Formation Of Methylene Bridges Research Articles

Mechanism of BPh3 -Catalyzed N-Methylation of Amines with CO2 and Phenylsilane: Cooperative Activation of Hydrosilane.

BPh3 catalyzes the N-methylation of secondary amines and the C-methylenation (methylene-bridge formation between aromatic rings) of N,N-dimethylanilines or 1-methylindoles in the presence of CO2 and PhSiH3 ; these reactions proceed at 30-40 °C under solvent-free conditions. In contrast, B(C6 F5 )3 shows little or no activity. 11 B NMR spectra suggested the generation of [HBPh3 ]- . The detailed mechanism of the BPh3 -catalyzed N-methylation of N-methylaniline (1) with CO2 and PhSiH3 was studied by using DFT calculations. BPh3 promotes the conversion of two substrates (N-methylaniline and CO2 ) into a zwitterionic carbamate to give three-component species [Ph(Me)(H)N+ CO2 - ⋅⋅⋅BPh3 ]. The carbamate and BPh3 act as the nucleophile and Lewis acid, respectively, for the activation of PhSiH3 to generate [HBPh3 ]- , which is used to produce key CO2 -derived species, such as silyl formate and bis(silyl)acetal, essential for the N-methylation of 1. DFT calculations also suggested other mechanisms involving water for the generation of [HBPh3 ]- species.

TiO2 Nanoparticle-Loaded Poly(NIPA-co-NMA) Fiber Web for the Adsorption and Photocatalytic Degradation of 4-Isopropylphenol.

A TiO2 nanoparticle-loaded polymer fiber web was developed as a functional material with the ability to adsorb and photo-catalytically degrade organic pollutants in aquatic media. A linear copolymer of N-isopropylacrylamide (primary component) and N-methylol acrylamide (poly(NIPA-co-NMA)) was prepared, and composite fibers were fabricated by electrospinning a methanol suspension containing the copolymer and commercially available TiO2 nanoparticles. The crosslinking of the polymer via the formation of methylene bridges between NMA units was accomplished by heating, and the fiber morphology was analyzed by electron microscopy. 4-Isopropylphenol generated by the degradation of bisphenol A—one of the endocrine-disrupting chemicals—was used as the model organic pollutant. As poly(NIPA) is a thermosensitive polymer that undergoes hydrophilic/hydrophobic transition in water, the temperature-dependence of the adsorption and photocatalytic degradation of 4-isopropylphenol was investigated. The degradation rate was analyzed using a pseudo-first-order kinetic model to obtain the apparent reaction rate constant, kapp. The enhancement of the photocatalytic degradation rate owing to the adsorption of 4-isopropylphenol onto thermosensitive poly(NIPA)-based fibers is discussed in terms of the ratio of the kapp of the composite fiber to that of unsupported TiO2 nanoparticles. Based on the results, an eco-friendly wastewater treatment process involving periodically alternated adsorption and photocatalytic degradation is proposed.

Effect of Impregnated Phenolic Resins on the Cellulose Membrane for Polymeric Insulator

In this study, a cellulose membrane (CM) was chemically treated with phenolic (PF) resin to improve its performance as a polymeric insulator. The CM was prepared from kenaf pulp, and the PF was synthesized from oil palm empty fruit (EFB) fibre. Four different concentrations of synthesized PF resin (5, 10, 15, and 20 wt.%) were impregnated under wet or dry conditions. Thermal analysis of the phenolic cellulose membrane (PCM) showed that the samples had good chemical interaction and compatibility. The PF uptake in the wet phenolic cellulose membrane (PCMW) was higher than in the dry phenolic cellulose membrane (PCMD). During the PF uptake, the CM underwent solvent exchange and absorption in wet and dry membranes, respectively. This difference also affected the crosslinking of PCM samples via the formation of methylene bridges. Due to the PF treatment, the PCM showed lower water absorption than CM. The PF concentrations also affect the surface roughness and electrical properties of PCM samples. These findings prove that PCM can be used as a renewable and green polymer electrical insulator.

