Solution and solid-state polymerizations of substituted p-quinodimethanes and p-quinone methides

This article describes the polymerization behavior of electron-accepting and electron-donating p-quinonoid monomers such as substituted p-quinodimethanes and p-quinone methides in solutions and solid states. In the solution polymerizations, the electron-accepting substituted p-quinodimethanes and p-quinone methides with same substituents such as cyano, ester, and sulfonyl groups at the exocyclic positions are not homopolymerizable, but copolymerizable with donor monomers like styrene in an alternating fashion. Their spontaneous polymerizations with donor monomers have been explained with the bond-forming initiation mechanism. The substituted p-quinodimethanes with ester groups are a first example to show an amphoteric behavior in alternating copolymerizations. Some electron-accepting substituted p-quinodimethanes and p-quinone methides with different substituents such as cyano, ester, acyl, alkylthio, and/or phenyl groups are homopolymerizable, and an anionic polymerization initiated with a butyllithium initiator proceeded in a living manner. Equilibrium polymerization behavior have been found in their radical homopolymerizations, and on the basis of the thermodynamic parameters determined for their polymerizations, it has been concluded that homopolymerizabilities of the electron-accepting substituted p-quinodimethanes and p-quinone methides are determined exclusively by a steric hindrance effect arising from the substituents at the exocyclic positions. A new concept for the radical alternating copolymerization have been proposed on the basis of the change in modes (random and alternating copolymerizations) for their copolymerizations with styrene and the cross-propagation step analysis by linear free energy relationship. The polymerizations of the electron-donating substituted p-quinodimethanes take place only in the presence of oxygen molecules. In the solid-state polymerizations, some electron-accepting substituted p-quinodimethanes with same ester substituents polymerized topochemically in vacuo, and the strict requirements of topochemical polymerization for substituted p-quinodimethane and p-quinone methide monomers have been determined on the basis of their crystallographic data. Topochemical alternating copolymerization with molecular oxygen in solid state was discovered for the first time.     

The effects of the time-dependent on the characteristic parameters of polypyrrole/p-type Si/Al diode

A detailed study of the effects of the time-dependent or aging on the characteristic parameters of polypyrrole/p-type Si/Al structure has been presented. The polypyrrole film has been formed on a p-type Si substrate by means of an anodization process. The polypyrrole/p-Si contact has demonstrated clearly rectifying behavior by the current-voltage curves studied at room temperature. The current-voltage (I-V) curves of the diode have been measured immediately, 7, 15, 30, 60 and 90 days after fabrication of the polypyrrole/p-Si contact. It has been seen that the characteristics parameters such as barrier height, ideality factor and series resistance of polypyrrole/p-type Si/Al structure have changed with increasing ageing time. Furthermore, the density distribution of interface states of the device was obtained from the forward bias I-V characteristics. The fact that the diode shows non-ideal I-V behavior with increasing ageing time may be ascribed to a slow replacement of the initial doping agent by oxygen and this process certainly plays a role in the aging of the diode.     

Scale-up of a fixed bed electrochemical reactor consisting of parallel screen electrode used for p-aminophenol production

The behavior of a fixed bed consisting of amalgamated copper screens has been investigated for the electrolytic reduction of nitrobenzene to p-aminophenol under potentiostatic condition (controlled potential). The preparative electrolysis of nitrobenzene was carried out using supporting electrolytes consisting of 2 M H₂SO₄ in a solution of 50% 2-propanol/50% water (v/v). The criterion for scale-up (?_n) was determined through application of one-dimensional model. The polarization curves that describe the reduction of nitrobenzene to p-aminophenol were obtained experimentally by using a pilot scale for different nitrobenzene concentrations and flow rates of catholyte.It was found that the effectiveness factor (?_n) increases with increasing flow rate, and decreasing nitrobenzene concentration. An optimum thickness of bed equal to 0.6 cm was obtained, in which the effectiveness factor not less than 0.588, to ensure a well distribution of current and potential.     

Competitive electrochemical oxidation of p-chlorophenol and p-nitrophenol on Bi-doped PbO2

The initial stages of oxidation of aqueous solutions of p-chlorophenol (pcp) and p-nitrophenol (pnp) on Bi-doped PbO₂ electrodes have been studied. From deconvolution and analysis of UV-Vis spectra of the solutions obtained during electrochemical oxidation, benzoquinone and aliphatic acids were identified as the primary oxidation intermediates; oxidation of benzoquinone was found to be the slow step during the early stages of the electrochemical combustion process. The effect of competing adsorption of pcp and pnp on Bi-PbO₂ was also examined, and the presence of pnp in solution was found to inhibit the rate of oxidation of pcp during concurrent oxidation of both phenols.     

