Types Of End Groups Research Articles

Effect of the End Group of Regioregular Poly(3-hexylthiophene) Polymers on the Performance of Polymer/Fullerene Solar Cells

In this report, we have studied the influence of the nature of the end groups (bromine or hydrogen) of regioregular poly(3-hexylthiophene) (P3HT) polymers on the performance of polymer solar cells made with blend films of P3HT and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM). Films and devices were studied before and after annealing at 140 °C for 2 h. The effects of the end-group type on the properties of pristine polymers and blend films were examined using optical absorption and emission spectroscopy, transient absorption spectroscopy, and measurements of photovoltaic device performance. It was observed that hydrogen end groups result in slightly higher absorption coefficients, higher photoluminescence intensities, faster and less dispersive charge recombination, and superior solar cell performance (notably a higher fill factor) compared to bromine end groups. The results are attributed to more-ordered polymer-chain packing in blend films made with hydrogen-capped P3HT, on account of the smaller size and weaker electrostatic interactions resulting from hydrogen compared to bromine. Some influence of the C−Br group on exciton quenching may also be present. The effect of the bromine end group on solar-cell performance became more pronounced with reducing incident-light intensity. Comparison of the polymer transport characteristics in organic field-effect transistor configuration indicated that the bromine end group enhances hole trapping.

A Computational Study of End‐Group Conformational Energy Barriers in Carotenoids

Introduction Carotenoids possessing different end-groups are known to be selectively metabolized in biological systems and there is evidence demonstrating that end-groups affect the ability of carotenoids to adopt specific geometries within larger biological structures such as membranes. In order to further assess the importance of the end-group structure in carotenoids we have investigated the influence of the end-group type and functionality on the conformational energy barrier. Methods We used the density functional method and the program GAUSSIAN to calculate the conformational energy curves for rotation of the beta-ring or the epsilon-ring relative to short polyene chains around the C6-C7 single bond. Results Epsilon rings have been shown to possess a dramatically larger conformational energy barrier to rotation than beta rings. The rotational energy barrier of the epsilon ring relative to the polyene chain around the C6-C7 bond has been found to range from 8.65 to 9.05 kcal/mole whereas the barrier for rotation around the beta-ring varies from 3.06 to 3.40 kcal/mole. Conclusions Conjugation of the beta-ring has no influence on the conformational energy barrier to rotation in carotenoid systems possessing this end-group. The length of the polyene chain (extent of conjugation) and substitution on the ring have little influence on the rotational energy barrier. The most important factor influencing the conformational energy for ring rotation about the C6-C7 bond is the steric interaction between the ring and polyene methyl groups. The high barrier to rotation for the epsilon ring provides a physical basis for the observed differences in the metabolism and properties of carotenoids possessing this end-group in biological systems.

Conformation Effects on the Dissociation of Ionized Polymers

The collision-induced dissociation (CID) of a series of oligomer chain lengths of four polymer samples have been quantified according to their observed total relative fragment ion abundances. The CID mass spectra of oligomers of ionized poly(methyl methacrylate) (PMMA), with three different types of end groups and polystyrene (PS), were obtained at fixed center-of-mass collision energies and collision numbers. For the PMMA polymers, the total fragment ion abundance increases with increasing chain length, consistent with an increase in internal energy deposition with the size of the ion. A discontinuity in the increase in total fragment ion abundance appears to correspond with a change in conformation of the polymer ions from linear (at short chain lengths) to cyclic (at long chain lengths). Ionized PS does not exhibit this change in conformation as all chain lengths show compact structures and accordingly the total fragment ion abundance does not change with increasing chain length.

Development of DSM's hybrane® hyperbranched polyesteramides

AbstractThe development of DSM's Hybrane® hyperbranched poly(ester amides) is described. The monomer (1) for the hyperbranched polyester is obtained from the reaction of a cyclic anhydride with diisopropanol amine, yielding a tertiairy amide with one COOH and two OH groups. Polycondensation takes place via an oxazolinium intermediate in bulk at relatively mild conditions in the absence of catalyst. The reaction has been scaled up to ton scale. By varying and combining anhydrides, and modification with several types of end groups, a large variety of structures with concomitant properties and industrial applications has been realized. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3110–3115, 2004

Surface, Core, and Structure Modifications of Phosphorus-Containing Dendrimers. Influence on the Thermal Stability.

