Macromonomers are synthesized from AGET activation of poly(n-butyl acrylate), P(nBuA), synthesized via ATRP. Reactivation of the polymer chains is achieved at 140 degrees C using Sn(EH)(2) as reducing agent. Under such conditions, P(nBuA) macroradicals undergo fast chain transfer reactions forming midchain radicals, MCR, followed by beta-scission reactions leading to unsaturated macromonomers. Furthermore, intermolecular transfer-to-polymer reactions lead to considerable amounts of hydrogen-terminated products. Soft ionization mass spectrometry of product samples reveals that under very dilute conditions the macromonomer formation is predominant over the hydrogen transfer reaction of the macroradicals, and macromonomers with overall 80% end-group purity can be achieved, which is, however, accompanied by a reduction in molecular weight from 2100 to 1300 g mol(-1) and an increase in polydispersity from 1.33 to roughly 1.5. Furthermore, we demonstrate not only that macromonomers under these reaction conditions are formed through simple backbiting/beta-scission via six-membered ring transition structures but that also random transfer plays a considerable role. Finally, due to the observation of a size-selective reaction pathway that favors generation of macromonomers with only odd numbers of monomer units on the backbone, a new MCR migration mechanism is postulated, which allows MCRs to move along the backbone.