CO2 based waterborne polyurethane (CO2-WPU), especially high CO2 content one, suffered from the poor flexibility at low temperature due to high content of rigid carbonate segment. Here a microphase separation idea was proposed by introducing poly(butyl acrylate) (PBA) as soft phase to toughen CO2-WPU via an organic solvent free route, i.e., the polymerizable butyl acrylate acted as a pseudo-solvent during CO2-WPU prepolymer synthesis, it then experienced radical polymerization after formation of CO2-WPU, and finally forming a material with multi-phase structure. Benefiting from the separation of soft PBA phase and hard CO2-WPU phase in micro-size, a 368% increase in elongation at break (epsilon(b)) and nearly 65 times increase in toughness at -10 degrees C was observed when PBA loading increased from blank to 65 wt%. As a comparison, the tensile strength (sigma(m)) and epsilon(b) of the PBA/CO2-WPU blending film at room temperature was only 4.4 MPa and 383% when PBA loading was 50 wt%, while the PBA/CO2-WPU multi-phase film showed sigma(m) of 18.8 MPa and epsilon(b) of 489%, respectively. The toughening effect was attributed to the more uniform micro-phase separation structure caused by molecular level inter-migration between PBA and CO2-WPU during preparation process. (C) 2018 Elsevier Ltd. All rights reserved.