Three new series of phosphorus-containing dendrimers are described. Their solubility depends on the type of end groups they bear. Perfluoroalkyl chains give dendrimers soluble in chlorofluorocarbons, whereas guanidinium and pyridinium derivatives give watersoluble compounds. The thermal stability of these compounds, as well as of 19 other dendrimers of various generations, having various cores, or various end groups, or branching points is studied. The main feature of this study is that the internal structure of these dendrimers is thermally stable at least up to 3768C. The number of the generation has practically no influence, whereas the principal criterion influencing the thermal stability is the type of end groups. The water-soluble cationic dendrimers are the least stable, but even those are stable up to 2258C. For most of these dendrimers, an important percentage of mass (around 50%) is retained even at a temperature as high as 10008C. In the best case, up to 70% of the initial mass is retained at 10008C.

Surface, core, and structure modifications of phosphorus-containing dendrimers. Influence on the thermal stability

Three new series of phosphorus-containing dendrimers are described. Their solubility depends on the type of end groups they bear. Perfluoroalkyl chains give dendrimers soluble in chlorofluorocarbons, whereas guanidinium and pyridinium derivatives give water-soluble compounds. The thermal stability of these compounds, as well as of 19 other dendrimers of various generations, having various cores, or various end groups, or branching points is studied. The main feature of this study is that the internal structure of these dendrimers is thermally stable at least up to 376°C. The number of the generation has practically no influence, whereas the principal criterion influencing the thermal stability is the type of end groups. The water-soluble cationic dendrimers are the least stable, but even those are stable up to 225°C. For most of these dendrimers, an important percentage of mass (around 50%) is retained even at a temperature as high as 1000°C. In the best case, up to 70% of the initial mass is retained at 1000°C.

Substituted propenyl end groups as reactive intermediates in radical polymerization

AbstractThe addition of propagating radicals of methyl acrylate (MA) and styrene (St) to CH2C(CO2CH3)CH2 and CH2C(C6H5)CH2 ω‐end groups of poly(methyl methacrylate) (PMMA) and polystyrene (PSt) was investigated. The end groups were as reactive as MA and St toward the poly(methyl acrylate) (PMA) and PSt radicals, respectively. The adduct radical derived from the two types of PMMA end groups and PMA radicals underwent β fragmentation exclusively to yield PMMA radicals and end groups bound to PMA chains. The addition of PSt radicals to PMMA with CH2C(CO2Me)CH2 end groups resulted in adduct radicals that underwent β fragmentation and addition to St or coupling with PSt radicals. Adduct radicals formed by the addition of PMA radicals to both types of end groups of PSt exclusively formed CC bond by coupling with PMA radicals to form branched structures or by addition to MA monomer to give a copolymer. The fate of the adduct radicals was highly dependent on the type of polymer chain and the substituent bound to the end group. Steric congestion of the adduct radical arising from the α‐methyl group of the PMMA chain was considered to be crucial for fragmentation to expel the PMMA radical. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 645–654, 2003

Fast and Slow Dynamics of Water-Soluble Dendrimers Consisting of Amido-Amine Repeating Units by Neutron Spin−Echo

Neutron spin−echo (NSE) experiments were performed at room temperature on D2O solutions of fifth generation poly(amido amine) (PAMAM) dendrimers with different types of end groups. Intermediate correlation functions I(Q,t)/I(Q,0) at different scattering vectors Q as a function of time t show different dynamics for dendrimer concentrations of 1.0 and 10 wt %, while the dynamics was independent of the chemical species of the end groups, hydroxyl or glucopeptide. The NSE results for a 10 wt % solution followed a single-exponential decay. With increasing Q, the evaluated effective diffusion coefficients Deff decreased and converged to the translational diffusion coefficient D0, which was obtained from dynamic light scattering. It is assumed that the Q-dependence of Deff is caused by the dendrimer-dendrimer interaction, which is observed as the inter-dendrimer structure factor on small-angle neutron scattering. On the other hand, NSE results for a 1.0 wt % solution were well-fitted to a double-exponential function with two decay rates (fast and slow modes). It was observed that the diffusion coefficient of the slow mode corresponds to D0. The fast mode originates from the motion of amido-amine segments in the dendrimer, which is a common internal unit for the two dendrimers.

