Single Radiation‐Induced Grafting Method for the Preparation of Two Proton‐ and Lithium Ion‐Conducting Membranes

Summary: Two distinct types of polymer electrolyte membranes for conducting protons and lithium ions have been prepared by a radiation‐induced grafting method. The polymer electrolyte precursor (PVDF‐g‐PS) is obtained by the simultaneous grafting of styrene onto poly(vinylidene fluoride) (PVDF) followed by one of two specific treatments. This includes sulfonation with a chlorosulfonic acid/dichloromethane mixture to obtain proton (H⁺)‐conducting membranes, or activation with LiPF₆/EC/DC liquid electrolyte to obtain lithium ion (Li⁺)‐conducting membranes. The chemical structure of the obtained electrolyte membranes is verified by FT‐IR spectroscopy. Differential scanning calorimetry is used to examine the changes in the crystallinity and the thermal properties of both electrolyte membranes during the preparation process. The thermal stability of both electrolyte membranes is also evaluated using thermal gravimetrical analysis. The obtained polymer electrolyte membranes achieve superior conductivity values: 1.61 × 10^?3 S · cm^?1 for Li⁺ and 5.95 × 10^?2 S · cm^?1 for H⁺ at room temperature at a polystyrene content of 50%. The results of this work suggest that high quality H⁺‐ and Li⁺‐conducting membranes can be obtained using a single radiation‐induced grafting method.

Schematic representation of the single root for preparation of Li⁺‐ and H⁺‐conducting membranes started by radiation‐induced grafting of styrene onto a PVDF film followed by chemical treatment.     

Benzylic Fragmentation (BFR), an Aromatic Analogue of E1cB Including a Homolytic Variant,as either Nucleofugal or Homolytic Path to Aryl Stabilized Carbanions or Ketyls,respectively

BFR is the decisive step of novel reductions at benzyl‐type carbons that may complement available methods. BFR is mechanistically important for understanding and control of curious findings in radical anion chemistry. Driving force of BFR is rearomatization of a dihydroarene species usually derived from anthracene. This intermediate 1 arises (solvent THF) either from coupling of anthracenidyl A ^°− with a benzyl radical or from nucleophilic reaction of anthracene hydride AH ⁻ with a benzylic electrophile followed by CH ₂ deprotonation (excess AH ⁻ ) of the generated 9‐benzyl‐dihydroanthracene. 1 spontaneously undergoes BFR either heterolytically or homolytically depending on the stabilities of carbanion 2 (or even dianion 2 , e.g. from chalcone) and ketyl 3 . BFR is more rapid with counterion Na⁺ than with Li⁺ . The overall reaction is a selective one electron (innersphere eletron transfer) or two electron reduction of a benzylic electrophile, sometimes under expected subsequent rearrangement. This account reports on mechanism and scope of these reductions; a few related cases are described.     

Counter‐ion‐specific helix formation of poly(γ‐methyl L‐glutamate) derivatives containing sulfonate group in aqueous alcohols

Charged polypeptides containing sulfonate groups were prepared by transesterification of poly(γ‐methyl L ‐glutamate) with isethionic acid. The coil–helix transition of the sulfonated polypeptides was investigated in aqueous alcohols. Marked counter‐ion specificity was observed for helix formation: Li⁺ < Na⁺ < Cs⁺ ≦ Rb⁺ ≦ K⁺; this was different to that for poly(L ‐glutamate) (PLG): Cs⁺ ? K⁺ < Li⁺ < Na⁺. Specific helix stabilization by counter‐ion mixing, which has been found for the PLG system, was not observed for the sulfonated polypeptides. The counter‐ion‐ and solvent‐specific helix formation is discussed and compared with that in PLG. © 2001 Society of Chemical Industry     

Electrical conductivity properties of poly[(maleic acid)‐co‐(vinyl pyrrolidone)] salts in aqueous solutions

