Temporal Control in Atom Transfer Radical Polymerization Using Zerovalent Metals

Polymer chain growth can be controlled in a spatiotemporal manner by external stimuli including chemical, redox, light, electrical current, and mechanical procedures. In this paper, atom transfer radical polymerization (ATRP) was investigated in the presence of zerovalent metals, such as copper or silver wire, as chemical stimuli to assert temporal control over ATRP reactions. The ATRP activator, L/Cu-I complex, was (re)generated, or catalyst was switched "on", in the presence of the metal wires to start the reaction whereas removing the wire from the solution stopped activator (re)generation. In the absence of the metal zero wires, the residual activator in the solution was consumed by irreversible radical termination processes converting activators to deactivator species-catalyst switched "off"-and hence polymerization stopped. However, the nature of the ligand played a crucial role in defining the concentrations of deactivator and activator species, [L/Cu-II]/[L/Cu-I], present in the polymerization medium. More active catalysts shifted the ATRP equilibrium toward a higher concentration of L/Cu-II and hence lower concentration of activator L/Cu-I. In this case the reaction quickly stopped in the absence of metal wires. On the other hand, lower activity catalysts provided a higher concentration of the L/Cu-I species that took a longer time to be consumed by radical termination processes so that the reactions continued for longer times in the absence of the wires.