On the protonation of partially quaternized poly-4-vinylpyridines in aqueous solution

The ionic equilibria for free pyridine rings in water-soluble macromolecules of poly-4-vinylpyridine (PVP), partially quaternized with either benzylchloride (PP_yBz) or bromoacetone (PP_yAc), were studied by potentiometry and spectrophotometry. Peculiarities of protonation in aqueous solution were observed for the pyridine rings of these polymers compared with the analogue 4-ethylpyridine (EtP_y). The values of intrinsic pK₀ in the absence of protonated pyridine residues for PP_yBz and PP_yAc were found to be 2·4–3·3, and are abnormally low compared with pK_a of EtP_y (6·1). The pK₀ depends insignificantly on the degree of quaternization in PVP but more markedly on ionic strength. However, increase of NaCl concentration (from 0 to 0·1 M) does not cause pK₀ to rise to pK_a of the analogue. The titration data of the base group and keto-enol tautomerism of acetomethylene fragments in PP_yAc allow one to assume the effect of positively charged atoms in α-position of the ring on the proton binding to nitrogen atom of the same ring, thereby decreasing the value of pK₀. The intensification of electrostatic interactions between these charges in the polymer pyridine compared with that of the analogue is likely to be due to partial dehydration of the microregion near to nitrogen atom (microenvironmental effect).     

EPR studies of copper(II) complexes with poly-2-methyl-5-vinyl-pyridine and poly-2-vinylpyridine

Cu(II) complexes of poly-2-vinylpyridine (P2VP) and poly-2-methyl-5-vinylpyridine (P2M5VP), partially quaternized by dimethylsulphate, and of the analogues (2-methyl-5-ethylpyridine, 2-ethylpyridine) were studied by EPR spectroscopy in a mixture of methanol and water. Peculiarities of the complex formation reaction were observed for the polymers compared to the analogues. At ratios of [Py]:[Cu²⁺] > 40, the predominant formation of tetrapyridinate-Cu(II) species [(CuL₄)]²⁺ was found for P2M5VP. However, differences were found between the parameters of EPR-spectra for the [CuL₄]²⁺ in the polymer from that of the [Cu(2M5EPy)₄]²⁺. It was suggested that, in the polymer, [CuL₄]²⁺ complexes with structure intermediate between square planar and tetrahedral are formed. Moreover, the maximum value of the pyridine fraction forming [CuL₄]²⁺ in P2M5VP was found to be about 10% and it is appreciably less the value of the fraction in P4VP (about 40%). For P2VP at [Py]:[Cu²⁺] > 40, an insignificant amount of [CuL₁]²⁺ and [CuL₂]²⁺ are formed in the solution. It follows that the main chain position relative to the ligand nitrogen atom in these polyvinylpyridines affects profoundly the complexation between the macromolecules and Cu(II) ions. The steric hindrances due to the chain are likely to change the [CuL₄]²⁺ structure and to prevent complex formation for P2VP.     

Solvation effects and reactivity of free pyridine residues in macromolecules of poly-4-vinylpyridine derivatives

Water-soluble derivatives of poly-4-vinylpyridine (PVP) were prepared by means of quaternization of PVP with ethyl bromide, butyl bromide, hexyl-bromide and bromacetic acid. In addition, hydrolysis of p-nitrophenylacetate (NPA) and 3-nitro-4-acetoxybenzoic acid (NABA) catalyzed by these polymers and 4-ethylpyridine as analogue were studied spectrophotometrically both in water and alcohol-water mixtures at 25° and pH 7·8.The second-order rate of NPA hydrolysis (K_t) was independent of the length of alkyl groups and the salt concentration. While an increase of the degree of quaternization (β) for PVP derivatives resulted in a reduction of the value of K_i in water, in 37–50 vol per cent alcohol-water there was the opposite effect. At large values of β, K_i did not depend on the content of ethanol.The bell-shaped dependence of K_i on β was shown in the case of amionic NABA. The value of K_i proved to be very sensitive to both the nature of salt and the salt concentration in aqueous solution.The use of viscosity, turbidimetry and potentiometry for studying these polymers allowed the role of the quaternized pyridine residue to be elucidated. The peculiarities of the kinetic behaviour of the free basic group were explained by the hydrophilic nature of the quaternized PVP changing as the degree of quaternization is increased so causing an exclusion of the substrate molecules from the polyion.     