Depolymerization behaviour of poly-(p-phenylene diacrylic acid diethyl ester)

The photochemical and thermal behaviour of poly(p-phenylene diacrylic acid diethyl ester) [poly(p-PDA-Et)] has been investigated. It was observed that both photo and thermal depolymerization occurs with cleavage of the cyclobutane ring. Photochemically poly(p-PDA-Et) in solution depolymerizes selectively into oligomer, which undergoes cis-trans isomerization without further depolymerization to monomer. On the other hand, it depolymerizes thermally at random into low molecular weight polymer and monomer.     

Polarographic behaviour of p-nitrotoluene and p-nitroaniline in water-alcohol mixtures

The polarographic behaviour at the dme of p-nitrotoluene and p-nitroaniline is investigated in water-methanol, water-ethanol and water-isopropanol, in presence of 0.1 M LiCl as indifferent electrolyte. In water-methanol solutions one wave is observed. In essentially aqueous solutions (> 80 wt-% H₂O) and essentially alcoholic solutions (> 80 wt-% alcohol) of water-ethanol and water-iso-propanol, one wave is also obtained. At moderate ethanol and iso-propanol concentrations, (20–80 wt-% alcohol) splitting into two waves takes place. The reduction mechanism of the nitro compounds and the nature of the waves are discussed.     

Polymer whiskers based on p-mercaptobenzoyl and p-oxybenzoyl blocks

Preparation of polymer whisker based on p-mercaptobenzoyl (S) and p-oxybenzoyl (O) blocks was examined by the polymerization of S-acetyl-4-mercaptobenzoic acid (AMBA) and the following addition of 4-acetoxybenzoic acid (ABA). Polymerization of AMBA was carried out in liquid paraffin at 300 °C for 3 h yielding poly(p-mercaptobenzoyl) (PMB) whisker, and then ABA was added stepwise at 3-hour intervals into the solution. Initial polymerization concentration was 1.98%. The length of the whisker increased with the addition and it became 26.7 μm from 20.1 μm after third ABA addition. The width increased very slightly. Both length and width increased with polymer yield but the length increased preferentially rather than the width. The number of whiskers was constant even after the addition of ABA and poly(p-oxybenzoyl) (POB) whisker was not newly formed by the addition. Microscopic FT-IR and selected area electron diffraction measurements revealed that polymer molecules were aligned along the long axis of the whiskers, and the center and tip parts were comprised of PMB crystal and POB crystal, respectively. The polymer whisker based on S and O blocks was successfully prepared as grafting POB crystal on PMB whisker with stepwise addition of ABA. Thermal property of the resulting whiskers was also discussed.     

Fabrication of tridoped p-ZnO thin film and homojunction by RF magnetron sputtering

Tridoping (Al–As–N) into ZnO has been proposed to realize low resistive and stable p-ZnO thin film for the fabrication of ZnO homojunction by RF magnetron sputtering. The tridoped films have been grown by sputtering the AlN mixed ZnO ceramic targets (0, 0.5, 1 and 2 mol%) on GaAs substrate at 450 °C. Here, Al and N from the target, and As from the GaAs substrate (back diffusion) takes part into tridoping. The grown films have been characterized by Hall measurement, X-ray diffraction, photoluminescence, time-of-flight secondary ion mass spectroscopy and X-ray photoelectron spectroscopy. It has been found that all the films showed p-conductivity except for 2 mol% AlN doped film. The obtained resistivity (8.6×10⁻² Ω cm) and hole concentration (4.7×10²⁰ cm⁻³) for the best tridoped film (1 mol% AlN) is much better than that of monodoped and codoped ZnO films. It has been predicted that [(As_Zn−2V_Zn)+N_O] acceptor complex is responsible for the p-conduction. The homojunction fabricated using the best tridoped ZnO film showed typical rectifying characteristics of a diode. The junction parameters have been determined for the fabricated homojunction by Norde's and Cheung's method.     

