Abstract: On the basis of a novel mono(phosphinoamide) rare-earth yttrium complex, (2,4,6-Me₃C₆H₂NPPh₂)Y(CH₂C₆H₄NMe₂-o)₂, the copolymerizations of ethylene brassylate (EB) with δ-valerolactone (δ-VL) and ε-caprolactone (ε-CL) were realized, respectively. Copolymers with different compositions were synthesized through a one-pot ring-opening polymerization approach with a monomers/catalyst molar ratio of 200. Such an approach is featured by solvent-free and mild conditions (for 24 h at room temperature). The average sequence lengths and randomness characteristics of the copolymers were analyzed by nuclear magnetic resonance (NMR). Furthermore, the crystallization behaviors and thermal properties were investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and thermogravimetric analysis (TG). Results showed that the yields of the copolymers could reach 80%—90% when the reactions were conducted for 24 h. The composition ratios in products were in good agreement with the feed ratios. The catalyst was efficient for the copolymerization, resulting in the copolyesters with high molecular weights (M_n ＞ 3×10⁴) and narrow molecular weight distributions. The number average length and randomness of copolymers indicated that the copolymers had a block structure and presented a deviation from the random distribution of sequences (R = 0.25—0.36). The resulting block copolymers were able to crystallize over the entire range of composition, and the existence of EB sequences and the block structure had a significant effect on the thermal stability of the copolymers.