Effect of ionization by proton transfer on propagation rate coefficients: a PLP-SEC study of methacrylic acid-amine monomers

Abstract

Previously, we reported the dependence of the propagation rate coefficient (k_p) for methacrylic acid (MAA) and sodium methacrylate (MAANa) on monomer concentration, degree of ionization, and temperature (I. Lacík, L. Učňová, S. Kukučková, M. Buback, P. Hesse and S. Beuermann, Macromolecules, 2009, 42, 7753–7761). In this study, we extend this work to investigate the ionization of MAA via a proton transfer mechanism in the presence of primary isobutylamine (IBA) and tertiary triethylamine (TEA), which differ in their affinity for the carboxylic proton. An advantage of these systems lies in their solubility in both water and non-polar solvents due to the presence of a hydrophobic group in the cation moiety. NMR and FTIR spectroscopy showed that complete proton transfer occurs for both monomers in water, and for MAA-IBA in DMSO. In contrast, MAA-TEA forms a hydrogen-bonded molecular complex in DMSO. The Kamlet–Taft α parameter was determined as a measure of the hydrogen bond donor ability of these systems. The k_p values for these MAA-amine monomers were determined in water and DMSO over a monomer concentration range of 0.45–1.82 mol L⁻¹ and a temperature range of 20–60 °C, using pulsed laser polymerization coupled with size-exclusion chromatography. In both solvents, the k_p values for MAA-IBA are lower than those for MAA-TEA, with the difference being modest in water (up to a factor of 2) and more pronounced in DMSO (up to a factor of 4). The influence of monomer concentration on k_p is less significant than for MAANa. Activation energies, E_A(k_p), increase from 19.3 ± 1.5 and 17.8 ± 0.3 kJ mol⁻¹ in water to 28.2 ± 1.6 and 26.9 ± 1.7 kJ mol⁻¹ in DMSO for MAA-IBA and MAA-TEA, respectively. The pre-exponential factor ∼0.9 × 10⁶ L mol⁻¹ s⁻¹ is similar for both monomers in water and is increased by an order of magnitude in DMSO. These results demonstrate that k_p depends on monomer speciation and a complex interplay between electrostatic, hydrogen bonding, and hydrophobic interactions.

---

Introduction

Understanding the general principles of radical polymerization mechanisms has been a subject of research for many years. The main emphasis in kinetic studies of radical polymerization is the determination of the propagation rate coefficient, k_p. The pulsed-laser polymerization in conjunction with size-exclusion chromatography (PLP-SEC) method, introduced by Olaj et al. in 1987,^1,2 has become the method of choice for the determination of k_p. This method relies on conducting the polymerization induced by cyclic laser pulses generating primary radicals responsible for both chain growth initiation and termination, which allows controlling the course of propagation and obtaining a specific PLP-structured molar mass distribution (MMD). Characteristic inflection points in the first derivative of the MMD allow the estimation of the k_p values for the monomers under investigation. Because of its high accuracy and broad applicability, PLP-SEC has become the IUPAC-recommended method for determination of k_p values in both bulk and solution polymerizations, providing the benchmark k_p values for styrene,³ methacrylate and acrylate monomers,^4–6 vinylacetate,^7,8 and methacrylic acid in aqueous solution.⁹ Availability of reliable k_p values enables the determination of termination and transfer rate coefficients. Consequently, the understanding of kinetics and mechanism of radical polymerization has been significantly enhanced, including polymerizations carried out in bulk and organic solvents,¹⁰ in aqueous solutions,¹¹ and in ionic liquids.¹²

The course of polymerization, with particular emphasis on propagation, depends on many factors. The most fundamental determinant of k_p is the monomer structure. This has been demonstrated for polymerizations in the organic phase for acrylate¹³ and methacrylate¹⁴ monomers, where k_p increases with the length of the ester group, for nitrogen-containing methacrylate monomers,¹⁵ for (meth)acrylate monomers containing a heteroatom in the ester group,¹⁶ and for aqueous-phase polymerizations of acrylate, methacrylate, N-vinylamide, and zwitterionic water-soluble monomers.¹¹ Another interesting example of the influence of the monomer structure on propagation kinetics is the distinctively low k_p observed for itaconic monomers.^17–20

The reaction medium is another crucial factor influencing k_p. Numerous studies have highlighted the role of solvents in propagation kinetics in both organic^12,21–23 and aqueous media.^11,24 Monomer concentration also affects k_p, particularly for monomers that exhibit intermolecular monomer–solvent interactions. In organic solvents, this effect has been demonstrated in alcohols, where hydrogen bonding interactions lead to increased k_p at lower monomer concentration for non-functional monomers such as methyl methacrylate,¹² methyl acrylate and methoxyethyl acrylate,²⁵ as well as for functional monomers such as N-vinylpyrrolidone.²⁶ In alcohol–water mixtures, this effect is further amplified by hydrogen bonding from water molecules.^25,27 However, a much stronger influence of monomer concentration on k_p is observed for polymerization in aqueous solutions.¹¹ For both non-ionized functional^28–32 and non-functional monomers,²⁵k_p values increase by an order of magnitude in dilute aqueous solution relative to bulk polymerization. This phenomenon is primarily attributed to the enhanced rotational freedom of the transition state (TS) structure for propagation upon replacement of the monomer molecules by water molecules.¹¹ Finally, the presence of charge in the monomer structure is another factor that affects k_p. Repulsive electrostatic interactions between identical charges on monomers and growing radical chains reduce the k_p value, which can be compensated by adding salts that provide counterions capable of shielding the repulsion. Such behavior has been reported for various charged monomers, including anionic sodium acrylate³³ and methacrylate (MAANa),³⁴ permanently charged cationic monomers [2-(methacryloyloxyethyl)]trimethylammonium chloride (TMAEMC) and [3-(methacryloylaminopropyl)]tri-methylammonium chloride (MAPTAC),³⁵ and zwitterionic sulfobetaine methacrylate monomers.³⁶

Unsaturated carboxylic acids, such as methacrylic (MAA) and acrylic (AA) acids, belong to the class of ionizable monomers. Ionization of the carboxyl group can occur through neutralization and proton transfer reactions, as illustrated in Scheme 1. Neutralization proceeds via the reaction of equimolar amounts of an acid and a base (Scheme 1a), yielding a salt and water. This process does not involve the transfer of the acidic proton to the base; instead, the hydrogen atom is replaced by a cation, either metallic or organic, originating from the base. Proton transfer reactions, in contrast, follow the Brønsted–Lowry acid–base theory and involve either complete (Scheme 1b) or partial (Scheme 1c) transfer of the acidic proton from the carboxyl group to the base. The result is either ion pairs consisting of a carboxylate anion and a protonated base or hydrogen-bond-stabilized acid–base complexes.

Scheme 1 Schematic representation of the ionization of a weak carboxylic acid by (a) neutralization with sodium hydroxide, (b) complete proton transfer, and (c) partial proton transfer mechanisms, with primary or tertiary amines, respectively.

An essential aspect of the ionization of vinyl carboxylic acids is the modification of the type of intermolecular interaction occurring in systems consisting of ionized acids. Structural modification of the carboxyl group enhances the role of electrostatic interactions and eliminates the hydrogen bonding donor ability of the acid, while preserving its hydrogen bonding acceptor capacity. This modification is expected to increase the barrier towards the rotational mobility in the TS for propagation, as evidenced by a significant decrease in the pre-exponential Arrhenius parameter for MAANa³⁴ as well as TMAEMA and MAPTAC.³⁵ The ionization mechanism via proton transfer is directly influenced by the structure of the cation, typically a protonated amine or other organic bases.^37,38 The hydrophobic moiety of the protonated amine (Scheme 1b and c) suggests a solvation mechanism distinct from that of metal cations (e.g., sodium) of hydroxide used for the neutralization reaction (Scheme 1a). In aqueous solutions, salts with metallic cations dissociate, allowing water molecules to form hydration shells around ions. In contrast, acid–amine ion pairs behave more like quasi non-polar molecules and undergo hydrophobic hydration, i.e., an aqueous cage surrounding the ion pair is formed, thus making the separation of ions forming this ion pair difficult.³⁹ The presence of a hydrophobic group in the cation moiety enhances the solubility of the acid–amine salt in non-polar solvents. Furthermore, the choice of amines with varying affinity for the carboxylic proton enables the formation of ion pairs based either on the full proton transfer or on the hydrogen bonded acid–base molecular complexes.⁴⁰

Thus, the two pathways (Scheme 1bvs.Scheme 1c) generate structurally distinct species, characterized by either ion pairs or hydrogen-bonded complexes, depending on the type of amine. This outcome can be further modulated by solvent choice. In the case of vinyl carboxylic acids as proton donors, such as MAA, the structural form is expected to influence the reactivity of the double bond, which should be reflected in the k_p values.