Determination des constantes d'association des paires d'ions et des ions triples au sein de solutions de poly (vinyl-2 pyridine) vivantes

The association constants K_A and k_A for ion-pairs and triple-ions in living polymer solutions of 2-vinyl pyridine have been measured. The values of K_A and k_A in tetrahydrofuran are respectively 4·6 × 10⁸ and 5·2 × 10⁴ with Na' as counter ion, and 6·6 × 10⁸ and 1·6 × 10² with Cs⁺ as counter ion. These values fit in very well with kinetics recently published.In order to evaluate K_A and k_A more accurately, the translation diffusion coefficients of living or dead poly(2-vinyl pyridine) have been measured, monitoring the concentration gradient along an optical cell. In both cases for DP = 8, the value is 0366 cm²/sec. Such a value implies that the equivalent conductance of the poly 2-vinyl pyridine anions is lower than that of polystyrene anions of the same DP.     

Ion-specific swelling behaviors of partially quaternized poly(4-vinyl pyridine) gel

Swelling degrees of partially quaternized poly(4-vinyl pyridine) (p.q.P4VP) were measured as functions of the degree of quaternization (D.Q.: 5, 10, 15, 30, and 50 mol%), salt concentration (0–4 M), and salt species (KF, LiCl, NaCl, KCl, CsCl, MgCl₂, KBr, LiSCN, KSCN, Na₂SO₄, and MgSO₄) to study ion effects on the three kinds of interactions involved with p.q.P4VP in aqueous systems, i.e., electrostatic interaction, hydrogen bonding to the nitrogen of the pyridine ring, and hydrophobic interaction, as well as to estimate the relative contributions of the respective ion effects to gel swelling. Upon addition of salts at the lowest salt concentration studied (0.01 M), p.q.P4VP gels, except for one with D.Q.=10%, significantly deswelled depending on the counterion species; the lowest swelling degree was observed for the thiocyanates and the highest one for KF. In the higher salt concentration region (≧1 M) where electrostatic interactions might well be screened, however, some salt systems (e.g., LiCl, KSCN) showed swelling with increased salt concentration, contrary to the common behavior of usual ionic gels. These ion-specific swelling behaviors were interpreted as being caused by additive ionic effects on the three kinds of interactions.     

Quantitative analysis of hydrogen bonding in electrospun fibers of poly(4-vinyl pyridine)/(4,4′-biphenol) complexes by ATR using liquid blends as models

To understand the characteristics of electrospun fibers of poly(4-vinyl pyridine) (P4VP) that are potentially crosslinked by hydrogen-bonding when blended with small molecules like 4,4′-biphenol (BiOH), it is necessary to determine the proportion of hydrogen-bonded pyridine rings (f_b) and to contrast it with F_OH, the mole percent of OH groups available for interacting with the pyridine rings. While this can be done by Fourier transform infrared spectroscopy (FTIR), two practical difficulties must be overcome. First, the correct intensities of the overlapped bands of free and hydrogen-bonded pyridine must be obtained, which is possible using the second derivative spectra. Second, the band absorption coefficient ratio (a) of the pair of bands must be known. In the P4VP/BiOH system, only the pair of free and hydrogen-bonded pyridine ring bands at 993 and 1007 cm⁻¹ can be used for quantitative analysis. We determined, by analysis of liquid blends of BiOH as well as phenol with a model compound, 4-ethylpyridine (EtPy), using the attenuated total reflection mode (ATR), that a = 0.40. This led to values of f_b = F_OH, indicative of full complexation of the OH groups to pyridine, in the EtPy/BiOH liquid blends up to the BiOH solubility limit in EtPy (F_OH = 60%) and in the EtPy/phenol blends up to F_OH = 100%. In the electrospun P4VP/BiOH fibers prepared from solutions with F_OH up to about 120%, f_b averages 0.76F_OH, and full complexation of the pyridine groups is achieved only at about F_OH = 160%. In both P4VP/BiOH and EtPy/BiOH, the complexation occurs between one pyridine ring and one hydroxyl group. This work thus shows a new method to quantify the relative number of hydrogen-bonded pyridine rings in P4VP; it is expected to be applicable to other polymer-small molecule and polymer-polymer blends having suitable liquid models.     

