1,721,004 research outputs found
Solution behaviour of poly(N-acryloyl-L-leucine) and its copolymers with N-isopropylacrylamide
No full textA vinyl polymer carrying COOH groups and the groups (amido and isopropyl) present in the temperature-sensitive poly(N-isopropylacrylamide) (pNIPAAm) was synthesized along with two of its copolymers incorporating NIPAAm units. The protonation of COO- groups, studied in aqueous media over a range of temperatures (25-35°C) that included the lower critical solution temperature of pNIPAAm, showed polyelectrolyte behaviour with a decreasing pattern of the basicity constant (log K) and enthalpy (-ΔHo) changes in relation to the degree of protonation (α). Beyond a critical α value (α = 0.4), the log K/α slope drastically changed and the reduced viscosity (η/C) sharply decreased. Under these conditions the Fourier transform infra-red spectra revealed a strong band at 1625 cm-1 presumably reflecting the interaction of the amido groups of several monomer units. For the copolymers the decreasing pattern of log K values decreased with increasing NIPAAm content, and beyond the critical α, the log K/α slope increased again, showing a greater endothermic effect superimposed on the protonation of the COO- group. The phenomenon was attributed to hydrophobic forces between isopropyl groups outweighing the repulsive electrostatic interactions of the ionized polymer. At higher temperatures, the -ΔHo values decreased in a sensitive manner and the critical α shifted to a lower value because of enhanced hydrophobic interactions. A reduced amount of structured water molecules on the polymer was revealed by the lower entropy changes (ΔSo). A hydroxo-complex species of Cu(OH)2L2 2- (L- is the repeating unit of the polymer) stoichiometry was revealed at low pH with a linear decrease in stability constants (log β) in relation to pH. The pH range for a single complex species was consistent with the electronic spectra. © 1997 Elsevier Science Ltd
Redox-active polymers: synthesis and exchange reaction of amino compounds containing a cyclic disulfide
No full textTwo monoamidated amino compounds, bearing the cyclic disulfide moiety of lipoic acid, were prepared by imidazole group transfer with aliphatic diamines (1,3-diaminopropane and piperazine). The products, characterized by spectroscopic methods (H-1 nuclear magnetic resonance and Fourier-transform infra-red), were studied in solution. Basicity constants for the protonation of the basic nitrogens were evaluated in aqueous media, and redox properties in dimethylsulfoxide, by cyclic voltammetry. Insoluble polymers were obtained by polymer-analogue exchange reaction of the monoamidated compound with a benzotriazole residue present in preformed poly(N-acryloylbenzotriazole). A copolymer of the same monomer with N-vinylpyrrolidinone was soluble in solvents, giving a response in cyclic voltammetry. Its positive anode peak was slightly higher than that of the simple monoamidated compounds
Redox and pH-Sensitive polymer-grafted membranes: thermodynamic characteristics in drug delivery
No full textPoly(amido-amine)s with novel molecular architecture: synthesis and thermodynamic studies of protonation and metal (Zn, Cu) ion complexes
No full textNew short chain poly(amido-amine)s (SPAAs) carrying amido- and tertiary amino groups along the polymer chain separated by a single methylene group have been synthesized by polycondensation of N,N'-bis-(2-chloro- or bromo-acetyl)-piperazine with N,N'-dimethyl-hexamethylenediamine or 2-methyl-piperazine. The basicity of the SPAAs was lower compared to the PAAs obtained previously by polyaddition of bis-amines to bis-acrylamides. The shortening of the methylene chain from two (PAAs) to one (SPAAs) reduced the logK values of both tertiary amino groups present in the repeating unit due to inductive effects. Furthermore, the shortening resulted in basicity constants, that depend on pH, with respect to the first step of protonation. The real changes in enthalpy (-Delta H-0) of protonation of the SPAAs are greater than those found for PAAs due to the fact that protonated nitrogens and amido C=O groups are closer, favouring the formation of five-member rings. Five-member rings are formed also with metal(II) ions, such as Cu(II) and Zn(II). giving complex species more stable than those of PAAs, with higher log beta and larger -Delta H-0 values. Low entropy changes (Delta S-0) and molar absorption coefficient (epsilon) values support strongly a well ordered chelate structure involving three condensed five-membered rings in an octahedral distorted geometry
The protonation thermodynamics of ferulic acid/gamma-cyclodextrin inclusion compounds
No full textThe physico-chemical properties of ferulic acid (FA) and its inclusion compounds with the -cyclodextrin (-CD) have been studied at
25 ◦C in aqueous solution (0.15M NaCl) by potentiometry and solution calorimetry. The two basicity constants of the ionized FA, log K1
8.94 and logK2 4.48, in agreement with those of compounds of similar structure, were attributed to the protonation of hydroxyl and carboxyl
ionized groups, respectively. Unlike log K2, the first basicity constant showed a slight decreasing pattern on increasing the amount of -CD.
