Radical/Cation Transformation Polymerization and Its Application to the Preparation of Block Copolymers

Abstract

ESR study on the primary radicals obtained by decomposition of azo-compounds showed that primary radicals with electron donating substituents were transformed to the corresponding cations in the presence of electron acceptors such as ph₂I⁺PF^? ₆. Accordingly, propagating radicals are transformed to the corresponding cations in the polymerization of p-methoxy-styrene (MOS), n-butyl vinyl ether (BVE), and N-vinylcarbazole (VCZ) with azoinitiators such as AIBN in the presence of electron acceptors such as Ph₂I⁺PF^? ₆. In the case of BVE, the polymer formation was caused by cationic species produced by the transformation of the initiating radical. The polymerizations of MOS and VCZ were ascribed to the transformation of the growing radical to the corresponding cation during the propagation step which was classified as the radical/cation transformation polymerization. Block copolymers of MOS/cyclohexene oxide (CHO) and VCZ/CHO were effectively prepared by the radical/cation transformation polymerization of the appropriate monomers in the presence of AIBN, electron acceptor and CHO. The formation of block copolymers was characterized by turbidimetry, thin-layer chromatography, and solubility tests.     

Collisionally activated dissociation of 2,4,6-triphenylpyridinium cations

Thresholds for the appearance of fragment ions allowed the estimation of threshold fragmentation energies (TFE) for the collisionally activated dissociation (CAD) in the gas phase of laser-desorbed pyridine-ring substituted N-benzylpyridinium cations to form pyridine and a carbocation. p-Methylbenzylpyridinium cation underwent an alternative CAD into pyridinium cation and the p-quinodimethane. The TFE are discussed in comparison with the energy differences (ΔΔH_f = ΔH_f(Py) + ΔH_f(R⁺) ? ΔH_f(Py ⁺R) calculated by the AM1 method to provide strong evidence for benzyl to tropylium cation rearrangement in an ion-molecule pair.     

Voltammetric Sensing of Mercury and Copper Cations at Poly(EDTA‐like) Film Modified Electrode

Complexing polymer‐coated electrodes have been synthesized by oxidative electropolymerization of ethylenediamine tetra‐N‐(3‐pyrrole‐1‐yl)propylacetamide (monomer L ). The presence of four polymerizable pyrrole fragments on the same EDTA skeleton was thought to confer enhanced rigidity and controlled dimensionality to the resulting complexing materials, which were used for the electrochemical detection of Hg(II), Cu(II), Pb(II) and Cd(II) ions by means of the chemical preconcentration‐anodic stripping technique. The polyamide electrode material showed particularly a significant selectivity towards mercury ions, even in the presence of a large excess of other metal cations. Moreover, the use of imprinted polymer‐coated electrodes prepared by electropolymerization of L in the presence of metal cations turned out to significantly improve the detection limits, down to 5×10^?10 mol L^?1 for Hg(II) and Cu(II) species.     

Catalytic Carbocation Generation Enabled by the Mesolytic Cleavage of Alkoxyamine Radical Cations

A new catalytic method is described to access carbocation intermediates via the mesolytic cleavage of alkoxyamine radical cations. In this process, electron transfer between an excited state oxidant and a TEMPO‐derived alkoxyamine substrate gives rise to a radical cation with a remarkably weak C?O bond. Spontaneous scission results in the formation of the stable nitroxyl radical TEMPO^. as well as a reactive carbocation intermediate that can be intercepted by a wide range of nucleophiles. Notably, this process occurs under neutral conditions and at comparatively mild potentials, enabling catalytic cation generation in the presence of both acid sensitive and easily oxidized nucleophilic partners.     

Graphite disk Lanthanum(III)-selective electrode based on Kryptofix-22DD

ABSTRACT

A novel ion-selective electrochemical sensor based on kryptofix-22DD (1,10-didecyl-1,10-diaza-18-crown-6) as an ionophore was constructed for determination of lanthanum(III) cation in solutions. The D-Optimal Mixture Design(D-OMD) was used as an experimental design strategy for optimal formulation of the membrane ingredients, and the actual values of the ingredients were compared with their predicted values. The amounts of the ionophore, plasticizer, PVC, and also the graphite powder which influence the performance of the fabricated electrode were analyzed using D-OMD. A non-linear relationship was observed between the slope of the electrochemical response of the electrode and these variables and the interaction between the variables as well. The experimental results showed that the amount of the ionophore has the most positive effect on the electrochemical response of the electrode. The constructed electrode exhibits a Nernstian slope (20.60 ± 0.21 mv.decade^?1) in a wide linear concentration range from 1.0 × 10^?7 to 1.0 × 10^?1 M with respect to La³⁺ cation at pH range of 4.0 to 6.0, and its detection limit was found to be:0.8 × 10^?7M. The selectivity of the proposed electrode towards some of the metal cations present in lanthanum(III) cation solutions was tested, and the experimental results showed a good selectivity towards this metal cation.     