The interaction of alkaline ions with an anionic dicarboxylic polyelectrolyte is studied by means of electrical conductivity measurements. The results show that lithium, sodium and potassium counter‐ions interact in a very different way with this polyelectrolyte. The Kohlrausch plots bend downwards in the high‐dilution zone. The limiting equivalent conductivities of these polymers were then determined according to Vink's method by a polynomial adjustment of specific conductivity data. Whereas the dissociation order follows the sequence Li⁺ > Na⁺ > K⁺, the trend in equivalent conductivity follows the reverse order, and apparently is mainly determined by the equivalent conductivity of the counter‐ions. The interaction parameter f was determined using the Manning theory for the limiting equivalent conductivity of the polyion species. From the values of the latter parameters, the average distance between charges agrees well with that calculated from simple geometrical calculations for a mesomeric configuration. An alternative method was used to obtain the relative interaction parameter f* as a function of polymer concentration. The results can be explained in terms of the hydration parameters and the limiting equivalent conductivities of the counter‐ions. © 2001 Society of Chemical Industry     

Extraction relationship of Li+ and H+ using tributyl phosphate in the presence of Fe(III)

ABSTRACT

During the extraction of lithium from high Mg-containing salt lake brines by tributyl phosphate (TBP) in the presence of Fe(III), H⁺ is used to stabilize Fe(III). However, the distribution ratio of H⁺ (D_H) is 4–6 times higher than that of Li⁺ (D_Li), which affects the extraction of Li⁺ significantly. In this study, the competition mechanism between H⁺ and Li⁺ was investigated by spectral analysis and thermodynamic equilibrium. The extracted species are determined as HFeCl₄ · 2TBP and LiFeCl₄ · 2TBP for H⁺ and Li⁺, respectively. The apparent equilibrium constants are K_H = 799.8 and K_Li = 120.6, respectively. Both equilibrium constants and the distribution ratios for H⁺ and Li⁺ extraction show that extraction of H⁺ is stronger than Li⁺.     

Synthesis and characterization of triazine‐based dendrimers and their application in metal ion adsorption

This article described the synthesis of triazine‐based dendrimers with poly(ethylene glycol) core by convergent method. Compound 1 was prepared by coupling of amino group of diethanolamine with cyanuric chloride in dry THF (tetrahydrofuran). Reaction of compound 1 with p‐aminobenzylamine resulted in compound 2 . Compound 4 was synthesized using coupling reaction of amino group of compound 2 with cyanuric chloride, then coupling of amine groups of p‐aminobenzylamine with compound 3 in the hybrid solvents. The final dendrimer (den‐OH) were synthesized using reaction of dendron 4 with compound 5. Ethylene diaminetetraacetic acid modified final dendrimer were successfully prepared via coupling ethylene diaminetetraacetic acid dianhydride and den‐OH. The growth of dendrons and their structures were investigated by using usual spectroscopy methods and elemental analysis. The chelating behavior and sorption capacities of triazine dendrimers were determined in relation to pH dependency for some metal ions such as Cu⁺², Ni⁺², and Zn⁺² using atomic absorption methods. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Pervaporation separation of dimethyl carbonate/methanol mixtures with regenerated perfluoro‐ion‐exchange membranes in chlor‐alkali industry

The waste perfluoro‐ion‐exchange membranes (PFIEMs) in chlor‐alkali industry were regenerated and used to the separation of dimethyl carbonate (DMC)/methanol (MeOH) mixtures by pervaporation process. The energy‐dispersive spectrum (EDS) demonstrates that the impurities on the surfaces of waste PFIEMs can be effectively cleared by the regeneration process. The degree of swelling, sorption, and pervaporation properties of the regenerated PFIEMs with different counter ions were investigated. The results indicate that the counter ions of PFIEMs conspicuously influence the degree of swelling, sorption, and pervaporation properties for DMC/MeOH mixtures. The degree of swelling and solubility selectivity both decreases with the alkali metal counter ions in the sequence: Li⁺ > Na⁺ > K⁺ > Cs⁺. The degree of swelling increases with MeOH concentration increasing in feed liquid. The pervaporation measurements illustrate that the permeation flux decreases and the separation factor increases with the rising in ion radius of counter ions. The increase of feed concentration (MeOH) and feed temperature is advantageous to improve permeation flux while at the cost of separation factor decreasing. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Nanostructured high‐energy xLi2MnO3·(1‐x)LiNi0.5Mn0.5O2 (0.3 ≤ x ≤ 0.6) cathode materials

