Poly(N-isopropylacrylamide) (PNIPAM) microgels were prepared through soap-free emulsion polymerization using 2, 2'-azobisobutyronitrile and potassium persulfate as initiator respectively. The thermal response of microgels was researched by measuring the transmittance and the hydrodynamic diameter of the microgels at different temperatures. The result shows that the different structure of the end groups of polymer that come from residues of initiator result in the different thermal response of PNIPAM microgels. The LCST (lower critical solution temperature) of AIBN-initiator microgels is 5 degrees C lower than that of the KPS-initiator microgels, whereas the AIBN-initiated PNIPAM microgels have better thermal response sensitivity. The scanning electron microscope characterization shows that the morphology of AIBN-initiated PNIPAM microgels is more regular than that of KPS-initiated. Furthermore, the T-g of the microgels was measured by differential scanning calorimeter and the result indicates that the end groups influences the T-g of microgels severely. This work demonstrated that the hydrophobic end group coming from initiators can decreases the LCST of PNIPAM microgels and increases the thermal response sensitivity, which providing a newly simple but effective method to regulate the thermal response of PNIPAM microgels. (C) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1164-1171, 2013