Obtaining a copolymer of polyhexamethylene guanidine hydrochloride and polyvinyl alcohol

The possibility of obtaining a polymer gel based on polyhexamethylene guanidine hydrochloride and polyvinyl alcohol under conditions of acid catalysis is considered. It was shown that the use of formaldehyde as a crosslinking agent leads to the formation of methylene bridges between the hydroxyl groups of polyvinyl alcohol and the amino groups of polyhexamethylene guanidine hydrochloride. The study of samples obtained by IR spectroscopy revealed the presence of corresponding signals of functional groups at 2780 cm-1 (isolated methylene group between nitrogen and oxygen atoms) and 1580 cm-1 (characteristic band of the guanidine group).

Approaching Highly Leaching-Resistant Fire-Retardant Wood by In Situ Polymerization with Melamine Formaldehyde Resin.

The objective of the work was to improve the leaching resistance of fire-retardant (FR) modified wood by the incorporation of a thermoset resin. Here, Scots pine (Pinus sylvestris L.) sapwood was impregnated with melamine formaldehyde (MF) resin and hydrophilic FRs guanyl-urea phosphate/boric acid by a vacuum-pressure treatment. Resistance to leaching of FR-modified wood was evaluated, after conducting an accelerated aging test according to European standard EN 84. Inductively coupled plasma analysis showed that the incorporation of MF resin significantly reduced the leachability of FRs. Scanning electron microscopy/energy-dispersive X-ray spectrometry revealed that the mechanism of water resistance was by doping the FRs into MF resin microspheres. Fourier transform infrared spectra showed the chemical functionality changes of FR-modified wood such as the formation of methylene bridges by drying the modified wood specimens. An increase in the thermal stability of FR-modified wood was confirmed by thermal gravimetric analysis. Excellent fire performance of FR-modified wood after leaching was affirmed by the limiting oxygen index and cone calorimeter tests.

Antibody Identification for Antigen Detection in Formalin-Fixed Paraffin-Embedded Tissue Using Phage Display and Naïve Libraries.

Immunohistochemistry is a widely used technique for research and diagnostic purposes that relies on the recognition by antibodies of antigens expressed in tissues. However, tissue processing and particularly formalin fixation affect the conformation of these antigens through the formation of methylene bridges. Although antigen retrieval techniques can partially restore antigen immunoreactivity, it is difficult to identify antibodies that can recognize their target especially in formalin-fixed paraffin-embedded tissues. Most of the antibodies currently used in immunohistochemistry have been obtained by animal immunization; however, in vitro display techniques represent alternative strategies that have not been fully explored yet. This review provides an overview of phage display-based antibody selections using naïve antibody libraries on various supports (fixed cells, dissociated tissues, tissue fragments, and tissue sections) that have led to the identification of antibodies suitable for immunohistochemistry.

Adhesive properties of urea-formaldehyde resins blended with soy protein concentrate

Commercial urea-formaldehyde (UF) resin was blended with a renewable and environmentally friendly soy protein concentrate (SP) adhesive in order to reduce the emission of volatile organic compounds. The UF resin was replaced by SP in different proportions (10 wt%, 20 wt%, and 30 wt%). Rheology showed pseudoplastic behavior of UF/SP slurries, even at low SP content, caused by the entanglements between polymer chains. Nevertheless, viscosity of UF/SP slurries dropped to 100 mPa·s at high shear rate, reaching the viscosity value of the UF resin alone. Fourier transform infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy confirmed a chemical reaction between the two components. Shear strength in wet conditions of UF/SP blends was 29% higher than SP alone. Water extraction showed an increase of 55% in the insoluble fraction caused by the formation of methylene bridges between UF and SP. Finally, the incorporation of soy protein slurries to a urea-formaldehyde resin led to the development of adhesives with high solid content and appropriate viscosity to be used by the spray method, and environmentally friendlier than the resins currently used by the wood industry.