Chain transfer for vinyl monomers polymerized in N-allylstearamide

Apparent transfer constants have been determined for styrene, methyl methacrylate vinyl acetate, and diethyl maleate polymerized in N-allylstearamide at 90°C. Regression coefficients for transfer were: methyl methacrylate, 0.301 × 10^?3; styrene, with no added initiator, 0.582 × 10^?3; styrene, initiated with benzoyl peroxide, 0.830 × 10^?3; vinyl acetate, 62.01 × 10^?3; and diethyl maleate, 2.24 × 10^?3. Rates of polymerization were retarded for both styrene and methyl methacrylate. Vinyl monomer and comonomer disappearance followed an increasing exponential dependence on both initiator and monomer concentration. Although degradative chain transfer probably caused most of the retardation, the cross-termination effect was not eliminated as a contribution factor. Rates for the vinyl acetate copolymerization were somewhat retarded, even though initiator consumption was large because of induced decomposition. The kinetic and transfer data indicated that the reactive monomers added radicals readily, but that rates were lowered by degradative chain transfer. Growing chains were terminated at only moderate rates of transfer. Unreactive monomers added radicals less easily, producing reactive radicals, which transferred rapidly, so that molecular weights were lowered precipitously. Although induced initiator decomposition occurred, rates were still retarded by degradative chain transfer. A simple empirical relation was found between the reciprocal number-average degree of polymerization, 1/X?_n1 and the mole fraction of allylic comonomer entering the copolymer F₂, which permitted estimation of the molecular weight of copolymers of vinyl monomers with allylic comonomers. This equation should be applicable when monomer transfer constants for each homopolymer are known and when osmometric molecular weights of one or two copolymers of low allylic content have been determined.     

Poly(Vinyl Chloride) Containing Dibenzo-18-crown-6 as an Ion-Selective Membrane for Hyamine 1622

Abstract

A poly(vinyl chloride) membrane ion-selective electrode for the Hyamine 1622 cation is proposed. The active substance of the electrode was the neutral carrier dibenzo-18-crown-6 and di-iso-octylphthalate was used as plasticizer. The electrode had a Nernstian behaviour between 6.0 × 10^?6 and 1.6 × 10^?3 mol/dm³, a pH working range of 2 – 12 and high selectivity towards inorganic cations. Among the organic cations tested, only those having surfactant properties did interface. The electrode was suitable for determining the critical micelle concentration and as end-point detector in the potentiometric titration of the cationic surfactants.     

Analytical Study of New Creatininium and Tetramethyl-ammonium Cation Selective Membrane Electrodes

Abstract

Two new liquid membrane electrodes which respond to creatininium and tetramethylammonium cations are described. The creatininium cation electrode exhibits rapid and near Nernstian response to creatininium cation activity, at pH 3, in the 10^?3?10^?1 mol/L range. The useful concentration range extends to 10^?4 mol/L. The tetramethyl-ammonium cation electrode exhibits rapid and near Nernstian response to tetramethylammonium cation activity, at pH 2–11.5, in the 2x10^?5? 10^?1 mol/L range. Major interferences for the creatininium electrode are Na⁺, K⁺, NH⁺ ₄ and creatine. The pK_a of the creatininium cation was calculated. A method is described for the potentiometric precipitation titration of tetramethylammonium cation with sodium tetraphenylboron. Amounts of tetramethylammonium in the range 20–200 μol have been determined using Gran's plots, with an average error of about 0.6%.     

Electrooxidation and Amperometric Detection of Ascorbic Acid at GC Electrode Modified by Electropolymerization of N,N‐Dimethylaniline

