To initiate critical evaluation of propagation rate coefficients for water-soluble monomers polymerized in the aqueous phase. Data from pulsed-laser initiated polymerization in conjunction with polymer molecular weight analysis by size-exclusion chromatography will be considered. The specific role of polymerization conditions with water being the polymerization medium will be emphasized.
Knowing the kinetics and mechanism of polymerization provides a thorough understanding of polymerization processes and allows for the simulation of polymer properties and polymerization rates. The precise knowledge of rate coefficients, as is documented by the activities of the IUPAC Subcommittee on Modeling of Polymerisation Kinetics and Processes (former Working Party IV.2.8), was highly insufficient until recently. The situation has been improved by employing the IUPAC-recommended PLP-SEC technique which combines pulsed-laser initiated polymerization (PLP) with size-exclusion chromatography (SEC). This technique provides benchmark propagation rate coefficient, kp, of free-radical polymerization that enable quantification also of the individual termination and transfer rate coefficients for homopolymerization and copolymerization of a number of hydrophobic monomers, including styrene, several alkyl methacrylates, functional methacrylates, and, more recently, butyl acrylate.
The information on individual rate coefficients for free-radical polymerization of water-soluble monomers polymerized in the aqueous phase is significantly lagging behind the one for hydrophobic monomers due to several serious difficulties encountered in applying the PLP-SEC technique to these polymerization systems. On the other hand, precise rate coefficients are badly needed, in particular as aqueous-phase free-radical polymerizations of water-soluble monomers are important industrial polymerization process, by which polymers for hydrogels, superabsorbents, cosmetics, membranes, thickeners, flocculants, etc. are produced.
Recent activities of several groups around the world demonstrate that careful experiments can provide access to precise kp values for a wide variety of aqueous-phase polymerization conditions. Polymerization is strongly influenced by solvent effects of the hydrogen-bonded monomer, polymer and water species. Further, it has been recognized that polymerization process is controlled by monomer concentration, ionic strength, pH (in case of ionizable monomers), and that characterization of the molecular weight distribution by SEC may pose problems. Moreover, in case of the acrylate family of monomers, e.g. acrylic acid and acrylamide, intra- and intermolecular transfer reactions need to be taken into account, in particular under technically relevant polymerization conditions of temperature and conversion.
It is the aim of this project to provide reliable information on propagation rate coefficients for free-radical polymerizations in the aqueous phase. The discussion will start with the critical evaluation of recently obtained PLP-SEC kp data for methacrylic acid and will be extended to the analysis of kp values for polymerization of other monomers in aqueous phase. General recommendations for studying the aqueous phase kinetics of water-soluble monomers by the PLP-SEC technique will be another important outcome of this project. Furthermore, the discussion will provide information on aqueous-phase SEC analysis.
Jan 2005 – project announcement published inÂ Chem. Int. Jan/Feb 2005
The project has resulted in the publication of a 6th paper in the series of benchmark data sets of critically evaluated propagation rate coefficients: “Critically evaluated rate coefficients for free-radical polymerization Part 6: Propagation rate coefficient of methacrylic acid in aqueous solution”, S. Beuermann, M. Buback, P. Hesse, F.-D. Kuchta, I. LacÃk, and A.M. van Herk, Pure Appl. Chem. 79(8), 1463-1469, 2007 [doi:10.1351/pac200779081463]