A series of temperature-sensitive microgels based on N-isopropylacrylamide as the main monomer, tert-butyl acrylate (tBA) as the comonomer, and N'N'-methylene-bis(acrylamide) as the crosslinker were synthesized with a modified surfactant-free emulsion polymerization method. The chemical structure and global shape with an excellent monodispersity of the microgels were confirmed by Fourier transform infrared spectroscopy and scanning electron microscopy, respectively. The temperature-sensitive behavior of the microgels was investigated by dynamic light scattering and ultra violet-visible spectrophotometric analysis. The results show that the volume phase-transition temperature of the poly(N-isopropylacrylamide-co-tert-butyl acrylate) [poly(NIPAM-co-tBA)] microgels were tuned over a broad range by the incorporated amount of tBA comonomer and their temperature sensitivity decreased with increasing content of tBA units incorporated into the microgel network. Furthermore, the swelling ratios of the poly (NIPAM-co-tBA) microgels were lowered gradually with increasing tBA unit content within the microgel network. (c) 2006 Wiley Periodicals, Inc.