This work aims to elucidate how the mode of ionization affects the k_p values for MAA. The effect of MAA neutralization using sodium hydroxide on k_p (Scheme 1a) has been thoroughly described.^34,41 However, no such studies exist on the use of amines as organic bases for MAA ionization. The only available report is that from the 1970s, which examined the effect of isobutylamine on the kinetics of radical polymerization of MAA in dioxane.⁴² As a motivation for this work, we propose that the k_p values for MAA-amine monomers polymerized in various solvents should be of considerable interest for controlling the propagation and termination kinetics of ionized monomers. Different ionization modes of MAA, according to Scheme 1, are expected to clarify how propagation for ionized MAA can be tuned by the degree of electrostatic interactions.

In this study, we investigate the effect of proton transfer on k_p for MAA, a monomer with well-established k_p values for polymerization in aqueous solutions in both non-ionized³² and ionized (neutralized)³⁴ forms. Isobutylamine (IBA) and triethylamine (TEA) were selected as carboxylic proton acceptors. Because the amine structure dictates the extent of proton transfer from the carboxyl group to nitrogen, the use of amines of different orders yields MAA-amine monomers with distinct proton transfer properties. In general, primary amines such as IBA (Scheme 1b) fully detach the acidic proton, leading to complete ionization of the carboxyl group, whereas tertiary amines such as TEA (Scheme 1c) form equilibrium systems between the molecular complex formed by the hydrogen bond and the ion pair.⁴⁰ The ionization states of MAA-IBA and MAA-TEA monomers were studied in water, a protic solvent that promotes proton transfer, and in DMSO, an aprotic solvent that acts as a hydrogen bond acceptor but lacks the proton transfer inducing properties. Notably, this represents the first PLP-SEC study on an ionized monomer in an organic solvent, enabled by using hydrophobic organic bases as cations. The speciation of both monomers was determined by NMR and FTIR in both solvents, and correlated with k_p values as a function of monomer concentration and temperature. The data clearly demonstrate that the degree of MAA ionization can be tuned through the choice of amine and solvent, which is directly reflected in the corresponding k_p values.

Experimental section

Materials

Methacrylic acid (MAA, 99%, stabilized with 250 ppm MEHQ, Sigma-Aldrich), isobutylamine (IBA, 99%, Sigma-Aldrich), triethylamine (TEA, ≥99.5%, Sigma-Aldrich), dimethyl sulfoxide (DMSO, anhydrous ≥99.9%, Sigma-Aldrich), diethyl ether (≥99.5%, Merck), Amberlite IRC120H (H⁺ form, Supelco), pyridine-N-oxide (PNO, 95%, Sigma-Aldrich), 2-hydroxy-2,2-dimethyl acetophenone (D1173, 97%, Ciba Specialty Chemicals Inc.), and hydroquinone monomethyl ether (Sigma-Aldrich) were used as received. Ultrapure water was obtained from an Ultrapure Water System NW Series (Heal Force Bio-Meditech Holdings Ltd).

Synthesis of monomers

Isobutylammonium methacrylate (MAA-IBA). An appropriate amount of IBA was dissolved in diethyl ether in a round-bottom flask placed in an ice bath and sealed with a rubber septum. MAA was then added dropwise from a syringe equipped with a needle until an equimolar ratio of acid to amine was reached. The resulting salt crystals were isolated by filtration through filter paper and dried under vacuum for 24 hours.

Triethylammonium methacrylate (MAA-TEA). TEA was added dropwise at an equimolar amount to MAA in a sealed vessel immersed in a cold-water bath. The resulting homogeneous liquid mixture was stirred at room temperature for approximately one hour.

¹H NMR and ¹³C NMR spectroscopy

¹H NMR and ¹³C NMR analyses were performed on a 500 MHz JOEL JNM-ECZR500 RS1 (JOEL Ltd, Japan) spectrometer at 22 °C. Spectra of solutions in D₂O were calibrated using the residual water signal, while those in DMSO-d₆ were calibrated using tetramethylsilane (TMS) as the chemical shift standard.

Hydrogen bonding ability

The hydrogen bonding donor ability, expressed by the Kamlet–Taft α parameter (α), was determined according to eqn (1) based on the chemical shifts of ¹³C NMR signals of PNO used as a molecular probe. PNO was dissolved in MAA-IBA and MAA-TEA solutions at a concentration of 0.25 mol L⁻¹ and the α parameter was obtained from eqn (1):^43–45

α = 2.32 − 0.15 × (δ_4PNO − δ_2PNO) (1)

where δ_4PNO and δ_2PNO represent the chemical shifts of PNO carbons.

Pulsed laser polymerization (PLP)

Solutions were prepared in volumetric flasks with a D1173 concentration of 20 mmol L⁻¹ and the selected monomer concentration. This relatively high D1173 concentration was necessary, as MAA-amine type monomers did not polymerize at lower concentrations. PLP experiments were conducted using an ExciStar XS 500 excimer laser (Coherent, Inc.) operating at 351 nm with corona preionization and an all-solid-state pulser. A BXUV-10.0-3X beam expander (CVI Melles Griot) was placed between the laser and the cell to evenly illuminate the solution. Approximately 1 mL of the monomer solution was transferred to a quartz cuvette (Hellma 110 OS, 10 mm path length), purged with nitrogen for 2 minutes, and thermostatted for 10 minutes. The experimental temperature ranged from 20 to 60 °C. The pulse frequency varied from 1 to 16 Hz, and 100–200 pulses were applied to achieve a polymerization conversion of approximately 5%. After the PLP experiment, a few crystals of MEHQ were added to the samples to prevent further polymerization. Samples were then purified by dialysis (Spectra/Por, MWCO 3000 g mol⁻¹) to remove unreacted MAA and associated amines. Subsequently, ∼500 mg of a strong cation exchange resin in the protonated form (Amberlite IR-120, H⁺ form) was added and the mixture was stirred for 60 minutes at room temperature. This step converted the resulting poly(MAA-IBA) and poly(MAA-TEA) polymers from their salt forms to the non-ionized poly(MAA) form. This was done for two reasons: (1) the presence of IBA and TEA hydrophobic cations may interfere with the size-based exclusion of polymers during the SEC analysis due to interactions with the column packing, and (2) the partial exchange of IBA and TEA cations for sodium during the SEC analysis would make the molar mass of the monomer unit ambiguous, reducing the accuracy of k_p values. The dialysed solutions were filtered, frozen, and lyophilized. Monomer conversion was determined gravimetrically.

Size-exclusion chromatography (SEC)

SEC analysis was performed using a Waters system comprising a degasser, 515 pump, 717 autosampler, column heater and 2414 refractive index detector. The setup included a Suprema guard column and three Suprema analytical columns (Polymer Standards Service, Mainz) with 10 µm particle size and pore sizes of 100, 1000, and 3000 Å.⁴⁶ Analyses were carried out at 50 °C using an eluent of 0.1 mol L⁻¹ Na₂HPO₄ and 200 ppm of NaN₃ in ultrapure water. The flow rate was 1 mL min⁻¹ with ethylene glycol as a flow marker. The injected sample concentration was ∼1 mg mL⁻¹ with a 100 μL injected volume. Calibration was performed using poly(MAANa) narrow-distributed standards (Polymer Standards Service, Mainz). Data acquisition and evaluation were carried out using PSS WinGPC UniChrom software (Polymer Standards Service, Mainz).