Polymorphism of syndiotactic poly(p-fluoro-styrene)

Syndiotactic poly(p-fluoro-styrene) (s-PPFS) has been prepared with a polymerization procedure which allows reaching high average molecular masses and satisfactory yields. The polymorphic behavior of the polymer has been mainly studied by X-ray diffraction, calorimetric and infrared analyses. The main crystalline phase of s-PPFS, obtained by melt processing or cold-crystallization, exhibits trans-planar chains, is orthorhombic (a = 9.5 Å, b = 28.7 Å, c = 5.1 Å) and melts at nearly 320 °C. The X-ray analysis shows a strict analogy of this orthorhombic phase with the β phase of s-PS, also as for the occurrence of two limit ordered (β″) and disordered (β′) modifications, which differ for the intensity of reflections characterized by h + k = 2n + 1. A metastable crystalline phase, also exhibiting trans-planar chains, has been observed for as-polymerized samples as well as for amorphous samples crystallized by sorption of toluene or 1,4-difluoro-benzene. Mainly on the basis of solvent sorption and desorption experiments, it is suggested that this metastable phase is a co-crystalline phase with the low-molecular-mass guest molecules.     

Modeling of bubble column slurry reactor for reductive alkylation of p-phenylenediamine

A bubble column slurry reactor (BCSR) model has been developed for the reductive alkylation of p-phenylenediamine (PPDA) with methyl ethyl ketone (MEK) to N,N^′-di-secondary-alkyl-p-phenylenediamine (Di-amine). This particular reaction system is commercially relevant and involves a combination of parallel and consecutive reactions comprising equilibrium non-catalytic (homogeneous) and catalytic (heterogeneous) steps. The proposed model is based on the ‘mixing cell approach’. In this work the mixing cell approach has been extended by including a liquid backflow stream from all but the bottommost mixing cell. The model incorporates the contributions of gas-liquid and liquid-solid mass transfer, heat effects, and complex multistep reaction kinetics. CFD model is used to estimate the extent of backflow among mixing cells and its dependence on operating parameters. The effect of gas and liquid velocities, catalyst loading, inlet PPDA concentration, and temperature on the conversion, selectivity, global rate of hydrogenation, and temperature rise is discussed. The comparison of the current approach with the traditional mixing cell model is discussed. The BCSR model presented here will be useful to provide guidelines for designing and improving overall performance of bubble column reactors.     

Reactivity and mechanism in the cationic polymerization of p-methoxystyrene initiated by cycloheptatrienyl hexachloroantimonate

The cationic polymerization of p-methoxystyrene initiated by cycloheptatrienyl hexachloroantimonate in dichloromethane solution has been studied in some detail. Reactions proved to be highly exothermic, and rates of monomer consumption were measured using an adiabatic calorimetric technique. Termination was deduced to be insignificant during kinetic lifetimes, and $\text{M}\text{?}{}_{\mathrm{n}}$ values in the range 10000–50000 showed chain breaking to occur by transfer mechanisms. Appropriate analysis of conversion/time curves allowed computation of enthalpies of polymerization and rate coefficients for propagation, k_p (obs), under various conditions. Data for k_p (obs) were found to vary with the initial concentrations of initiator and monomer employed, and these dependences are discussed in terms of current theories regarding ion pair/free ion equilibria in nonaqueous solvents. In particular values of 3.6 × 10³ M^?1s^?1 and 4.8 × 10³ M^?1s^?1 at 0° and +10°C respectively for the rate constant for propagation by free poly (p-methoxystyryl) cation have been deduced, and a tentative value of 450 M^?1s^?1 at 0°C has been estimated for the rate constant for propagation by the corresponding hexachloroantimonate ion pairs. These data and the related activation parameters are compared with independent results in the literature. Polymerizations carried out in the presence of excess common ion salt, dimethyl benzyl phenyl ammonium hexachloroantimonate, showed rate depressions far in excess of those predicted by a simple mass law effect, arising possibly as a result of a more dramatic ionic association than simple ion pairing.     

Acylation of p-xylene over zeolites

Zeolites differing in structure and acidity were tested in liquid phase acylation of p-xylene. Hexanoyl chloride, propionic anhydride and isobutyric anhydride were used as acylating agents. The highest conversions of acylating agent were achieved over large pore zeolites USY (66.3%) and Beta (58.2%). It was found that acylation of p-xylene proceeds only over large pore zeolites Beta and USY. Selectivities to monoacylated p-xylene obtained over USY zeolite decreased in the order: propionic anhydride 78.0% > hexanoyl chloride 67.2% > isobutyric anhydride 33.1%. Diacylated product was formed over zeolite USY with all acylating agents tested but only with hexanoyl chloride over zeolite Beta. It was found that the optimum Si/Al ratio of zeolite Beta for p-xylene acylation with propionic anhydride is 25, while for isobutyric anhydride is 19. Conversion of isobutyric anhydride decreased with increasing isobutyric anhydride concentration and increased with increasing amount of catalyst.     

