The use of terpene-based oxides derived from β-elemene is reported in the context of new functional semiaromatic polyester development. In particular, poly(β-elemene monoxide-alt-phthalic anhydride) featuring two double bonds per repeating unit having distinct reactivity is conveniently prepared by iron- and aluminum-mediated ring-opening polymerization (ROCOP). Sequential and selective transformation of each of these double bonds is addressed by epoxidation allowing one to precisely manipulate the degree and nature of the functionality in the polymer backbone while modulating the T_g from 68 to 121 °C. Direct access to cross-linked polyesters is demonstrated by ROCOP of bifunctional β-elemene dioxide and phthalic anhydride furnishing a material with a T_g of 125 °C.