Results and discussion

Speciation of monomers

A crucial issue in this work is evaluating the degree of proton transfer from the acid (HA) to the amine (B) in the studied systems. The simplest approach is to approximate the proton transfer using the difference in pK_a values, ΔpK_a = pK_a,BH⁺/B − pK_a,HA/A⁻, based on the pK_a values of the acid and the conjugate base (BH⁺) in aqueous solutions. The larger the ΔpK_a, the stronger the acid–base interactions and the higher the degree of ionization.⁴⁷ A practical rule of thumb is that primary and secondary amines are generally capable of complete proton transfer from the acid, whereas tertiary amines are weaker proton acceptors and often show only a partial proton transfer. This results in an equilibrium between the molecular complex formed by the hydrogen bond and the ion pair.⁴⁰ In this work, MAA (pK_a = 4.6) was used, with IBA (pK_a = 10.7) as a primary amine and TEA (pK_a = 10.8) as a tertiary amine, giving ΔpK_a values ~6 in both systems. According to the studies of similar acid–base systems,⁴⁰ for primary amine-based mixtures, ΔpK_a > 4 is sufficient to achieve >99% ionization. In contrast, for tertiary amines, ΔpK_a ≫ 6 is required to ensure complete proton transfer. It should be noted that this ΔpK_a approach is valid only for bulk (neat) systems.

A more complex situation arises when neat acid–base mixtures are combined with a solvent, as the solvent significantly influences the proton transfer reaction and can substantially affect the degree of ionization. Therefore, in this study, two solvents with different dielectric permittivity (ε) values were chosen: the dipolar aprotic solvent DMSO (ε = 46), which does not favour the proton transfer, and water (ε = 80), which promotes the proton transfer reaction.⁴⁸ Additionally, and importantly from an experimental perspective, both DMSO and water readily dissolve both the photoinitiator, the MAA-amine monomers under investigation, and the PMAA-amine polymer formed during the PLP experiments.

In the first stage, the monomer–solvent systems (MAA-IBA and MAA-TEA in water or DMSO) were characterized by NMR spectroscopy. The complete NMR datasets (¹H and ¹³C NMR) are provided in the SI (Fig. S1–S8). As an example, Fig. 1 shows representative ¹H NMR spectra of DMSO-d₆ solutions of MAA, MAA-TEA, and MAA-IBA. A highly deshielded carboxyl singlet at 12.4 ppm with an integral area close to 1 is clearly visible for MAA. In contrast, for MAA-IBA, the acid proton signal disappears and a new signal appears at 8.75 ppm with a normalized integral area of 3, characteristic of primary ammonium salts.⁴⁹ This phenomenon is also observed for more diluted MAA-IBA solutions (Fig. S2), clearly indicating the formation of the isobutylammonium cation (R–NH₃⁺) via complete transfer from the acid to the amine.

Fig. 1 Representative ¹H NMR spectra of 1.82 mol L⁻¹ solutions of MAA, MAA-IBA and MAA-TEA in DMSO-d₆ at 22 °C, including the corresponding monomer structures and annotated proton signals. Labile hydrogens are highlighted in red.

A different behaviour is observed for the MAA-TEA system. In this case, a significant downfield shift of the acidic proton singlet from 12.4 ppm to 13.6 ppm is observed, without a change in its integral area. The extent of deshielding decreases with decreasing monomer concentration, reaching 11.4 ppm at 0.45 mol L⁻¹ (Fig. S4). This shift results from interactions between the carboxylic proton and TEA, which may include partial proton transfer and formation of the triethylammonium cation. It should be noted that the NH⁺ chemical shift for triethylammonium salts typically falls within the range of 8.8–11.9 ppm and does not correlate with acid strength.⁵⁰ Therefore, ¹H NMR analysis does not definitely resolve the extent of proton transfer in the MAA-TEA system in DMSO. Notably, previous NMR studies have shown that triethylamine forms hydrogen-bonded molecular complexes with carboxylic acids of similar pK_a to MAA (e.g., propionic and acetic acids), without complete proton transfer.^50,51

As shown in Table 1, the chemical shift of the carboxyl carbon is sensitive to the ionization state of the monomer. The presence of amines leads to a downfield shift of the signal from 168.8 ppm for MAA to approximately 170.6 ppm and 172.1 ppm for MAA-TEA and MAA-IBA, respectively. This deshielding of the carboxyl carbon results from the ionization of the carboxyl group. The ¹³C chemical shift for MAA-IBA in DMSO-d₆ falls within the range of 172–174 ppm, which is typical for carboxylate salts of strong organic bases, such as tetrabutylammonium acetate⁵² and 1-butyl-3-methylimidazolium acetate.⁵³ This confirms complete ionization of MAA in the MAA-IBA monomer. In contrast, the slight downfield shift observed for MAA-TEA suggests weaker acid–base interactions and an equilibrium between hydrogen bonding and salt formation within the MAA-TEA molecular complex. Upon the addition of TEA to MAA, hydrogen bonding between MAA dimers is disrupted and replaced by hydrogen bonding with TEA molecules. A schematic representation of such an equilibrium is provided in Scheme 2, which compiles the information from the literature^38,50 and incorporates findings from this work.

Scheme 2 Complex acid–base equilibria in the MAA-TEA system in DMSO, illustrating transitions between the (a) MAA-TEA hydrogen-bonded complex, (b) MAA-TEA intermediate proton-sharing complex, and (c) MAA-TEA hydrogen-bonded ion pair.^38,50

Table 1 Average chemical shifts (ppm) of carboxyl carbon in MAA, MAANa and MAA-amine monomer solutions in D₂O and DMSO-d₆ determined by ¹³C NMR measurements over a monomer concentration range of 0.45–1.82 mol L⁻¹

Monomer	D2O	DMSO-d6
a MAANa is insoluble in DMSO.
MAA	171.5	168.8
MAANa	177.4	n.d.^a
MAA-IBA	177.1	172.1
MAA-TEA	176.7	170.6

---

The second solvent used in this study is water. Due to the rapid hydrogen–deuterium exchange of carboxylic protons, ¹H NMR spectroscopy is not suitable for studying MAA-TEA and MAA-IBA in D₂O, as the carboxylic proton is not observed in the spectra (Fig. S1 and S3). For this reason, only ¹³C NMR spectroscopy (Fig. S5 and S7) was used to assess the ionization of MAA in MAA-TEA and MAA-IBA according to previously published data.^54,55Table 1 presents the chemical shifts of the carboxyl carbon for the MAA-TEA and MAA-IBA systems, as well as for MAA (non-ionized) and MAANa (completely ionized) in D₂O. As expected, ionization of the carboxyl group results in a downfield shift from 171.5 ppm for MAA to 177.4 ppm for MAANa. A similar degree of deshielding is observed for MAA-IBA (177.1 ppm) and MAA-TEA (176.7 ppm), confirming that MAA-amine ion pairs are fully ionized in aqueous solution, analogous to MAANa.

Fig. 2 shows the FTIR spectral region corresponding to C=O, C=C, and N–H absorption frequencies for MAA, MAA-IBA and MAA-TEA solutions in both D₂O and DMSO. A strong C=O stretching vibration associated with carboxylic acid cyclic dimers appears between 1710 and 1680 cm⁻¹ for MAA (in both solvents) and for the DMSO solution of MAA-TEA. In contrast, this band disappears entirely in the spectra of MAA-IBA (in both solvents) and MAA-TEA in D₂O. Instead, asymmetric stretching bands of the methacrylate anion appear in the 1580–1550 cm⁻¹ range.⁵⁶ These bands are characteristic of fully ionized MAA salts, such as tetraethylammonium methacrylate (Fig. 2b) and MAANa in D₂O (Fig. 2a). In the DMSO solution of MAA-TEA, only a weak and broad band is observed in this region, which is attributed to hydrogen bonding between the carboxylic acid and the amine. In addition to the C=O vibrations, bands corresponding to asymmetric and symmetric NH₃⁺ bending modes are expected in the 1600–1500 cm⁻¹ region.⁵⁷ The presence of both bands in Fig. 2b suggests the partial proton transfer resulting in an equilibrium between the molecular complex formed by the hydrogen bond and the ion pair, as illustrated in Scheme 2.

