Computer modeling of the formation of radical pairs in amorphous high density polyethylene (HDPE)

Abstract

Hydrogen atom ejection and subsequent radical pair formation have been modeled in a simple atomistic study employing a BIOSYM amorphous polyethylene macrocell. Mean radical pair distances have been obtained for various maximum hydrogen displacement vibrational cone angles (o) in the model. o angles extrapolated from these data, which correspond to experimentally determined mean radical pair distances of Dubinskii et al. (ca. 5.6 A) and Iwasaki et al. (ca. 5.75 A), are found to be close to o angles calculated from hydrogen atom ejection theory. The Dubinskii et al. mean is thought to be the best determination, because the associated model o angle (ca. o = 15°) is the closest to o * angles calculated for excited states of methane. The simple computer model thus supports the mechanism of radical pair formation in solid n-hydrocarbons and polyethylenes. In corroborating the theory for radical pair formation, the theory for polyene crosslinking termination reactions in amorphous polyethylenes irradiated in the presence of acetylene is also supported, because the mechanism requires the prior formation of radical pairs that are separated by distances of the order of those found by Dubinskii et al. The model is transferable to the study of radical-pair reactions in solid n-hydrocarbons irrespective of branching and density variations. A distribution function of radical pair distances from this model, which corresponds to the Dubinskii et al. experimentally determined mean distance, is given for amorphous HDPE.