Poly­(3-hydroxyburtyrate) (P3HB) is the simplest but most important member of the large biodegradable polyhydroxyalkanoate family. However, bacterial P3HB, a pure isotactic crystalline material, is brittle, thus limiting its broad applications. Considering that biodegradable poly­(ε-caprolactone) (PCL) and poly­(4-hydroxybutyrate) (P4HB) are much more ductile than P3HB, here we utilize metal-catalyzed stereoselective copolymerization of racemic eight-membered dimethyl diolide (rac-8DLMe) with ε-caprolactone (ε-CL) and γ-butyrolactone (γ-BL) to toughen P3HB through the formation of isotactic P3HB-based synthetic copolyesters. Notably, catalyst’s strong kinetic preference for polymerizing rac-8DLMe over ε-CL enables one-pot copolymerization of the 1/1 comonomer mixture to afford diblock copolymer P3HB-b-PCL with two crystalline domains (T m1 ∼ 55 °C, T m2 ∼ 164 °C). Semicrystalline random copolymers, P­(3HB-co-CL) and P­(3HB-co-4HB), can also be obtained by simply adjusting the comonomer feed ratio and other polymerization conditions. Mechanical testing showed that P3HB-b-PCL is a hard and tough plastic that synergistically combines isotactic P3HB’s high modulus with PCL’s high ductility.