Catalytic aminolysis of acrylic copolymers in solution and in the melt. II. Comparison between styrenic and ethylenic copolymers

The aminolysis of poly(styrene-co-methyl acrylate) (SMA) by octadecylamine in solution and in the melt has been reported in Part I. We now have studied the aminolysis of poly(ethylene-co-methyl acrylate) (EMA) with the same amine in the melt and compared the two sets of data in this paper. With EMA, the data confirmed and precised the catalytic mechanism proposed in Part I. The best tautomeric catalysts are the ones which form an eight-atom ring structure with the ester and amine groups. With EMA aminolysis is faster than with SMA because of the steric hindrance of phenyl groups in SMA. But EMA aminolysis remains a rather slow reaction. In a corotating twin-screw extruder the conversion was only around 4% at 220°C with a mean residence time of 150 s. It was also shown that the EMA/octadecylamine/catalyst system, like the SMA system, is homogeneous in the molten state at temperatures around 200°C.     

偶氮染料吸附和光催化氧化动力学

以甲基橙和酸性大红两种偶氮染料为模拟污染有机物,对它们的暗吸附和光催化氧化行为进行研究.实验结果表明,两种偶氮染料的吸附受溶液酸碱度影响很大,酸性（pH=3）条件下,两种染料吸附量都很大,酸性大红吸附量更大；近中性（pH≈6）时两种染料的吸附显著减少；碱性（pH=9）条件下两种染料不发生吸附.光催化反应结果显示,碱性条件或酸性条件下两种染料降解速度都很快.说明在不同酸碱度条件下,光催化反应按不同机理进行.酸性条件下,反应在催化剂表面进行,在碱性介质中,光催化氧化在溶液中进行.提出了一个碱性条件下的动力学方程,经过进一步简化,可以得到表观一级方程,形式上和准一级L-H方程十分相似,但其含义不同.     

Multistate properties of 7-(N,N-diethylamino)-4'-hydroxyflavylium. An example of an unidirectional reaction cycle driven by pH

The synthetic flavylium salt 7-(N,N-diethylamino)-4'-hydroxyflavylium tetrafluoroborate gives rise in aqueous solution to a complex network of chemical reactions driven by pH. The system was studied by 1H NMR, single crystal X-ray diffraction, steady state and transient UV-Vis spectrophotometry as well as stopped flow. The crystal structure shows a high degree of coplanarity between the pyrylium system and the phenyl group in position 2. Thermodynamic and kinetic constants for the pH dependent network of chemical reactions were obtained. The introduction of an amino group in position 7 allows formation of protonated species leading, in particular, to a tautomeric form of the protonated cis-chalcone, H+, whose absorption spectra is rather red shifted, in comparison with the correspondent protonated trans-chalcone, H+. The H+ species can be rapidly converted into the flavylium cation through a first order process with lifetime of 0.2 s at pH = 2.35. This new reaction channel confers this compound a peculiar behaviour in acidic media, allowing to define an unidirectional pH driven reaction cycle.     

Exchange and free radical grafting reactions in reactive extrusion

The potential of two types of reactions for reactive extrusion was evaluated in our laboratory: exchange reactions of copolyesters and free radical grafting of monomers onto polyolefins. Specifically, the alcoholysis of ethylene and vinyl acetate copolymers, the transesterification of ethylene and alkyl acrylate copolymers (EAA) and the aminolysis of alkyl acrylate copolymers were investigated as the exchange reactions while the grafting of maleic anhydride (MA) onto polypropylene (PP) as the free radical reactions. The exchange reactions are very slow without catalysts. Tin derivatives, such as dibutyl tin dilaurate (DBTDL) and dibutyl tin oxide (DBTO), are good catalysts for the alcoholysis and the transesterification, but not for the aminolysis. For the latter, tautomeric compounds, such as 2-pyridone and 2-mercaptopyridone, are efficient. Comparative kinetic studies showed that, for all the three exchange reactions, the reacting medium (solution or the homogeneous melt) does not affect their reaction mechanisms nor their intrinsic rate constants. Additionally, mechanical mixing does not contribute to the overall reactions. As for the free radical grafting of MA, the presence of electron-donating monomers and styrene (ST) in particular was shown to be very effective in improving the grafting yield while minimising the degradation of PP. This was proven to be due to the formation of a charge transfer complex between MA and such a monomer.     

