Determination of intramolecular chain transfer and midchain radical propagation rate coefficients for butyl acrylate by pulsed laser polymerization

A novel method to extract individual free-radical polymerization rate coefficients for butyl acrylate intramolecular chain transfer (backbiting), k(bb), and for monomer addition to the resulting midchain radical, k(p)(t), is presented. The approach for measuring k(bb) does not require knowledge of any other rate coefficient. Only the dispersion parameter of SEC broadening has to be determined by fitting measured MWDs or should be available from separate experiments. The method is based upon analysis of the shift in the position of the inflection point of polymer molecular weight distributions produced by a series of pulsed-laser polymerization (PLP) experiments with varying laser pulse repetition rate. The coefficient k(bb) is determined from the onset of the sharp decrease of the apparent propagation rate coefficient (k(p)(app)) with decreasing repetition rate, an approach verified by simulation. With experiments performed between -10 and +30 degrees C, the estimated values are fitted well by an Arrhenius relation with pre-exponential factor A(k(bb)) = (4.84 +/- 0.29) x 10(7) s(-1) and activation energy E-a(k(bb)) = (31.7 +/- 2.5) kJ center dot mol(-1). At low pulse repetition rates, the experimental k(p)(app) values are related to an averaged propagation rate coefficient, k(p)(av), that is dependent on the relative population of chain-end and midchain radicals. Evaluated by comparing simulated and experimental molecular weight distributions, k(p)(av) provides an estimate for k(p)(t). The Arrhenius parameters are: A(k(p)(t)) = (1.52 +/- 0.14) x 10(6) L center dot mol(-1)center dot s(-1) and E-a(k(p)(t)) = (28.9 +/- 3.2) kJ center dot mol(-1).