Reversible addition-fragmentation chain transfer (RAFT) emulsion polymerization is becoming an important technique to synthesize the latex of block copolymers. A previous study showed that in the synthesis of polystyrene-b-poly(butyl acrylate)-b-polystyrene triblock copolymer via RAFT emulsion polymerization using amphiphilic oligo(acrylic acid-styrene) macroRAFT as surfactant and mediator, the molecular weight distribution could be much broadened to PDI higher than 2. In this study, an in-depth investigation was performed to decrease PDI. It was found that long-chain branches could be formed in the synthesis of triblock block copolymer, leading to the appearance of a higher molecular weight shoulder in the GPC curve of the final product. The lower neutralization degree of acrylic acid (AA) units on the macroRAFT and shorter AA chains would help to suppress the formation the long-chain branches, leading to PDI around 1.5. It is evidenced that the successful suppression is due to the promotion of radical entry as a result of decreased interfacial transport impedance. It is also evidenced that the presence of styrene during the polymerization of butyl acrylate could promote the formation of long chain branches. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1464-1473