Several well-controlled polystyrene-block -poly(styrene-alt -maleic anhydride) (PS-b -P(St-alt -MA)) functionalized block copolymers (BCs) and their sodium salt ionomers with different block ratios and molecular weights are synthesized through two-step reversible additionfragmentation chain transfer (RAFT) polymerization. The atomic force microscopy (AFM) images and differential scanning calorimetry (DSC) results reveal that the prepared BCs and their ionomers microphase separate into various nanostructures depending on the block ratios. By taking advantage of the formation of amphiphilic nanostructures via the microphase, the block ionomers serve as nucleation agents to improve the crystallization rate of poly(ethylene terephthalate) (PET). The crystallization behavior of PET upon the addition of block ionomers is investigated isothermally and non-isothermally by DSC. The results show that the block ionomers can effectively accelerate the crystallization rate of PET, which strongly depends on the molecular weights and block ratios interaction energy and the chain stretching and mainly depend on the molecular weight fraction of the constituent blocks and segregation strength denoted by χN , where χ is the Flory-Huggins interaction parameter and N is the polymerization index. [2][3][4][5] For BCs in bulk, self-assembled, periodic structures such as body-centered-cubic-packed (bcc) spheres, hexagonal-close-packed (hcp) cylinders, lamellae, and more complex phases have been theoretically predicted and experimentally observed. [5][6][7] An important application of BCs is as a compatibilizer for immiscible homopolymers. This makes the systems compatible by taking advantage of the amphiphilic characteristics to interact with both phases. For instance, functionalized BCs consisting of polar maleic anhydride (MA) units often serve as commercial compatibilizers in most immiscible polymer blends. The MA-grafted poly(styreneb -(ethylene-co -butylene)-b -styrene) block copolymer (SEBS-MA) improves the dispersion of nanomagnesium hydroxide (nano-MH) in isotactic polypropylene (PP)