Poly(pent-1-ene) Synthesized with the Syndiospecific Catalyst i-Pr(Cp)(9-Flu)ZrCl2/MAO

1-Pentene was polymerized with the syndiospecific catalyst system i-PrC(Cp)(9-fluorenyl)ZrCl2/MAO. The molar mass of the resulting polymers depends strongly on the reaction temperature and decreases from M̄w = 126 000 at 0°C to M̄w = 46 000 at 100°C, but is more or less independent of the monomer and the MAO concentration. The influence of reaction temperature and concentrations of MAO and monomer on the type of end-groups generated during the chain termination, as well as on the type of stereoerror, was investigated. The degree of tacticity was dependent on the polymerization temperature with [rrrr] > 0.99 at 0°C and [rrrr] = 0.75 at 100°C.

Effect of End Groups on the Thermotropic Behavior of Linear Poly[11-(4‘-cyanophenyl-4‘ ‘-phenoxy)undecyl acrylate]s Prepared by ATRP and Their Topological Blends

Linear poly[11-(4‘-cyanophenyl-4‘ ‘-phenoxy)undecyl acrylate]s with 9−77 (GPC relative to linear polystyrene) or 7−101 (GPCLS) repeat units were synthesized by atom transfer radical polymerization of 11-(4‘-cyanophenyl-4‘ ‘-phenoxy)undecyl acrylate using a mesogenic initiator, 11-(4‘-cyanophenyl-4‘ ‘-phenoxy)undecyl 2-bromopropionate. The biphasic regions of the smectic A to isotropic (sA−i) transition of the linear polymers are extremely narrow, with full widths at half of the maximum peak intensity (fwhm) = 4.52−7.74 °C, in contrast to that of the corresponding polymer prepared by conventional radical polymerization (fwhm = 17.0 °C). The breadths of this transition do not vary with chain length, in contrast to the corresponding linear polymers with methyl propionate end groups. The breadth of the sA−i biphasic region of 1:1 binary blends of poly[11-(4‘-cyanophenyl-4‘ ‘-phenoxy)undecyl acrylate]s increases relatively linearly with the difference in end group (and therefore branching) density of the two components, regardless of the combination of molecular architectures (linear, three-arm star, or comb) or the type of end groups (bromine and hydrogen or methyl propionate).

Thermal stability of aliphatic hyperbranched polyesters

The influence of the molar mass (generation), number and type of end groups on the thermal stability of aliphatic hyperbranched polyesters are presented in this study. Different end groups were obtained by modification of the samples with chlorides of propionic acid and stearic acid. The thermal stability of the hyperbranched polyesters was determined by thermogravimetry using a NET-ZSCH TG 209 instrument in nitrogen atmosphere at a heating rate of 10?C/min. A comparison of the temperatures obtained for mass losses of 5 20 and 40 wt% for unmodified samples showed that the thermal stability increased up to the fourth generation and then remained practically constant. An increase in the thermal stability of modified samples of the second, third and fourth generation was observed.

Reaction kinetics for solid‐state polymerization of poly(ethylene terephthalate)

AbstractA simple kinetic model capable of describing the behaviors of solid‐state polymerization (SSP) of poly(ethylene terephthalate) (PET) has been developed. According to this model, there are two types of end groups, namely active end groups and inactive end groups, and the solid‐state polycondensation reaction is a second‐order reaction with respect to the active end group concentration. The inactive end groups include chemically dead end groups and functional end groups that are immobilized by the crystalline structure. The inactive end group concentration determines the ultimate intrinsic viscosity (IV) or molecular weight achievable in the SSP—the lower the inactive end group concentration, the higher the ultimate IV. Four pulverized PET prepolymers with IV values ranging from 0.20 to 0.35 dL/g were solid‐state polymerized in a fluid‐bed reactor at temperatures between 200 and 230°C to generate data to test the model. The experimental results fit the proposed rate equations very well, confirming the adequacy of the proposed kinetic model. Under these SSP conditions, the activation energy is about 23.6 kcal/mol and the average SSP rate about doubles with each 10°C increase in temperature. The rate constant increases, while the inactive end group concentration decreases, with increasing temperature and prepolymer IV. This explains why the reaction rate and the ultimate IV increase with increasing prepolymer IV as well as temperature in the SSP of PET. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1748–1761, 2001

Varying Topology of Dendrimers − A New Approach toward the Synthesis of Di-Block Dendrimers

A new way to synthesise di-block dendrimers having two types of end groups located in two different areas of their surface is reported. It consists in growing, step-by-step, a second dendritic wedge from the core of a first dendron. This method is mainly applied to the synthesis of di-block dendrimers having phosphanyl groups on one part of the surface.