Radical copolymerization of N-vinylcarbazole and p-bromostyrene: determination of monomer reactivity ratios by SEC-multidetection

N-Vinylcarbazole (A)/p-bromostyrene (B) copolymers were prepared by radical copolymerization. Size exclusion chromatography (SEC) equipped with a refractometer and UV-vis spectrophotometer was found to be a very convenient technique to follow copolymerizations and to determine monomer conversions, copolymer composition, average molecular weights, polydispersity indexes versus time. The monomer reactivity ratios r_A (N-vinylcarbazole) and r_B (p-bromostyrene) were determined by using the Finemann-Ross (FR), the inverted Finemann-Ross (IFR), the Kelen-Tüdos (KT), and the fitting curve graphical methods. The four methods were in good agreement and led to very different values for r_A (0.55) and r_B (12.3) which induces a preference for the incorporation of B in the copolymer structure. Eventually, with these results the influence of initial feed on the microstructure of the copolymer has been predicted.     

One-step synthesis of α-p-vinylphenylalkyl-ω-hydroxy poly(ethylene oxide) macromonomers by anionic polymerization initiated from p-vinylphenylalkanols

ω-(p-Vinylphenyl)alkanols, including methanol, ethanol, propanol, pentanol, and hexanol, have been partially alkoxidated with potassium naphthalene to initiate anionic polymerization of ethylene oxide (EO) in order to directly prepare the corresponding α-p-vinylphenylalkyl-ω-hydroxy poly(ethylene oxide) (PEO) macromonomers. p-Vinylphenylmethanol, i.e. p-vinylbenzyl alcohol (VBA) afforded the expected well-defined macromonomer via living polymerization mechanism and the kinetics have been examined as a function of extent of potassium-alkoxidation. Other alcohols such as p-vinylphenylpropanol (VPP), -pentanol (VPPT), and -hexanol (VPH) were also successful to afford the corresponding PEO macromonomers, while p-vinylphenylethanol (VPE) alkoxide polymerized EO to give p-divinylbenzene and poly(ethylene glycol) without p-vinylphenylethoxy end group, which were supposed to form by a very facile intramolecular chain transfer of the activated oligomeric alkoxide chain end to abstract a benzylic proton of the initiating fragment.     

Benzoxazines with tolyl, p-hydroxyphenyl or p-carboxyphenyl linkage and the structure-property relationship of resulting thermosets

Three benzoxazines (7-9) with a tolyl, p-hydroxyphenyl or p-carboxyphenyl structure, respectively, were successfully synthesized by a three-pot or two-pot procedure. In the three-pot approach, the first step is the condensation of 2-hydroxybenzaldehyde with p-toluidine, 4-aminophenol and 4-aminobenzoic acid, respectively, forming intermediates (1-3) with an imine linkage. The second step is the addition of 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) on the imine linkage, resulting in intermediates (4-6) with a secondary amine linkage. The last step is the ring closure condensation of (4-6), leading to benzoxazines (7-9). In the simplified two-pot approach, the first two steps-the condensation and imine reduction were carried out in one reactor, so the yield of (4-6) was increased. The structures of monomers (1-9) were characterized and confirmed by 1D and 2D NMR spectra. The synthesized benzoxazines were thermally self-cured or copolymerized with a commercial benzoxazine, bis(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-yl)methane (F-a). IR analysis was utilized to monitor the ring-opening reaction of (7-9) and to propose the structures of P(7-9). The microstructure and the structure-property relationship of the resulting homopolymers and copolymers are studied.     

Correlation between polymer architecture, mesoscale structure and photovoltaic performance in side-chain-modified poly(p-arylene-ethynylene)-alt-poly(p-arylene-vinylene): PCBM bulk-heterojunction solar cells

Recent investigations have shown that an anthracene containing poly(p-arylene-ethynylene)-alt-poly(p-arylene-vinylene) statistical copolymer consisting of a well defined conjugated backbone, along which linear and branched alkoxy side chains are attached in a random manner, yields, compared to its counterparts with regular side chain substitution, an improved performance in polymer [6,6]:-phenyl-C₆₁-butyric acid methyl ester (PCBM) bulk-heterojunction solar cells. The microscopic origin for the improved power conversion efficiency (η ≈ 3.8%) of the statistical copolymer - which is the best in its material class - has not been resolved. We conducted grazing incidence wide-angle x-ray scattering investigations in order to correlate the nanomorphology of the active layers to the photovoltaic performance of the device. A comparison of the results obtained for the statistical copolymer to those obtained for the corresponding regular copolymers shows that the improved performance of the former may be attributed to a combination of the following structural characteristics: 1. well, ordered stacked domains that promote backbone planarization, 2. partly face-on alignment of domains relative to the electrodes for an improved active layer-electrode charge transfer, and 3. a more isotropic domain orientation throughout the active layer that ensures that the backbone alignment direction has components perpendicular and parallel to the electrodes in order to compromise between light absorption and efficient intra-chain charge transport. The regular copolymers exhibiting inferior performance lack either sufficient stacking order or face-on alignment of the domains. None of them shows isotropic domain orientation.     

