Abstract <p>Methacrylate derivatives of methoxypolyethylene glycol (mPEG) (5 kDa) have been synthesized in supercritical carbon dioxide (SC-CO₂) using urethane formation reaction. The use of SC-CO₂ has been shown to allow one to effectively run the reaction in two steps (with the preparation of an isocyanate derivative of mPEG in the first step and a methacrylate derivative in the second step) or combine the two steps into one. We have examined the effect of the reactant ratio and reaction conditions on the composition of the reaction products. Running the reaction in two steps has made it possible to obtain a methacrylate derivative of mPEG with a yield of up to 80%. The preparation of the isocyanate derivative of the polyester and the attachment of methacrylate residues were carried out at different parameters of SC-CO₂: isocyanate groups were introduced under “harsh” conditions (65°C, 20–24 MPa), and methacrylate groups, under “mild” conditions (40°C, 12–13 MPa). The resultant monomers were polymerized to give three-dimensionally cross-linked systems based on both individual modified mPEG and its mixtures with methacrylated polylactide. In the future, such systems can be used as scaffolds for cell immobilization and fabrication of implants.</p>