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The dispersion reversible addition-fragmentation chain transfer (RAFT) polymerization of 4-vinylpyridine in toluene in the presence of the polystyrene dithiobenzoate (PS-CTA) macro-RAFT agent with different chain length is discussed. The RAFT polymerization undergoes an initial slow homogeneous polymerization and a subsequent fast heterogeneous one. The RAFT polymerization rate is dependent on the PS-CTA chain length, and short PS-CTA generally leads to fast RAFT polymerization. The dispersion RAFT polymerization induces the self-assembly of the in situ synthesized polystyrene-b-poly(4-vinylpyridine) block copolymer into highly concentrated block copolymer nano-objects. The PS-CTA chain length exerts great influence on the particle nucleation and the size and morphology of the block copolymer nanoobjects. It is found, short PS-CTA leads to fast particle nuclea-tion and tends to produce large-sized vesicles or large-compound micelles, and long PS-CTA leads to formation of smallsized nanospheres. Comparison between the polymerizationinduced self-assembly and self-assembly of block copolymer in the block-selective solvent is made, and the great difference between the two methods is demonstrated. The present study is anticipated to be useful to reveal the chain extension and the particle growth of block copolymer during the RAFT polymerization under dispersion condition. V C 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1573-1584 Self-assembly or micellization of amphiphilic block copolymer in the block-selective solvent is a general method to prepare block copolymer nano-objects. [36][37][38][39][40][41][42][43][44][45][46] By tuning the chain length of the hydrophilic or the hydrophobic block and/or the solvent character such as the solvent polarity, 36-38,41-43 temperature, 44,45 and pH, 45,46 various block copolymer nano-objects including micelles, vesicles, rods and large compound micelles (LCMs) can be prepared in the Additional Supporting Information may be found in the online version of this article.