Mechanistic analysis and thermochemical kinetic simulation of the pathways for volatile product formation from pyrolysis of polystyrene, especially for the dimer

Abstract

Simulations of the initial distribution of volatiles from pyrolysis of polystyrene were based on propagation rate constants estimated by thermochemical kinetic procedures. The voluminous database exhibits a disturbing lack of consistency with respect to effects of conversion level, temperature, and reactor type. It therefore remains difficult to assign the true primary distribution of the major products, styrene, 2,4-diphenyl-1-butene (“dimer”), 2,4,6-triphenyl-1-hexene (“trimer”), 1,3-diphenylpropane, and toluene, and its dependence on conditions. Probable perturbations by secondary reactions and selective evaporation are considered. The rate constant for 1,3-hydrogen shift appears much too small to accommodate the commonly proposed “back-biting” mechanism for dimer formation. Dimer more likely arises by addition of benzyl radical to olefinic chain-ends, followed by β-scission, although ambiguities remain in assigning rate constants for the addition and β-scission steps. With this modification, the major products can be successfully associated with decay of the sec-benzylic chain-end radical. In contrast, the minimal formation of allylbenzene, 2,4-diphenyl-1-pentene, and 2,4,6-triphenyl-1-heptene suggests a minimal chain-propagating role for the prim chain-end radical. Compared with polyethylene, the much enhanced “unzipping” to form monomer from polystyrene and the more limited depth of “back-biting” into the chain arise from an enthalpy-driven acceleration of β-scission coupled with a kinetically driven deceleration of intramolecular hydrogen transfer. In contrast, the greater “unzipping” of poly(isobutylene) compared with polyethylene is proposed to result from relief of steric strain.