Physico-chemical properties of the copper(II)-poly-4-vinylpyridine complexes

Water-soluble Cu(II) complexes of poly-4-vinylpyridine, partially quaternized by methyl bromide or dimethylsulphate (PP, Mt) and of the analogue (4-ethylpyridine) were studied by physicochemical technique such as visible spectrophotometry, viscosity, speed sedimentation and EPR spectroscopy. Peculiarities of the complex formation reaction were observed for the polymer compared to the analogue. The predominant formation of tetrapyridinate-Cu(II) species [(CuL₄)]²⁴ was found to take place in aqueous solution for PP, Mt with degree of quaternization (β) 20,24,37,49,57,65 per cent over a wide range of P_YJ Cu(II) molar ratios. In addition, not all the free polymer pyridine residues are capable of forming [Cu L4 ]² complexes even though there is a large excess of Cu(II) ions. The maximum value of the pyridine fraction forming [Cu L₄ ]²¹ in PP_YMt-20, 24, 37, 49 (obtained by spectroscopic titration) was found to be 45–50 per cent. Addition of Cu(NO₃)₂ to the aqueous polymer solution causes strong reduction of specific viscosity and increase of sedimentation coefficient from - 1 S to - 6 S. These data lend to the suggestion of the existence of macromolecular associates bound by Cu(II) ions and approximate estimation of their average molecular weight. It was shown also that Bjerrum's procedure cannot be used to account for the polymer stability constants. The differences between complex formation for polymer pyridine and monomer pyridine seem to be due to the high local pyridine concentration in the coil of the macromolecule and the hydration effects of the polymer chain on the characteristics of the pyridine residues.     

Wholly aromatic polyamides by the direct polycondensation reaction using triphenyl phosphite in the presence of poly(4-vinylpyridine)

The polycondensation reaction of aromatic dicarboxylic acids and diamines by using triphenyl phosphite were carried out in N-methylpyrrolidone (NMP) in the presence of poly(4-vinylpyridine) (P4VP). The reaction, especially of terephthalic acid (TPA), was markedly facilitated to give the absence of P4VP. The reaction promoted by P4VP was further favored by the addition of various pyridine derivatives; of the pyridines examined, pyridine was most effective, giving the best results at a high level (pyridine/P4VP values up to 26). P4VP of the molecular weight in the range of 1.3 × 10⁴?3.0 × 10⁵ did not affect the viscosity of the resulting polymer. These favorable additive effects of P4VP on the reaction of TPA were not observed in the reactions of isophthalic acid, and m -and p-aminobenzoic acids.     

Photoregulation of polymer conformation by photochromic moieties. II. Cationic and neutral moieties on an anionic polymer

Abstract— Photoregulation of the conformation of poly (methacrylic acid) (PMA) in the range 4·5 < pH > 6·5 has been achieved by means of the cationic photochromic ligand, p-phenyl-azophenyl trimethyl ammonium iodide (PTA), as well as by means of copolymerization of methacrylic acid (MA) with the neutral photochromic co-monomer, p-phenyl azomethacrylanilid (PM). At degrees of ionization α, corresponding to the conformational unfolding of PMA, the pK_app is 0·2 units higher when the ligand PTA is trans than when PTA is in the photostationary 80%“cis” state. Equilibrium dialysis shows that the binding of PTA is independent of the cis/trans ratio at high α (after the conformational unfolding), but at lower αtrans-PTA binds positively and cis-PTA negatively. In the transition region, the presence of non-polar trans-azo ligand is therefore held responsible for a shift of the conformational equilibrium toward the more compact, folded form, which is harder to ionize than the unfolded form. Because of the relation pK_app# pH + log [(1 –α)/α], the degree of ionization of PMA, and thus the polymer conformation, as reflected in the reduced viscosity, can be photochemically regulated in pH buffers. At pH # 5·5 (α# 0·20) a maximal relative shift of Δα/α= 20 per cent was found, with a corresponding relative shift in reduced viscosity of 20·5 per cent. In the photochromic copolymer a pK_app switch of similar magnitude was found. In this case the azo-moiety is permanently affixed to the PMA so that the photosensitivity of the pK_app in the range of the conformational unfolding is due to a shift in the conformational equilibrium induced by the presence of non-polar trans-azo and polar “cis” azo, respectively. In both cases only about 1 photochromic moiety per 100 monomer units is capable of appreciably shifting the conformational manifold of PMA and/or the degree of ionization in constant pH buffers, provided one stays close to the unfolding transition. Mention is made of the implications for photoregulation of the permeability of, and the potential across. model membranes made of photochromic PMA molecules.     

