Free‐radical propagation rate coefficients

Abstract

AbstractThe pulsed laser polymerization‐size‐exclusion chromatography (PLP‐SEC) technique has become the International Union of Pure and Applied Chemistry (IUPAC)‐recommended method for the accurate determination of propagation rate coefficients, kp, for radical polymerization. The fundamental insight provided by Heuts, Gilbert, and Radom into the relevance of transition state dynamics allows for an adequate understanding of the dependence of kp on radical chain length, solvent environment, and monomer conversion. The associated entropic effects are particularly pronounced in aqueous solution, but are also found in less polar media and with non‐polar monomers. Moreover, these arguments are applicable towards the interpretation of copolymerization reactivity ratios. The addition of sodium hydroxide to aqueous solutions of (meth)acrylic acid yields partially and even fully ionized systems with clearly reduced kp, which indicates the action of repulsive forces between the equally charged monomer and radical species. Increasing the concentration of monomer as well as the addition of salts may reverse this effect and increase kp due to counterion activity.