The unseeded batch-emulsion polymerisation of methyl methacrylate (MMA) was investigated using the chain transfer agent iso-octyl-3-mercaptopropionate (iOMP), which exhibits a low environmental impact. The commercial iOMP is a mixture of over 10 isomers that proved adequate to control the molecular weights of poly(methyl methacrylate) with a decreasing evolution of both the number- and weight-average molecular weights along the reactions. The iOMP also affects the polymerisation kinetics as a consequence of the lower reactivity of the iOMP radicals with respect to the MMA-ended radicals. Experimental results were interpreted with the help of a first principles mathematical model. (c) 2013 Canadian Society for Chemical Engineering