Nanostructured lithium‐manganese‐rich nickel‐manganese‐oxide xLi₂MnO₃·(1‐x)LiNi_0.5Mn_0.5O₂ (0.3 ≤ x ≤ 0.6) composite materials were synthesized via spray pyrolysis using mixed nitrate precursors. All the materials showed a composite structure consisting of Li₂MnO₃ (C2/m) and LiNi_0.5Mn_0.5O₂ components, and the amount of Li₂MnO₃‐phase appeared to increase with x, as observed from XRD analysis. These composite materials showed a high‐discharge capacity of about 250 mAhg^?1. In the range of x considered, the layered 0.5Li₂MnO₃·0.5LiNi_0.5Mn_0.5O₂ materials displayed the highest capacity and superior cycle stability. Nonetheless, voltage suppression from a layered‐spinel phase transition was observed for all the composites produced. This voltage suppression was dependent of the amount of Li₂MnO₃ phase present in the composite structure. © 2013 American Institute of Chemical Engineers AIChE J 60: 443–450, 2014     

Metal‐solvating and self‐association of amphiphilic polyamides containing poly(oxyethylene) blocks

An amphiphilic poly(ether amide) consisting of hydrophilic poly(oxyethylene) amide blocks was prepared from the copolymerization of sebacic acid and two poly(oxyalkylene) diamines including a poly(oxyethylene) diamine (POE‐amine at 2000 M_w) and a poly(oxypropylene) diamine (POP‐amine at 230 M_w). The copolymer was estimated to have an average molecular weight of 15,000 M_w (GPC) or approximately three hydrophilic POE segments per strain. The presence of POE segments rendered polymer hydrophilicity and complexing ability for Li⁺, K⁺, Ca⁺², Ni⁺², Pd⁺², and Cu⁺² salts. In particular, lithium perchlorate affected the copolymer to the greatest extent in enhancing electrostatic dissipation or reducing surface resistivity as low as 10^5.0 Ω/sq (cross‐sectional area) at 1/180 Li⁺/EO from 10^7.2 Ω/sq (without metal ion). In such a metal complexation, the copolymer showed a new POE segmental crystalline phase at a melting temperature between ?10.4 and ?14°C, accompanied with the metal‐free original phase of ?31°C. In static toluene/water, the metal ions had no effect on the copolymer surfactancy in lowering the interfacial tension, reaching 4.4 dyn/cm at a critical micelle concentration (cmc) of 0.01 wt %. When mixing toluene and water, the lithium or nickel ions were found to be detrimental to the emulsifying process. Without the metal ion, fine droplets at average sizes of 4.5–5.0 μm were observed in the copolymer/ toluene/water emulsification. These amphiphilic behaviors of the POE‐segmented polyamide with or without metal ions were explained by the competing noncovalent bonding interactions among POE/metal ion/water. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 612–621, 2002     

Facile synthesis of 5,8‐linked quinoline‐based copolymers

Coupling quinoline at its 5,8‐positions provides a facile way to copolymerize quinoline with other conjugated moieties without breaking conjugation. Model compounds containing quinoline units, either at the periphery or in the core, were synthesized and one of them was characterized by X‐ray diffraction. Copolymers of quinoline and 9,9‐dialkylfluorene (R = n‐octyl or 2‐ethylhexyl) were synthesized in good yield by palladium‐catalysed Suzuki cross‐coupling. The copolymers are highly soluble in common organic solvents and gel permeation chromatography measurements against polystyrene standards showed the number‐average molecular weight to be in the range (1.02–2.06) × 10⁴ g mol^?1. Both copolymers emit in the blue region with an emission maximum centred at 435 nm. Copyright © 2012 Society of Chemical Industry     

Microwave assisted hydrothermal synthesis of MnO2·0.5H2O ion-sieve for lithium ion selective adsorption

Li_1.6Mn_1.6O₄ (LMO) synthesized by microwave assisted hydrothermal method were used to obtain MnO₂·0.5H₂O ion sieves (HMO) after acid treatment. The HMO-1 prepared with a Li/Mn molar ratio of 4, at 100ºC for 1 h under microwave irradiation, exhibits an effective Li⁺ adsorption capacity of 5.6 mmol·g^?1 and a high selectivity to Li⁺ with an equilibrium distribution coefficient of 12366.44 mL·g^?1. Moreover, it shows almost saturated ion-exchange capacity (> 95%) for Li⁺ extracted/inserted process. Thus HMO ion-sieves with high adsorption capacity and selectivity to Li⁺ are expected to be a promising application for Li⁺ selective adsorption from brine, seawater, and aqueous lithium generated in industries.     

