Macromonomers are synthesized from thermal reactivation of three different precursors, Poly(n-butyl acrylate) P(nBuA), Poly(tert-butyl acrylate) P(tBuA), and Poly(2-ethylhexyl acrylate) P(EHA), synthesized via nitroxide mediated polymerization (NMP). Reactivation of the polymer chains is carried out at 140 C, whereby the polyacrylate macroradicals undergo chain transfer reactions forming midchain radicals, MCR, followed by β-scission reactions leading to unsaturated macromonomers. Soft-ionization mass spectrometry of product samples reveals that in all cases predominantly macromonomers that carry a hydrogen end group on the other chain end are formed, which is also accompanied by small reductions in molecular weight (-200 g·mol-1) and slight increases in polydispersity (+0.2). Furthermore, we demonstrate that macromonomers under these reaction conditions are not only formed through simple backbiting/β-scission via six-membered ring transition structures but also that complex addition-fragmentation equilibria must play a considerable role. As observed before for macromonomers made from activation of atom transfer radical polymerization (ATRP)-made precursors, a size-selective reaction pathway is observed that favors the generation of macromonomers with only odd numbers of monomer units on the backbone, supporting an MCR migration mechanism, which allows MCRs to move along the backbone.