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The effect of cadmium chloride (CdCl₂) on the phase behavior of polystyrene-block-poly(4-vinylpyridine) copolymer (S4VP) was investigated by using rheometry, small-angle X-ray scattering, and transmission electron microscopy. For this purpose, symmetric S4VPs with various molecular weights were prepared by anionic polymerization. We found that with the addition of CdCl₂ the order-to-disorder transition of S4VP was significantly increased because of the intermolecular coordination connecting different P4VP block chains. We also studied the change of the domain spacing (D) upon addition of CdCl₂ as well as gelation behavior. At a relatively low content of CdCl₂ (γ < 6, where γ is the number of coordinated CdCl₂ per chain), D remains unchanged. However, when the amount of CdCl₂ increases further, D begins to decrease due to many coordinations of CdCl₂ that strongly perturb block copolymer …