Effect of Li+ ions on structure,properties, and actuation of cellulose electro‐active paper actuator

We have reported an electro‐active paper actuator from regenerated cellulose. After dissolving cellulose fibers with a solution of lithium chloride in N,N‐dimethylacetamide, cellulose was regenerated by combining distillation of cellulose solution along with washing with the mixture of deionized water, isopropyl alcohol, and running water. However, the effect of Li⁺ ions on structure, properties, and the actuation behavior of the actuator was not studied. This article describes the changes in these parameters when the Li⁺ ions are removed by subjecting it to different running water exposure time. The structure and properties of cellulose electro‐active paper and its actuation behavior were studied. As Li⁺ ions content reduced from 4354.17 to 10.26 ppm by increasing the exposure time of running water, crystallinity, Young's modulus, and bending displacement decreased. Details about the investigation have been explained. This elimination of ions is important to increase the piezoelectric effect in EAPap by decreasing the ion migration effect. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

High‐Q microwave ceramics of Li2TiO3 co‐doped with magnesium and niobium

In order to solve the problems of acceptor/donor individual doping in Li₂TiO₃ system and clarify the superiority mechanism of co‐doping for improving the Q value, Mg + Nb co‐doped Li₂TiO₃ have been designed and sintered at a medium temperature of 1260°C. The effects of each Mg/Nb ion on structure, morphology, grain‐boundary resistance and microwave dielectric properties are investigated. The substitution of (Mg_1/3Nb_2/3)⁴⁺ inhibits not only the diffusion of Li⁺ and reduction in Ti⁴⁺, but also the formation of microcracks in ceramics, which promotes the enhancement of Q value. The experiments reveal that Q × f value of Li₂TiO₃ ceramics co‐doped with magnesium and niobium is 113 774 GHz (at 8.573 GHz), which is increased by 113% compared with the pure Li₂TiO₃ ceramics. And the co‐doped ceramics have an appropriate dielectric constant of 19.01 and a near‐zero resonance frequency temperature coefficient of 13.38 ppm/°C. These results offer a scientific basis for co‐doping in Li₂TiO₃ system, and the outstanding performance of (Mg + Nb) co‐doped ceramics provides a solid foundation for widespread applications of microwave substrates, resonators, filters and patch antennas in modern wireless communication equipments.     

Optimization of Lithium Content and Sintering Aid for Maximized Li+ Conductivity and Density in Ta‐Doped Li7La3Zr2O12

Ta‐doped cubic phase Li₇La₃Zr₂O₁₂ (LLZ) lithium garnet received considerable attention in recent times as prospective electrolyte for all‐solid‐state lithium battery. Although the conductivity has been improved by stabilizing the cubic phase with the Ta⁵⁺ doping for Zr⁴⁺ in LLZ, the density of the pellet was found to be relatively poor with large amount of pores. In addition to the high Li⁺ conductivity, density is also an essential parameter for the successful application of LLZ as solid electrolyte membrane in all‐solid‐state lithium battery. Systematic investigations carried out through this work indicated that the optimal Li concentration of 6.4 (i.e., Li_6.4La₃Zr_1.4Ta_0.6O₁₂) is required to obtain phase pure, relatively dense and high Li⁺ conductive cubic phase in Li_7?xLa₃Zr_2?xTa_xO₁₂ solid solutions. Effort has been also made in this work to enhance the density and Li⁺ conductivity of Li_6.4La₃Zr_1.4Ta_0.6O₁₂ further through the Li₄SiO₄ addition. A maximized room‐temperature (33°C) total (bulk + grain boundary) Li⁺ conductivity of 3.7 × 10^?4 S/cm and maximized relative density of 94% was observed for Li_6.4La₃Zr_1.4Ta_0.6O₁₂ added with 1 wt% of Li₄SiO₄.     

