A novel class of homogeneous nickel(II) catalysts, i.e [Ni{o,o′(CH2NMe2)2C6H3}Br], denoted as Ni(NCN′)Br, is reported to mediate in the presence of activated alkyl halides, e.g., CCl4 or R-halocarbonyl compounds, a well-controlled radical polymerization of methacrylic monomers [methyl and n-butyl methacrylate), (MMA, n-BuMA)] at rather low temperatures (<100°C). The number-average molecular weight of the polymer gradually increased with the monomer conversion and was inversely proportional to the initiator concentration of alkyl halides. The molecular weight distribution (MWD) remained very narrow during the whole course of the polymerization (MWD < 1.3). All the experimental data including a successful block copolymerization (n-BuMA-b-MMA) experiment were in agreement with a living polymerization process, and remarkably enough, poly(methyl methacrylate) (PMMA) with molecular weight up to at least 10 5 g/mol was synthesized in a controlled fashion. Increased thermal stability of the PMMA is a further indication of the high regioselectivity and the virtually absence of termination reactions. Owing to the compatibility of the Ni(II) complexes toward water, extension to aqueous suspension polymerization was attempted successfully as attested by the promising preliminary results. Indications on the mechanism let us suggest that the reactive alkyl halide or the corresponding growing chain end is reversibly activated/deactivated by single electron transfer together with the halogen transfer.

No abstract

The controlled/living polymerization of methyl methacrylate was carried out by atom transfer radical polymerization (ATRP) in a water-borne system. Both the direct and reverse processes were performed in the presence of a copper halide/4,4′-di(5-nonyl)-2,2′-bipyridine (dNbpy) complex and a nonionic surfactant. In direct ATRP, ethyl 2-bromoisobutyrate (EBiB) was used as the oil-soluble initiator. Several surfactants with a large range of hydrophile-lypophile balance (HLB) values were tested. The critical micellar concentration (cmc) and the pH of the medium were also measured to determine their effects on the emulsion stability. The direct ATRP most likely proceeds via the suspension mechanism. In reverse ATRP, 2,2′-azobis[2-(2-dimidazolin-2-yl)propane] dihydrochloride (VA-044) was used as the water-soluble initiator, and the results suggest an emulsion polymerization mechanism. In both approaches, the kinetics were followed along with the particle size (measured by laser diffraction and SEM), and the influence of the halogen atom associated with the catalyst (i.e., CuBr or CuCl) was examined. It was demonstrated that utilizing the halide exchange could improve the control over both the molecular weight and end functionality of the polymers.

Appendix 1 -1 H-NMR spectroscopic data for the major product X resulting from the reaction of diisobutylaluminum hydride with dipyridine in deuterated toluene at room temperature 6.15 (1H) dt J 7 8 = 9.89, J 7 -9 = 1.96