Anti-diabetic effects of trans-resveratrol byproducts induced by plasma treatment

Cold plasma processing has emerged a promising green technology with great potential to improve the quality and microbial safety of various minimally processed foods and materials. However, studies on non-thermal plasma-induced chemical interactions between major food ingredients that might change chemical structure and biological properties are very sparse. Thus, the objective of this study was to evaluate the influence of a dielectric barrier discharge (DBD) treatment on principal trans-resveratrol (TR) in several food stuffs by spectroscopic (HPLC, NMR, MS) and biological analyses. TR was dissolved in methanol and directly exposed to atmospheric non-thermal plasma field at 250 W for different durations (10, 20, 40, and 60 min), 40% relative humidity, and 25 °C. TR treated with plasma for 40 min showed greatly enhanced inhibitory activities for α-glucosidase and α-amylase than parent TR. Newly generated unusual compounds (1, 2) and known compounds (3–6) from plasma treated TR for 40 min were characterized using chromatographic and spectroscopic methods. The predominant reaction of TR induced by cold plasma followed by typical dimerization of products included methylene bridge formation and cyclization of TR. Among predominantly generated products, new compounds 1 and 2 showed more potent α-glucosidase and α-amylase inhibition capacities than parent TR. These results might be used to modify structures and enhance biological property of TR during food processing using DBD plasma treatment.

Effects of Molar Ratio and pH on the Condensed Structures of Melamine-Formaldehyde Polymers.

The base-catalyzed melamine-formaldehyde (MF) reactions were studied in both diluted and concentrated solutions. The influences of F/M molar ratio and pH on the polymer structures were investigated based on the quantitative 13C-NMR analysis. The results show that both F/M molar ratio and pH influence the competitive formation of ether and methylene bridges. For the cases of F/M = 2.0, and 3.0, methylene bridge formation is minor in contrast to ether bridges either at pH = 9.3–9.8 or at 7.3–7.8. When the molar ratio was lowered to 1.0, methylene bridges became competitive with ether bridges at pH = 9.3–9.8, but the latter is still more favorable. When the lower molar ratio overlaps with the lower pH, significant changes were found. The content of methlylene bridges was over three times that of ether bridges with M/F = 1.0 and at pH = 7.3–7.8. The results in this study were compared with those previously obtained for base-catalyzed urea-formaldehyde reactions. It was found that molar ratio and pH influence the structures of the MF and UF polymers in similar ways. The different synthesis conditions of UF and MF resin were also addressed by comparing the structures of UF polymers with MF polymers.

Competitive formation of the methylene and methylene ether bridges in the urea–formaldehyde reaction in alkaline solution: a combined experimental and theoretical study

The competitive formation of the methylene and methylene ether bridges in the urea–formaldehyde reaction in alkaline solution was investigated by using 13C NMR and quantum chemistry calculations. Despite that the classical theory states that the condensations of methylolureas only form methylene ether bridges at alkaline pH, the competing formation of methylene bridges was observed in this study. The NMR spectra show that the methylene ether bridges are exclusively formed under conditions of 80 and 90 °C with F/U = 2/1. At 80 °C with F/U = 1/1, the linear methylene bridge (–NH–CH2–NH–) began to compete with ether bridges, but it was minor. At 90 °C with F/U = 1/1, the methylene bridges were found to be much more competitive. The theoretically calculated energy barriers for the formation of different types of methylene ether bridges are lower than those for methylene bridges by 12–26 kJ/mol. The steric hindrance was proposed to be another important effect that suppresses the condensations between di- and tri-methylolureas to form the branched methylene bridges [–(HOCH2)N–CH2–NH– or –(HOCH2)N–CH2–N(CH2OH)–]. However, the reaction between a free amino group –NH2 and a methylol group –CH2OH was proposed to encounter no steric hindrance. Hence, once the higher temperature and lower F/U ratio were guaranteed, the competitive formation of linear methylene bridges was observed.

