EPR Study of Backbiting in the Aqueous‐Solution Polymerization of Acrylamide

Abstract

Via electron paramagnetic resonance (EPR) spectroscopy, the type of radicals occurring during acrylamide (AAm) homopolymerization in aqueous solution is investigated between -5 and +100 °C. The radicals are produced photochemically under stationary conditions. Midchain AAm radicals (MCRs) are clearly identified by EPR which demonstrates that secondary propagating AAm radicals (SPRs) undergo backbiting reactions. Above 50 °C, the fraction of MCRs even exceeds the one of SPRs. The extent of backbiting is however well below the one in butyl acrylate polymerization at identical temperature.

Photopolymerization of acrylamide in the very low monomer concentration range

“…The slight decrease in the polymerization rate with conversion agrees more with the trend of the ratio k p /k t obtained for various water-soluble acrylic monomers (acrylic acid, methacrylic acid, acrylamide and methacrylamide) assined to the chain transfer events [7,13] and some references mentioned herein. Herein the polymerization process is accompanied by the transformation of a secondary propagating radical (sPR, k p s ) into a tertiary midchain radical (MCR, k p t ).…”

“…Figure 2 shows that the polymerization rate slightly decreases with conversion in the interval 2. Indeed the ratio k p /k t obtained for various water-soluble acrylic monomers (acrylic acid, methacrylic acid, acrylamide and methacrylamide) corrected with the chain transfer steps was observed to slightly decrease with conversion [7,13] and some references mentioned herein. The observed decrease in R p with conversion in interval 2 points out on the increased termination.…”

“…The side group is denoted by R, which is a ConH 2 ester moiety. [7] indicate that the "encapsulation" of radicals within the polymer matrix is responsible for the observed behavior (the formation of less reactive growing radicals by the chain transfer reactions).…”

“…The two types of radicals, sPRs and MCRs, differ distinctly in propagation and termination rate, which results in a strong impact of backbiting on the overall polymerization kinetics. [14] Thus, an increase in MCRs, which accompanies decreased monomer concentration, is associated with a decrease of the effective rate coefficient of propagation, k eff p , according to equation (7).…”

“…[7] By comparing the values of t pngel , t prop , and t ter , it was concluded that the propagation and termination events in the aqueous phase are the most probable fate of radicals. Herein, the formation of encapsulated radicals is less pronounced.…”