Reversible-Deactivation Radical Polymerization of Methyl Methacrylate and Styrene Mediated by Alkyl Dithiocarbamates and Copper Acetylacetonates

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(lifa)(2)] provided better control than [Cu(acac)(2)] for the polymerization of MMA, leading to PMMA with narrow molecular weight distribution, M-w/M-n similar to 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 M-w/M-n 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/M-n. 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/Cu-III 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.