Cure properties and adhesive performances of cure-accelerated phenol-urea-formaldehyde resins

The cure properties of cure-accelerated phenol-urea-formaldehyde (PUF) resins with different catalysts [calcium oxide (CaO), sodium carbonate (Na2CO3), zinc oxide (ZnO), and magnesium oxide (MgO)] were investigated by gelation test and differential scanning calorimetry (DSC) analysis. The results indicated that catalysts such as Na2CO3, ZnO, and MgO were capable of increasing the curing rate and decreasing the curing temperature of PUF resins, however, the CaO inhibited the cure reaction. The formation of methylene bridges was considered to be the main reaction during curing. For the ZnO- and MgO-accelerated PUF resins, the addition reaction of formaldehyde with free phenolic site may act as subsidiary reaction. The activation energies (E a ) of cure-accelerated PUF resins other than CaO-acceleration were much lower than the control resin. The effects of catalysts and hot press temperature on adhesive performances of PUF resins were also discussed by plywood test. The PUF resins with Na2CO3, ZnO, and MgO had higher wet shear strength than the control resin. Hot press temperature had a strong influence on the wet shear strength as well as the catalysts. Among the catalysts, MgO had more significant improving effect on both the curing process and the wet shear strength of PUF resin.

Evidence of char formation during wood heat treatment by mild pyrolysis

The behaviour of wood polymers during heat treatment carried out under inert atmosphere at 240 °C has been reinvestigated to understand the important decrease of the O/C ratio observed in a previous study using X-ray photoelectron spectroscopy (XPS). Heat treatment was performed not only on beech sawdust but also on its lignin and holocellulose fractions obtained after acidic hydrolysis of polysaccharides or delignification with sodium chlorite. CP/MAS 13C NMR spectra indicate as previously reported an important degradation of hemicelluloses after thermal treatment. However, assignments of the signals appearing in the range of 125–135 ppm and 35 ppm attributed up to now to thermal crosslinking of lignin and formation of methylene bridges should be reconsidered. Indeed, heat treatment of the holocellulose fraction indicates quite similar signals showing that these latter are not due to lignin modification. According to the literature, these new signals have been attributed to the beginning of char formation. Determination of Klason lignin and HPLC analysis of the sugars contained in the hydrolysate support the hypothesis of formation of carbonaceous materials within the wood structure during heat treatment by mild pyrolysis.

Polymerization of resole resins with several formaldehyde/phenol molar ratios: Amine catalysts against sodium hydroxide catalysts

AbstractTriethylamine and sodium hydroxide catalyzed phenol/formaldehyde resole resins were investigated in terms of their behavior during both addition and polymerization reactions. Amine‐catalyzed prepolymers were mainly ortho‐substituted structures, whereas the sodium hydroxide catalyst directed the addition reaction to para reactive sites. During polymerization, triethylamine led to dimethylene ether bridges as the principal linkages between aromatic structures, increasing their final concentration as the starting hydroxymethyl group concentration increased. In contrast, the use of sodium hydroxide reduced dramatically the dimethylene ether bridge concentration, favoring methylene bridge formation. The influence of hydroxyl ions on the stability of quinone methide intermediates could be the reason for those differences. Despite the formation of dimethylene ether bridges, at higher curing temperatures, more oxidized groups started to appear in cured resoles when the formaldehyde/phenol molar ratio was higher. The presence of infrared bands associated with quinones, aldehydes, and/or carbonyl groups, mainly in high‐formaldehyde‐content resins, could indicate a direct oxidation process from dimethylene ether bridges and/or residual hydroxymethyl groups, without the formation of methylene bridges. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2623–2631, 2006

Blends of cellulose ester/phenolic polymers – chemical and thermal properties of blends with polyvinyl phenol

A series of miscible cellulose ester/poly (vinyl phenol) (CE/PVP) blends containing a latent formaldehyde source were prepared. Due to the low molecular weight of the PVP, the maximum PVP content ID the films was 50 wt %. The blends were then thermally cross-linked ID an attempt to create semi-interpenetrating polymer networks (semi-IPN). The blends were characterized with differential scanning calorimetry, swelling experiments, pyrolysis molecular beam mass spectrometry (py-MBMS), and Fourier transform infrared (FTIR) spectroscopy. The results from the swelling experiments, py-MBMS and FTIR showed that the PVP component did react with the formation of methylene bridges. Blends that contained 50% PVP and high levels of formaldehyde formed semi-IPN structures.