Selective gas-phase oxidation of pyridine derivatives with hydrogen peroxide

The interfering kinetics of the synchronous reactions of hydrogen peroxide decomposition and the oxidation of pyridine derivatives have been studied experimentally. The regions of the selective oxidation of the pyridine derivatives have been found, and the optimal conditions for the production of 4-vinylpyridine, 4-vinylpyridine N-monoxide, 2,2-dipyridyl, and pyridine have been determined. The most probable synchronization mechanism is suggested for hydrogen peroxide decomposition and the free-radical chain oxidation of pyridine derivatives. The HO ₂ ^· radical plays the key role in this mechanism. The activation energies are calculated for the elementary steps of 4-ethylpyridine dehydrogenation.     

Synthesis and characterization of AgBr nanocomposites by templated amphiphilic comb polymer

A novel amphiphilic graft copolymer, poly(vinylidene fluoride-co-chlorotrifluoroethylene)-g-poly(4-vinyl pyridine) (P(VDF-co-CTFE)-g-P4VP) at 65:35 wt.%, respectively, was synthesized via atom transfer radical polymerization (ATRP), as confirmed by nuclear magnetic resonance (¹H NMR) and transmission electron microscopy (TEM). Silver bromide (AgBr) nanoparticles were in situ generated within the self-assembled P(VDF-co-CTFE)-g-P4VP graft copolymer. TEM, UV–visible spectroscopy and X-ray diffraction (XRD) analyses support the successful formation of P(VDF-co-CTFE)-g-P4VP nanocomposites consisting of stabilized AgBr nanoparticles mostly 20–40 nm in size, which is presumably due to the capping action of the coordinating pyridine groups of the graft copolymer. The wavenumber of pyridine nitrogen in FT-IR spectra and the glass transition temperature (T_g) of the graft polymer measured by DSC shifted upon the formation of AgBr nanoparticles, indicating specific interactions between the nanoparticles and the graft copolymer matrix.     

Poly(oxyethylene methacrylate)–poly(4-vinyl pyridine) comb-like polymer electrolytes for solid-state dye-sensitized solar cells

Poly(oxyethylene methacrylate)–poly(4-vinyl pyridine) (POEM–P4VP) comb-like copolymers with 3:7, 5:5, and 6:4 wt ratio were synthesized via atom transfer radical polymerization and confirmed by ¹H-NMR and FT-IR spectroscopy. The copolymers were quaternized with 1-iodopropane to convert the pyridine groups into pyridinium ions, i.e., POEM–qP4VP. Transmission electron microscopy showed that strongly segregated microphase separation in POEM–P4VP was less prominent upon quaternization due to interactions between the ether oxygens of POEM and the quaternized pyridine groups of qP4VP, as confirmed by FT-IR spectroscopy. The energy conversion efficiencies of dye-sensitized solar cells (DSSCs) with quaternized polymer electrolytes were always greater than those with pristine electrolytes due to greater ionic conductivity and concentrations of free iodide ions. The maximum energy conversion efficiency of a DSSC employing POEM–qP4VP electrolyte reached 3.0% at 100 mW/cm² when a 6:4 wt.% of POEM–qP4VP was used.     

Voltammetric properties of electrodes modified by poly-[N-(9,10-anthraquinone-2-carbonyl)ethylenimine] and used in aqueous solution