Fig. 2 FTIR-ATR spectra in the 1800–1450 cm⁻¹ region for 1.82 mol L⁻¹ solutions of monomers in (a) D₂O and (b) DMSO. Spectra for other monomer concentrations are shown in Fig. S10 and S11.

The degree of proton transfer ionization of MAA in the MAA-TEA system in DMSO was estimated using a classical approach⁵⁸ based on comparing the integral area of the C=O band in MAA-TEA with that of non-ionized MAA in DMSO at identical molar concentration. The resulting data (Fig. S9) show that the degree of ionization of MAA-TEA in DMSO is only slightly dependent on monomer concentration, ranging between 55% and 63%. In contrast, a different behaviour is observed for the primary amine system. The formation of the isobutylammonium salt is clearly visible in both DMSO and D₂O (Fig. 2a and b) as a broad absorption band. For the D₂O solutions of MAA-TEA, the peak at 1540 cm⁻¹ could be assigned to the carboxylate stretching vibration, as the N–H bending vibrations of tertiary amine salts are typically weak.⁵⁷ Another notable feature of the FTIR spectra is the position of the C=C stretching band, which depends on the degree of acid ionization. For MAA and MAA-TEA solutions in DMSO, this band appears at around 1631 cm⁻¹, whereas in ionized systems, such as tetraethylammonium methacrylate and MAANa, the band shifts to 1641–1643 cm⁻¹. This suggests that ionization may affect the C=C bond strength.

Next, we determined the hydrogen bonding characteristics of MAA-amine monomers in both solvents by evaluating the Kamlet–Taft α parameter. This parameter reflects the hydrogen bond donor ability of a medium, including contributions from both solute and solvent molecules, using pyridine-N-oxide (PNO) as a hydrogen bond acceptor molecular probe.⁵⁹ Typical α values range from 0.1 to 0.2 for aprotic solvents such as acetone, DMF, or DMSO, and from 1.0 for neat carboxylic acids to 1.36 for strong hydrogen bond donors such as water.^43–45Fig. 3 shows the α parameter as a function of monomer concentration for both solvents. In aqueous solutions (Fig. 3a), α values remain constant for both MAA-IBA and MAA-TEA monomers. The measured values are close to that of pure water (α_H2O = 1.32) and show no dependence on the concentration of either non-ionized MAA or fully ionized MAANa. Since water acts as both a hydrogen bond donor and an acceptor and is present at a much higher molar concentration (approx. 40–50 mol L⁻¹) than the monomers (between 0.4 and 2 mol L⁻¹), the data suggest that water molecules primarily form hydrogen bonds with the PNO probe. As a result, any contribution from the monomers, including MAA-IBA and MAA-TEA, to hydrogen bonding is effectively masked at the concentrations used. In contrast, a significant effect of MAA on the α value is observed in DMSO solutions (Fig. 3b), with an increase of about 0.2 compared to pure DMSO (α_DMSO = 0.27). However, no appreciable enhancement of the α value is observed for either MAA-amine monomers in DMSO, suggesting that the carboxyl protons are unable to form hydrogen bonds with the PNO probe. In the MAA-TEA monomer, this proton is shared within the MAA-TEA molecular complex (Scheme 2), preventing the formation of additional hydrogen bonds with PNO. Similarly, in the case of MAA-IBA, complete proton transfer from MAA to the amine occurs, thereby eliminating its hydrogen bond donating ability.

Fig. 3 Dependence of the α parameter on monomer concentration for (a) D₂O and (b) DMSO-d₆ monomer solutions at 22 °C. Dots at zero concentration represent the α values determined for the neat solvents.

To summarize, the presented NMR and FTIR spectroscopic data, together with the analysis of α values, demonstrate that the amine structure and solvent type govern the protonation state of MAA in MAA-amine monomers. In aqueous solutions, both monomers exist predominantly as ion pairs resulting from complete proton transfer, where amines act as bases and abstract the acidic proton. Unlike alkali metal cations, amines contain hydrophobic organic components that enable their solubility across a range of solvents differing in polarity, including DMSO. In contrast, the ionization behaviour of MAA-amine monomers in DMSO depends on the amine structure. With the primary amine (IBA), an ion pair is formed, similar to aqueous solutions. However, with the tertiary amine (TEA), molecular complexes are formed via partial proton transfer, resulting in an acid–base equilibrium.

PLP-SEC experiments in aqueous solutions

PLP experiments were conducted at initial monomer concentrations, c_M, between 0.45 and 1.82 mol L⁻¹ for both MAA-IBA and MAA-TEA, in the temperature range from 20 to 60 °C. The pulse frequency was set between 1 and 16 Hz, applying 100 and 200 laser pulses to keep monomer conversion below 5%. Representative molar mass distributions (MMDs) and the associated first derivative curves from the PLP experiments are shown in Fig. 4 and Fig. S12. The PLP-structured MMDs exhibit well-resolved primary, secondary, and often tertiary inflection points, showing ratios M₁/M₂ and M₁/M₃ close to 0.50 and 0.33, respectively.

Fig. 4 Molar mass distributions (solid lines) and the associated first derivative curves (dotted lines) from PLP-SEC experiments of 1.82 mol L⁻¹ (a and c) MAA-IBA and (b and d) MAA-TEA in aqueous solution at 20 and 60 °C. A pulse frequency of 4 Hz and a photoinitiator concentration c_D1173 of 20 × 10⁻³ mol L⁻¹ were applied.

In this work, we investigate the previously unexplored effect of MAA ionization via proton transfer in the presence of IBA and TEA amines on k_p and compare the results with existing data for non-ionized MAA and MAA ionized by neutralization with NaOH.^32,34 Experimental conditions and individual k_p values are summarized in Table S1. Multiple experiments were performed at the same temperature and monomer concentration using different pulse frequencies and numbers of pulses. Arithmetic mean k_p values determined for MAA-IBA and MAA-TEA in aqueous solutions as a function of c_M and temperature are listed in Table 2. The standard deviations of the averaged k_p values were consistently below 10%.

Table 2 Averaged k_p values for MAA-IBA and MAA-TEA in aqueous solution and in DMSO at various monomer concentrations (c_M) and temperatures. For comparison, k_p values from previous studies are included for methacrylic acid (MAA), sodium methacrylate (MAANa), and methyl methacrylate (MMA)

Solvent	Monomer	c M (mol L^−1)	k p (L mol^−1 s^−1)
			20 °C	30 °C	40 °C	50 °C	60 °C
Water	MAA-IBA	0.45	237 ± 6	308 ± 9	411 ± 17	553 ± 30	651 ± 20
		0.91	326 ± 25	417 ± 11	531 ± 13	677 ± 34	813 ± 15
		1.36	361 ± 14	437 ± 31	594 ± 10	751 ± 34	890 ± 28
		1.82	356 ± 26	434 ± 42	573 ± 26	735 ± 11	896 ± 25
	MAA-TEA	0.45	432 ± 16	546 ± 17	708 ± 15	892 ± 32	1075 ± 33
		0.91	545 ± 27	685 ± 53	886 ± 30	1110 ± 32	1329 ± 31
		1.36	501 ± 50	585 ± 62	792 ± 40	986 ± 22	1174 ± 35
		1.82	406 ± 41	487 ± 61	645 ± 44	801 ± 13	960 ± 31
DMSO	MAA-IBA	0.45	81 ± 1	103 ± 3	152 ± 8	223 ± 8	312 ± 3
		0.91	66 ± 2	95 ± 2	134 ± 7	193 ± 8	274 ± 13
		1.36	64 ± 6	92 ± 4	132 ± 5	186 ± 12	255 ± 9
		1.82	63 ± 4	88 ± 4	131 ± 8	198 ± 8	257 ± 8
	MAA-TEA	0.45	297 ± 33	415 ± 22	567 ± 35	792 ± 50	991 ± 91
		0.91	281 ± 23	433 ± 25	566 ± 102	852 ± 24	1123 ± 60
		1.36	250 ± 26	359 ± 35	514 ± 31	668 ± 31	925 ± 23
		1.82	225 ± 9	344 ± 31	447 ± 39	685 ± 67	899 ± 53
DMSO	MAA^60	3.5	470		990	1200	1400
Water	MAA^32	1.4			5800
	MAANa^34	1.4			1300
Bulk	MAA^32				780
	MMA^4				490
NMP	MMA^61	2.81			615