Infrared and Raman Studies on Films of Organosiloxane Networks Produced by PECVD

The effect of the incorporation of oxygen and nitrogen on the structure of films obtained by PECVD of hexamethyldisiloxane (HMDSO)-He-N 2 and HMDSO-He-O 2 mixtures is investigated using infrared and Raman spectroscopies. From transmittance spectra of films deposited onto single-crystal KBr disks, the transverse optical (TO) and longitudinal optical (LO) functions in the mid-infrared region were calculated. To correlate structural aspects with the observed LO-TO splittings, an identification analysis of functional group based on the infrared and Raman literature was made. It was concluded that the structure of the films deposited from HMDSO-He-O 2 discharges was strongly dependent on the proportion of oxygen in the gas feed. In the absence of oxygen, i.e., for a discharge of a HMDSO-He mixture, the resulting film consisted of a network of interconnected siloxane and carbosilane units. Addition of O 2 precluded the formation of methylene bridges and induced the formation of a material enriched with Si-O-Si groups. Films formed from the HMDSO-He-N 2 plasmas, on the other hand, consisted mainly of interconnected siloxane and carbosilane units in addition to a small quantity of silazane units. On the basis of these results, we propose an interpretation for the variation of the LO-TO splitting amplitude for the asymmetrical stretching mode (AS1) of Si-O in Si-O-Si groups as a function of the oxygen or nitrogen incorporation into the films.

Polymerization of Formaldehyde and Phenol at Different Pressures

Phenol-formaldehyde resole resins were investigated as regards their cure behaviour using both atmospheric and high-pressure conditions. Results obtained using Fourier transform infrared spectroscopy and solid-state 13C (cross-polarization/magic angle spinning) nuclear magnetic resonance showed different polymerization pathways of the resin into the usually used differential scanning calorimetry pressure resistant crucibles and under atmospheric pressure. Under high pressure conditions a competition between oxidation and polymerization reactions was detected, leading to less methylene bridge formation and more remaining free ortho -positions, with a consequent lower degree of polymerization.

A new approach to the determination of the heat of phenolic resol synthesis

A new method has been developed for the determination of the heat of phenolic resol synthesis carried out in an aqueous solution. This heat is determined as the difference in the heats of syntheses of the resitol from the mixture of raw materials and from the resol studied. Heats of resitol syntheses were determined by DSC at constant heating rate using high-pressure crucibles, i.e. under isochoric conditions for the liquid–vapour sample. The heat of synthesis of resols, obtained at formaldehyde-to-phenol molar ratio equal to 3.0 and using NaOH as catalyst, were determined. The relevant values were calculated on the basis of the chemical composition of the resols and the molar enthalpies of the aromatic ring substitution and methylene bridge formation reactions. The experimentally determined heats were consistent with the calculated ones, not when free formaldehyde (CH 2O) but its monohydrate, CH 2O·H 2O (methylene glycol, HO–CH 2–OH) was assumed to be the reactive form of formaldehyde in an aqueous medium (formalin).

