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

The ESR spectrum of the propagating radical of cyclohexyl acrylate (CHA) ov erlapping 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 a lpha- and beta-hydrogens of the propagating radical of CHA were assigned ba sed on the spectrum of the polymer radical of CHA-alpha-d. The polymerizati on of CHA was found to proceed in the presence of two types of radical spec ies with different reactivities, the propagating radical and the mid-chain radical. The content of the midchain radical produced by abstraction of the or-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 C-13 NMR sp ectroscopy 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 lo w conversion, which was obtained by extrapolation, to be 0.65% at 1 mol/l o f CHA. The absolute rate coeffficients for the propagation (k(p)) and termi nation (k(t)) of CHA were determined using kinetic data obtained by FT-near infrared spectroscopy and the concentration of the propagating radical ext rapolated to 0% conversion to minimize the influence of the mid-chain radic als: k(p) = 1224 1/mol s and k(t) = 5.7 x 10(5) 1/mol s at 60 degrees C. Th ese seemed to be apparent values because the propagating radical is repeate dly converted to the less reactive mid-chain radical by the intramolecular hydrogen abstraction during the lifetime of each radical. Slow initiation b y the mid chain radical would result in the propagating radical which propa gates further and accompanies the conversion to the mid-chain radical. (C) 2000 Elsevier Science Ltd. All rights reserved.