Minimized residual monomer is an essential requirement particularly in hygienic, pharmaceutical, and food packaging polymer products such as superabsorbent polymers (SAPs). The present article is the first non-patent report on the study of a highly effective polymerization variable (e.g. initiator) on the residual monomer of SAPs based on partially neutralized acrylic acid. Two persulfate systems, i.e. ammonium persulfate (APS)/sodium metabisulfite (SMBS) and APS/tetramethyl ethylenediamine (TMEDA), were examined to initiate the free-radical polymerization at room temperature. It was shown that chromatographically measured residual monomer and swelling capacity was strongly dependent on the type and concentration of the initiator. A kinetic model was also derived to describe the experimental results. Dissociation rate of initiation system was recognized to be a key factor to obtain a SAP with low residual monomer. It was found that, in aqueous solution polymerization, the effect of a slowly dissociating system such as APS/TMEDA on decreasing the residual monomer was much higher than that of a rapidly dissociating system like APS/SMBS. Under selected conditions, residual monomer could be reduced up to 5327 +/-138 and 1715 +/- 44 ppm for APS/SMBS and APS/TMEDA initiating systems, respectively. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2533-2540, 2009