Novel thin-film nanocomposite membrane with water-soluble polyhydroxylated fullerene for the separation of Mg2+/Li+ aqueous solution

Despite the prosperity of membrane technology, the separation efficiency for Mg²⁺/Li⁺ mixture is still far from satisfactory. Herein, a novel thin-film nanocomposite (TFN) membrane was developed by loading polyhydroxylated fullerene (PHF) via interfacial polymerization. The effects of the PHF dosages on the as-developed membranes were investigated comprehensively by XPS, SEM, AFM, contact angle measurements, as well as nanofiltration tests. The results revealed the TFN membrane containing 0.01% (w/v) PHF exhibited the optimum performances. The membrane showed a pure water flux of 6.7 L·m⁻²·h⁻¹·bar⁻¹ and salt rejections with the order of Na₂SO₄ (95.6%) > MgSO₄ (93.6%) > MgCl₂ (89.9%) > NaCl (22.6%) > LiCl (16.3%). The membrane not only presented a separation factor of 13.1 in separating Mg²⁺/Li⁺ mixtures, but also demonstrated excellent antifouling ability, which enables membrane regeneration without operation break, suggesting its great potentials in the recovery of Li⁺ from brine or seawater. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48029.     

Synthesis,characterization, and gamma radiation effects over well‐defined poly(vinylsiloxanes) copolymers

Anionic ring‐opening polymerization (AROP) was employed for the controlled synthesis of linear model block copolymers of 1,3,5,7‐tetrametil‐1,3,5,7‐tetravinyl(cyclotetrasiloxane) (V₄) and 1,3,5‐dimethyl(cyclotrisiloxane) (D₃) monomers by using sec‐butyl lithium (sec‐Bu^?Li⁺) as initiator, and high‐vacuum anionic polymerization techniques. V₄ copolymerization was promoted by employing D₃ and sec‐Bu^?Li⁺ producing living silanolates that open the stable V₄ ring. For this purpose, two strategies were applied: (a) sequential addition of monomers, and (b) one‐step copolymerization at different reaction temperatures. According to the experimental results, higher levels of V₄ incorporation (～ 18.14 mol %) were obtained by mixing both co‐monomers and performing the reaction at high temperature (80°C). This strategy allowed the control of the V₄ incorporation into the copolymer structure, giving the opportunity of synthesizing model vinyl‐siloxane polymers. The gamma radiation of these materials showed that lower doses are needed to achieve the same gel content as in a model poly(dimethylsiloxane) (PDMS). In such a sense, these results constitute one of the first reports regarding the effect of gamma radiation on vinyl‐containing silicon polymers, and may be of fundamental importance if a biomedical cross‐linked rubber‐type PDMS is needed at earlier doses of sterilization. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

On the difference in cycling behaviors of lithium-ion battery cell between the ethylene carbonate- and propylene carbonate-based electrolytes

Density functional theory (DFT) calculations and classical molecular dynamics (MD) simulations have been performed to gain insight into the difference in cycling behaviors between the ethylene carbonate (EC)-based and the propylene carbonate (PC)-based electrolytes in lithium-ion battery cells. DFT calculations of the lithium solvation, Li⁺(S)_i (S = EC or PC; i = 1–4) with and without the presence of the counter anion showed that the desolvation energy to remove one solvent molecule from the first solvation shell of the lithium ion was significantly reduced by as much as 70 kcal mol⁻¹ (293.08 kJ mol⁻¹) in the presence of the counter anion, suggesting the lithium ion is more likely to be desolvated at high salt concentrations. The thermodynamic stability of the ternary graphite intercalation compounds, Li⁺(S)_iC₇₂, in which Li⁺(S)_i was inserted into a graphite cell, was also examined by DFT calculations. The results suggested that Li⁺(EC)_iC₇₂ was more stable than Li⁺(PC)_iC₇₂ for a given i. Furthermore, some of Li⁺(PC)_iC₇₂ were found to be energetically unfavorable, while all of Li⁺(EC)_i=1–4C₇₂ were stable, relative to their corresponding Li⁺(S)_i in the bulk electrolyte. In addition, the interlayer distances of Li⁺(PC)_iC₇₂ were more than 0.1 nm longer than those of Li⁺(EC)_iC₇₂. MD simulations were also carried out to examine the solvation structures at a high salt concentration of LiPF₆: 2.45 mol kg⁻¹. The results showed that the solvation structure was significantly interrupted by the counter anions, having a smaller solvation number than that at a lower salt concentration (0.83 mol kg⁻¹). We propose that at high salt concentrations, the lithium desolvation may be facilitated due to the increased contact ion pairs so as to form a stable ternary GIC with less solvent molecules without destruction of graphite particles, followed by solid–electrolyte-interface film formation reactions. The results from both DFT calculations and MD simulations are consistent with the recent experimental observations.     

