<p>Ring-opening alternating copolymerization (ROAC) of cyclic anhydrides and epoxides is a promising approach for the well-controlled synthesis of polyesters with versatile structures. ROAC typically yields amorphous polyesters owing to challenges in controlling the main-chain stereochemistry, whereas most applied polyesters are crystalline. Isobutylene oxide (IBO), a highly symmetric epoxide, can overcome this issue, thus facilitating the synthesis of crystalline polyesters via ROAC. However, low reactivity and potential side reactions remain major obstacles to achieving an IBO-based ROAC in a controlled manner. In this study, we successfully developed a well-controlled ROAC of cyclic anhydrides and IBO using environmentally benign organobase catalysts. Using <i>t-</i>BuP₂ as a catalyst, the ROAC of phthalic anhydride (PA) and IBO proceeds smoothly, yielding crystalline P(PA-<i>alt</i>-IBO) with narrow molecular weight distribution (<i>Đ</i> ≤ 1.1). This method was extended to other cyclic anhydrides and initiators, enabling the synthesis of polyesters with diverse structures. Additionally, this ROAC system was applied to the synthesis of copolymers, including statistical copolymer of P(PA-<i>alt</i>-IBO) and P(PA-<i>alt</i>-butylene oxide), as well as block copolymer comprising P(PA-<i>alt</i>-IBO) and poly(trimethylene carbonate). This study not only expands the scope of epoxides in ROAC but also provides a straightforward and precise strategy for synthesizing crystalline polyesters and further macromolecular designs.</p>