The radical polymerization of methyl methacrylate (MMA) by the thermal initiator tert-butyl peroxypivalate (tBPPiv) and in the presence of various thiol-containing chain transfer agents (CTAs) has been studied to quantitatively determine the chemical selectivity and fate of initiator radicals. Deuterated MMA-d 5 was used to follow polymerization reactions by 1D and 2D NMR spectroscopy and to determine the identity and quantity of small molecule byproducts, together with the distribution of polymer end-groups. These data have resulted in a detailed understanding of initiator decay pathways, radical selectivity, and of key importance to the design of well-defined polymers for commercial applications, the efficiency of so-called end-group functionalization. In the presence of a thiol-containing small molecule, chain transfer to CTA leads predominantly to initiation by the resulting thiyl radical; however, direct initiation by initiator radicals and a variety of other reactions reduce the total number of CTA-functionalized chain-ends. The approach reported here gives fundamental insights into the fates of radicals in the polymerization of a model methacrylate, increased control over end-groups, and can be extended to a range of commercially relevant polymer compositions. Moreover, these results provide a quantitative framework that can potentially open the door to the design of end-functional polymers by industrially relevant methods, avoiding the need for more timeconsuming and complex reversible-deactivation radical polymerization (RDRP) techniques.