The reactivity of catechols with radicals was applied for the first time to synthesize cross-linked nanostructures, i.e. microgels, without addition of any other cross-linker. Stable microgels with narrow size distribution were successfully obtained via surfactant free emulsion polymerization (SFEP) of acrylamide-type main monomers, namely, acrylamide (AM), N,N-dimethylacrylamide (DMAA), N-vinylpyrrolidone (NVP), N-vinylcaprolactam (VCL), N-isopropylacrylamide (NIPAM), and (dimethylamino)propylmethacrylamide (DMAPM), and vinyl comonomer bearing unprotected catechol in aqueous solution at 70 degrees C. The formation mechanism of cross-linking network structures was mainly attributed to the reactions between unprotected catechol groups of polymer chains and the radicals of propagating chains during SFEP. With catechol chemistry, microgels with fully water-soluble polymers as scaffolds were achieved without using any surfactant stabilizer.