We demonstrate here a novel method to synthesize poly(ethylene terephthalate)-block-poly(tetramethylene oxide) multiblock copolymers (PET-b-PTMO-b-PET)_x by the one pot melt polymerization of cyclic oligo(ethylene terephthalate)s (COETs) using poly(tetramethylene oxide) (PTMO) as a macroinitiator. Two-dimensional and one-dimensional nuclear magnetic resonance (2D and 1D-NMR) techniques, including ¹H–¹³C Heteronuclear Single Quantum Coherence (HSQC) and ¹H–¹H Correlation Spectroscopy (COSY), have been used to characterize and reveal the multiblock copolymer structures and absolute molecular weights. It was found that the COETs were consumed completely within 15 minutes, and the molecular weights of the block copolymers increased linearly with reaction time. Based on the polymerization kinetic studies, a two-stage polymerization mechanism is proposed: the ring-opening polymerization of COETs by a PTMO macroinitiator to PET-b-PTMO-b-PET triblock copolymers at the first stage, followed by the in situ condensation polymerization of triblock copolymers to (PET-b-PTMO-b-PET)_x multiblock copolymers at the second stage. The structures of the multiblock copolymers are further characterized and confirmed by viscometry and gel permeation chromatography (GPC). These multiblock copolymers show improved thermal stability when compared to PTMO homopolymers, and double crystalline properties from PTMO and PET segments, as revealed by thermogravimetric analysis and differential scanning calorimetry, respectively.