Double crystalline multiblock copolymers exhibit two well-defined melting temperatures associated with the two phases formed by their constituent blocks. The crystalline superstructure formed in these copolymers is complex and depends on the miscibility of both comonomers. In this work, an innovative series of double crystalline poly(ethylene terephthalate)-mb-poly(oxyhexane) multiblock copolymers were prepared in one pot. Previously synthesized low molecular weight poly(oxyhexane)telechelic diol and poly(ethylene terephthalate) (PET) oligomers were reacted by transesterification using an organic catalyst (1,8-diazabicyclo[5.4.0]-undec-7-ene/benzoic acid), in a solvent-free process. The copolymerization was demonstrated by H-1 NMR and C-13 NMR spectroscopies, and the random distribution of poly(oxyhexane) chains was confirmed. We found that all compositions exhibited double crystallinity, but the crystallization of the copolymers was strongly affected by the PET/poly(oxyhexane) composition. When the PET content in the copolymer decreases, the crystallization and melting temperatures of the poly(oxyhexane) phase decrease as well as its crystallization rate. The poly(oxyhexane) content increases induce similar changes in the PET phase. PET and poly(oxyhexane) chain segments form a one-phase melt according to small-angle X-ray scattering (SAXS). When the material is cooled from the melt, the PET phase crystallizes first (at higher temperatures), forming superstructural (micrometer size spherulites) templates. Upon further cooling, the crystallization of poly(oxyhexane) lamellae occurs, within the interlamellar regions of PET spherulitic templates. Furthermore, during crystallization of the copolymer, the amorphous regions of both components undergo phase separation, as evidenced by the presence of two Tgs. Polarized light optical microscopy/atomic force microscopy studies were performed and demonstrated the presence of microspherulitic morphology in the whole composition range. Considering all results, including temperature-dependent synchrotron SAXS/wide-angle X-ray scattering, we demonstrate the ability of poly(oxyhexane) to crystallize upon cooling within the previously formed PET spherulitic templates. Hence, these copolymers form complex double crystalline spherulitic superstructures which contain two amorphous and two crystalline interlamellar phases.