Li3AlN2—a self‐activated yellow light emitting wide‐bandgap semiconductor used for LEDs

In allusion to the problems existing in the defect‐related luminescent materials, a series self‐activated light emitting semiconductors of Li₃AlN₂: R (R=0, Na⁺, Mg²⁺, Si⁴⁺, Tb³⁺, Eu³⁺) have been successfully synthesized by sample‐pressure sintering. Under the excitation at 422 nm, a yellow light peaked at 580 nm have been observed in the host lattice of Li₃AlN₂. The crystal structure and the electron structure of Li₃AlN₂ have been measured to investigate the defect‐related luminescent properties of Li₃AlN₂ using the Rietveld refinement on the basic of X‐ray diffraction data and the density functional theory (DET). The results show that Li₃AlN₂ crystallizes in cubic phase with full filled edge‐shared (Al/Li)N₄ tetrahedrons and is a wide‐bandgap semiconductor. The impurity defects produced by ions substitution have also been investigated, which leads to the red‐shift of the emission peak. Finally, the photoluminescence excitation (PLE) spectrum of Li₃AlN₂ with two excitation bands peaked at 300 nm and 422 nm has been detected, and the latter matches well with near‐UV LED chips. The thermal stability shows that integral intensity of Li₃AlN₂ at 150°C still has 60% of the initial intensity at room temperature. The results indicate their potential applications as the LED used phosphors.     

Oxime‐Derived Palladium Complexes as Very Efficient Catalysts for the Heck–Mizoroki Reaction

Oxime‐derived, chloro‐bridged palladacycles 16 are efficient complexes for the Heck vinylation of aryl halides. The isolated catalysts are thermally stable, not sensitive to air or moisture and easily accessible from inexpensive starting materials. The reaction can be performed under aerobic conditions, with aryl iodides, bromides and chlorides with acrylic esters and olefins displaying turnover numbers (TON) of up to 10¹⁰ for phenyl iodide and turnover frequencies (TOF) of 1.4×10⁸ h⁻¹. Deactivated aryl bromides undergo the Heck reaction with styrene with TON and TOF values up to 97,000 and 6063 h⁻¹, respectively. Even aryl chlorides undergo the coupling reaction with olefins with TON up to 920. Complexes 16 catalyze the synthesis of 2,3‐disubstituted indenones and indoles in good yields via annulation reaction of internal alkynes with o‐bromo‐ or o‐chlorobenzaldehyde and o‐iodoaniline, respectively.     

N‐alkylation of chitosan by β‐halopropionic acids in the presence of various acceptors

N‐carboxyethylation of chitosan by β‐halopropionic acids in the presence of various proton and halogen ion acceptors was investigated. It has been observed that carboxyethylation of chitosan in aqueous medium is accompanied by the by‐processes of hydrolysis and dehydrohalogenation of the β‐halopropionic acids yielding β‐hydroxypropionic acid, bis(2‐carboxyethyl) ether, and acrylic acid. Degree of carboxyethyl substitution (DS) of chitosan and the relative rates of the by‐processes varied significantly depending on the conditions used and nature of the proton or halogen ion acceptor. At carboxyethylation of chitosan with the alkaline β‐bromopropionates, the DS increased in the order Cs⁺ < Rb⁺ < K⁺ ～ Na⁺ < Li⁺. For alkaline earth salts BrCH₂CH₂COOM_0.5 (M = Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺), the highest DS was obtained with strontium and barium salts, which could be subsequently removed from the reaction mixture by precipitation as sulfates. Among the organic bases applied (tetrabutylammonium hydroxide, triethylamine, trimethylamine, pyridine, 4‐N,N‐dimethylaminopyridine, 2,6‐lutidine, and 1,5‐diazabicyclo[4.3.0] non‐5‐ene), the highest DS was obtained using a moderately strong base triethylamine. For the halogen acceptors (Pb²⁺, Ag⁺, Tl⁺), the stoichiometrically highest DS was achieved in a system comprising iodopropionic acid plus Tl⁺ and a comparable conversion rate was obtained using also a combination of chloropropionic acid and Ag⁺. A novel alternative preparative approach—gel‐state synthesis—was suggested that provides for the highest DS at the optimum reaction conditions. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008