Single-pulse pulsed laser polymerization-electron paramagnetic resonance investigations into the termination kinetics of n-butyl acrylate macromonomers

The termination of model mid-chain radicals (MCRs), which mimic radicals that occur in acrylate polymerization over a broad range of reaction conditions, has been studied by single-pulse pulsed laser polymerization (SP-PLP) in conjunction with electron paramagnetic resonance spectroscopy. The model radicals were generated by initiator-fragment addition to acrylic macromonomers that were preformed prior to the kinetic experiments, thus enabling separation of termination from the propagation reaction, for these model radicals propagate sparingly, if at all, on the timescale of SP-PLP experiments. Termination rate coefficients of the MCRs were determined in the temperature range of 0-60°C in acetonitrile and butyl propionate solution as well as in bulk macromonomer over the range of 0-100 °C. Termination rate coefficients slightly below those of the corresponding secondary radicals were deduced, demonstrating the relatively high termination activity of this species, even when undergoing MCR-MCR termination. For chain length of 10, a reduction by a factor of 6 is observed. Unusually high activation energies were found for the termination rate coefficient in these systems, with 35 kJ mol ^-1 being determined for bulk macromonomer.