMP have closed shell singlet ground state. 2D-PMP systems have higher ionization potential (IP), electron affinity (EA) and lower band gap (Eg) than 1D-PMP of the similar size. The ground state of 1D-PA is a single reference singlet while the nature of 2D-PA ground state depends strongly on the size. The small members of 2D-PA series have singlet ground states (single- or multireferencial). The larger members have single reference high spin ground states with number of unpaired electrons increasing with their size and reaching 12 for the largest member of 2D-PA series according to both B3LYP and RAS methods. Similar to PMP, Eg decreases with size more rapidly for 2D-PA than for 1D-PA and ionization potentials are higher for 2D-PA of the similar size.Unlike 1D-PMP and 2D-PMP the electron affinities of 1D-PA are consistently higher then these of 2D-PA.

Robust ethylenedioxythiophene-vinylene oligomers from fragile thiophene-vinylene cores: synthesis and optical, chemical and electrochemical properties of multicharged shapes.

Oligomers of ethylendioxythiophene-vinylene have been prepared. Their optical, electrochemical and chemical properties have been studied in detail by absorption and emission spectroscopy as well as cyclic voltammetry, Raman techniques and spectroelectrochemistry complemented with quantum chemical calculations. A comparison with their non-ethylendioxy and non-vinylene parents has been done. The inclusion of the EDO plus the vinylene function generates more robust electronic ground states regarding the largely flexible thiophene-vinylene (n TV) oligomeric homologues. The redox features of the new compounds are also rich of oxidative processes arising as an interesting stabilising balance effect between the oxygen fragment in the EDO groups (mesomeric effect) and the linear π-conjugated structure. The oxidised species have been characterised, which show the ability for the formation of mixed valence charge-transfer complexes and π dimers of different oxidation states, in particular, in the electrochemical medium, resulting that the electrochemical response is accounted for a succession of aggregation and electron-transfer steps. With this work, a full understanding of the optical and electronic properties of these new oligomers in the context of the oligomer approach has been proposed.

Diradicals acting through diamagnetic phenylene vinylene bridges: Raman spectroscopy as a probe to characterize spin delocalization

We present a complete Raman spectroscopic study in two structurally well-defined diradical species of different lengths incorporating oligo p-phenylene vinylene bridges between two polychlorinated triphenylmethyl radical units, a disposition that allows sizeable conjugation between the two radicals through and with the bridge. The spectroscopic data are interpreted and supported by quantum chemical calculations. We focus the attention on the Raman frequency changes, interpretable in terms of: (i) bridge length (conjugation length); (ii) bridge conformational structure; and (iii) electronic coupling between the terminal radical units with the bridge and through the bridge, which could delineate through-bond spin polarization, or spin delocalization. These items are addressed by using the "oligomer approach" in conjunction with pressure and temperature dependent Raman spectroscopic data. In summary, we have attempted to translate the well-known strategy to study the electron (charge) structure of π-conjugated molecules by Raman spectroscopy to the case of electron (spin) interactions via the spin delocalization mechanism.

High transition temperature shape memory polymers (SMPs) by telechelic oligomer approach

This work describes the synthesis and comparative shape memory properties of cross-linked networks derived from epoxy and cyanate ester monomers containing polyether oligomers as reactive shape memory segments. The hydroxy telechelic oligomers viz. polyethyleneglycol (PEG), polypropyleneglycol (PPG), and polytetramethyleneglycol (PTG) are reacted with epoxy–cyanate ester matrix resulting in shape memory polymers with high transition temperatures. The soft oligomer segments act as flexible linker unit which interconnect oxazolidone, isocyanurate and triazine ring structures in the cross-linked polymer. The resultant cyclomatrix SMPs exhibit high transition temperatures 132, 178 and 161°C respectively for PEG, PPG and PTG integrated SMPs. The Eg/Er ratios are increased in the order PEG<PTG<PPG. The PTG and PPG based SMPs show shape retention of 99% and shape recovery of >98% with recovery time <100s. All the SMPs display good thermal stabilities (both inert and oxidative) above 275°C.

Theoretical investigations on the electronic and optical characteristics of fused-ring homopolymers: Comparison of oligomer method and PBC−DFT method

The electronic and optical characteristics of conjugated polymers are the curial parameters for their photovoltaic and photonic applications. Here, the electronic and optical properties of six selected homopolymers were studied using two methods: oligomer method and PBC−DFT method. The calculated results show, PBC/B3LYP/6-31G* is preferable to weakly conjugated homopolymers, while the oligomer approach using linear fit at B3LYP/6-31G* level, reproduced well band gaps for stronger conjugated homopolymers. The results also indicate that the calculated absorption spectra using TD-B3LYP/6-31G* at trimer model are in agreement with the available experiments, however the Meier fit of oligomers highly overestimates the experimental data.

Folding of Alternating Dialkylsilylene‐Spaced Donor–Acceptor Copolymers: The Oligomer Approach

A series of oligmers with donor-acceptor pairs separated by diisopropylsilylene (iPr(2)Si) spacers, composed of monomer 4b, dimer 5, trimer 6, and tetramer 7, were synthesized to scrutinize the folding behavior. Monomer 4a with a dimethylsilylene (Me(2)Si) spacer was also prepared for comparison. The 4-aminostyrene moiety was used as the donor and the stilbene moiety as the acceptor. Both steady-state and time-resolved fluorescence spectroscopic measurement were made. Regardless of the substituents on the silicon atom, the emission spectra of 4a and 4b exhibit both local excited (LE) emission of the acceptor chromophore and emission from the charge-separated state (CT emission), which are similar to that of the corresponding Me(2)Si-spaced copolymer 2a with the same donor and acceptor chromophores, but different from that of the copolymer with the iPr(2)Si spacer 2b. Dimer 5 behaves like 4 and 2a. As the chain length of the oligomers increases, the emission properties of the higher homologues become prone to that of 2b. Thus, tetramer 7 exhibits emission from the charge-transfer complex, which is essentially same as that of 2b. Moreover, charge-transfer absorptions emerge in 6 and 7. These results suggest that the folding nature of oligomers approaches that of the corresponding polymer, as the degree of oligomerization increases, and the electronic interactions between adjacent donor-acceptor pairs are controlled by the steric effect of the substituents on the silicon atoms and concomitant amplification of the stabilizing energy by extending the distance of the folding structure.

