EPR Analysis of n-Butyl Acrylate Radical Polymerization

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 °C and by the 7‐line spectrum of MCRs at +70 °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 allows for estimating the rate coefficient k pt for monomer addition to a mid‐chain radical. For 70 °C, k pt is obtained to be 65.5 L · mol−1 · s−1. Temperature dependence of mid‐chain radical (MCR) mole fraction, xMCR, during polymerization of butyl acrylate (black symbols). The fractions were obtained by analyzing EPR spectra using two different approaches. One assumes a 3‐line MCR conformation in addition to the MCR 7‐line in the BA EPR spectrum (filled black squares). The contribution of the 3‐line conformation to the MCR part of the EPR signal strongly decreases with temperature (filled blue squares).