Tautomerism of 4-phenyl-2,4-dioxobutanoic acid. Insights from pH ramping NMR study and quantum chemical calculations

Aryldiketo acids (ADKs) exhibit the variety of biological activities, mainly due to large affinity toward divalent metal ions. Metal complexation ability of ADKs, as well as interactions with proteins, depend on tautomeric form present in solution. The main aim of this study was to fully explore the tautomeric preferences of 4-phenyl-2,4-dioxobutanoic acid (4PDA), as ADKs representative, in aqueous media at different pH values. 1D and 2D NMR spectroscopy in combination with quantum chemical calculations was applied in order to better understand the tautomeric preferences of 4PDA. The data in highly acidic media are especially interesting since there are no such findings in the literature due to low solubility of ADKs in molecular form. At low pH values, where 4PDA is unionized, the most abundant tautomeric form is enol with keto group closer to phenyl ring. At higher pH values, mixture of two 4PDA ionic forms coexists in solution. Their ratio calculated according to NMR data fits the values predicted using two experimentally determined pK _a values. Based on the complexity of ¹H NMR spectrum of monoanionic 4PDA form, coexistence of two stable rotamers was assumed. In an alkaline media, 4PDA is mostly present in dianionic form. As π-electrons of dianion are delocalized over an entire keto-enol moiety, spectral distinction between tautomers was not possible. Quantum chemical calculations were used to predict relative stability of tautomers. The predictions were in good accordance with experimental results only in case when explicit water molecule was included in calculations.     

The classic "brown-ring" reaction in a new medium: kinetics, mechanism, and spectroscopy of the reversible binding of nitric oxide to iron(II) in an ionic liquid

To elucidate the applicability and properties of ionic liquids (ILs) to serve as chemical reaction media for the activation of small molecules by transition-metal complexes, detailed kinetic and mechanistic studies were performed on the reversible binding of NO to FeCl(2) dissolved in the IL 1-ethyl-3-methylimidazolium dicyanamide ([emim][dca]) as a solvent. We report, for the first time, the application of laser flash photolysis at ambient and high pressure to study the kinetics of this reaction in an IL. The kinetic data and activation parameters for the "on" and "off" reactions suggest that both processes follow a limiting dissociative (D) ligand substitution mechanism, in contrast to that reported for the same reaction in aqueous solution, where this well-known "brown-ring" reaction follows an interchange dissociative (I(d)) ligand substitution mechanism. The observed difference apparently arises from the participation of the IL anion as a N-donor ligand, as evidenced by the formation of polymeric [Fe(dca)(3)Cl](x)[emim](2x) chains in the solid state and verified by X-ray crystallography. In addition, infrared (IR), Mo?ssbauer, and EPR spectra were recorded for the monomeric reaction product [Fe(dca)(5)NO](3-) formed in the IL, and the parameters closely resemble those of the {FeNO}(7) unit in other well-characterized nitrosyl complexes. It is concluded that its electronic structure is best described by the presence of a high-spin Fe(III) (S = 5/2) center antiferromagnetically coupled to NO(-) (S = 1), yielding the observed spin quartet ground state (S(t) = 3/2).     

Reactions of trifluoro- and hexafluoroacetylacetones with thiobenzoylhydrazine

The reaction of thiobenzoylhydrazine with trifluoroacetylacetone leads to the product of condensation at the CH₃C=O bond with the 5-hydroxy-2-pyrazoline structure. The tautomeric transition to the 2,3-dihydro-1,3,4-thiadiazole isomer occurs in solution. This isomer undergoes the cleavage to 2-methyl-5-phenyl-1,3,4-thiadiazole and trifluoroacetone. The reaction of thiobenzoylhydrazine with hexafluoroacetylacetone affords one of diastereomers of [3,5-dihydroxy-3,5-bis(trifluoromethyl)pyrazolidin-1-yl](phenyl)thioketone, which at equilibrium is transformed in solution into the second diastereomer of the 3,5-dihydroxypyrazolidine form followed by the elimination of water elements and formation of a tautomeric 5-hydroxy-2-pyrazoline-2,3-dihydro-1,3,4-thiadiazole mixture.     

