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

The electron spin resonance (ESR) spectrum of the propagating radical of ph enyl acrylate (PM) was successfully recorded in benzene as a non-polar solv ent. The hyperfine coupling constants for the alpha and beta-protons were e valuated oil the basis of the spectra of the propagating radicals of PhA an d phenyl acrylate-alpha-d. The simulated spectrum satisfactorily fits those observed during polymerization, and the spectrum of the poly(PhA) radicaI 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 radica l produced by abstraction of the or-hydrogen of the monomeric unit. The con tent of branching in the polymers as a result of the formation of a mid-cha in radical was found to be 1-3% by C-13 NMR spectroscopy The absolute rate constants for propagation (k(p)) and termination (k(t)) of PhA at low conve rsions were determined based on the quantification of the propagating radic al by ESR spectroscopy at 60 degrees C: k(p) = 3580 dm(3) . mol(-1) . s(-1) and k(1) = 6.8 x 10(6) dm(3) . mol(-1) . s(-1). However, these seemed to b e apparent values because the propagating radical is expected to be convert ed to the mid-chain radical by intra- and intermolecular hydrogen abstracti on before the loss of its activity by bimolecular termination. Conversion F rom the propagating radical to the mid-chain radical followed ed by reiniti ation was estimated to occur more than twenty times during the lifetime of each polymer chain. First order plots for short polymerization times to est imate the slope at an infinitely low conversion: a, 480 s; b. 600; c, 1800 s; d, 3600 s.