Characterization of melt-polymerized polycarbonate: preparative fractionation, branching distribution and simulation

Melt-polymerized bisphenol-A polycarbonate materials characterized by a low degree of branching were fractionated according to molecular weight by the continuous polymer fractionation (CPF) method. The distribution of two types of end-groups and in-chain salicylate moieties arising from thermal rearrangement reactions were assessed across the molecular weight distribution by analysis of the fractions. Experimentally determined branching densities of the fractions agreed well with a molecular simulation based on a random sampling polycondensation model. Both simulation and experiments showed that the branching density increases with molecular weight in the experimentally accessible range.

Chromatographic behaviour of low molar-mass polyesters in normal-phase high-performance liquid chromatography

The normal-phase chromatographic retention behaviour of polyesters on bare silica and on a polymer-based polyamine (PA) column has been studied with a variely of binary mobile phases under isocratic conditions. The dependence of experimental retention data on the degree of polymerization (p) and on mobile phase composition (φ) was characterized by to an approach developed by Jandera et al. The bulky repeating unit and the relatively highly polar end groups of the polyesters both had a large influence on retention behaviour. The two effects in combination explain the molar-mass-independent retention observed experimentally at a particular mobile phase composition for all the mobile phase—stationary phase combinations investigated. These conditions were found to be independent of the type of end group. End group separation on a silica column improves when the polarity of the less polar solvent is increased. End group separation is better on the PA column because of a greater difference between the adsorption energy of the alcohol and acid end groups. Better prediction of retention data on the PA column was achieved by use of an approach which assumes two different types of adsorption site. Results enabled further understanding of retention behaviour in normalphase gradient polymer-elution chromatography (NPGPEC) and explained both the dependence of the order of elution onp and differences between the end-group selectivity of different systems.

A theoretical framework for the experimental determination of host–guest interaction energies in solid inclusion compounds

A mathematical model is developed to provide the framework of an experimental approach for determining host–guest interaction energies in solid inclusion compounds with one-dimensional tunnel host structures. The approach considers the competitive inclusion of two different types of potential guest molecules X(S)qX and X(S)rX (q≠r) within the tunnel host structure, where X represents a given type of end group (e.g., CH3, halogen, etc.) and S represents an appropriate spacer unit (e.g., CH2, CH, etc.). Sequential and simultaneous models for the growth of the guest substructure within the host tunnel are considered. The relative proportions (m and 1−m) of the two types of guest molecule included within the host tunnel depend on the relative proportions (γ and 1−γ) of the two types of guest molecule in the external “pool” of potential guest molecules and the relative “affinities” (χ and 1/χ) of the host tunnel structure for including the two different types of guest molecule. Expressions linking χ, m, and γ are developed, and can be applied to determine χ directly from experimental measurements of m for a series of inclusion compounds prepared for different (known) values of γ. Fundamentally, the value of χ depends on the intermolecular interaction energies per unit length of tunnel for the two different types of guest molecule, and χ may be expressed in terms of the host–guest interaction energies for the spacer units S and the end groups X, and the guest–guest (X⋯X) interaction energy. The mathematical model provides a framework for assessing these different energy contributions from experimental measurements of m for sets of inclusion compounds prepared for different values of the parameters q, r, and γ, and with the same host tunnel structure.

End-Group Effects on the Structure and Spectroscopy of Oligoazines

Oligoazines with three different types of end groups, ketone, hydrazone, and 2-pyridine, are studied in solution. When the end groups are ketone or 2-pyridine, the 1H and 13C NMR spectra are simple and show that the oligomers maintain 2-fold symmetry up to the longest chain lengths investigated (five azine units for the 2-pyridine end groups). In contrast, when the end groups are hydrazones, the 1H NMR spectra show evidence for as many as five conformers present in solution for each oligomer while the 13C NMR spectra are still simple, indicative of a single compound. Semiempirical AM1 calculations suggest that all of the oligoazines should be severely twisted but give no insight about the origin of the large number of conformers found for the materials with hydrazone end groups. The end groups have a significant effect on the UV spectra. All the oligoazines show two absorptions close in energy, but the relative intensities change depending upon the end group. For ketone end groups the higher energy transi...

