A novel technique to synthesize a macroporous poly(N-isopropylacrylamide) hydrogel with ultrarapid swelling-deswelling properties by using dodecyl dimethyl benzyl ammonium bromide (DDBAB) during the polymerization / crosslinking reaction at -8 degrees C was proposed. A hydrophobic initiator dodecyl dimethyl benzyl ammonium persulfate (APS) was proved to be produced in situ by the reaction of DDBAB and the water-soluble initiator APS and a heterogeneous initiation mechanism was proposed. The scanning electron microscope and environmental scanning electron microscope images showed that the pore size of the hydrogel increased with increase of the amount of DDBAB. The macroporous structure of hydrogel induced a large swelling ratio and rapid swelling-deswelling rate, since the free water could diffuse very quickly through the macroporous network. The time required from equilibrium swollen state to equilibrium shrunken state was only about 1 min and vice versa. The oscillating swelling-deswelling measurements showed that this macroporous hydrogel could be used repeatedly. These properties of ultrarapid response and reversibility are advantageous for several technical applications, as a kind of biomaterials, both in medicine and in biotechnology. (c) 2007 Wiley Periodicals, Inc.