Thermal decomposition of 1-ethyl-5-iodotetrazole

The thermal decomposition of 1-ethyl-5-iodotetrazole in a melt and in solutions was studied using thermogravimetry, manometry, pyrolytic mass spectrometry, and IR spectroscopy. The kinetic and activation parameters of the process and the nature of the decomposition products were determined. The reaction mechanism is assumed to involve equilibrium tautomeric rearrangement of the tetrazole to azidoazomethine form followed by homolytic cleavage of the C-I bond.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1923–1926, November, 1994.The authors are grateful to O. A. Ivashkevich and P. N. Gaponic (Belarussian State University), who provided the samples of the tetrazole.     

Tautomeric forms of Oxy- and Oxo-derivatives of 1,3,5-Triazine. Part VI. Isomerization mechanism and thermal stability of the methyl esters of cyanuric acid

Previous investigations of the kinetics and the mechanism of the thermal isomerization C₃N₃(OMe)₃ → C₃(NMe)₃O₃ have been continued with a study of the isomerization mechanism of the mixed O,O,N- and O,N,N-trimethyl esters of cyanuric acid. A full discussion of the overall process is given, in which it is assumed that the above mixed esters are intermediates. It is shown that the tautomeric transformation occurs as the consequence of the attack of Me⁺ cations on the ring nitrogen of the neutral molecules. The presence of such cations in the melt was established by ionic exchange with the H⁺ of the molten acids. It is also shown that the tautomeric transformation in solution proceeds with the same mechanism. An explanation is proposed for the thermal instability of the mixed esters (which isomerize even in the solid state, more than 20° below their melting point), which is based on the high polarity of these molecules. This property is discussed in terms of electronic structure calculations carried out with the Pariser-Parr-Pople approximation of the A.S.M.O., S.C.F., method.     

Synthesis and characterization of poly(butadiene-b-sulphone) by block-copolycondensation—II. Synthesis from α,ω-dicarboxylic oligobutadienes and α,ω-diepoxy oligosulphones

The synthesis of poly(butadiene-b-sulphone) by two methods was described in part I (Eur. Polym. J. 20, 305 [1]). The results obtained by reaction of α,ω-dicarboxylic oligobutadiene with α,ω-diepoxy oligosulphones are reported here. The kinetics of the reaction are studied on models: reaction orders, rate constants, activation parameters are determined. Their values fit Madec's kinetic pattern [2] and are close to those obtained by Crépeau [6] for the grafting of hydroxybenzoic acid onto a copolymer bearing epoxy side-groups. Contribution of the side-reaction [2] is quantified. Block-copolycondensation of the reactive oligomers are carried out both in solution and in the melt. Contribution of the side-reaction is higher in the melt than in solution, but in both cases block-copolymers are obtained and characterized by GPC.     

Chemical reactions between immiscible polymers in the melt: Transesterification of poly(ethylene-co-methyl acrylate) with mono-hydroxylated polystyrenes