The polymer film of N,N‐dimethylaniline (DMA) is deposited on the electrochemically pretreated glassy carbon (GC) electrode by continuous electrooxidation of the monomer. This poly N,N‐dimethylaniline (PDMA) film‐coated electrode can be used as an amperometric sensor of ascorbic acid (AA). The polymer film (thickness (?): 0.3±0.02 μm) having positive charge in its backbone attracts the anionic species AA. Thus, the anodic peak potential (350 mV vs. Ag|AgCl|NaCl_(sat)) for the oxidation of AA at the bare electrode is largely shifted to the negative value (150 mV) at this electrode. The PDMA film‐coated electrode is stable in acidic, alkaline and neutral media and can sense AA at different pH's. The diffusion coefficients of AA in solution (D) and in film (D_s) were estimated by rotating disk electrode voltammetry: D=(5.5±0.1)×10^?6 cm² s^?1 and D_s=(6.3±0.2)×10^?8, (6.0±0.2)×10^?8 and (4.7±0.2)×10^?8 cm² s^?1 for 0.5, 1.5 and 3.0 mM AA, respectively. A permeability of AA through the PDMA film was found to decrease with increasing the concentration of AA in the solution. In the chronoamperometry, the current response for the oxidation of AA at different times elapsed after potential‐step application is linearly increased with the increase in AA concentration in a wide range of its concentration from 25 μM to 1.65 mM. In the hydrodynamic amperometry, a successive addition of 10 μM AA caused the successive increase in current response with equal amplitude and the sensitivity was calculated as 0.178 μA cm^?2 μM^?1. So, the fouling of the electrode surface caused by the oxidized product of AA is markedly eliminated at this PDMA film‐coated electrode. A flow injection analysis based on the present electrode was performed to estimate the concentration of vitamin C in fruit juice.     

Co‐adsorption of Cations as the Cause of the Apparent pH Dependence of Hydrogen Adsorption on a Stepped Platinum Single‐Crystal Electrode

The successful deployment of advanced energy‐conversion systems depends critically on our understanding of the fundamental interactions of the key adsorbed intermediates (hydrogen *H and hydroxyl *OH) at electrified metal–aqueous electrolyte interfaces. The effect of alkali metal cations (Li⁺, Na⁺, K⁺, Cs⁺) on the non‐Nernstian pH shift of the step‐related voltammetric peak of the Pt(553) electrode is investigated over a wide pH window (1 to 13) by means of experimental and computational methods. The co‐adsorbed alkali cations along the step weaken the OH adsorption at the step sites, causing a positive shift of the potential of the step‐related peak on Pt(553). Density functional calculations explain the observations on the identity and concentration of alkali cations on the non‐Nernstian pH shift, and demonstrate that cation–hydroxyl co‐adsorption causes the apparent pH dependence of “hydrogen” adsorption in the step sites of platinum electrodes.     

A Copper(II) Ion‐Selective Potentiometric Sensor Based on N,N′,N″,N′′′‐Tetrakis(2‐pyridylmethyl)‐1,4,8,11‐tetraazacyclotetradecane in PVC Matrix

The simple PVC‐based membrane containing N,N′,N″,N′′′‐tetrakis(2‐pyridylmethyl)‐1,4,8,11‐tetraazacyclotetradecane (tpmc) as an ionophore and dibutyl phthalate as a plasticizer, directly coated on a glassy carbon electrode was examined as a new sensor for Cu²⁺ ions. The potential response was linear within the concentration range of 1.0×10^?1–1.0×10^?6 M with a Nernstian slope of 28.8 mV/decade and detection limit of 7.0×10^?7 M. The electrode was used in aqueous solutions over a wide pH range (1.3–6). The sensor exhibited excellent selectivity for Cu²⁺ ion over a number of cations and was successfully used in its determination in real samples.     

Tetraalkylammonium salt as photoinitiator of vinyl polymerization in organic and aqueous media: A mechanistic and laser flash photolysis study

N‐Dimethyl‐N‐[2‐(N,N‐dimethylamino)ethyl]‐N‐(1‐methylnaphthyl)ammonium tetrafluoroborate ( I ) was synthesized with the aim of obtaining a versatile photoinitiator for vinyl polymerization in organic solvents and water. Salt I was able to trigger the polymerization of acrylamide, 2‐hydroxyethylmethacrylate and styrene even at very low concentrations of the salt (～1.0 × 10^?5 M). Using laser flash photolysis and fluorescence techniques and analyzing the photoproduct distribution, we were able to postulate a mechanism for the photodecomposition of the salt. With irradiation, I undergoes an intramolecular electron‐transfer reaction to form a radical ion pair (RIP). The RIP intermediate decomposes into free radicals. The RIP and the free radicals are active species for initiating the polymerization. Depending on the concentration of the vinyl monomers studied, the initiation mechanism of the polymerization reaction changes. At large monomer concentrations, the RIP state is postulated to trigger the reaction by generating the anion radical of the olefin substrate. At a low monomer concentration, the free radicals produced by the decomposition of I are believed to start the chain reaction. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 901–913, 2002; DOI 10.1002/pola.10166     

The influence of alkali metal cations on the rate of the Fe(CN)64−/Fe(CN)63− electrode process

The rate of the hexacyanoferrate redox system shows a first order dependence on the concentration of the cationic component of the supporting electrolyte. The catalytic influence of the alkali metal cations on the electrode process increases in the order Li⁺<Na⁺<K⁺～Cs⁺. The temperature dependence of the rate constant of the electrode process in KF and LiNO₃ has been measured and the results show that the activated complex is formed by the collision or association of a cation of the supporting electrolyte with the reactant anion, which may already be paired with one cation. It is suggested that this mechanism may be applicable to other electrode reactions involving highly charged species.     

