Further considerations of the sources of the volatiles from pyrolysis of polystyrene

Formation of the radical precursor to trimer (T) during pyrolysis of polystyrene features a 1,5-hydrogen shift. However because 1,3-shift is so much slower, the sources of the less abundant dimer (D) and tetramer (Te) remain unclear. While we and others have proposed addition of small radicals to olefinic polymer end-groups as a route to oligomer precursor radicals, others recently suggested that such addition is also too slow and proposed a third alternative: 1,7-shift followed serially by 7,3-shift to give the precursor for D. Although considerable evidence suggests that 1,7-shift would be much slower than 1,5-shift, this alternate kinetic model assigned them as comparably rapid. We apply a computational method to predict initial product distributions based on estimated, and empirically varied, propagation rate constants for 1,x-shift, β-scission, hydrogen transfer, and addition radical steps. The addition mechanism successfully predicted the relative amount of D but systematically underestimated Te. This deficiency could be removed by empirical inclusion of a small amount of 1,7-shift, although the other literature evidence still causes this to remain a questionable hypothesis.