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 degrees 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 degrees 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. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 174-180, 2012