Applying the Oligomer Approach to Luminescent Conjugated Organoboranes

A series of highly luminescent monodisperse fluoreneborane oligomers (n=1-6) were prepared using a new iterative synthetic procedure that takes advantage of the highly selective and differential reactivity of bromoboranes with arylsilanes and arylstannanes. Cyclic and square wave voltammetry revealed a gradual decrease of the LUMO energy levels with increasing chain length, while absorption and emission data showed a bathochromic shift and increase in quantum efficiency. An extended conjugation length of n(ECL)=5 was derived.

A Comparative Ab Initio and DFT Study of Polyaniline Leucoemeraldine Base and Its Oligomers

Ab initio Hartree-Fock (HF) and density functional theory (DFT) calculations are being performed to investigate the geometric, vibrational, and electronic properties of the polyaniline leucoemeraldine base (PANI-LB). Vibrational spectra of PANI-LB have been analyzed using the DFT oligomer approach, and complete assignments are being reported. Lower region spectral assignments of the PANI-LB which were not being reported earlier are being done in the present work. DFT calculations with the 6-31G** basis set produce very good results of not only vibrational modes but also of energy band gap.

All‐aromatic liquid crystalline thermosets with high glass transition temperatures

AbstractWe have synthesized and characterized a new family of low melting all‐aromatic ester‐based liquid crystal oligomers end‐capped with reactive phenylethynyl end groups. In a consecutive, high‐temperature step, the reactive end groups were thermally activated and polymerization was initiated. This reactive oligomer approach allows us to synthesize liquid crystal thermosets with outstanding mechanical and thermal properties, which are superior to well‐known high‐performance polymers such as PPS and PEEK. We have modified an intractable LC formulation based on hydroquinone and terephthalic acid, with Mn = 1000, 5000, and 9000 g mol−1, and varied the backbone composition using isophthalic acid, resorcinol, 4‐hydroxy‐benzoic acid, 6‐hydroxy‐2‐naphthoic acid, and chlorohydroquinone. All fully cured polymers showed glass transition temperatures in the range of 164–275 °C, and high storage moduli at room temperature (∼ 5 GPa) and elevated temperature (∼ 2 GPa at 200 °C). All oligomers display nematic mesophases and in most cases, the nematic order is maintained after cure. Rheology experiments showed that the phenylethynyl end group undergoes predominantly chain extension below 340 °C and crosslinking above this temperature. Highly aligned fibers could be spun from the nematic melt, and we found that the order parameter 〈P2〉 was not affected by the chain extension and crosslink chemistry. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1368–1380, 2009

Toward the synthesis of strongly photoluminescing polyesters by entropically driven ring‐opening polymerizations

AbstractA range of strongly fluorescent polyesters incorporating distyrylarylene moieties have been prepared. The polyesters have the potential to be prepared from the corresponding macrocyclic oligomers (MCOs) by entropically driven ring‐opening polymerizations (ED‐ROP). This reactive macrocyclic oligomer approach to the polymers has the advantage of allowing the deposition of conjugated materials by solution deposition techniques, such as ink‐jet, since the macrocycles after deposition can be converted directly on the substrate, in the absence of any external initiator, into solvent intractable thin polymeric films. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Oligomer Approaches for Solid-State Dye-Sensitized Solar Cells Employing Polymer Electrolytes

Three different approaches utilizing oligomers, termed the “oligomer approaches”, followed by in situ self-solidification, were applied successfully to prepare solid-state dye-sensitized solar cells. The oligomer approach employs (1) supramolecules containing quadruple hydrogen-bonding sites at both chain ends, (2) oligomer blends with high-molecular-weight polymers, and (3) nanocomposites of oligomer with SiO2 nanoparticles. The overall energy conversion efficiency was as high as 4.5% at 1 sun condition, which may be primarily due to both the increased ionic conductivity and the better interfacial contact between the TiO2 layer and the electrolyte due to the deep penetration of the small-sized liquid oligomer electrolyte. A threshold ionic conductivity of ∼1 × 10-4 S cm-1 was also observed, below which the ionic conductivity may play a key role in determining the photocurrent and the energy conversion efficiency. Above the threshold ionic conductivity, the ionic conductivity may play a less-important rol...

Optical Bandgaps of π‐Conjugated Organic Materials at the Polymer Limit: Experiment and Theory

AbstractIn this Review, the extreme care that must be taken when predicting the optical properties of conjugated polymers via the oligomer approach, and when comparing theoretical and experimental data, is illustrated. In the first part, conceptual strategies for the correct determination of optical transitions from experimental spectra and relevant extrapolation procedures at the polymer limit are introduced. The impact of conformational, substitution, solvent, and solid‐state effects on the optical properties is discussed in light of experimental data reported for molecular backbones based on phenylene, phenylenevinylene, and thiophene repeat units. A comparison is then made between experimental results and those provided by standard quantum‐chemical methods, to assess their reliability.