---

Data from Table 2 are replotted in Fig. 5. The magnitude and concentration dependence of k_p differ slightly between MAA-IBA and MAA-TEA. For MAA-IBA (Fig. 5a), k_p increases gradually by ∼30–50% with increasing c_M from 0.45 to 1.82 mol L⁻¹ (i.e., 3.6 to 28.5 wt%), with a tendency to level off at higher monomer concentrations. In contrast, the k_p for MAA-TEA (Fig. 5b) shows only a weak dependence on c_M, with similar values at the lowest and highest concentrations and a slight maximum at an intermediate concentration of 0.91 mol L⁻¹. Within the studied c_M and temperature ranges, the k_p range for MAA-IBA is 200–800 L mol⁻¹ s⁻¹, whereas for MAA-TEA it is 400–1400 L mol⁻¹ s⁻¹.

Fig. 5 Dependence of averaged k_p values on monomer concentration for (a) MAA-IBA and (b) MAA-TEA monomers in aqueous solution at different temperatures. Dashed lines for k_p of MAANa at 60 °C were calculated using eqn (7) from ref. 34.

Despite similar speciation for both of these monomers in an aqueous medium, as determined by NMR and FTIR as a function of c_M, their k_p values differ. Since both systems are fully ionized in water, the counterion type and size account for these differences. TEA, being less water soluble than IBA, provides stronger charge shielding attributed to stronger hydrophobic hydration³⁹ than that in more water soluble IBA. Hence, more effectively reduced electrostatic repulsion between the monomer and the growing radical chain-end for MAA-TEA than for MAA-IBA monomers is proposed to result in higher k_p values for MAA-TEA. This also likely explains the reduced dependence of k_p on c_M for MAA-TEA. The slight maximum at intermediate c_M cannot be explained with the current data. A weaker shielding by IBA, due to its higher water solubility and weaker ion pair stability due to hydrophobic hydration, likely increases the sensitivity of k_p to c_M. As c_M increases, IBA counterion concentration also increases, enhancing k_p with c_M until k_p values converge at two highest concentrations. This behaviour resembles that of fully ionized MAANa,³⁴ shown for comparison in Fig. 5. However, the concentration effect on k_p is the strongest for MAANa, intermediate for MAA-IBA, and the weakest for MAA-TEA. Sodium counterions are much more mobile than IBA and TEA counterions, and the effective shielding for MAANa is only achieved at counterion concentrations close to 5 mol L⁻¹, where k_p values for non-ionized and ionized forms are similar. Such concentrations were not studied here due to the solubility limits of MAA-IBA and MAA-TEA. Since the k_p values of MAA-IBA and MAA-TEA fall within or are somewhat below (up to a factor of 2) the range observed for MAANa (Fig. 5), it can be concluded that ionization via proton transfer reduces the k_p of MAA similarly to ionization by neutralization. The repulsive interactions and, thus, k_p values in aqueous MAA-amine systems compared to MAANa are additionally influenced by the level of shielding depending on the type of amine hydrophobic moiety.

In the next step, the averaged k_p values were used to determine Arrhenius parameters by linear fitting (Fig. 6a and b). The fitted lines are nearly parallel for all concentrations for both monomers, indicating that the initial monomer concentration does not significantly influence E_A(k_p). For MAA-IBA, a mild increase in k_p with concentration, levelling off at higher c_M, is reflected in the fit for the lowest concentration (0.45 mol L⁻¹), which lies below the fits for higher concentrations (Fig. 6a). For MAA-TEA (Fig. 6b), no systematic trend is observed. The Arrhenius fitting is complemented by 95% joint confidence intervals (JCIs),⁶² shown in Fig. 6c and d. The extensive overlap of the JCIs further demonstrates that monomer concentration does not significantly affect E_A(k_p) and A(k_p) under the studied polymerization conditions for the respective MAA-amine monomers.

Fig. 6 Linear Arrhenius fits (a and b) and 95% joint confidence intervals (JCIs) (c and d) of k_p values for (a and c) MAA-IBA and (b and d) MAA-TEA in aqueous solution at different monomer concentrations. Symbols within JCIs represent the best-fit Arrhenius parameters.

The calculated Arrhenius parameters are listed in Table 3. Although E_A(k_p) and A(k_p) are correlated parameters through Arrhenius fitting, they represent distinct physical quantities that can be separately discussed. The E_A(k_p) values vary only slightly with c_M, allowing representative averaged values of 19.3 ± 1.5 and 17.8 ± 0.3 kJ mol⁻¹ for MAA-IBA and MAA-TEA, respectively. The trend toward lower E_A(k_p) for MAA-TEA compared to MAA-IBA is also visible from the relative positions of the JCIs in Fig. 6c and d. For comparison, E_A(k_p) ∼ 15 kJ mol⁻¹ was reported for non-ionized MAA,³² while E_A(k_p) ∼ 11–12 kJ mol⁻¹ was obtained for fully ionized MAANa.³⁴ This reduction in E_A(k_p) for MAA upon neutralization was attributed to the increased polarity of the polymerization medium due to the presence of ionized MAA³⁴ and stronger hydration via hydrogen bonding interactions between water molecules and ionized species,⁶⁴ which polarize the double bond and reduce the E_A(k_p). In contrast, ionization by proton transfer increases E_A(k_p) compared to that for non-ionized MAA polymerized, yielding values intermediate between those of non-ionized MAA in water and methacrylate monomers polymerized in organic media, i.e., E_A(k_p) = 22.4 kJ mol⁻¹ for methyl methacrylate.⁴ This indicates that the cation structure strongly influences propagation: compared to sodium, protonated amines with hydrophobic alkyl chains reduce hydrogen bonding with water and lower the polar effects of the medium, thereby increasing E_A(k_p).

Table 3 Arrhenius parameters for MAA-IBA and MAA-TEA polymerizations in aqueous and DMSO solutions, determined from averaged k_p values at different c_M values and 20 and 60 °C. For comparison, Arrhenius parameters are listed for methacrylic acid (MAA), sodium methacrylate (MAANa), and methyl methacrylate (MMA) from previous studies. k_{p,25 °C} values were calculated from Arrhenius parameters

Solvent	Monomer	c M (mol L^−1)	E A(kp) (kJ mol^−1)	A(kp) × 10^−6 (L mol^−1 s^−1)	k p,25 °C (L mol^−1 s^−1)
Water	MAA-IBA	0.45	20.9 ± 0.6	1.24 ± 0.2	270
		0.91	18.7 ± 0.2	0.7 ± 0.1	370
		1.36	18.6 ± 0.7	0.7 ± 0.2	390
		1.82	19.2 ± 0.7	0.9 ± 0.2	390
	MAA-TEA	0.45	18.8 ± 0.9	1.0 ± 0.1	510
		0.91	18.1 ± 0.4	0.9 ± 0.1	600
		1.36	17.7 ± 0.9	0.7 ± 0.2	600
		1.82	17.7 ± 0.7	0.6 ± 0.1	470
DMSO	MAA-IBA	0.45	26.6 ± 1.3	4.5 ± 1.8	98
		0.91	28.6 ± 0.6	8.1 ± 1.7	79
		1.36	28.4 ± 0.2	7.2 ± 0.5	76
		1.82	29.3 ± 0.6	10.2 ± 2.0	75
	MAA-TEA	0.45	25.2 ± 0.7	9.1 ± 2.2	350
		0.91	28.0 ± 0.9	28.6 ± 8.0	360
		1.36	26.5 ± 0.7	13.1 ± 2.8	300
		1.82	28.1 ± 0.7	22.7 ± 5.5	270
DMSO	MAA^60	3.5	21.4	3.4	603
Water	MAA^32	1.7	14.5	1.33	3822
	MAANa^34	2.4	12.4	0.20	1341
Bulk	MAA^32		16.1	0.38	572
	MMA^63		22.4	2.67	316
NMP	MMA^61	2.8	26.8	19.64	394

