Reversible-deactivation radical polymerization of methyl methacrylate and styrene mediated by alkyl dithiocarbamates and copper acetylacetonates

Abstract

Reversible-deactivation radical polymerization (RDRP) of methyl methacrylate (MMA) and styrene (St) was successfully mediated by copper(II) acetylacetonate, [Cu(acac)2], or copper(II) hexafluoroacetylacetonate, [Cu(hfa)2], in conjunction with 1-cyano-1-methylethyl diethyldithiocarbamate (MAN-DC) or 2-(N,N- diethyldithiocarbamyl)ethyl isobutyrate (EMA-DC) initiators/transfer agents in the absence of additional reducing agents. Linear first-order kinetic plots were obtained for the polymerization of MMA in the presence of [Cu(hfa)2] or [Cu(acac)2] and MAN-DC. [Cu(hfa)2] provided better control than [Cu(acac)2] for the polymerization of MMA, leading to PMMA with narrow molecular weight distribution, Mw/Mn ∼ 1.1. Polymerization of St was successfully carried out with either MAN-DC or EMA-DC in the presence of [Cu(hfa)2], also resulting in polymers with low Mw/Mn values. In the absence of alkyl dithiocarbamates or copper acetylacetonates, the polymerizations resulted in only trace amounts of polymers or polymers with high values of M w/Mn. Thus, the combination of alkyl dithiocarbamates and copper(II) acetylacetonates provides a convenient way to prepare well-controlled PMMA and PSt. NMR analysis of low-MW polymers reveals the presence of DC groups as chain ends. DFT calculations show that DC group transfer from a H-MMA-DC model of the growing chain to the Cu(II) complexes is energetically accessible and more favorable than Br atom transfer, thus rationalizing the need for the Cu(II)/dithiocarbamate combination for successful control and suggesting that the process takes place by reversible DC group transfer involving a Cu II/CuIII couple. Attempts to synthesize complexes [Cu(acac)2(DC)] and [Cu(hfa)2(DC)], in combination with DFT calculations, suggest that these complexes are thermodynamically unstable relative to the bis(diketonate)copper(II) and dithiuram disulfide, but this does not preclude the involvement of the Cu(III) species as a spin trap in RDRP controlled by DC group transfer. © 2013 American Chemical Society.