Via electron paramagnetic resonance (EPR) spectroscopy, concentrations of secondary propagating radicals (SPRs) and tertiary mid-chain radicals (MCRs) in n-butyl acrylate solution polymerization were measured. The EPR spectrum is dominated by the 4-line spectrum of SPRs at -50 degrees C and by the 7-line spectrum of MCRs at +70 degrees C. At intermediate temperatures, a third spectral component is seen, which is assigned to an MCR species with restricted rotational mobility. The MCR components are produced by 1,5-hydrogen shift (backbiting) of SPRs. The measured ratio of MCRs to SPRs U allows for estimating the rate coefficient k(p)(t) for monomer addition to a mid-chain radical. For 70 degrees C, k(p)(t) is obtained to be 65.5 L . mol(-1) . s(-1).