Thermosetting Polyetherimides: The Influence of Reactive Endgroup Type and Oligomer Molecular Weight on Synthesis, Network Formation, Adhesion Strength and Thermal Properties

The influence of the reactive endgroup on the synthesis, cure behavior and network properties of thermosetting polyetherimides was investigated. Reactive phenylethynyl, ethynyl and maleimide terminated etherimide oligomers were prepared and characterized. Optimal reaction conditions were established to produce fully endcapped oligomers with imidized structures and controlled molecular weight. The phenylethynyl and ethynyl endcapped systems were synthesized by ester-acid methods. The maleimide endcapped system utilized an amic-acid route. Phenylethynyl endcapped oligomers had good processibility and were thermally cured at high temperatures (350–380°C). The networks exhibited good thermal and hydrolytic stability and good adhesion strength, and are candidates for “primary'' bonding adhesives. In contrast, more reactive ethynyl and maleimide endcapped systems were prepared as “secondary'' bonding materials, which could be cured at temperatures lower than that of the T g of the primary structure. Lap shear test results obtained from NMP-cast/methanol-extracted scrim-cloth-supported precursors confirmed that good adhesion to titanium at both room temperature and at 177°C was achieved when cured at 250°C-280°C. High glass transition temperatures and good thermal stability were achieved as determined by thermal analysis (DSC, TGA and DMA). Solvent extraction measurements confirmed that very high gel fractions were obtained, which is consistent with good chemical resistance. The influence of molecular weight between crosslinks (Mc) on thermal and mechanical behavior was also investigated for 2,3,5,7 and 10k initial M n values. Lower molecular weight oligomers exhibited lower T g and cure temperatures, but higher cured network crosslink densities afforded higher T g and higher gel fractions, but with reduced toughness.

Adsorption of serum albumin to thin films of poly(lactide-co-glycolide)

Protein adsorption has been implicated in the variability of drug release from biodegradable microspheres. We used optical reflectometry to measure the extent and kinetics of bovine serum albumin (BSA) adsorption to smooth spin-cast films prepared from two poly(lactide-co-glycolide) (PLG) samples that have different end-groups, one being a hydrophilic carboxylic end group and the other a hydrophobic ester end group. One of us has previously shown that these end-groups influence microsphere degradation (Tracy et al., 1998, Factors affecting the degradation rate of poly(lactide-co-glycolide) microspheres in vivo and in vitro. Biomaterials: submitted for publication.). Both films were moderately hydrophobic, and their wettability was independent of the type of end-group. BSA adsorbed readily to both native PLG films, attaining as much as 50% surface coverage by area and was insensitive to the type of end-group. Aging the films in water for 24 h prior to BSA exposure decreased the hydrophobicity of the films and this in turn correlated with a significant decrease in the initial BSA adsorption rate. This was consistent with the often-observed trend that surface hydrophobicity favors protein adsorption. In spite of the lower adsorption affinity revealed by this decreased initial adsorption rate, the final adsorbed amounts on the aged films exceeded those attained on native films, presumably due to the increase in total surface area produced by partial PLG erosion.

Chain end analysis of bisphenol A polysulfone and its relation to molecular weight

The preparation of amine-terminated polysulfone by step polymerization of the monomers bisphenol A and dichlorodiphenyl sulfone in the presence of end-capping reagent 4-aminophenol was investigated. A persistent problem with end-capping strategy as applied to step polymerization is the presence of end groups other than those introduced by the end-capping reagent. These unintended end groups, which can persist in the polymer product even when 100% of the end-capping reagent has reacted, are associated with a proportionate decrease in polymer chain length. This situation renders quantitative analysis of a single type of end group invalid as a method for molecular weight determination. The presence of unintended end groups does not appear to correlate with a particular set of reaction conditions; unintended end groups were found to occur in polymerizations conducted under strong base conditions (NaOH), under weak base conditions (K2CO3), and with a wide range of monomer feed ratios. A scheme for unambiguous quantification of chain ends and molecular weight for end-capped polysulfone is described. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1309–1316, 1998