A Detailed investigation of the experimental conditions for the reversible addition fragmentation chain transfer‐mediated copolymerization of acrylonitrile and butadiene

The synthesis of acrylonitrile‐butadiene rubbers (NBRs) via trithiocarbonate‐mediated reversible addition fragmentation chain transfer (RAFT) polymerization of acrylonitrile (ACN) and 1,3‐butadiene (BD) in solution under azeotropic conditions (38/62) was investigated for a broad range of common solvents: N,N‐dimethylacetamide (DMAc), chlorobenzene, 1,4‐dioxane, tert‐butanol, isobutyronitrile, toluene, trimethylacetonitrile, dimethyl carbonate, acetonitrile, methyl acetate, acetone, and tert‐butyl methyl ether. The gravimetrically determined conversions for the free radical polymerizations of ACN/BD after 22 h at 100 °C were in the range of 15% for methyl acetate to 35% for DMAc. The origin of the differences in conversion is attributed to the unequal decomposition behavior of the employed azo initiator 2,2′‐azobis(N‐butyl‐2‐methylpropionamide) ( 1 ) in the solvents under investigation, as determined by ultraviolet–visible (UV–vis) spectroscopy. Relative decomposition of 1 in solution (0.1 mol L^?1) at 100 °C was calculated from the UV–vis spectra for selected solvents. 90% of 1 in DMAc was decomposed after 22 h, 83% in tert‐butanol, 57% in 1,4‐dioxane, 53% in isobutyronitrile, 45% in chlorobenzene, and 21% in toluene. The evolution of molecular weight with conversion using the initiator 1 was in accordance with the theoretically expected values, regardless of the solvent studied. Moreover, the RAFT‐mediated copolymerization of ACN/BD in DMAc with azo initiators 1 , 1‐[(1‐cyano‐1‐methylethyl)azo]formamide ( 2 ) and 1,1′‐azobis(cyclohexanecarbonitrile) ( 3 ) was investigated. A strong deviation from the linear evolution of molecular weight due to a fast decomposition of these initiators – congruent with high primary radical delivery rates – at the selected temperature was observed when using 2 and 3 . The deviation was not observed when using 1 . © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

CH Alkenylations with Alkenyl Acetates,Phosphates, Carbonates,and Carbamates by Cobalt Catalysis at 23 °C

Inexpensive cobalt catalysts with N‐heterocyclic carbene ligands enable direct arene alkenylations with easily accessible alkenyl acetates through regioselective C? H/C? O functionalizations in a stereoconvergent fashion. The versatile cobalt catalyst was broadly applicable and thus also allowed for the efficient conversion of alkenyl phosphates, carbonates, and carbamates at ambient temperature.     

Oxygen Reduction with a Bifunctional Iridium Dihydride Complex

The iridium dihydride [Ir(H)₂(HPNP)]⁺ (PNP=N(CH₂CH₂PtBu₂)₂) reacts with O₂ to give the unusual, square‐planar iridium(III) hydroxide [Ir(OH)(PNP)]⁺ and water. Regeneration of the dihydride with H₂ closes a quasi‐catalytic synthetic oxygen‐reduction reaction (ORR) cycle that can be run several times. Experimental and computational examinations are in agreement with an oxygenation mechanism via rate‐limiting O₂ coordination followed by H‐transfer at a single metal site, facilitated by the cooperating pincer ligand. Hence, the four electrons required for the ORR are stored within the two covalent M? H bonds of a mononuclear metal complex.     

Stabilization of Liquid Foams through the Synergistic Action of Particles and an Immiscible Liquid

Liquid foams are familiar from beer, frothed milk, or bubble baths; foams in general also play important roles in oil recovery, lightweight packaging, and insulation. Here a new class of foams is reported, obtained by frothing a suspension of colloidal particles in the presence of a small amount of an immiscible secondary liquid. A unique aspect of these foams, termed capillary foams, is the particle‐mediated spreading of the minority liquid around the gas bubbles. The resulting mixed particle/liquid coating can stabilize bubbles against coalescence even when the particles alone cannot. The coated bubbles are further immobilized by entrapment in a network of excess particles connected by bridges of the minority liquid. Capillary foams were prepared with a diverse set of particle/liquid combinations to demonstrate the generality of the phenomenon. The observed foam stability correlates with the particle affinity for the liquid interface formed by spreading the minority liquid at the bubble surface.