This article presents a unique example dealing with how chemical reactions between immiscible polymers in the melt behave differently than they would do in solution. Specifically, a model reaction was chosen: the transesterification between poly(ethylene-co-methyl acrylate) (EMA) and polystyrene mono-hydroxylated at the chain end (PSOH). It was carried out in the melt in a batch mixer. The overall rate of this reaction has a similar dependence of temperature, composition of reactants, and the nature and concentration of catalyst as in solution. The reactivity of PSOH decreases drastically with increasing molecular weight, and it becomes very weak when the molecular weight exceeds 8000 g/mol. As opposed to a reaction in solution or in a homogeneous melt, mechanical mixing increases the reaction rate since it generates interfacial area and reduces the diffusion length. The EMA-g-PS graft copolymer formed at the interfaces reduces the interfacial tension, and increases the miscibility of the reaction mixture. However, its occupation of the interfaces reduces contact between the reactive moieties, thus decreasing the overall reactivity. More importantly and much to our surprise, adding 1 to 2 wt % of an inert solvent increased greatly the overall reaction rate. While an increased interfacial mixing and diffusion by the presence of minor amounts of solvent are thought to be the major factors contributing to the drastic increase in reactivity, numerous questions still remain. Nevertheless, this study clearly showed that as opposed to a reaction in solution, mechanical mixing and the presence of minor amounts of solvent are two additional and critical means to control chemical reactions between immiscible polymer melts. © 1995 John Wiley & Sons, Inc.     

Imino-enamine tautomerism and dynamic prototropy in 1-imino-3-amino-1H-indens

The tautomeric structures and dynamic prototropic behavior of the products 1 and 2 obtained in the condensation reaction of 1,3-indandione and 2-pyridyl-1,3-indandione with p-toluidine, respectively, were investigated by ¹H NMR spectroscopy and X-ray analysis. In the solid state, compound 1 is in an imino-enamine tautomeric form, whereas in solution it coexists with an imino-imino tautomeric form. Dynamic 1,5-prototropic interconversion of the imino-enamine form was revealed to be very fast at room temperature by temperature-dependent ¹H NMR spectra. For 2, the imino-enamine form is the only species present in solution. The hydrogen of the enamine NH is hydrogen-bonded intramolecularly with the nitrogen in the pyridine ring. When the temperature is raised, the NH proton enters into dynamic 1,5-migration, which is accompanied by internal rotation around the pivot bond, which changes the hydrogen-bonding sites. For the condensation product 3 of 2-(2-quinolyl)-1,3-indandione with p-butylaniline, dynamic behavior similar to that found in 2 was observed also in ¹³C NMR spectra.     

Mechanism of the Reaction between the Aqua Complexes of Palladium(II) and Iron(II) in a Solution of HClO4

The kinetics of the reaction of an aqua complex of iron(II) with a tetraaqua complex of palladium(II) in a perchloric acid solution was studied over the temperature range 20–55°C, and a reaction mechanism was proposed. It was found that the reaction is autocatalytic, and it occurs by a two-step one-electron transfer mechanism with the formation of iron(III) and palladium black. Based on the kinetic data, the nature of the autocatalysis in the test reaction was hypothesized.     

Multiband fluorescence spectral properties of QMOM

The spectral characteristics of the 1-methyl-2-(4-methoxyphenyl)-3-hydroxy-4(1H)-quinolone (QMOM) dye with dual fluorescence in acetonitrile were studied under selective excitation in a wide temperature range. This dye is a structural analog of 3-hydroxyflavone and exhibits excited-state proton transfer, which forms a fluorescent tautomeric form, while the solution is characterized by dual fluorescence. The thermal behavior of the relative band intensities revealed the kinetic character of the proton transfer. The third form showed itself as a maximum between the bands of the normal and tautomeric forms upon excitation in several regions of the absorption spectrum and became dominant in solution at 60–80°C. The characteristics of the third form were studied. Additional experiments showed that this was possibly the anionic form of the dye.     