Characterisation of thermally modified wood: molecular reasons for wood performance improvement

Relatively mild thermal treatments of wood according to a two step process which leads to improved dimensional stability and improved timber performance were investigated by solid phase CP-MAS 13C-NMR to understand at molecular level the reasons for the improvements reported. All the occurrences described appear to be the consequence of reactions which are known in wood chemistry. These are the formation of acetic acid liberated from the hemicelluloses, which further catalyses carbohydrates cleavage, causing a reduction of degree of polymerisation of the carbohydrates. Acid catalysed degradation results in the formation of formaldehyde, furfural and other aldehydes as well as some lignin cleavage at Cα and O4 and believed to cause some aldehyde production from lignin units Cγ, all occurring in the first reaction step. Lignin autocondensation through the cleaved, positively charged benzylic Cα to form some methylene bridges presumably starts already to occur in this first phase. The increase in the number of free reactive sites on the aromatic ring of some lignin units already occurs in this phase but continues into the next. In the second treatment step completion of the autocondensation of lignin is believed to occur through the formation of methylene bridges connecting aromatic rings. The aromatic nuclei sites are released by demethoxylation and through the cleaved, positively charged benzylic Cα. Reactions of some of the aldehyde groups formed in the first step phase occur with lignin aromatic nuclei sites to connect aromatic rings through methylene bridges. The extent of these reactions is mild, but nonetheless they lead to an increase in cross-linking with consequent improvement in dimensional stability and decreased hygroscopicity of wood.

The Chemistry of Phenol-Formaldehyde Resin Vulcanization of EPDM: Part I. Evidence for Methylene Crosslinks

Abstract The application of phenol-formaldehyde resins as crosslinking agents is increasing in importance due to the good high temperature properties of the corresponding vulcanizate and the use in thermoplastic vulcanizates. With respect to the chemistry of phenol-formaldehyde cure (reaction mechanism and structure of crosslink) there are still problems that have to be resolved. The reaction products of the phenol-formaldehyde resin curing of EPDM, contain 2-ethylidene norbornene (ENB) as the third monomer, have been studied. Since such an investigation is rather difficult to perform for the polymer system, a low molecular weight model for EPDM was used: 2-ethylidene norbornane (ENBH). Reaction of ENBH and a resole results in scission of the dimethylene ether bridges, i.e. in degradation of the resole into mono-, bis- and terisooctylphenol units. These are consequently converted into products, consisting of two ENBH molecules linked by mono-, bis- and terisooctylphenol units. The solid resole seems to be a technological solution for storing phenol in combination with formaldehyde. These results support the use of 2-hydroxymethylphenol (HMP) as a low molecular weight model for the resole. At low temperatures and/or short reaction times HMP oligomers (= resoles) and HMP oligomers linked to one ENBH molecule are formed, which are converted into ENBH/HMP (1:1) condensation products. The reaction products of ENBH with both the resole and HMP are shown to contain methylene linked structures, as demonstrated by the formation of monisooctylphenol crosslinks and the presence of residual unsaturation and hydroxyl groups, besides chroman linked structures. This is the first experimental evidence that during phenol-formaldehyde resin cure of rubber, formation of methylene bridges occurs.

Studies of coals and binder systems for smokeless fuel briquettes

Three coals are compared as potential briquetting feedstocks. A variety of chemical, physical and spectroscopic techniques have been used to elucidate their very different performance in a molasses-bound briquetting process. The results demonstrate that the elemental composition of the surface may differ markedly from the bulk chemistry of the coals. While further differences in petrology, mineralogy, and physical properties were evident, the immediate cause of poor briquetting performance was a high mesoporosity resulting in a greater propensity for particle breakage under shear stress. Fractures appear to take place at boundaries with mineral inclusions. 13C NMR spectroscopy was used to optimise the curing regimes (temperature/time) for molasses and modified starch binders. The spectroscopic data could be interpreted in terms of aliphatic, methoxy and aromatic carbon types. Crosslinking, due to the formation of methylene bridges, reached a maximum at ≈ 250°C. It resulted in an increase in the proportion of aliphatic carbon, and a corresponding reduction in methoxy carbon, as low molecular weight oxygenates were lost in condensation reactions. At higher temperatures, charring was accompanied by increasing aromatisation of the ring systems.