Synthesis of the ABA triblock copolymer nanoparticles of poly(N,N-dimethylacrylamide)-block-polystyrene-block-poly(N,N-dimethylacrylamide) (PDMA-b-PS-b-PDMA) by seeded RAFT polymerization is performed, and the effect of the introduced third poly(N,N-dimethylacrylamide) (PDMA) block on the size and morphology of the PDMA-b-PS-b-PDMA triblock copolymer nanoparticles is investigated. This seeded RAFT polymerization affords the in situ synthesis of the PDMA-b-PS-b-PDMA core-corona nanoparticles, in which the middle solvophobic PS block forms the compacted core, and the first solvophilic PDMA block and the introduced third PDMA block form the solvated complex corona. During the seeded RAFT polymerization, the introduced third PDMA block extends, and the molecular weight of the PDMA-b-PS-b-PDMA triblock copolymer linearly increases with the monomer conversion. It is found that, the size of the PS core in the PDMA-b-PS-b-PDMA triblock copolymer core-corona nanoparticles is almost equal to that in the precursor of the poly(N,N-dimethylacrylamide)-block-polystyrene diblock copolymer core-corona nanoparticles and it keeps constant during the seeded RAFT polymerization, and whereas the introduction of the third PDMA block leads to a crowded complex corona on the PS core. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1777-1784