Altering the effectiveness of radical traps in atom transfer radical coupling reactions of polymer chains

Abstract

Atom transfer radical coupling (ATRC) reactions assisted by radical traps were studied under various conditions, with the extent of coupling of polymer chains monitored as a function of time. When using a radical trap that tended to aggregate into dimers, such as 2-methyl-2-nitrosopropane (MNP), the rate of coupling was enhanced when conditions were adjusted to favor its de-aggregation to the monomeric form: less polar solvent mixtures and/or the addition of β-cyclodextrin (β-CD). With radical traps that did not aggregate, such as nitrosobenzene (NBz) and N-tert-Butyl-α-phenylnitrone (tBuPN), coupling was generally accelerated under conditions consistent with higher KATRP values. MALDI-TOF analysis was used to confirm the incorporation of the radical trap into the polymer dimers, supporting the mechanistic pathway proposed for radical trap-assisted ATRC. The effect of aggregation of the radical trap was further probed by using fractional equivalents of NBz in the coupling reaction, with a full equivalent leading to end-capped polymer chains that were unable to dimerize. By reducing the equivalents of NBz, the extent of coupling could be controlled by the temperature, with higher temperatures (80 °C) allowing ATRC to compete with RTA-ATRC and higher extents of coupling were observed.