---

The pre-exponential factor A(k_p) reflects the rotational potentials of hindered internal motions in the transition state (TS).^65,66 As shown in Table 3, A(k_p) varies only slightly with monomer concentration and can be represented by an average of 0.9 × 10⁶ L mol⁻¹ s⁻¹ for both MAA-IBA and MAA-TEA monomers. This indicates that the TS configurations for propagation are similar for the two systems, despite differences in the organic counterion type. These values are comparable to those of the non-ionized form of MAA at c_M ∼ 30–40 wt%,³² but are an order of magnitude higher than those for ionized MAANa.³⁴ Thus, the friction to internal rotations in the TS of proton-transfer ionized MAA more closely resembles that of non-ionized MAA than that of MAA ionized by neutralization. This reflects the speciation of MAA-amine monomers in the polymerization systems and supports the earlier discussion that hydrophobic hydration affords stronger shielding and, therefore, MAA-IBA and MAA-TEA experience weaker repulsive electrostatic interactions than MAANa. It should be reminded that this apparent similarity in A(k_p) values between non-ionized MAA and MAA-amine monomers does not originate from the same type of interaction: for non-ionized MAA polymerized in water and in bulk, A(k_p) values are governed by hydrogen bonding,³² whereas in MAA-amine systems the repulsive interactions and the level of their shielding dominate. Finally, the higher k_p values of MAA-TEA compared to MAA-IBA in aqueous solution can be attributed to the lower E_A(k_p) for MAA-TEA.

PLP-SEC experiments in DMSO solutions

PLP experiments for MAA-IBA and MAA-TEA in DMSO were carried out at the same concentration and temperature ranges as in aqueous solutions. Representative MMDs and the associated first derivative curves are shown in Fig. 7 and Fig. S13. As in water, the inflection points M₁, M₂ and, occasionally, M₃ are clearly visible, with M₁/M₂ ∼ 0.5 and M₁/M₃ ∼ 0.33.

Fig. 7 Molar mass distributions (solid lines) and the associated first derivative curves (dotted lines) obtained from PLP-SEC experiments of 1.82 mol L⁻¹ (a and c) MAA-IBA and (b and d) MAA-TEA in DMSO at 20 and 60 °C. The laser repetition rate was 2 Hz and the photoinitiator concentration c_D1173 was 20 × 10⁻³ mol L⁻¹.

Experimental conditions and individual k_p values are summarized in Table S2; averaged values are listed in Table 2 and re-plotted in Fig. 8. As in aqueous solutions, k_p is higher for MAA-TEA than for MAA-IBA, but the relative difference is much larger in DMSO, with MAA-TEA showing k_p values 3–4 times higher. k_p in DMSO is largely independent of c_M and even shows a slight decrease at higher concentrations. This suggests that although MAA-IBA and MAA-TEA exist as ion pairs in DMSO, i.e., 100% for MAA-IBA (Fig. 1) and ∼55% for MAA-TEA (Fig. S9), their increased concentration does not enhance the shielding of electrostatic repulsion, which would otherwise result in an increased k_p.

Fig. 8 Dependence of averaged k_p values on monomer concentration for (a) MAA-IBA and (b) MAA-TEA in DMSO solution at different temperatures.

Fig. 9 shows the comparison of the k_p values of the MAA-amine monomers polymerized in both solvents at 20 and 60 °C (re-plotted from Table 2) to better visualize the effects of monomer type, solvent, and temperature. The figure shows that the k_p values for MAA-IBA polymerized in aqueous solutions are 3–6 times higher than those obtained in DMSO, particularly at the lower polymerization temperature. For MAA-TEA, the k_p values in DMSO are only slightly (∼10%) lower than those in water at 60 °C, but are up to two times lower at 20 °C. Qualitatively, this highlights the importance of hydrogen bonding with water, which becomes more pronounced at lower temperatures. Interestingly, the k_p values of MAA-TEA in DMSO are approximately 2-fold and 1.5-fold lower at 20 °C and 60 °C, respectively, compared to those reported for non-ionized MAA polymerized in DMSO.⁶⁰ This suggests that the MAA-TEA monomer behaves similarly to non-ionized MAA, where the propagation step is influenced by a balance of intramolecular hydrogen bonding interactions (via the proton-shared complex) and intramolecular electrostatic repulsion (via the ion pair; Scheme 2). In contrast, the 6- to 8-fold decrease in k_p for MAA-IBA compared to that for non-ionized MAA in DMSO⁶⁰ as well as in bulk³² indicates that repulsive electrostatic interactions dominate when hydrogen bonding is absent.

Fig. 9 Dependence of averaged k_p values on monomer concentration for MAA-IBA and MAA-TEA polymerized in aqueous and DMSO solutions at (a) 20 °C and (b) 60 °C. For comparison, the dashed lines represent the dependence of k_p on monomer concentration reported for MAANa in water³⁴ and MAA in DMSO,⁶⁰ respectively.

Linear Arrhenius fits based on averaged k_p values are shown in Fig. 10a and b with the resulting parameters in Table 3. As in aqueous solution, the fits are nearly parallel and close together for different concentrations, consistent with the overlapping JCIs shown in Fig. 10c and d. Averaged E_A(k_p) values are 28.2 ± 1.6 for MAA-IBA and 26.9 ± 1.7 kJ mol⁻¹ for MAA-TEA. These high values (24–32 kJ mol⁻¹ within JCIs) exceed those for non-ionized MAA in water³² or DMSO,⁶⁰ and even surpass the E_A(k_p) for methyl methacrylate (MMA) polymerized in bulk.⁴ Interestingly, the Arrhenius parameters for MAA-amine monomers polymerized in DMSO are similar to the values reported for MMA polymerized in N-methyl-2-pyrrolidone (NMP),⁶¹ an aprotic dipolar solvent similar to DMSO. This suggests that despite the presence of intramolecular hydrogen bonding (MAA-TEA) and intramolecular electrostatic (MAA-IBA) interactions in DMSO, these do not significantly polarize and activate the double bond, as reflected in the unusually high E_A(k_p). A mechanism similar to that proposed for MMA in NMP,⁶¹i.e., stabilization of a radical–solvent complex counterbalanced by increased A(k_p) due to monomer–solvent complexation, may also apply here, although further investigation is required. Notably, the A(k_p) values of MAA-amine monomers in DMSO are about an order of magnitude higher than those in water and approach or exceed those for MMA in bulk (Table 3). This implies reduced hindrance to rotational mobility in the TS, consistent with suppressed intermolecular interactions in the aprotic solvent.

Fig. 10 Linear Arrhenius fits (a and b) and 95% joint confidence intervals (JCIs) (c and d) of k_p values for (a and c) MAA-IBA and (b and d) MAA-TEA in DMSO at different monomer concentrations. Symbols within JCIs represent the best-fit Arrhenius parameters.

Conclusion

The effect of ionization on the propagation rate coefficient, k_p, is an important topic in ongoing efforts to better understand the kinetics and mechanisms of radical polymerization. The ionization of methacrylic acid (MAA) by neutralization to its sodium salt (MAANa) is considered as a model system for such studies. In this work, we investigated the effect of MAA ionization via proton transfer, using two model amines – primary isobutylamine (IBA) and tertiary triethylamine (TEA) – on k_p in water and DMSO. Ionization of MAA by proton transfer and the presence of hydrophobic amine moieties facilitate the solvation of MAA-amine monomers in both aqueous and organic solvents, creating a unique opportunity to study the k_p values for an ionized monomer under two highly distinct solvent conditions. To our knowledge, this represents the first pulsed laser PLP-SEC study of an ionized monomer in an organic solvent.