High activity of iron containing metal-organic-framework in acylation of p-xylene with benzoyl chloride

Catalytic behavior of metal-organic-frameworks (MOFs) Cu₃(BTC)₂, MIL-100(Fe) and MIL-100(Cr) was investigated in p-xylene acylation with benzoyl chloride and compared with the behavior of large pore zeolites Beta and USY. It has been clearly shown that MIL-100(Fe) exhibit much higher conversion of benzoyl chloride with excellent selectivity in p-xylene acylation. The performance of MIL-100(Fe) catalyst was favorable with those of other conventional heterogeneous catalysts like zeolites. Conversion of benzoyl chloride equal to 100% was achieved over MIL-100(Fe) with 20-30 min under optimized reaction conditions. It is assumed that unsaturated Lewis acid sites associated with the presence of Fe exhibit optimum acid strength for catalyzing this acylation reaction.     

The behaviour of a fixed bed porous flow-through electrode during the production of p-aminophenol

The behaviour of a fixed bed porous flow-through monel electrode has been investigated for the reduction of nitrobenzene to p-aminophenol in an aqueous acidic electrolyte and under essentially activation controlled conditions. Chemical yields have been determined experimentally as well as calculated from polarisation curves for a plane electrode. Potential and current distributions have been interpreted by means of a simple one-dimensional model.     

Mott—Schottky analyses on n- and p-GaAs/room temperature chloroaluminate molten-salt interfaces

Mott—Schottky analyses were performed on n- and p-GaAs electrodes in room temperature molten-salt electrolytes comprising mixtures of AlCl₃ and n-butyl pyridinium chloride (BPC) in approx. the 2:1molar ratio. These analyses revealed that the two interfaces, n-GaAs/AlCl₃BPC and p-GaAs/ AlCl₃-BPC, conformed with the behavior expected from the simple depletion layer model after suitable etching of the electrode surfaces. The flat-band potentials (V_fb) thus determined for interfaces were: ?0.20 ± 0.03 V (vs Al^{$\text{0}\text{3}$+}) for n-GaAs and +1.22 ± 0.01 V (vs Al^{$\text{0}\text{3}$}+) for p-GaAs, respectively. The doping densities obtained from the slope of the Mott—Schottky plots, however, were 2–3 times higher than those predicted from the Hall data supplied by the manufacturer. Possible reasons for these discrepancies are discussed. The difference between the V_fb values for the n- and p-type GaAs correspond to the energy band-gap of the semiconductor—a behavior consistent with the absence of significant band-edge unpinning effects in the two cases. Alternatively, the data were seen to be consistent with a behavior such that the band-edges are fixed at the surface for n- and p-GaAs electrodes and all or most of the applied voltage drops across the depletion layer in the semiconductor. The present data are compared with results from cyclic voltammetry obtained in a previous study. In this study, photocurrents were sustained on n-GaAs at potentials significantly positive of the valence band-edge in the presence of certain electroactive species in the AlCl₃BPC electrolyte. This behavior may be understood in terms of accumulation at the interface such that the semiconductor band-edges no longer remain fixed, unlike in the situation above in the absence of redox species. The mechanisms underlying charge accumulation could involve the presence of either a high density of surface states or an inverted p-type surface region caused by hole injection from highly oxidizing redox species as discussed by previous authors.     

Electron emission from CVD diamond p-i-n junctions with negative electron affinity during room temperature operation

We successfully observed electron emission from hydrogenated diamond p-i-n junction diodes with negative electron affinity during room temperature operation. The emissions started when the applied bias voltage produced flat-band conditions, where the capacitance-voltage characteristics showed carrier injection in the i-layer. In this low current injection region, the electron emission efficiency (η) of the p-i-n junction diodes (p is top layer) was about 5 × 10^− 5, while that of the n-i-p diodes (n is top layer) was about 10^− 8. With increasing diode current, both diodes showed an increase in η and a nonlinear increase in emission current. In the high current injection region with high diode current of 5-50 mA, both diodes had an emission current of almost 10 μA, where η of a p-i-n junction diode was 0.18%, while that of a n-i-p junction diode was 0.02%.Note that η, which corresponds to the electron emission mechanism, depended on the diode current level.