Effect of (pseudo)halide initiators and copper complexes with non-halogen anions on the atom transfer radical polymerization

Atom transfer radical polymerization (ATRP) relies on an activation/deactivation cycle where a lower oxidation state metal, such as Cu(I)YL, abstracts a (pseudo)halogen from an activated alkyl (pseudo)halide, RX, to generate an active radical and the higher oxidation state metal, Cu(II)XYL [L = ligand = 4,4'-di-(5-nonyl)-2,2'-bipyridine (dNbpy)]. This investigation focused on the effect of X on the alkyl compound and Y on the copper(l) species in the ATRP of styrene and methyl acrylate. Using either symmetrical combinations of copper species and initiators, where X Y, or unsymmetrical combinations, where X 4 Y, the polymerization kinetics were monitored to determine the difference in reactivity between halogens like chlorine, bromine, or iodine and (pseudo)halogens such as the thiocyanate group (SCN). The results show that pseudo(halogens) can be used to initiate an ATRP reaction, but the polymerization is only controlled in the presence of a copper(l) halide complex. This suggests that there is an exchange between X and Y once the Cu(II)XY complex is formed to generate a CuX species and Y terminated chain end and that the carbon-centered radical will preferentially bond to a halogen as opposed to the pseudo(halogen). This was confirmed by using Cu(l) species with non-exchangeable anions like CuPF6, Which led to poorly controlled ATRP.