Amine type and solvent environment were shown to control the degree of MAA ionization, which is reflected in k_p values. Prior to PLP-SEC experiments, the speciation analyses of MAA-amine monomers were carried out as a function of amine type (IBA vs. TEA) and solvent polarity and hydrogen-bonding characteristics (water vs. DMSO). ¹H and ¹³C NMR and FTIR spectroscopy revealed complete ionization of MAA-IBA, forming ion pairs in both solvents, while MAA-TEA forms ion pairs in water and molecular complexes via partial proton transfer in DMSO.

k _p values were successfully determined for both MAA-amine monomers across a wide range of concentrations (0.45 and 1.82 mol L⁻¹) and temperatures (20 to 60 °C) in both water and DMSO. In water, k_p is primarily governed by electrostatic repulsion. Although both monomers show similar speciation in water, their k_p values differ: the k_p for MAA-TEA is about twice that of MAA-IBA, which is attributed to stronger shielding of electrostatic interactions due to hydrophobic hydration. For MAA-IBA, k_p slightly increases as a function of increased monomer concentration, although this effect is weaker than that for ionized MAANa. MAA-TEA does not show dependence of k_p on monomer concentration. The activation energy, E_A(k_p), for both monomers lies between the values reported for methacrylates in an organic environment and those for non-ionized and ionized MAA in water, reflecting reduced polar effects due to hydrophobic alkyl chains of the protonated amines. The pre-exponential factors fall between those of ionized and non-ionized MAA, consistent with effective shielding of electrostatic repulsion in MAA-amine monomers.

The influence of amine type on k_p is more pronounced in DMSO than in water. The k_p values for MAA-TEA are 3–4 times higher than those for MAA-IBA, while the k_p values for MAA-IBA are 3–6 times lower in DMSO than in water. These differences depend on monomer concentration and temperature. In DMSO, k_p is essentially independent of monomer concentration, showing a slight decrease at higher concentrations. For MAA-TEA, k_p is governed by a balance of intramolecular repulsion and hydrogen-bonding interactions, whereas MAA-IBA is dominated by repulsive interactions. Notably, E_A(k_p) values in DMSO are ∼10 kJ mol⁻¹ higher than those in water, suggesting that the interactions present in these systems do not contribute to C=C bond activation during propagation.

Overall, these findings show that the propagation kinetics for acidic vinyl monomers are jointly governed by the ion pair structure and the solvent environment, providing the framework for tuning the propagation rate in both aqueous and organic media. Further work should focus on polymerization of MAA-amine monomers in organic solvents, systematically varying amines and solvents to clarify whether the trends reported here are general and can be used to control the kinetics of functional monomers, such as MAA. Promising directions include tuning solvent polarity with mixed organic/aqueous systems, as recently demonstrated for k_p values^25,27 and copolymerization^67,68 of functional and non-functional monomers.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available and contains individual k_p values for MAA-IBA and MAA-TEA polymerized in aqueous (Table S1) and DMSO (Table S2) solutions, ¹H NMR spectra of MAA-IBA in D₂O (Fig. S1) and DMSO-d₆ (Fig. S2), ¹H NMR spectra of MAA-TEA in D₂O (Fig. S3) and DMSO-d₆ (Fig. S4), ¹³C NMR spectra of MAA-IBA in D₂O (Fig. S5) and DMSO-d₆ (Fig. S6), ¹³C NMR spectra of MAA-TEA in D₂O (Fig. S7) and DMSO-d₆ (Fig. S8), the degree of ionization by proton transfer for MAA-TEA from FTIR in DMSO (Fig. S9), FTIR-ATR spectra for MAA-IBA (Fig. S10) and MAA-TEA (Fig. S11) in water and DMSO, MMDs and associated first-derivative curves for MAA-IBA and MAA-TEA polymerized in aqueous (Fig. S12) and DMSO (Fig. S13) solutions at 40 °C. See DOI: https://doi.org/10.1039/d5py01000d.

Acknowledgements

The authors acknowledge the support of the Polish National Agency for Academic Exchange (NAWA) in the Exchange Programme for Students and Scientists as part of bilateral cooperation (No. BPN/BIL/2021/1/00068/U/00001) (SW). This work was also supported by the Slovak Scientific Grant Agency VEGA (project no. 2/0143/23) (IL) and partially by the Slovak Academic Information Agency (SAIA) scholarship grant (ID 26879) (SB).