Tautomeric equilibria of 5-fluorouracil anionic species in water

It has long been postulated that rare tautomeric or ionized forms of nucleic acid bases may play a role in mispair formation. Therefore, ab initio quantum chemical investigations on the tautomeric equilibrium in 5-fluorouracil (5FU) and its anions (deprotonated from N1, AN1, and from N3, AN3) and their tautomeric forms in water were performed. The effect of the water as solvent was introduced using solute-solvent clusters (four water molecules). The influence of the water molecules on the tautomeric reactions between different forms was considered by multiple proton transfer mechanisms. We show that when a water dimer is located in the reaction site between the two pairs of N-H and C═O groups, the assistive effect of the water molecules is strengthened. All calculations of the solute-water complexes were carried out at an MP2 level of theory and supplemented with correction for higher order correlation terms at CCSD(T) level, using the 6-31+G(d,p) basis set. The ab initio calculated frequencies and Raman intensities of 5FU and its anions AN1, AN3, and dianion are in good agreement with the experimental Raman frequencies in aqueous solution at different pH. In order to establish the pH-induced structural transformation in the molecule of 5FU, further (1)H, (19)F, and (13)C NMR spectra in water solution for pH = 6.9-13.8 were acquired and the chemical shift alterations were determined as a function of pH. On the basis of NMR spectroscopic data obtained for 5FU in aqueous solution at alkaline pH, we suggest the existence of a mixture of the anionic tautomeric forms predicted by our theoretical calculations.     

Nature of cationic poly(propylenimine) dendrimers in aqueous solutions as studied using versatile indicator dyes

Aqueous solutions of four cationic poly(propylenimine) low-generation dendrimers of different architecture and hydrophobicity have been examined as media for acid-base reactions of indicator dyes. The cationic dendrimers in solution can be considered as oligomers of cationic polyelectrolytes, or surfactant-like species, able to form micelles through self-association or sometimes even as unimolecular micelles. The dendrimers influence the ionization constants, tautomeric equilibria, and absorption/emission/excitation spectra of indicator dyes. The p K a values of the majority of the indicator dyes decrease in dendrimer solutions, often by 1-2 p K a units, similar to effects registered in micellar solutions of cationic surfactants. Analogously, the shifts of absorption band maxima indicate that the microenvironments of the dyes bound to the dendrimers are less polar than in water. However, some spectral effects denote the specificity of the dendrimers. The greatest difference between the dendrimers and spherical surfactant micelles is revealed by kinetic processes, especially of bromophenol blue alkaline fading in a dendrimer solution but not in a micellar surfactant solution. Within the dendrimer series, the most significant differences were observed for substances possessing n-dodecyl tails on the one hand and those without such hydrophobic portions on the other. For the last-named, the decrease in p K a's of indicators, band shifts of their anions, and in particular displacement of tautomeric equilibria compared with aqueous solutions are much smaller than for more hydrophobic dendrimers.     

Ionization-induced proton transfer in model DNA base pairs: a theoretical study of the radical ions of the 7-azaindole dimer.

Proton-transfer reactions of the radical anion and cation of the 7-Azaindole (7AI) dimer were investigated by means of density functional theory (DFT). The calculated results for the dimer anion and cation were very similar. Three equilibrium structures, which correspond to the non-proton-transferred (normal), the single-proton-transferred (SPT) and the double-proton-transferred (tautomeric) forms, were found. The transition states for proton-transfer reactions were also located. The calculations showed that the first proton-transfer reaction (normal-->SPT) is exothermic and almost barrier-free; therefore, it should occur spontaneously in the period of a vibration. In contrast, the second proton-transfer reaction (SPT-->tautomer) was found to be far less-probable in terms of reaction energy and barrier. Hence, it was concluded that both (7Al)2*- and (7Al)2*+ exist in the SPT form. The conclusion was further confirmed by the calculated electron vertical detachment energy (VDE) of the SPT form of (7Al)2*-, 1.33 eV, which is very close to the experimental measurement of 1.35 eV. The calculated VDEs of the normal and tautomeric (7Al)2*- forms were too small compared to the experimental value. The proton transfer process was found to be multidimensional in nature involving not only proton motion but also intermolecular rocking motion. In addition, IR spectra were calculated and reported. The spectra of the three structures showed very different features and, therefore, can be considered as fingerprints for future experimental identifications. The implications of these results to biology and spectroscopy are also briefly discussed.     

