Free radical polymerization of cyclohexyl acrylate involving interconversion between propagating and mid-chain radicals

Abstract

The ESR spectrum of the propagating radical of cyclohexyl acrylate (CHA) overlapping with a three-line spectrum of a mid-chain radical was recorded in benzene as a non-polar solvent. The hyperfine coupling constants for the α- and β-hydrogens of the propagating radical of CHA were assigned based on the spectrum of the polymer radical of CHA-α-d. The polymerization of CHA was found to proceed in the presence of two types of radical species with different reactivities, the propagating radical and the mid-chain radical. The content of the mid-chain radical produced by abstraction of the α-hydrogen of the monomeric unit of the polymer was found to increase up to 25% with increasing conversion. The content of branching in the polymers arising from the hydrogen abstraction, 1.2–3.5%, determined by 13C NMR spectroscopy was also found to increase with progress of the polymerization. The rate of the intramolecular formation of the mid-chain radical relative to propagation was estimated from the branching content at an infinitely low conversion, which was obtained by extrapolation, to be 0.65% at 1 mol/l of CHA. The absolute rate coefficients for the propagation ( k p) and termination ( k t) of CHA were determined using kinetic data obtained by FT-near infrared spectroscopy and the concentration of the propagating radical extrapolated to 0% conversion to minimize the influence of the mid-chain radicals: k p =1224 l/mol s and k t =5.7×10 5 l/mol s at 60°C. These seemed to be apparent values because the propagating radical is repeatedly converted to the less reactive mid-chain radical by the intramolecular hydrogen abstraction during the lifetime of each radical. Slow initiation by the mid chain radical would result in the propagating radical which propagates further and accompanies the conversion to the mid-chain radical.