Three differently loaded anthraquinone polymers have been prepared by condensation of 2-anthraquinonecarbonylchloride and poly(ethylenimine). They have been adsorbed on vitreous carbon or mercury electrodes by dipping the electrode in a solution of pyridine or methylene chloride containing 0.01–0.05% polymer. The influence of adsorption parameters and of electrochemical variables on the voltammetric behavior of the polymer-coated electrodes in aqueous buffer solutions is described. Changing the dip time in pyridine from 30 s to 5 min and changing the polymer concentration have very little effect on the apparent coverage (=1.3-1.8×10^?9 mol cm^?2) and peak shape. In contrast, an increase of the loading (the number of monomer units loaded with quinone) or the use of methylene chloride as a dip-coating solvent instead of pyridine affected the shape of the cathodic and anodic peaks which broaden and tend to separate. The peak shape is characterized by a tailing which tends to disappear at slow scan rate. The modifications of the peak shape and position when the scan rate is changed, have been shown to fit qualitatively with a proposed polylayer model in which electron (proton) transfer to the sublayer nearest the underlying conductor is slow. It has been observed that the weak acid dissociation constants for the dihydroanthraquinone and hydroanthraquinone anion are both different for the corresponding monomer in solution (pK_A=8.65 and 11.6) and the adsorbed polymer (pK_A=9 and 13).     

Etude de la nature des especes ioniques intervenant au cours de la polymerisation anionique de la vinyl-4 pyridine

Initiation of anionic polymerization of 4-vinyl-pyridine (4-VP) in tetrahydrofuran solution, using organo-metallic compounds of Na and K, involves the same type of side reactions as the polymerization of 2-vinyl pyridine (2-VP). The living polymers of 2- and 4-VP have similar u.v. spectra. The values of the dissociation constants of ionic species of 2- and 4-VP are similar when the chain [poly(styrene-b vinyl-4 pyridine)] contains one 4-VP unit (K_n ≈ 10^?9). But when the size of 4-VP block increases, the conductivity goes up to the value observed for living polystyrene (K_D ≈ 10^?7). This phenomenon could be due to a solvation of the counter-ion by the poly 4-vinyl pyridine chain, acting as a polydentate complexing agent.     

Correlations of 95Mo NMR chemical shifts with ligand basicity in substituted pyridine pentacarbonylmolybdenum(O) complexes

Molybdenum-95 NMR chemical shifts are reported for a series of Mo(O) compounds of the type Mo(CO)₅L (L = pyridine derivatives). A good correlation is found between the δ(⁹⁵Mo) values and the Hammett sigma constant of the pyridine substituent or the pK_a of the substituted pyridine. The chemical shift values, which range from −1366 ppm (3-CN, σ = 0.62, pK_a = 1.35) to −1433 ppm (4-NMe₂, σ = −0.83, pK_a = 9.61), directly reflect the electronic properties of the pyridine derivatives even though the substituent is four or five bonds away from the molybdenum atom.     

Solubility of oxides in the KBr-BaBr2 eutectic melt (0.495: 0.505) at 973 K

Solubility products of MnO (pK _s,MnO = 6.32 ± 0.1), NiO (pK _s,NiO = 8.06 ± 0.2), and PbO (pK _s,PbO = 4.04 ± 0.2) in KBr-BaBr₂ (0.495: 0.505) melt at 973 K were determined by potentiometric titration using a Pt(O₂)|ZrO₂(Y₂O₃) membrane oxygen electrode. A significant increase in oxide solubilities compared to those of 2CsBr · KBr melt is attributed to the enhancement of acidic properties of cations in the series Cs⁺, K⁺-Ba²⁺, ensuring a greater degree of binding of oxide ions that results from dissociation of the oxide under study to the cationic framework of the melt. The solubilities of the investigated oxides in chloride and bromide melts with similar cationic compositions are virtually identical.     

Kinetic Properties of Two Rhizopus Exo-polygalacturonase Enzymes Hydrolyzing Galacturonic Acid Oligomers Using Isothermal Titration Calorimetry

The kinetic characteristics of two Rhizopus oryzae exo-polygalacturonases acting on galacturonic acid oligomers (GalpA) were determined using isothermal titration calorimetry (ITC). RPG15 hydrolyzing (GalpA)₂ demonstrated a K _m of 55 μM and k _cat of 10.3 s^?1 while RPG16 was shown to have greater affinity for (GalpA)₂ with a K _m of 16 μM, but lesser catalytic activity with a k _cat of 3.9 s^?1. Both enzymes were inhibited by the product, galacturonic acid, with _app K _i values of 886 and 501 μM for RPG15 and RPG16, respectively. RPG15 exhibited greater affinity for (GalpA)₃ with a K _m of 9.2 μM and a similar k _cat at 10.7 s^?1 relative to (GalpA)₂. Catalytic constants for RPG16 hydrolyzing (GalpA)₃ could not be determined; however, single-injection ITC assays suggest a distinct preference and catalytic rate for (GalpA)₃ relative to (GalpA)₂. Thermodynamic parameters of a series of galacturonic acid oligomers binding to RPG15 were determined and exhibited some distinct differences from RPG16 binding thermodynamics, providing potential clues to the differing kinetic characteristics of the two exo-polygalacturonase enzymes.     

