Enzymatic ring-opening polymerization (eROP) of epsilon-caprolactone (epsilon-CL) and atom-transfer radical polymerization (ATRP) of methyl methacrylate (MMA) were integrated into a one-step chemoenzymatic route to yield poly(epsilon-CL-block-MMA) (PCL-b-PMMA) in supercritical carbon dioxide (scCO2). The interaction between eROP and ATRP during the copolymerization was studied: Cu(I) Br/bpy as ATRP catalyst was compatible with eROP, while Ni(PPh)(3)Br-2 inhibited enzyme activity; under the reaction conditions, both epsilon-CL and PCL improved the solubility of poly(MMA) (PMMA) in the reaction system, and thus the ATRP proceeded successfully. The propagation of PMMA block exhibited living kinetics during the copolymerization, ensuring the molecular weight was well controlled. PCL-b-PMMA copolymers with varying molecular weight were synthesized with good control of the composition of the block copolymer. The one-step chemoenzymatic polymerization provides a very simple and versatile route for the synthesis of block copolymers.