A well-controlled atom transfer polymerization of methyl acrylate (MA) was realized by mechanical mediation (mechanoATRP) in dimethyl sulfoxide (DMSO, 50% v/v). High conversions of typically over 90% were achieved. The resulting polymers had well-controlled molecular weights and very low dispersities (D = 1.03-1.09). No polymerization of MA was observed under various conditions from bulk up to 33.3% DMSO (v/v) solution. It was found that adding an equivalent volume of DMSO with respect to MA activated the polymerization. This finding suggested that DMSO played a crucial role in the mechanoATRP of MA. DMSO not only improved the solubility of CuBr(2 )complex but also facilitated an electron transfer process in the mechanical reduction of CuBr2. For a proof of the concept, a DMSO analogue acrylate, 2-(methylsulfinyl)ethyl acrylate (MSEA), was also polymerized. In addition, the high chain-end functionality of the polymers collected at similar to 95% conversion was confirmed by H-1 NMR, MALDI-ToF-MS, and in-situ chain extension experiments. The extended chain polymers were characterized and found to have predicted molecular weights and low dispersities (D = 1.06). Chain extension in the presence of residual oxygen still yielded a well-controlled molecular weight but a slightly higher dispersity of 1.11. This work provided an in-depth insight into the mechanoATRP and demonstrated its good potential in producing well-defined polymers at high conversions.