Copolymers containing nonhomopolymerizable gamma-butyrolactone (BL) and c-caprolactone (CL) units of different compositions were synthesized by the ring-opening polymerization method using the SmI2/Sm initiator system. A maximum of 33 mol % of BL was incorporated into polycaprolactone (PCL) backbone. The microstructure of these copolymers was analyzed by one- and two-dimensional NMR spectroscopies. The overlapped methylene proton signals of CL and BL were resolved using 2D H-1-C-13 heteronuclear multibond correlation (HMBC) and homonuclear H-1-H-1 total correlation spectroscopy (TOCSY) techniques. The triad sequence distribution centered at the carbonyl carbon and methylene carbon of CL and BL has been obtained from C-13 NMR spectroscopy. Further, 1D and 2D NMR spectroscopic techniques were used to confirm the change of microstructure from PCL blocks separated by discrete BL units to random copolymer structure having BL-BL units with increase in the mole percent of BL units in the copolymer.