Typical ring-opening copolymerization (ROCOP) of cyclic anhydrides (A) and epoxides (B) catalyzed by most metal complexes underwent strict alternating monomer insertions giving (AB)(n) polyesters. In this contribution, a novel polyester containing (ABB)(n) polymer sequences was prepared via the ROCOP of cyclic anhydrides and cyclohexene oxide (CHO) using a tin(II) alkoxide complex. Quantum calculations revealed that the preference for the (ABB)(n) sequence was enhanced by the energetic preference of the second CHO insertion. The copolymer with (ABB)(n) content as high as 70% was obtained from the ROCOP of succinic anhydride and CHO with an enhanced T-g of 65 degrees C (>20 degrees C higher than the (AB)(n) analogue). The ROCOP of maleic anhydride and CHO also gave a copolymer with high (ABB)(n) content at 48% and improved T-g of 89 degrees C (38 degrees C higher). The improved thermal properties were a result of the more rigid (ABB)(n) polymer backbone compared with conventional alternating (AB)(n) copolymer analogues. In addition, terpolymerization with epsilon-caprolactone (epsilon-CL) was achieved by sequential addition of epsilon-CL affording the block copolymer with the (ABB)(n) backbone.