On the other hand, calorimetric data showed greater exothermicity during the protonation of the COO− group in FA when in the presence of
the -CD. Any increase of the latter led to greater enthalpy (–H◦) and lower entropy (S◦) changes. The increase of –H◦ and the decrease
of S◦ values reached a constancy only beyond -CD/FA molar ratios of 1. Thermodynamic data were consistent with the hypothesis of the
inclusion complexation obtained by the penetration of the neutralized guest FA into the -CD host cavity
Amphoteric poly(amido-amine) polymers containing the N,N\'-ethylenediaminediacetic acid moiety: stability of copper(II) and calcium(II) chelates
No full textProtonation thermodynamics of membranes grafted with polyelectrolytes controlling solute permeability
No full textThe protonation of two polyelectrolytes with carboxyl groups, grafted on porous cellulose and polyurethane supports, was evaluated by potentiometry, titration calorimetry and Fourier transform IR (FT-IR) spectroscopy in aqueous media. The large potentiometric hysteresis loops observed were indicative of interactions between functional groups of the graft chains and the substrate. This was confirmed by FT-IR spectroscopic and calorimetric analysis. The thermodynamic properties are reviewed in relation to applications concerning solute permeability. The pH-sensitive porous membrane, having a ''chemical valve'' function, self-regulated insulin release in response to the pH of the surrounding solution. The porosity of the membrane was controlled by the charge state conformations of grafted polyacids
The thermodynamics of protonation and copper(II) complex formation of a-aminomalonic acid
No full textThe thermodynamic functions (ΔG⊖, ΔH⊖, and ΔS⊖) for the protonation of α-aminomalonic acid were determined in aqueous solution (25°C, l = 0.1 mol dm-3 NaCl). The basicity constants and enthalpy changes were evaluated from potentiometric and calorimetric data for both the primary amino and the carboxylate groups. On the basis of the thermodynamic changes a closed structure is hypothesized with the protonated amino nitrogen hydrogen-bonded to both the carboxylate groups to form two five-membered rings. The protonation of the CO2- groups provokes opening of the rings. A relatively high stability constant for complexation with Cu2+ was determined, however in a narrow range of low pH. The co-ordination to the copper(II) occurs both with nitrogen and the two oxygen donor atoms, to form two five-membered chelate rings
Spectroscopic and calorimetric studies on the protonation of polymeric aminoacids
No full textThe protonation of some poly(amido amines) has been studied in aqueous solution by FTIR spectrophotometry and by calorimetry. On the basis of the stretching bands of both the ionized and the un-ionized carboxyl groups, the structures of all four polymeric amino acids in their different protonation states are determined. Calorimetric studies show that the enthalpies of protonation do not change with pH and follow a trend similar to that observed in simple amino acids of comparable structure. An anomalously high value of |ΔH2°| observed for the polymer obtained by the polyaddition of ß-alanine to diacryloylpiperazine has been found and attributed to the formation of a very highly symmetrical structure when the carboxyl groups are in the un-ionized form. © 1986, American Chemical Society. All rights reserved
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