References

1. O. F. Olaj and I. Bitai, Angew. Makromol. Chem., 1987, 155, 177–190.
2. O. F. Olaj, I. Bitai and F. Hinkelmann, Makromol. Chem., 1987, 188, 1689–1702.
3. M. Buback, R. G. Gilbert, R. A. Hutchinson, B. Klumperman, F. D. Kuchta, B. G. Manders, K. F. O'Driscoll, G. T. Russell and J. Schweer, Macromol. Chem. Phys., 1995, 196, 3267–3280.
4. S. Beuermann, M. Buback, T. P. Davis, R. G. Gilbert, R. A. Hutchinson, O. F. Olaj, G. T. Russell, J. Schweer and A. M. van Herk, Macromol. Chem. Phys., 1997, 198, 1545–1560.
5. S. Beuermann, M. Buback, T. P. Davis, N. García, R. G. Gilbert, R. A. Hutchinson, A. Kajiwara, M. Kamachi, I. Lacík and G. T. Russell, Macromol. Chem. Phys., 2003, 204, 1338–1350.
6. J. M. Asua, S. Beuermann, M. Buback, P. Castignolles, B. Charleux, R. G. Gilbert, R. A. Hutchinson, J. R. Leiza, A. N. Nikitin, J.-P. Vairon and A. M. van Herk, Macromol. Chem. Phys., 2004, 205, 2151–2160.
7. C. Barner-Kowollik, S. Beuermann, M. Buback, R. A. Hutchinson, T. Junkers, H. Kattner, B. Manders, A. N. Nikitin, G. T. Russell and A. M. van Herk, Macromol. Chem. Phys., 2017, 218, 1600357.
8. R. A. Hutchinson and S. Beuermann, Pure Appl. Chem., 2019, 91, 1883–1888.
9. S. Beuermann, M. Buback, P. Hesse, F.-D. Kuchta, I. Lacík and A. M. van Herk, Pure Appl. Chem., 2007, 79, 1463–1469.
10. S. Beuermann and M. Buback, Prog. Polym. Sci., 2002, 27, 191–254.
11. M. Buback, R. A. Hutchinson and I. Lacík, Prog. Polym. Sci., 2023, 138, 101645.
12. S. Beuermann, Macromol. Rapid Commun., 2009, 30, 1066–1088.
13. A. P. Haehnel, B. Wenn, K. Kockler, T. Bantle, A. M. Misske, F. Fleischhaker, T. Junkers and C. Barner-Kowollik, Macromol. Rapid Commun., 2014, 35, 2029–2037.
14. A. P. Haehnel, M. Schneider-Baumann, L. Arens, A. M. Misske, F. Fleischhaker and C. Barner-Kowollik, Macromolecules, 2014, 47, 3483–3496.
15. K. B. Kockler, F. Fleischhaker and C. Barner-Kowollik, Polym. Chem., 2016, 7, 4342–4351.
16. A. P. Haehnel, M. Stach, A. Chovancová, J. M. Rueb, G. Delaittre, A. M. Misske, I. Lacík and C. Barner-Kowollik, Polym. Chem., 2014, 5, 862–873.
17. E. Meyer, T. Weege and P. Vana, Polymers, 2023, 15, 1345.
18. L. H. Yee, M. L. Coote, R. P. Chaplin and T. P. Davis, J. Polym. Sci., Part A: Polym. Chem., 2000, 38, 2192–2200.
19. Z. Szablan, M. H. Stenzel, T. P. Davis, L. Barner and C. Barner-Kowollik, Macromolecules, 2005, 38, 5944–5954.
20. P. Vana, L. H. Yee and T. P. Davis, Macromolecules, 2002, 35, 3008–3016.
21. S. Beuermann and N. García, Macromolecules, 2004, 37, 3018–3025.
22. P. Deglmann, K. D. Hungenberg and H. M. Vale, Macromol. React. Eng., 2017, 11, 1600037.
23. T. R. Rooney and R. A. Hutchinson, Ind. Eng. Chem. Res., 2018, 57, 5215–5227.
24. P. Deglmann, K. D. Hungenberg and H. M. Vale, Macromol. React. Eng., 2018, 12, 1800010.
25. M. Agboluaje, I. Refai, H. H. Manston, R. A. Hutchinson, E. Dušička, A. Urbanová and I. Lacík, Polym. Chem., 2020, 11, 7104–7114.
26. L. Uhelská, D. Chorvát, R. A. Hutchinson, S. Santanakrishnan, M. Buback and I. Lacík, Macromol. Chem. Phys., 2014, 215, 2327–2336.
27. O. J. Ajogbeje, I. Lacík and R. A. Hutchinson, Polym. Chem., 2024, 15, 2094–2103.
28. I. Lacík, A. Chovancová, L. Uhelská, C. Preusser, R. A. Hutchinson and M. Buback, Macromolecules, 2016, 49, 3244–3253.
29. I. Lacík, S. Beuermann and M. Buback, Macromolecules, 2003, 36, 9355–9363.
30. M. Stach, I. Lacík, D. Chorvát, M. Buback, P. Hesse, R. A. Hutchinson and L. Tang, Macromolecules, 2008, 41, 5174–5185.
31. M. Stach, I. Lacík, P. Kasák, D. Chorvát, A. J. Saunders, S. Santanakrishnan and R. A. Hutchinson, Macromol. Chem. Phys., 2010, 211, 580–593.
32. S. Beuermann, M. Buback, P. Hesse and I. Lacík, Macromolecules, 2006, 39, 184–193.
33. I. Lacík, S. Beuermann and M. Buback, Macromol. Chem. Phys., 2004, 205, 1080–1087.
34. I. Lacík, L. Učňová, S. Kukučková, M. Buback, P. Hesse and S. Beuermann, Macromolecules, 2009, 42, 7753–7761.
35. A. Urbanová, I. H. Ezenwajiaku, A. N. Nikitin, M. Sedlák, H. M. Vale, R. A. Hutchinson and I. Lacík, Macromolecules, 2021, 54, 3204–3222.
36. I. Lacík, P. Sobolčiak, M. Stach, D. Chorvát and P. Kasák, Polymer, 2016, 87, 38–49.
37. S. K. Mann, S. P. Brown and D. R. MacFarlane, ChemPhysChem, 2020, 21, 1444–1454.
38. M. Szafran, J. Mol. Struct., 1996, 381, 39–64.
39. E. Gojło, M. Śmiechowski and J. Stangret, J. Mol. Struct., 2005, 744–747, 809–814.
40. J. Stoimenovski, E. I. Izgorodina and D. R. MacFarlane, Phys. Chem. Chem. Phys., 2010, 12, 10341.
41. S. Beuermann, M. Buback, P. Hesse, S. Kukučková and I. Lacık, Macromol. Symp., 2007, 248, 23–32.
42. R. Z. Shakirov, D. A. Topchiev, V. A. Kabanov and L. P. Kalinina, Polym. Sci. U.S.S.R., 1972, 14, 42–52.
43. S. Bednarz, K. Mielczarek and S. Wierzbicki, J. Mol. Liq., 2023, 384, 122231.
44. H. Schneider, Y. Badrieh, Y. Migron and Y. Marcus, Z. Phys. Chem., 1992, 177, 143–156.
45. P. P. Madeira, H. Passos, J. Gomes, J. A. P. Coutinho and M. G. Freire, Phys. Chem. Chem. Phys., 2017, 19, 11011–11016.
46. I. Lacík, M. Stach, P. Kasák, V. Semak, L. Uhelská, A. Chovancová, G. Reinhold, P. Kilz, G. Delaittre, B. Charleux, I. Chaduc, F. D'Agosto, M. Lansalot, M. Gaborieau, P. Castignolles, R. G. Gilbert, Z. Szablan, C. Barner-Kowollik, P. Hesse and M. Buback, Macromol. Chem. Phys., 2014, 216, 23–37.
47. S. K. Davidowski, F. Thompson, W. Huang, M. Hasani, S. A. Amin, C. A. Angell and J. L. Yarger, J. Phys. Chem. B, 2016, 120, 4279–4285.
48. A. J. Parker, Q. Rev., Chem. Soc., 1962, 16, 163.
49. N. Bandopadhyay, K. Paramanik, G. Sarkar, S. Chatterjee, S. Roy, S. J. Panda, C. S. Purohit, B. Biswas and H. S. Das, New J. Chem., 2023, 47, 9414–9420.
50. L. E. Shmukler, I. V. Fedorova, M. S. Gruzdev and L. P. Safonova, J. Phys. Chem. B, 2019, 123, 10794–10806.
51. P. Judeinstein, C. Iojoiu, J. Y. Sanchez and B. Ancian, J. Phys. Chem. B, 2008, 112, 3680–3683.
52. J. C. Bieniek, B. Mashtakov, D. Schollmeyer and S. R. Waldvogel, Chem. – Eur. J., 2024, 30, e202303388.
53. Q. Xia, H. Peng, L. Yuan, L. Hu, Y. Zhang and R. Ruan, RSC Adv., 2020, 10, 11643–11651.
54. E. L. Ibarra-Montaño, N. Rodríguez-Laguna, A. Sánchez-Hernández and A. Rojas-Hernández, J. Appl. Solution Chem. Model., 2015, 4, 7–18.
55. R. Hagen and J. D. Roberts, J. Am. Chem. Soc., 2002, 91, 4504–4506.
56. P. R. Pike, P. A. Sworan and S. E. Cabaniss, Anal. Chim. Acta, 1993, 280, 253–261.
57. R. M. Silverstein, G. C. Bassler and T. C. Morrill, Spectrometric Identification of Organic Compounds, Wiley, 1991.
58. R. Lindemann and G. Zundel, J. Chem. Soc., Faraday Trans. 2, 1977, 73, 788–803.
59. M. J. Kamlet, J. L. M. Abboud, M. H. Abraham and R. W. Taft, J. Org. Chem., 2002, 48, 2877–2887.
60. F.-D. Kuchta, A. M. van Herk and A. L. German, Macromolecules, 2000, 33, 3641–3649.
61. M. D. Zammit, T. P. Davis, G. D. Willett and K. F. O'Driscoll, J. Polym. Sci., Part A: Polym. Chem., 1997, 35, 2311–2321.
62. A. M. van Herk, J. Chem. Educ., 1995, 72, 138.
63. S. Beuermann, M. Buback, T. P. Davis, R. G. Gilbert, R. A. Hutchinson, O. F. Olaj, G. T. Russell, J. Schweer and A. M. van Herk, Macromol. Chem. Phys., 1997, 198, 1545–1560.
64. M. Sedlák, Č. Koňák, P. Štepánek and J. Jakeš, Polymer, 1990, 31, 253–257.
65. J. P. A. Heuts, R. G. Gilbert and L. Radom, Macromolecules, 2002, 28, 8771–8781.
66. M. Buback, Macromol. Symp., 2009, 275–276, 90–101.
67. N. Rajabalinia, F. Salarhosseini and R. A. Hutchinson, Macromolecules, 2025, 58, 7732–7743.
68. M. Agboluaje, G. Kaur, E. Dušička, A. Urbanová, M. Pishnamazi, B. Horváth, M. Janata, V. Raus, I. Lacík and R. A. Hutchinson, Can. J. Chem. Eng., 2023, 101, 5300–5314.

---