Photoinitiated free radical polymerization of vinyl compounds in aqueous solution

A new method for the photochemical initiation of polymerization of vinyl compounds in aqueous solution is described. The photochemically active species is an ion pair complex of the formula Fe³⁺X⁻(X⁻ = OH⁻, CI⁻, N⁻₃, etc.). The light absorption by the ion pair leads to an electron transfer causing reduction of the cation and oxidation of the anion to an atom or free radical X. The latter leads to the initiation of polymerization in accordance with X + CH₂CHR→XCH₂ CHR . The kinetics of the reaction were studied by the measurement of: (a) ferrous ion formed (colorimetrically), (b) monomer disappearance (by titration and by weighting the polymer), (c) the chain length of the polymer (in the case of methyl methacrylate). The dependence of the quantum yield on the light intensity, light absorption fraction, and the concentration of vinyl monomer and ferrous ion added initially was investigated. A complete mechanism, both with regard to the formation of free radicals and the polymerization reaction, was put forward involving: (1) light absorption, (2) a primary dark back reaction, (3) dissociation of the primary product, (4) a secondary dark back reaction, (5) initiation of polymerization by free radicals, (6) propagation of polymerization, and (7) termination by recombination of active polymer endings. The mechanism was verified by the experimental results and some constant ratios were estimated quantitatively.     

Construction of a new Ho<Superscript>3+</Superscript> PVC-membrane electrochemical sensor based on <Emphasis Type="Italic">N,N</Emphasis>′-dipyridoxyl(1,4-butanediamine)

N,N′-dipyridoxyl(1, 4-butanediamine) (PYBA) (2 wt % ), nitrobenzene (66 wt %), sodium tetra phenyl borate (2 wt ), and poly vinyl chloride (30 wt %) were applied to create a PVC membrane in Ho³⁺ selective electrode (Ho-SE). Over the concentration range of 1.0 × 10^?5 to 1.0 × 10^?2 M in holmium(III) solution a linear response with lower detection of limit equal 4.0 × 10^?6 M was exhibited by new holmium(III) sensor. Ho-SE possessed a Nernstian slope of 20.1 ± 0.8 mV decade^?1 in very short time spans (~9 s). The potential response of electrode remains without any changes over a pH range of 2.77–8.71. The selectivity coefficient data which was considered by matched potential method for variety of common cations indicated no significant interference. An acceptable recovery for Ho³⁺ in the blend of several cations has been achieved. The utility of Ho-SE in potentiometric titration as an end point indicator electrode gave successfully results.     

Peptide cation-radicals. A computational study of the competition between peptide N-Calpha bond cleavage and loss of the side chain in the [GlyPhe-NH2 + 2H]+. cation-radical

Cation‐radicals and dications corresponding to hydrogen atom adducts to N‐terminus‐protonated N_α‐glycylphenylalanine amide (Gly‐Phe‐NH₂) are studied by combined density functional theory and Møller‐Plesset perturbational computations (B3‐MP2) as models for electron‐capture dissociation of peptide bonds and elimination of side‐chain groups in gas‐phase peptide ions. Several structures are identified as local energy minima including isomeric aminoketyl cation‐radicals, and hydrogen‐bonded ion‐radicals, and ylid‐cation‐radical complexes. The hydrogen‐bonded complexes are substantially more stable than the classical aminoketyl structures. Dissociations of the peptide N? C_α bonds in aminoketyl cation‐radicals are 18–47 kJ mol^?1 exothermic and require low activation energies to produce ion‐radical complexes as stable intermediates. Loss of the side‐chain benzyl group is calculated to be 44 kJ mol^?1 endothermic and requires 68 kJ mol^?1 activation energy. Rice‐Ramsperger‐Kassel‐Marcus (RRKM) and transition‐state theory (TST) calculations of unimolecular rate constants predict fast preferential N? C_α bond cleavage resulting in isomerization to ion‐molecule complexes, while dissociation of the C_α? CH₂C₆H₅ bond is much slower. Because of the very low activation energies, the peptide bond dissociations are predicted to be fast in peptide cation‐radicals that have thermal (298 K) energies and thus behave ergodically. Copyright © 2003 John Wiley & Sons, Ltd.     

