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Radical copolymerization of medium-chain-length poly-3-hydroxyalkanoate (PHA) with methyl acrylate (MA) assisted by high-energy sound wave was investigated. Successful graft copolymerization of poly-3-hydroxyalkanoate –graft– poly-methyl acrylate (PHA-g-PMA) was inferred from the increase in molecular weight of initial neat PHA. The presence of PMA backbone in the graft copolymer was also authenticated by two additional proton peaks at 1.85 and 3.7 ppm. The concentration of PHA, MA and hydrogen peroxide (H2O2) showed significant effects towards the change in the graft copolymer molecular weight. The ideal amount of PHA, MA and H2O2 for graft copolymerization under ultrasonication were determined at 20 g L−1, 0.12 M and 20 μL, respectively. The copolymerizing process between PHA chains and PMA was investigated by analyzing the physicochemical properties of the produced graft copolymers. To this end, dielectric spectroscopy (DS) was used in corroboration to conventional thermal analyses, i.e., differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). In addition, small-wide angle X-ray spectroscopy (SWAXS), field emission scanning electron microscopy (FESEM) and contact angle (CA) analysis were used to characterize the film morphology of the graft copolymers. From the results, it is suggested that PHA chains are subjected to pre-fragmentation prior to random grafting with PMA chains. [ABSTRACT FROM AUTHOR]