Radical polymerization kinetics of water-soluble monomers

Radical polymerization of monomers with functional groups such as carboxylic acid and amide moieties yields materials of significant technical importance. The reactions are mostly carried out in aqueous phase, which provides the additional advantage of using a cheap and benign solvent. In addition to varying monomer concentration, temperature and pressure, the kinetics and thus the polymer properties may be tuned by varying the degree of monomer ionization, by changing pH and ionic strength of the aqueous solution, and by addition of an organic cosolvent. These systems exhibit strong interactions via hydrogen bonds resulting in large effects on rate coefficients, even for propagation, which for long have been considered as almost insensitive towards solvent environment. The determination of rate coefficients in aqueous solution largely assists the understanding of the impact of intermolecular interactions on polymerization rate. Despite the enormous importance of polymers produced by radical polymerization in aqueous solution, the associated mechanism and the availability of accurate rate coefficients have been very limited. This situation has improved by applying pulsed-laser techniques, which enable the precise measurement of individual rate coefficients in aqueous solution as required for the simulation of radical polymerization processes.

This review primarily addresses the two most important rate coefficients, i.e., those for propagation and termination, with the diffusion-controlled termination step depending on radical chain length. Both rate coefficients have been studied over a wide range of reaction conditions. The enormous improvement in data quality reached by using methods such as pulsed-laser polymerization (PLP) – size-exclusion chromatography (SEC) and single pulse (SP) – PLP – electron paramagnetic resonance (EPR) spectroscopy is illustrated. Outlined are results for homopolymerizations of non-ionized monomers, subdivided into monomers which may or may not undergo backbiting. This reaction adds considerable complexity, as backbiting results in the formation of midchain radicals with reactivity differing largely from the one of chain-end radicals. The kinetic investigations have been extended to partially and fully ionized monomers. Examples are given of how the rate coefficients from PLP experiments are used to simulate polymerization kinetics and polymer properties of continuously-initiated systems. The review demonstrates that the basic kinetic concepts for conventional radical polymerization in organic media also apply towards polymerization of monomers in aqueous solution.