Effects of Nonionic, Cationic, and Mixed Nonionic-Cationic Surfactants on the Acid-Base Behavior of Phenyl Salicylate

The effects of Brij 35 micelles, CTABr micelles, and mixed Brij 35–CTABr micelles on the acid–base behavior of phenyl salicylate (PST) have been studied in aqueous solution containing 2% v/v acetonitrile. The apparent pK_b (pK^app_b) of PST is decreased by 1.5 pK units with the increase in [Brij 35] from 0 to 0.02 M which is attributed to micellar medium effect. The values of pK^app_b remain almost independent of [CTABr] within its range 0.01–0.03 M. The increase in [CTABr] from 0 to 0.03 M in aqueous solution containing 0.02 M Brij 35 has not resulted in a change in pK^app_b. This shows that the characeristic structural features of nonionic Brij 35 micelles remain essentially unchanged on addition of CTABr under the present experimental conditions.     

Photoregulation of polymer conformation by photochromic moieties. I. Anionic ligand to an anionic polymer

Abstract— Photoregulated biological processes appear to make use of membrane-bound photochromic macromolecules. In order to elucidate various physicochemical pathways by which these processes can be triggered, model studies have been undertaken employing photochromic moieties bound to synthetic macromolecules with a labile fold structure. Inspired by Lovrien's 1967 work, attention was first focused on the anionic dis-azo stilbene dye chrysophenine (CHP) and poly(methacrylic acid) (PMA) in aqueous solution. This ligand is found to bind to and unfold PMA only if the degree of ionization of the polymer is below 0·75. Viscosity as well as equilibrium dialysis data indicate that maximally one CHP per ten monomer units PMA is bound. The apparent degree of ionization conferred to PMA by the ionic CHP ligand leading to polymer unfolding is the same as the known real degree of ionization leading to polymer unfolding in the absence of CHP. Upon trans→cis photoisomerization, the ligand either desorbs or creates a higher local dielectric constant because of the large cis-azo dipole moments. As a result some refolding to a more compact hydrodynamic volume occurs, as deduced from the viscosity measurements at a low degree of ionization. Simultaneously a lowering of the pK_app by 0·1 pK unit is observed at degrees of ionization below 0·075. Photoregulation of conformation as well as of pK_app indicates two possible pathways for the regulation of ionic fluxes such as have been postulated for photobiological transducers.     

Complexation and eutectic crystallization in poly(2-vinyl pyridine)-block-poly(ε-caprolactone) and pentadecylphenol mixtures

Crystallization behavior via hydrogen bonding interaction in amphiphilic block copolymer/surfactant mixtures consisting of poly(2-vinyl pyridine)-block-poly(ε-caprolactone) (P2VP-PCL) and 3-pentadecylphenol (PDP) were investigated by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. The P2VP-PCL/PDP mixtures exhibit eutectic crystallization behavior; the eutectic composition is approximately at 70 wt.% PDP. Scanning probe microscopy (SPM) observation revealed the microphase structure in the P2VP-PCL/PDP mixtures and the unique eutectic morphology at the eutectic composition, which was further confirmed by small angle X-ray scattering (SAXS) results. To our knowledge, this is the first example of eutectic crystallization observed in amphiphilic block copolymer/surfactant systems. The FTIR study proved that there are competitive hydrogen bonding interactions between P2VP block/PDP and PCL block/PDP pairs in the P2VP-PCL/PDP mixtures. On the basis of the SPM results and FTIR study, a model describing the microstructure of the P2VP-PCL/PDP eutectic mixtures is proposed. The amorphous P2VP blocks are expelled from the ordered eutectic lamellae formed by the crystalline PCL blocks and PDP, which deviates remarkably from the existing structural model proposed by other authors for poly(vinyl pyridine)/PDP and poly(styrene-block-4-vinyl pyridine)/PDP mixtures.