Credit: Reprinted with permission from Langmuir 2021, 37, 151-159. Copyright (2021) American Chemical Society Aqueous free-radical precipitation polymerization is one of the most useful methods to prepare the uniformly sized hydrogel microspheres (microgels), and an understanding of the polymerization mechanism is crucial to control the structure or physicochemical properties of microgels. However, the details of the mechanism of precipitation polymerization remain unclear. Thus, first author Yuichiro Nishizawa, Prof. Daisuke Suzuki of the Graduate School of Textile Science & Technology, Shinshu University and Prof. Takayuki Uchihashi of Nagoya University set out to clarify the formation mechanism of microgels during precipitation polymerization by evaluating structural evolution and thermoresponsiveness of developing microgels during the polymerization and by visualization of polymerization directly.