Mass transfer effects on the electropolymerization current efficiency of 3-methylthiophene in the magnetic field

3-Methylthiophene was chosen as a representative conducting polymer precursor, whose electropolymerization proceeds through the coupling of cation radicals. The density of the solution that contains electrogenerated 3-methylthienyl radicals and early oligomers is higher than the density of the surrounding solution, and at low (<0.2 M) monomer concentrations the diffusion layer falls, compromising the electropolymerization current efficiency. Applying a homogeneous magnetic field (3.0 T) perpendicular to the electrode surface (θ=0°) produces concentration gradient paramagnetic forces (F _∇C) that hold the diffusion layer in contact with the electrode. This gives time for more oligomers to get oxidized and for more cation radicals to couple so that the current efficiency increases from 0.028 to 0.037 at 0.05 M of monomer concentration, and from 0.051 to 0.071 at 0.1 M. With the magnetic field parallel to the electrode surface (θ=90°) Lorenz forces causing magnetohydrodynamic convection, in combination with F _∇C forces keeping the flow pattern in contact with the electrode, increase the current efficiency even more, to 0.048 at 0.05 M of monomer concentration, and to 0.076 at 0.1 M. At higher monomer concentrations (>0.2 M), the rate of radical coupling is evidently fast enough so that, even in the absence of a magnetic field, no natural convection effects are observed and the current efficiency (0.7–0.8) is not affected by the magnetic field.

Polymerization of acrylamide initiated by the persulfate–thiosulfate redox couple

Conversion–time data were obtained for the polymerization of acrylamide initiated by the redox couple persulfate–thiosulfate by using a dilatometer. A plot of initial rate as a function of thiosulfate concentration shows a well-defined maximum and three distinct regions of behavior. In each region the shape of the conversion–time curves demonstrates the differences in apparent order with respect to monomer arising from changes in initiator concentration during an individual run. A reaction mechanism is proposed to explain the results, and a limiting form of the rate expression is derived for each of the three regions. The ranges of concentration studied are: persulfate, 9.5 × 10^?4?4.7 × 10^?2M; thiosulfate, 2 × 10^?5?2 × 10^?2M; initial monomer, 0.05–1.0M; and temperature, 30–50°C. Within these ranges the initial rate shows a halforder dependence on persulfate and a first-order dependence on initial monomer concentration.     

Detection of the Short‐Lived Radical Cation Intermediate in the Electrooxidation of N,N‐Dimethylaniline by Mass Spectrometry

The N,N‐dimethylaniline (DMA) radical cation DMA^.+, a long‐sought transient intermediate, was detected by mass spectrometry (MS) during the electrochemical oxidation of DMA. This was accomplished by coupling desorption electrospray ionization (DESI) MS with a waterwheel working electrode setup to sample the surface of the working electrode during electrochemical analysis. This study clearly shows that DESI‐based electrochemical MS is capable of capturing electrochemically generated intermediates with half‐lives on the order of microseconds, which is 4–5 orders of magnitude faster than previously reported electrochemical mass spectrometry techniques.     

Pulsed laser polymerization study of the propagation kinetics of acrylamide in water

Pulsed laser polymerization was used in conjunction with aqueous‐phase size exclusion chromatography with multi‐angle laser light scattering detection to determine the propagation rate coefficient (k_p) for the water‐soluble monomer acrylamide. The influence of the monomer concentration was investigated from 0.3 to 2.8 M, and k_p decreased with increasing monomer concentration. These data and data for acrylic acid in water were consistent with this decrease being caused by the depletion of the monomer concentration by dimer formation in water. Two photoinitiators, uranyl nitrate and 2,2′‐azobis(2‐amidinopropane) (V‐50), were used; k_p was dependent on their concentrations. The concentration dependence of k_p was ascribed to a combination of solvent effects arising from association (thermodynamic effects) and changes in the free energy of activation (effects of the solvent on the structure of the reactant and transition state). Arrhenius parameters for k_p (M^?1 s^?1) = 10^7.2 exp(?13.4 kJ mol^?1/RT) and k_p (M^?1 s^?1) = 10^7.1 exp(?12.9 kJ mol^?1/RT) were obtained for 0.002 M uranyl nitrate and V‐50, respectively, with a monomer concentration of 0.32 M. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1357–1368, 2005