This paper reports on the synthesis of polyester-grafted starch (starch-g-PCL) by in situ ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) catalyzed by a metal free organic catalyst (imidazole). This reaction has been carried out in bulk, without solvent. The structure of starch-g-PCL was characterized by IR, NMR, DSC, and SEM. A moisture sorption isotherm of modified starch revealed a more hydrophobic material. A water contact angle measurement also confirmed the hydrophobic character of starch-g-PCL. As expected, this chemical modification reduced the diffusion coefficient of water. The use of an organic catalyst instead of the traditional organometallic catalyst offers an interesting alternative for producing moisture tolerant thermoplastic starch-based materials.