Reaction mechanism and tautomeric equilibrium of 2-mercaptopyrimidine in the gas phase and in aqueous solution: a combined Monte Carlo and quantum mechanics study

A combined Monte Carlo and quantum mechanical study was carried out to analyze the tautomeric equilibrium of 2-mercaptopyrimidine in the gas phase and in aqueous solution. Second- and fourth-order M?ller-Plesset perturbation theory calculations indicate that in the gas phase thiol (Pym-SH) is more stable than the thione (Pym-NH) by ca. 8 kcal/mol. In aqueous solution, thermodynamic perturbation theory implemented on a Monte Carlo NpT simulation indicates that both the differential enthalpy and Gibbs free energy favor the thione form. The calculated differential enthalpy is DeltaH(SH)(-->)(NH)(solv) = -1.7 kcal/mol and the differential Gibbs free energy is DeltaG(SH)(-->)(NH)(solv) = -1.9 kcal/mol. Analysis is made of the contribution of the solute-solvent hydrogen bonds and it is noted that the SH group in the thiol and NH group in the thione tautomers act exclusively as a hydrogen bond donor in aqueous solution. The proton transfer reaction between the tautomeric forms was also investigated in the gas phase and in aqueous solution. Two distinct mechanisms were considered: a direct intramolecular transfer and a water-assisted mechanism. In the gas phase, the intramolecular transfer leads to a large energy barrier of 34.4 kcal/mol, passing through a three-center transition state. The proton transfer with the assistance of one water molecule decreases the energy barrier to 17.2 kcal/mol. In solution, these calculated activation barriers are, respectively, 32.0 and 14.8 kcal/mol. The solvent effect is found to be sizable but it is considerably more important as a participant in the water-assisted mechanism than the solvent field of the solute-solvent interaction. Finally, the calculated total Gibbs free energy is used to estimate the equilibrium constant.     

Change of polymer reactivity in the course of macromolecular reaction

For a macromolecular reaction in polymer solution or melt, the reactivity of the transforming unit is supposed to depend upon its microenvironment, including both the nearest neighbours of the same chain and units belonging to the other chains or low molecular components of the reaction mixture. The microenvironment changes with conversion. Hence, the reactivity of polymer functional groups should change during the reaction. Such a reaction belongs to Markovian processes with locally interacting components and time-dependent transient probabilities. To describe the reaction kinetics, it is necessary: (1) to analyse statistical properties of an individual transforming chain, (2) to derive the kinetic equations using relations based on these properties, (3) to determine the dependence of the transient probabilities upon time or upon degree of conversion. For a general reaction model, equations are derived to describe the kinetic curve, distribution of units and composition heterogeneity of the products. The calculations reveal the influence of an interchain interaction on the reaction rate and also on the distribution of units for reaction in a polymer melt.     

Identity double-proton transfer in (3Z)-3-hydroxy-1,4-di(quinolin-2-yl)but-3-en-2-one

Although there is a very fast (on the NMR timescale) double-proton transfer in (1Z,3Z)-3-hydroxy-4-quinolin-2-yl-1-quinolin-2(1H)-ylidenbut-3-en-2-one (the product of the condensation of ethyl oxalate with 2lithiomethylquinoline), it is the only species present in chloroform solution. Comparison of the product of condensation of ethyl oxalate with 2lithiomethyl derivatives of pyridine (recent studies) and quinoline (present studies) shows that benzoannulation considerably affects the tautomeric equilibrium. The observed changes are not only quantitative but also qualitative. Moreover, contrary to the proton transfer in the pyridine tautomers, this process is fast in the quinoline tautomers. Comparison of the experimental and ab initio/DFT GIAO-calculated (13)C and (15)N chemical shifts for the transition states in the proton-transfer reactions between (1Z,3Z)- 3-hydroxy-4-quinolin-2-yl-1-quinolin-2(1H)-ylidenbut-3-en-2-one and its tautomers support the theory that a concerted identity reaction takes place between the enolimine-enaminone and enaminone-enolimine tautomeric forms. As a consequence, the most stable tautomeric form, (1Z,3Z)-1,4-di(quinolin-2-yl)buta-1,3-diene-2,3-diol, is not present in the tautomeric mixture.