The central goal of this contribution is to introduce to the attendees of the workshop Charged Soft Matter: Bridging Theory and Experiment the topic of radical polymerization in aqueous solutions, particularly the issues connected with the propagation rate coefficient, kp, for charged monomers.
The kp values are of principal interest for understanding of radical polymerization, simply because they determine the rate of polymer chain growth, i.e. the polymer formation by addition of unsaturated monomer to the growing radical chain-end. Additionally, they are required to determine termination and transfer rate coefficients to ultimately enable modeling of polymerization kinetics and molar mass of resulting polymers. This principle genuinely requires the access to accurate kp values, which became available to the polymer community thanks to the pioneering work of Prof. O.-F. Olaj and his co-workers from the University of Vienna, who introduced the combination of pulsed-laser polymerization with size-exclusion chromatography (PLP-SEC) as a straightforward method for kp determination [1].
The kp values determined for a given monomer in different solvents provide the information on the effect of the solvent environment on the propagation step. This phenomenon is known as a solvent effect that is specifically manifested for polymerizations carried out in aqueous solutions. The presence of functional groups (e.g., hydroxy, carboxy, amino, ammonium, carboxylate, sulfate) in the monomer structure gives rise to intra and intermolecular specific interactions (hydrogen bonding, electrostatic, van der Waals) that are amplified by interactions with water used as a solvent. This greatly complicates the understanding of the polymerization mechanism and kinetics compared to the polymerizations in non-aqueous solutions. In the last two decades we achieved a significant understanding of radical polymerization for water-soluble monomers in aqueous solutions, with the comprehensive summary recently published by Buback et al [2]. For different monomer categories, the dependence of kp values on monomer concentration, monomer conversion, temperature, pressure, and for ionized and ionizable monomers, on degree of ionization and ionic strength became available along with understanding of fundamental mechanisms determining the magnitude of kp values. The mechanistic picture of chain growth, already challenging for non-ionized monomers due to the presence of predominantly hydrogen bonding interactions, is further complicated by the presence of electrostatic interactions in the case of charged monomers. The kp values were determined for ionized acrylic [3] and methacrylic [4] acids, zwiterionic monomers [5], and cationic monomers [6].
Our work focusing on the determination of the kp values for fully charged monomers by using the PLP-SEC method will be presented in more detail. The effect of polymerization conditions, including monomer concentration, concentration of added salt, temperature, pH, will be shown for cationically charged monomers [2-(methacryloyloxyl)ethyl]trimethylammonium chloride (TMAEMC) and [3-(methacryloylamino)propyl] trimethyl ammonium chloride (MAPTAC), cationically ionizable 2-(dimethylamino)ethyl methacrylate (DMAEMA), and for anionically charged sodium methacrylate (MAANa). The propagation step is governed by the molar concentration of counterions that are introduced to the polymerization system. At very low counterion content, repulsive electrostatic interactions dominate. Upon sufficient screening of these interactions by counterions, the kp values follow the trends seen for non-ionized monomers. A linear correlation between kp values and the molar concentration of counterions was found for polymerization of TMAEMC and MAPTAC. The comparison between kp for TMAEMC and MAPTAC monomers vs MAANa points at the importance of the spacer length between the charged group and the C=C double bond of a monomer. This spacer is shorter for MAANa than for TMAEMC and MAPTAC, which corresponds to higher repulsive interactions requiring a higher counterion content for effective screening in the case of MAANa than of TMAEMC and MAPTAC. These and other findings will be reported to present the current understanding of radical polymerization for charged monomers in aqueous solutions and the effect of polymerization conditions on kp values.
[1] Olaj, O.F., Bitai, I., Hinkelmann, F. Makromol. Chem. 1987, 188, 1689.
[2] Buback, M., Hutchinson, R. A., Lacík, I. Prog. Polym. Sci. 2023, 138, 101645.
[3] Lacík, I., Beuermann, S., Buback, M. Macromol. Chem. Phys. 2004, 205, 1080.
[4] Lacík, I., Učňová, L., Kukučková, S., Buback, M., Hesse, P., Beuermann S. Macromolecules 2009, 42, 7753.
[5] Lacík, I., Sobolčiak, P., Stach, M., Chorvát, D., Kasák P. Polymer 2016, 87, 38.
[6] Urbanová, A., Ezenwajiaku, I. H., Nikitin, A.N., Sedlák, M., Vale, H.M., Hutchinson, R.A.H., Lacík, I.Macromolecules 2021, 54, 3204.
Acknowledgement. This work was supported by the Slovak Scientific Grant Agency VEGA 2/0143/23 and by funding from BASF SE Ludwighshafen, Germany.