Ab initio reversible addition fragmentation chain transfer (RAFT) emulsion polymerization of butadiene was investigated by using the amphiphilic poly(acrylic acid(n)-b-styrene(5)) trithiocarbonate as both surfactant and mediator. The neutralization on acrylic acid (AA) units played significant influence on the gelation. When half of the AA units were neutralized, the gelation occurred in the early stage of the polymerization so that the highest accessible molecular weight of polybutadiene was as low as 5 kg mol (1). In the non-neutralized conditions, the gelation was much retarded so that the highest accessible molecular weight was increased up to 23 kg mol(-1). In the non-neutralized conditions, potassium persulfate could not initiate the polymerization. When azobisisobutyronitrile was used as initiator, the polymerization mediated by poly(acrylic acid(27)-b-styrene(5)) trithiocarbonate could proceed much faster than the solution polymerization did. The latex was stable. Before the gel point, molecular weight agreed well with the theoretical prediction while PDI was relatively high due to the branching reaction. The poly(butadiene-b-styrene) core/shell particles could obtained by extending polybutadiene. When the n value in poly (acrylic acid(n)-b-styrene(5)) trithiocarbonate was lower than 20, the coalescence would occur, leading to the formation of some coagulum. On the other hand, when n value was as high as 60, the molecular weight was out of control. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem49: 2980-2989, 2011