The percentage of two types of free radicals occurring within n -butyl acrylate pulsed laser polymerization (PLP) has been measured via ESR spectroscopy. Polymerization in solution of toluene, at monomer concentrations between 0.50 and 2.53 mol L-1 and temperatures between -50 and 70°C, was induced by 351 nm excimer laser pulses applied at a repetition rate of 20 Hz. The ESR spectra obtained at low degrees of monomer conversion may be adequately represented by superimposing a six-component four-line spectrum assigned to secondary propagating radicals (SPRs) and a nine-line spectrum assigned to tertiary midchain radicals produced by backbiting (MCR1s). The percentage contribution to total radical concentration of the latter species, which is produced from an SPR by a 1,5-H shift backbiting reaction, is negligible at -50°C and is close to 80% at 70°C. The transition of an SPR to a MCR1 species invalidates the linear correlation of time t after applying a laser pulse with the size of the growing radical. The observed high concentration of MCR1 species explains the known severe limitations met in acrylate PLP-SEC experiments directed toward measuring the propagation rate of acrylate radicals with chain-end functionality.