Radical polymerization of phenyl acrylate as studied by ESR spectroscopy: concurrence of propagating and mid-chain radicals

Abstract

The electron spin resonance (ESR) spectrum of the propagating radical of phenyl acrylate (PhA) was successfully recorded in benzene as a non-polar solvent. The hyperfine coupling constants for the α and β-protons were evaluated on the basis of the spectra of the propagating radicals of PhA and phenyl acrylate-α-d. The simulated spectrum satisfactorily fits those observed during polymerization, and the spectrum of the poly(PhA) radical obtained at low conversion (<15%) was assigned to the propagating radical. The spectra observed at higher conversions (>15%) indicated the presence of two types of radical species, a propagating radical and a mid-chain radical produced by abstraction of the α-hydrogen of the monomeric unit. The content of branching in the polymers as a result of the formation of a mid-chain radical was found to be 1–3% by 13C NMR spectroscopy. The absolute rate constants for propagation (kp) and termination (kt) of PhA at low conversions were determined based on the quantification of the propagating radical by ESR spectroscopy at 60°C: kp = 3 580 dm3·mol–1·s–1 and kt = 6,8×106 dm3·mol–1·s–1. However, these seemed to be apparent values because the propagating radical is expected to be converted to the mid-chain radical by intra- and intermolecular hydrogen abstraction before the loss of its activity by bimolecular termination. Conversion from the propagating radical to the mid-chain radical followed by reinitiation was estimated to occur more than twenty times during the lifetime of each polymer chain.