Poly(methyl methacrylate) (PMMA) nanoparticles with a sensitive CO2-responsive hydrophilic/hydrophobic surface that confers controlled dispersion and aggregation in water were prepared by emulsion polymerization at 50 degrees C under CO2 bubbling using amphiphilic diblock copolymers of 2-dimethylaminoethyl methacrylate (DMAEMA) and N-isopropyl acrylamide (NIPAAm) as an emulsifier. The amphiphilicity of the hydrophobic-hydrophilic diblock copolymer at 50 degrees C was triggered by CO2 bubbling in water and enabled the copolymer to serve as an emulsifier. The resulting PMMA nanoparticles were spherical, approximately 100 nm in diameter and exhibited sensitive CO2/N-2-responsive dispersion/aggregation in water. Using copolymers with a longer PNIPAAm block length as an emulsifier resulted in smaller particles. A higher concentration of copolymer emulsifier led to particles with a stickier surface. Given its simple preparation and reversible CO2-triggered amphiphilic behavior, this newly developed block copolymer emulsifier offers a highly efficient route toward the fabrication of sensitive CO2-stimuli responsive polymeric nanoparticle dispersions. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019